Chapter4 The Tissue Level of Organization
The body contains at least 200 distinct cell types. These cells contain essentially the same internal structures, yet they vary enormously in shape and function. The different types of cells are not randomly distributed throughout the body; rather, they occur in organized layers, a level of organization referred to as tissue. The variety in shape reflects the many different roles that cells fulfil in your body. The human body starts as a single cell at fertilization. As this fertilized egg divides, it gives rise to trillions of cells, each built from the same blueprint but organizing into tissues and becoming irreversibly committed to a developmental pathway.
4.1 Types of Tissues
The term tissue is used to describe a group of cells found together in the body. The cells within a tissue share a common embryonic origin. Microscopic observation reveals that the cells in a tissue share morphological features and are arranged in an orderly pattern that achieves the tissue’s functions. From the evolutionary perspective, tissues appear in more complex organisms. For example, multicellular protists, ancient eukaryotes, do not have cells organized into tissues.
Although there are many types of cells in the human body, they are organized into four broad categories of tissues: epithelial, connective, muscle, and nervous. Each of these categories is characterized by specific functions that contribute to the overall health and maintenance of the body. A disruption of the structure is a sign of injury or disease. Such changes can be detected through histology, the microscopic study of tissue appearance, organization, and function.
The Four Types of Tissues
Epithelial tissue, also referred to as epithelium, refers to the sheets of cells that cover exterior surfaces of the body, line internal cavities and passageways, and form certain glands. Connective tissue, as its name implies, binds the cells and organs of the body together and functions in the protection, support, and integration of all parts of the body. Muscle tissue is excitable, responding to stimulation and contracting to provide movement, and occurs in three major types: skeletal (voluntary) muscle, smooth muscle, and cardiac muscle in the heart. Nervous tissue is also excitable, allowing the propagation of electrochemical signals in the form of nerve impulses that communicate between different regions of the body (Figure 4.1).
The next level of organization is the organ, where several types of tissues come together to form a working unit. Just as knowing the structure and function of cells helps you in your study of tissues, knowledge of tissues will help you understand how organs function. The epithelial and connective tissues are discussed in detail in this chapter. Muscle and nervous tissues will be discussed only briefly in this chapter.
Figure 4.1 Four Types of Tissue: Body. The four types of tissues are exemplified in nervous tissue, stratified squamous epithelial tissue, cardiac muscle tissue, and connective tissue. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)
Embryonic Origin of Tissues
Each germ layer is identified by its relative position: ectoderm (ecto- = “outer”), mesoderm (meso- = “middle”), and endoderm (endo- = “inner”). The epithelial tissue originates in all three layers, whereas nervous tissue derives primarily from the ectoderm and muscle tissue from the mesoderm.
Tissue Membranes
A tissue membrane is a thin layer or sheet of cells that covers the outside of the body (e.g., skin), the organs (e.g., pericardium), internal passageways that lead to the exterior of the body (e.g., mucosa of the stomach), and the lining of the movable joint cavities. There are two basic types of tissue membranes: connective tissue and epithelial membranes (Figure 4.2).
Connective Tissue Membranes
The connective tissue membrane is formed solely from connective tissue. These membranes encapsulate organs, such as the kidneys, and line our movable joints. A synovial membrane is a type of connective tissue membrane that lines the cavity of a freely movable joint.
For example, synovial membranes surround the joints of the shoulder, elbow, and knee. Fibroblasts in the inner layer of the synovial membrane release hyaluronan into the joint cavity. The hyaluronan effectively traps available water to form the synovial fluid, a natural lubricant that enables the bones of a joint to move freely against one another without much friction. This synovial fluid readily exchanges water and nutrients with blood, as do all body fluids.
Epithelial Membranes
The epithelial membrane is composed of epithelium attached to a layer of connective tissue, for example, your skin. The mucous membrane (mucosa) is also a composite of connective and epithelial tissues. These epithelial membranes line the body cavities and hollow passageways that open to the external environment, including the digestive, respiratory, excretory, and reproductive tracts. Mucus, produced by the epithelial exocrine glands, covers the epithelial layer. The underlying connective tissue, called the lamina propria, helps support the fragile epithelial layer.
A serous membrane is an epithelial membrane that lines the cavities of the body that do not open to the outside. Serous membranes cover the organs located within those cavities. They are essentially membranous bags with mesothelium lining the inside and connective tissue on the outside. Serous fluid secreted by the cells of the thin squamous mesothelium lubricates the membrane and reduces abrasion and friction between organs. Serous membranes are identified according to location. Three serous membranes line the thoracic cavity: the two pleura that cover the lungs and the pericardium that covers the heart. A fourth, the peritoneum, is the serous membrane in the abdominal cavity that covers abdominal organs and forms double sheets of mesenteries that suspend many of the digestive organs.
Figure 4.2 Tissue Membranes. The two broad categories of tissue membranes in the body are (1) connective tissue membranes, which include synovial membranes, and (2) epithelial membranes, which include mucous membranes, serous membranes, and the cutaneous membrane, in other words, the skin.
The skin is an epithelial membrane also called the cutaneous membrane. It is a stratified squamous epithelial membrane resting on top of connective tissue. The apical surface of this membrane is exposed to the external environment and is covered with dead, keratinized cells that help protect the body from desiccation and pathogens.
4.2 Epithelial Tissue
Most epithelial tissues are essentially large sheets of tightly packed cells covering all the surfaces of the body exposed to the outside world and lining the outsides of organs. Epithelium also forms much of the glandular tissue of the body. Skin is not the only area of the body exposed to the outside. Other areas include the airways, the digestive tract, and the urinary and reproductive systems, all of which are lined by an epithelium. Hollow organs and body cavities that do not connect to the exterior of the body, which include blood vessels and serous membranes, are lined by endothelium (plural = endothelia), which is a type of epithelium.
Epithelial cells derive from all three major embryonic layers. The epithelia lining the skin, parts of the mouth and nose, and the anus develop from the ectoderm. Cells lining the airways and most of the digestive system originate in the endoderm. The epithelium that lines vessels in the lymphatic and cardiovascular system derives from the mesoderm and is called an endothelium.
All epithelia share some important structural and functional features. This tissue is highly cellular, with little or no extracellular material present between cells. Adjoining cells form a specialized intercellular connection between their cell membranes called a cell junction. The epithelial cells exhibit polarity with differences in structure and function between the exposed or apical-facing surface of the cell and the basal surface close to the underlying body structures. The basal lamina, a mixture of glycoproteins and collagen, provides an attachment site for epithelium, separating it from the underlying connective tissue. The basal lamina attaches to a reticular lamina, which is secreted by the underlying connective tissue, forming a basement membrane that helps hold it all together.
Epithelial tissues are nearly completely avascular—no blood vessels cross the basement membrane to enter the tissue, and nutrients must come by diffusion or absorption from underlying tissues or the surface. Many epithelial tissues are capable of rapidly replacing damaged and dead cells. Sloughing off damaged or dead cells is a characteristic of surface epithelium and allows our airways and digestive tracts to rapidly replace damaged cells with new cells.
Generalized Functions of Epithelial Tissue
Epithelial tissues provide the body’s first line of protection from physical, chemical, and biological wear and tear. The cells of an epithelium act as gatekeepers of the body by controlling permeability and allowing selective transfer of materials across a physical barrier. All substances that enter the body must cross an epithelium. Some epithelia often include structural features that allow the selective transport of molecules and ions across their cell membranes. Many epithelial cells are capable of secretion and release mucous and specific chemical compounds onto their apical surfaces.
Examples: The epithelium of the small intestine releases digestive enzymes. Cells lining the respiratory tract secrete mucous that traps incoming microorganisms and particles. Glandular epithelium contains many secretory cells.
The Epithelial Cell
Epithelial cells are typically characterized by the polarized distribution of organelles and membrane-bound proteins between their basal and apical surfaces. Particular structures found in some epithelial cells are adaptations to specific functions. Certain organelles are segregated to the basal sides, whereas other organelles and extensions, such as cilia, when present, are on the apical surface.
Cilia are microscopic extensions of the apical cell membrane that are supported by microtubules. They beat in unison and move fluids as well as trapped particles across their surface. Ciliated epithelium lines the ventricles of the brain, where it helps circulate the cerebrospinal fluid. The ciliated epithelium of your airway forms a mucociliary escalator that sweeps particles of dust and pathogens trapped in the secreted mucous toward the throat. It is called an escalator because it continuously pushes mucous with trapped particles upward. In contrast, nasal cilia sweep mucous down toward your throat. In both cases, the transported materials are usually swallowed and end up in the acidic environment of your stomach.
Cell-to-Cell Junctions
Cells of epithelia are closely connected and are not separated by intracellular material. Three basic types of connections allow varying degrees of interaction between the cells: tight junctions, anchoring junctions, and gap junctions (Figure 4.3).
Tight junctions occur when there is no extracellular space between two adjacent epithelial cells and the movement of some substances between cells is blocked. Hydrophilic ions and molecules cannot cross through this barrier. Note that hydrophobic molecules freely move through the cells themselves and are therefore not blocked by tight junctions. This enables the epithelia to act as selective barriers. An anchoring junction includes several types of cell junctions that help stabilize epithelial tissues. Anchoring junctions are common on the lateral and basal surfaces of cells, where they provide strong and flexible connections. There are three types of anchoring junctions: desmosomes, hemidesmosomes, and adherens. Desmosomes occur in patches on the membranes of cells. The patches are structural proteins on the inner surface of the cell’s membrane. The adhesion molecule is embedded in these patches and projects through the cell membrane to link with the same molecules of adjacent cells. These connections are especially important in holding cells together. Hemidesmosomes, which look like half a desmosome, link cells to the extracellular matrix (for example, the basal lamina). While similar in appearance to desmosomes, they include different types of adhesion proteins. Adherens junctions use adhesion proteins found in either desmosomes or hemidesmosomes, depending on whether they are linking to other cells or the matrix.
The junctions are characterized by the presence of the contractile protein actin, located on the cytoplasmic surface of the cell membrane. The actin can connect isolated patches or form a beltlike structure inside the cell. These junctions influence the shape and folding of the epithelial tissue.
In contrast with the tight and anchoring junctions, a gap junction forms an intercellular passageway between the membranes of adjacent cells to facilitate the movement of small molecules and ions between the cytoplasm of adjacent cells. These junctions allow electrical and metabolic coupling of adjacent cells, which coordinates function in large groups of cells.
Figure 4.3 Types of Cell Junctions. The three basic types of cell-to-cell junctions are tight junctions, gap junctions, and anchoring junctions.
Classification of Epithelial Tissues
Epithelial tissues are classified according to the shape of the cells and the number of cell layers formed (Figure 4.4). Cell shapes can be squamous (flattened and thin), cuboidal (boxy, as wide as it is tall), or columnar (rectangular, taller than it is wide). Similarly, the number of cell layers in the tissue can be one, which is a simple epithelium (where every cell rests on the basal lamina), or more than one, which is a stratified epithelium (only the basal layer of cells rests on the basal lamina). Pseudostratified describes tissue with a single layer of irregularly shaped cells that gives the appearance of more than one layer. Transitional describes a form of specialized stratified epithelium in which the shape of the cells can vary.
Figure 4.4 Cells of Epithelial Tissue. Simple epithelial tissue is organized as a single layer of cells, and stratified epithelial tissue is formed by several layers of cells.
Simple Epithelium
The shape of the cells in the single cell layer of simple epithelium reflects the functioning of those cells. The cells in simple squamous epithelium have the appearance of thin scales. Squamous cell nuclei tend to be flat, horizontal, and elliptical, mirroring the form of the cell. Endothelium is the epithelial tissue that lines vessels of the lymphatic and cardiovascular system, and it is made up of a single layer of squamous cells. Simple squamous epithelium, because of the thinness of the cell, is present where the rapid passage of chemical compounds is observed.
Example: The alveoli of the lungs, where gases diffuse; the segments of kidney tubules; and the linings of capillaries are also made of simple squamous epithelial tissue.
The mesothelium is a simple squamous epithelium that forms the surface layer of the serous membrane that lines body cavities and internal organs. Its primary function is to provide a smooth and protective surface. Mesothelial cells are squamous epithelial cells that secrete a fluid that lubricates the mesothelium.
In simple cuboidal epithelium, the nucleus of the boxlike cells appears round and is generally located near the centre of the cell. These epithelia are active in the secretion and absorption of molecules.
Example: Simple cuboidal epithelia are observed in the lining of the kidney tubules and in the ducts of glands.
In simple columnar epithelium, the nucleus of the tall columnlike cells tends to be elongated and located at the basal end of the cells. Like cuboidal epithelia, this epithelium is active in the absorption and secretion of molecules.
Example: Simple columnar epithelium forms the lining of some sections of the digestive system and parts of the female reproductive tract.
Ciliated columnar epithelium is composed of simple columnar epithelial cells with cilia on their apical surfaces. These epithelial cells are found in the lining of the fallopian tubes and parts of the respiratory system, where the beating of the cilia helps remove particulate matter.
Pseudostratified columnar epithelium is a type of epithelium that appears to be stratified but instead consists of a single layer of irregularly shaped and differently sized columnar cells. In pseudostratified epithelium, nuclei of neighbouring cells appear at different levels rather than clustered in the basal end. The arrangement gives the appearance of stratification, but in fact, all the cells are in contact with the basal lamina, although some do not reach the apical surface.
Example: Pseudostratified columnar epithelium is found in the respiratory tract, where some of these cells have cilia.
Both simple and pseudostratified columnar epithelia are heterogeneous epithelia because they include additional types of cells interspersed among the epithelial cells. A goblet cell is a mucous-secreting unicellular “gland” interspersed between the columnar epithelial cells of mucous membranes (Figure 4.5).
Figure 4.5 Goblet Cell. (a) In the lining of the small intestine, columnar epithelium cells are interspersed with goblet cells.
Stratified Epithelium
A stratified epithelium consists of several stacked layers of cells. This epithelium protects against physical and chemical wear and tear. The stratified epithelium is named by the shape of the most apical layer of cells, closest to the free space. Stratified squamous epithelium is the most common type of stratified epithelium in the human body. The apical cells are squamous, whereas the basal layer contains either columnar or cuboidal cells. The top layer may be covered with dead cells filled with keratin. Mammalian skin is an example of this dry, keratinized stratified squamous epithelium. The lining of the mouth cavity is an example of an unkeratinized stratified squamous epithelium. Stratified cuboidal epithelium and stratified columnar epithelium can also be found in certain glands and ducts but are uncommon in the human body.
Another kind of stratified epithelium is transitional epithelium, so called because of the gradual changes in the shapes of the apical cells as the bladder fills with urine. It is found only in the urinary system, specifically the ureters and urinary bladder. When the bladder is empty, this epithelium is convoluted and has cuboidal apical cells with convex, umbrella-shaped apical surfaces. As the bladder fills with urine, this epithelium loses its convolutions, and the apical cells transition from cuboidal to squamous. It appears thicker and more multilayered when the bladder is empty but more stretched out and less stratified when the bladder is full and distended.
Figure 4.6 summarizes the different categories of epithelial cell tissue cells.
Glandular Epithelium
A gland is a structure made up of one or more cells modified to synthesize and secrete chemical substances. Most glands consist of groups of epithelial cells. A gland can be classified as an endocrine gland, a ductless gland that releases secretions directly into surrounding interstitial fluid and blood, or an exocrine gland, whose secretions leave through a duct that opens directly, or indirectly, to the external environment. These secretions move into the lumens of hollow organs or onto epithelial surfaces.
Endocrine Glands
The secretions of endocrine glands are called hormones. Hormones are released into the interstitial fluid, diffused into the bloodstream, and delivered to targets—in other words, cells that have receptors to bind the hormones. The endocrine system is part of a major regulatory system coordinating the regulation and integration of body responses.
Figure 4.6 Summary of Epithelial Tissue Cells.
Example: The anterior pituitary, thymus, adrenal cortex, and gonads are all endocrine glands.
Exocrine Glands
Exocrine glands release their contents through a duct into the lumen of hollow organs or onto epithelial surfaces.
Example: Mucous, sweat, saliva, and breast milk are all examples of secretions from exocrine glands. They are all discharged through tubular ducts. Secretions into the lumen of the gastrointestinal tract, technically outside of the body, are of the exocrine category.
Glandular Structure
Exocrine glands are classified as either unicellular or multicellular. The unicellular glands are scattered single cells, such as goblet cells, found in the mucous membranes of the small and large intestine. The multicellular exocrine glands known as serous glands develop from simple epithelium to form a secretory surface that secretes directly into an inner cavity.
These glands line the internal cavities of the abdomen and chest and release their secretions directly into the cavities. Other multicellular exocrine glands release their contents through a tubular duct. The duct is single in a simple gland but in compound glands is divided into one or more branches. In tubular glands, the ducts can be straight or coiled, whereas tubes that form pockets are alveolar (acinar), such as the exocrine portion of the pancreas. Combinations of tubes and pockets are known as tubuloalveolar (tubuloacinar) compound glands. In a branched gland, a duct is connected to more than one secretory group of cells.
Methods and Types of Secretion
Exocrine glands can be classified by their mode of secretion and the nature of the substances released as well as by the structure of the glands and shape of the ducts (Figure 4.7). Merocrine secretion is the most common type of exocrine secretion. The secretions are enclosed in vesicles that move to the apical surface of the cell, where the contents are released by exocytosis.
Example: Watery mucous contains the glycoprotein mucin, a lubricant that offers some pathogen protection, and is a merocrine secretion. The eccrine glands that produce and secrete sweat are another example.
Apocrine secretion accumulates near the apical portion of the cell. That portion of the cell and its secretory contents pinch off from the cell and are released. Apocrine sweat glands in the axillary and genital areas release fatty secretions that local bacteria break down; this causes body odour. Both merocrine and apocrine glands continue to produce and secrete their contents with little damage caused to the cell because the nucleus and Golgi regions remain intact after secretion.
Figure 4.7 Modes of Glandular Secretion. (a) In merocrine secretion, the cell remains intact. (b) In apocrine secretion, the apical portion of the cell is released as well. (c) In holocrine secretion, the cell is destroyed as it releases its product, and the cell itself becomes part of the secretion.
In contrast, the process of holocrine secretion involves the rupture and destruction of the entire gland cell. The cell accumulates its secretory products and releases them only when it bursts. New gland cells differentiate from cells in the surrounding tissue to replace those lost by secretion. The sebaceous glands that produce the oils on the skin and hair are holocrine glands/cells (Figure 4.8).
Glands are also named after the products they produce. The serous gland produces watery, blood plasma–like secretions rich in enzymes such as alpha amylase, whereas the mucous gland releases watery to viscous products rich in the glycoprotein mucin. Both serous and mucous glands are common in the salivary glands of the mouth.
Figure 4.8 Sebaceous Glands. These glands secrete oils that lubricate and protect the skin. They are holocrine glands and are destroyed after releasing their contents. New glandular cells form to replace the cells that are lost. LM × 400. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Mixed exocrine glands contain both serous and mucous glands and release both types of secretions.
4.3 Connective Tissue Supports and Protects
As may be obvious from its name, one of the major functions of connective tissue is to connect tissues and organs. Unlike epithelial tissue, which is composed of cells closely packed with little or no extracellular space in between, connective tissue cells are dispersed in a matrix. The matrix usually includes a large amount of extracellular material produced by the connective tissue cells that are embedded within it. The matrix plays a major role in the functioning of this tissue. The major component of the matrix is a ground substance often crisscrossed by protein fibres. This ground substance is usually a fluid, but it can also be mineralized and solid, as in bones.
Connective tissues come in a vast variety of forms, yet they typically have in common three characteristic components: cells, large amounts of amorphous ground substance, and protein fibres. The amount and structure of each component correlate with the function of the tissue, from the rigid ground substance in bones supporting the body to the inclusion of specialized cells—for example, a phagocytic cell that engulfs pathogens and also rids tissue of cellular debris.
Functions of Connective Tissues
Connective tissues perform many functions in the body, but most importantly, they support and connect other tissues, from the connective tissue sheath that surrounds muscle cells to the tendons that attach muscles to bones and to the skeleton that supports the positions of the body. Protection is another major function of connective tissue, in the form of fibrous capsules and bones that protect delicate organs and, of course, the skeletal system.
Specialized cells in connective tissue defend the body from microorganisms that enter the body. Transport of fluids, nutrients, waste, and chemical messengers is ensured by specialized fluid connective tissues, such as blood and lymph. Adipose cells store surplus energy in the form of fat and contribute to the thermal insulation of the body.
Classification of Connective Tissues
The three broad categories of connective tissue are classified according to the characteristics of their ground substance and the types of fibres found within the matrix (Table 4.1).
Connective tissue proper includes loose connective tissue and dense connective tissue. Both tissues have a variety of cell types and protein fibres suspended in a viscous ground substance. Dense connective tissue is reinforced by bundles of fibres that provide tensile strength, elasticity, and protection. In loose connective tissue, the fibres are loosely organized, leaving large spaces in between. Supportive connective tissue—bone and cartilage—provides structure and strength to the body and protects soft tissues. A few distinct cell types and densely packed fibres in a matrix characterize these tissues. In bone, the matrix is rigid and described as calcified because of the deposited calcium salts. In fluid connective tissue—in other words, lymph and blood—various specialized cells circulate in a watery fluid containing salts, nutrients, and dissolved proteins.
Connective tissue proper | Supportive connective tissue | Fluid connective tissue |
|---|---|---|
Loose connective tissue • Areolar • Adipose • Reticular Dense connective tissue • Dense regular • Elastic • Dense irregular | Cartilage • Hyaline • Fibrocartilage • Elastic bone | Blood Lymph |
Connective Tissue Proper
Fibroblasts are present in all connective tissue proper (Figure 4.9). Fibrocytes, adipocytes, and mesenchymal cells are fixed cells, which means they remain within the connective tissue. Other cells move into and out of the connective tissue in response to chemical signals. Macrophages, mast cells, lymphocytes, plasma cells, and phagocytic cells are found in connective tissue proper but are actually part of the immune system, protecting the body.
Figure 4.9 Connective Tissue Proper. Fibroblasts produce this fibrous tissue. Connective tissue proper includes the fixed cells fibrocytes, adipocytes, and mesenchymal cells. LM × 400. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Cell Types
The most abundant cell in connective tissue proper is the fibroblast. Polysaccharides and proteins secreted by fibroblasts combine with extracellular fluids to produce a viscous ground substance that, with embedded fibrous proteins, forms the extracellular matrix. As you might expect, a fibrocyte, a less active form of a fibroblast, is the second-most common cell type in connective tissue proper.
Adipocytes are cells that store lipids as droplets that fill most of the cytoplasm. There are two basic types of adipocytes: white and brown. The brown adipocytes store lipids as many droplets and have high metabolic activity. In contrast, white fat adipocytes store lipids as a single large drop and are metabolically less active. Their effectiveness at storing large amounts of fat is witnessed in obese individuals. The number and type of adipocytes depend on the tissue and location and vary among individuals in the population.
The mesenchymal cell is a multipotent adult stem cell. These cells can differentiate into any type of connective tissue cell needed for the repair and healing of damaged tissue.
Connective Tissue Fibres and Ground Substance
Three main types of fibres are secreted by fibroblasts: collagen fibres, elastic fibres, and reticular fibres. Collagen fibre is made from fibrous protein subunits linked together to form a long and straight fibre. Collagen fibres, while flexible, have great tensile strength, resist stretching, and give ligaments and tendons their characteristic resilience and strength. These fibres hold connective tissues together, even during the movement of the body.
Elastic fibre contains the protein elastin along with lesser amounts of other proteins and glycoproteins. The main property of elastin is that after being stretched or compressed, it will return to its original shape. Elastic fibres are prominent in elastic tissues found in skin and the elastic ligaments of the vertebral column.
Reticular fibre is also formed from the same protein subunits as collagen fibres; however, these fibres remain narrow and are arrayed in a branching network. They are found throughout the body but are most abundant in the reticular tissue of soft organs, such as the liver and spleen, where they anchor and provide structural support to the parenchyma (the functional cells, blood vessels, and nerves of the organ).
All these fibre types are embedded in ground substance. Secreted by fibroblasts, ground substance is made of polysaccharides, specifically hyaluronic acid, and proteins. These combine to form a proteoglycan with a protein core and polysaccharide branches. The proteoglycan attracts and traps available moisture, forming the clear, viscous, colourless matrix known as ground substance.
Loose Connective Tissue
Loose connective tissue is found between many organs, where it acts to both absorb shock and bind tissues together. It allows water, salts, and various nutrients to diffuse through to adjacent or embedded cells and tissues.
Adipose tissue consists mostly of fat storage cells, with little extracellular matrix (Figure 4.10). A large number of capillaries allows rapid storage and mobilization of lipid molecules. White adipose tissue is most abundant. It can appear yellow and owes its colour to carotene and related pigments from plant food. White fat contributes mostly to lipid storage and can serve as insulation from cold temperatures and mechanical injuries. White adipose tissue can be found protecting the kidneys and cushioning the back of the eye. Brown adipose tissue is more common in infants, hence the term baby fat. In adults, there is a reduced amount of brown fat, and it is found mainly in the neck and clavicular regions of the body. Brown adipose tissue is thermogenic, meaning that as it breaks down fats, it releases metabolic heat rather than producing adenosine triphosphate (ATP), a key molecule used in metabolism.
Figure 4.10 Adipose Tissue. This is a loose connective tissue that consists of fat cells with a little extracellular matrix. It stores fat for energy and provides insulation. LM × 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Areolar tissue shows little specialization. It contains all the cell types and fibres previously described and is distributed in a random, weblike fashion. It fills the spaces between muscle fibres, surrounds blood and lymph vessels, and supports organs in the abdominal cavity. Areolar tissue underlies most epithelia and represents the connective tissue component of epithelial membranes, which are described further in a later section.
Reticular tissue is a meshlike, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver (Figure 4.11). Reticular cells produce the reticular fibres that form the network onto which other cells attach. It derives its name from the Latin reticulus, which means “little net.”
Figure 4.11 Reticular Tissue. This is a loose connective tissue made up of a network of reticular fibres that provides a supportive framework for soft organs. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Dense Connective Tissue
Dense connective tissue contains more collagen fibres than does loose connective tissue. As a consequence, it displays greater resistance to stretching. There are two major categories of dense connective tissue: regular and irregular. Dense regular connective tissue fibres are parallel to each other, enhancing tensile strength and resistance to stretching in the direction of the fibre orientations. Ligaments and tendons are made of dense regular connective tissue, but in ligaments, not all fibres are parallel. Dense regular elastic tissue contains elastin fibres in addition to collagen fibres, which allows the ligament to return to its original length after stretching. The ligaments in the vocal folds and between the vertebrae in the vertebral column are elastic.
In dense irregular connective tissue, the direction of fibres is random. This arrangement gives the tissue greater strength in all directions and less strength in one particular direction. In some tissues, fibres crisscross and form a mesh. In other tissues, stretching in several directions is achieved by alternating layers where fibres run in the same orientation in each layer, and it is the layers themselves that are stacked at an angle. The dermis of the skin is an example of dense irregular connective tissue rich in collagen fibres. Dense irregular elastic tissues give arterial walls the strength and the ability to regain their original shape after stretching (Figure 4.12).
Figure 4.12 Dense Connective Tissue. (a) Dense regular connective tissue consists of collagenous fibres packed into parallel bundles. (b) Dense irregular connective tissue consists of collagenous fibres interwoven into a meshlike network. From top, LM × 1000, LM × 200. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)
Supportive Connective Tissues
Two major forms of supportive connective tissue, cartilage and bone, allow the body to maintain its posture and protect internal organs.
Cartilage
The distinctive appearance of cartilage is due to polysaccharides called chondroitin sulfates, which bind with ground substance proteins to form proteoglycans. Embedded within the cartilage matrix are chondrocytes, or cartilage cells, and the spaces they occupy are called lacunae (singular = lacuna). A layer of dense irregular connective tissue, the perichondrium, encapsulates the cartilage. Cartilaginous tissue is avascular; thus all nutrients need to diffuse through the matrix to reach the chondrocytes. This is a factor contributing to the very slow healing of cartilaginous tissues.
The three main types of cartilage tissue are hyaline cartilage, fibrocartilage, and elastic cartilage (Figure 4.13). Hyaline cartilage, the most common type of cartilage in the body, consists of short and dispersed collagen fibres and contains large amounts of proteoglycans. Under a microscope, tissue samples appear clear. The surface of hyaline cartilage is smooth. Both strong and flexible, it is found in the rib cage and nose and covers bones where they meet to form movable joints. It makes up a template of the embryonic skeleton before bone formation. Plates of hyaline cartilage at the ends of bones allow continued growth until adulthood. Fibrocartilage is tough because it has thick bundles of collagen fibres dispersed through its matrix. Menisci in the knee joint and the intervertebral discs are examples of fibrocartilage. Elastic cartilage contains elastic fibres as well as collagen and proteoglycans. This tissue gives rigid support as well as elasticity. Tug gently at your ear lobes, and notice that the lobes return to their initial shape. The external ear contains elastic cartilage.
Figure 4.13 Types of Cartilage. Cartilage is a connective tissue consisting of collagenous fibres embedded in a firm matrix of chondroitin sulfates. (a) Hyaline cartilage provides support with some flexibility. The example is from dog tissue. (b) Fibrocartilage provides some compressibility and can absorb pressure. (c) Elastic cartilage provides firm but elastic support. From top, LM × 300, LM × 1200, LM × 1016. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)
Bone
Bone is the hardest connective tissue. It provides protection to internal organs, works with the skeletal muscle to produce movement, and allows us to remain upright. Bone’s rigid extracellular matrix contains mostly collagen fibres embedded in a mineralized ground substance containing hydroxyapatite, a form of calcium phosphate. Both components of the matrix, organic and inorganic, contribute to the unusual properties of bone. Without collagen, bones would be brittle and shatter easily. Without mineral crystals, bones would flex and provide little support. Osteocytes, bone cells like chondrocytes, are located within lacunae. The histology of transverse tissue from a long bone shows a typical arrangement of osteocytes in concentric circles around a central canal. Bone is a highly vascularized tissue. Unlike cartilage, bone tissue can recover from injuries in a relatively short time. (See Figure 6.12.)
Cancellous bone looks like a sponge under the microscope and contains empty spaces between trabeculae, or arches of bone proper. It is lighter than compact bone and found in the interior of some bones and at the ends of long bones. Compact bone is solid and has greater structural strength.
Fluid Connective Tissue
Blood and lymph are fluid connective tissues. Cells circulate in a liquid extracellular matrix. The formed elements circulating in blood are all derived from hematopoietic stem cells located in bone marrow (Figure 4.14). Erythrocytes, red blood cells, transport oxygen and some carbon dioxide. Leukocytes, white blood cells, are responsible for defending against potentially harmful microorganisms or molecules. Platelets are cell fragments involved in blood clotting. Some white blood cells have the ability to cross the endothelial layer that lines blood vessels and enter adjacent tissues. Nutrients, salts, and wastes are dissolved in the liquid matrix and transported through the body.
Figure 4.14 Blood: A Fluid Connective Tissue. Blood is a fluid connective tissue containing erythrocytes and various types of leukocytes that circulate in a liquid extracellular matrix. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Lymph contains a liquid matrix and white blood cells. Lymphatic capillaries are extremely permeable, allowing larger molecules and excess fluid from interstitial spaces to enter the lymphatic vessels. Lymph drains into blood vessels, delivering molecules to the blood that could not otherwise directly enter the bloodstream. In this way, specialized lymphatic capillaries transport absorbed fats away from the intestine and deliver these molecules to the blood.
4.4 Muscle Tissue and Motion
Muscle tissue is characterized by properties that allow movement. Muscle cells are excitable; they respond to stimuli. They are contractile, meaning they can shorten and generate a pulling force. When attached between two movable objects—in other words, bones—muscle contractions cause the bones to move. Some muscle movement is voluntary, which means it is under conscious control. For example, a person decides to open a book and read a chapter on anatomy. Other movements are involuntary, meaning they are not under conscious control, such as the contraction of your pupil in bright light. Muscle tissue is classified into three types according to structure and function: skeletal, cardiac, and smooth (Table 4.2).
Tissue | Histology | Function | Location |
|---|---|---|---|
Skeletal | Long cylindrical fibre, striated, many peripherally located nuclei | Voluntary movement, produces heat, protects organs | Attached to bones and around entrance points to the body (e.g., mouth, anus) |
Cardiac | Short, branched, striated, single central nucleus | Contracts to pump blood | Heart |
Smooth | Short, spindle-shaped, no evident striation, single nucleus in each fibre | Involuntary movement, moves food, involuntary control of respiration, moves secretions, regulates flow of blood in arteries by contraction | Walls of major organs and passageways |
Skeletal muscle is attached to bones, and its contraction makes possible locomotion, facial expressions, posture, and other voluntary movements of the body. Forty percent of your body mass is made up of skeletal muscle.
Skeletal muscles generate heat as a by-product of their contraction and thus participate in thermal homeostasis, and these functions are the result of a large number of mitochondria in each muscle cell. Shivering is an involuntary contraction of skeletal muscles in response to perceived lower-than-normal body temperature. The muscle cells, or myocytes, remain relatively constant in number throughout life. Skeletal muscle tissue is arranged in bundles surrounded by connective tissue. Under a light microscope, muscle cells appear striated with many nuclei squeezed along the membranes. The striation is due to the regular alternation of the contractile proteins actin and myosin, along with the structural proteins that couple the contractile proteins to connective tissues. The cells are multinucleated as a result of the fusion of the many myoblasts that form each long muscle fibre.
Cardiac muscle forms the contractile walls of the heart. The cells of cardiac muscle, known as cardiomyocytes, also appear striated under a microscope. Unlike skeletal muscle fibres, cardiomyocytes are single cells, typically with a single centrally located nucleus. A principal characteristic of cardiomyocytes is that they contract on their own intrinsic rhythms without any external stimulation. Cardiomyocytes attach to one another with specialized cell junctions called intercalated discs. Intercalated discs have both anchoring junctions and gap junctions. Attached cells form long, branching cardiac muscle fibres that are, essentially, a mechanical and electrochemical syncytium, allowing the cells to contract all at once. The cardiac muscle pumps blood through the body and is under involuntary control. The attachment junctions hold adjacent cells together across the dynamic pressure changes of the cardiac cycle.
Smooth muscle tissue contraction is responsible for involuntary movements in the internal organs. It forms the contractile component of the digestive, urinary, and reproductive systems as well as the airways and arteries. Each cell is spindle-shaped with a single nucleus and no visible striations (Figure 4.15).
Figure 4.15 Muscle Tissue. (a) Skeletal muscle cells have prominent striation and nuclei on their periphery. (b) Smooth muscle cells have a single nucleus and no visible striations. (c) Cardiac muscle cells appear striated and have a single nucleus. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)
4.5 Nervous Tissue Mediates Perception and Response
Nervous tissue is characterized as being excitable and capable of sending and receiving electrochemical signals that provide the body with information. Two main classes of cells make up nervous tissue: the neuron and neuroglia (Figure 4.16). Neurons propagate information via electrochemical impulses, called action potentials, which are biochemically linked to the release of chemical signals. Neuroglia play an essential role in supporting neurons and modulating their information propagation.
Neurons display distinctive morphology, well suited to their role as conducting cells, with three main parts. The cell body includes most of the cytoplasm, the organelles, and the nucleus. Dendrites branch off the cell body and appear as thin extensions. A long “tail,” the axon, extends from the neuron body and can be wrapped in an insulating layer known as myelin, which is formed by accessory cells. The synapse is the gap between nerve cells or between a nerve cell and its target—for example, a muscle or a gland—across which the impulse is transmitted by chemical compounds known as neurotransmitters. When a neuron is sufficiently stimulated, it generates an action potential that propagates down the axon toward the synapse. If enough neurotransmitters are released at the synapse to stimulate the next neuron or target, a response is generated.
Figure 4.16 The Neuron. The cell body of a neuron, also called the soma, contains the nucleus and mitochondria. The dendrites transfer the nerve impulse to the soma. The axon carries the action potential away to another excitable cell. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
The second class of neural cells comprises the neuroglia or glial cells, which have been characterized as having a simple support role. The word glia comes from the Greek word for glue. Recent research is shedding light on the more complex role of neuroglia in the function of the brain and nervous system. Astrocyte cells, named for their distinctive star shape, are abundant in the central nervous system. The astrocytes have many functions, including regulation of ion concentration in the intercellular space, uptake and/or breakdown of some neurotransmitters, and formation of the blood-brain barrier, the membrane that separates the circulatory system from the brain. Microglia protect the nervous system against infection but are not nervous tissue because they are related to macrophages. Oligodendrocyte cells produce myelin in the central nervous system (brain and spinal cord), while the Schwann cells produce myelin in the peripheral nervous system (Figure 4.17).
Figure 4.17 Nervous Tissue. Nervous tissue is made up of neurons and neuroglia. The cells of nervous tissue are specialized to transmit and receive impulses. LM × 872. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Key Terms
- adherens junction:
- Type of cell junction; uses adhesion proteins found in either desmosomes or hemidesmosomes, depending on whether they are linking to other cells or the matrix.
- adipocytes:
- Lipid storage cells.
- adipose tissue:
- Specialized areolar tissue rich in stored fat.
- anchoring junction:
- Mechanically attaches adjacent cells to each other or to the basement membrane.
- apocrine secretion:
- Release of a substance along with the apical portion of the cell.
- areolar tissue (also loose connective tissue):
- A type of connective tissue proper that shows little specialization with cells dispersed in the matrix.
- astrocyte:
- Star-shaped cell in the central nervous system that regulates ions and uptake and/or breakdown of some neurotransmitters and contributes to the formation of the blood-brain barrier.
- axon:
- A long “tail” that extends from the neuron body and can be wrapped in an insulating layer known as myelin.
- basal lamina:
- Thin extracellular layer that lies underneath epithelial cells and separates them from other tissues.
- basement membrane:
- In epithelial tissue, a thin layer of fibrous material that anchors the epithelial tissue to the underlying connective tissue; made up of the basal lamina and reticular lamina.
- cardiac muscle:
- Heart muscle under involuntary control; composed of striated cells that attach to form fibres; each cell contains a single nucleus and contracts autonomously.
- cell junction:
- Point of cell-to-cell contact that connects one cell to another in a tissue.
- chondrocytes:
- Cells of the cartilage.
- ciliated columnar epithelium:
- Tissue composed of simple columnar epithelial cells with cilia on their apical surfaces.
- collagen fibre:
- Flexible fibrous proteins that give connective tissue tensile strength.
- connective tissue:
- Type of tissue that serves to hold in place, connect, and integrate the body’s organs and systems.
- connective tissue membrane:
- Connective tissue that encapsulates organs and lines movable joints.
- connective tissue proper:
- Connective tissue containing a viscous matrix, fibres, and cells.
- cutaneous membrane:
- Skin; epithelial tissue made up of a stratified squamous epithelial cells that cover the outside of the body.
- dendrites:
- Branches off the neuron cell body and appear as thin extensions.
- dense connective tissue:
- Connective tissue proper that contains many fibres that provide both elasticity and protection.
- desmosomes:
- Type of cell junction that anchors cells together by linking adhesion molecules across their membranes.
- ectoderm:
- Outermost embryonic germ layer from which the epidermis and the nervous tissue derive.
- elastic cartilage:
- Type of cartilage, with elastin as the major protein, characterized by rigid support as well as elasticity.
- elastic fibre:
- Fibrous protein within connective tissue that contains a high percentage of the protein elastin, which allows the fibres to stretch and return to their original size.
- endocrine gland:
- Groups of cells that release chemical signals into the intercellular fluid to be picked up and transported to their target organs by blood.
- endoderm:
- Innermost embryonic germ layer from which most of the digestive system and lower respiratory system derive.
- endothelium:
- Tissue that lines vessels of the lymphatic and cardiovascular system, made up of a simple squamous epithelium.
- epithelial membrane:
- Epithelium attached to a layer of connective tissue.
- epithelial tissue:
- Type of tissue that serves primarily as a covering or lining of body parts, protecting the body; it also functions in absorption, transport, and secretion.
- exocrine gland:
- Group of epithelial cells that secrete substances through ducts that open to the skin or to internal body surfaces that lead to the exterior of the body.
- fibroblast:
- Most abundant cell type in connective tissue, secretes protein fibres and matrix into the extracellular space.
- fibrocartilage:
- Tough form of cartilage, made of thick bundles of collagen fibres embedded in chondroitin sulfate ground substance.
- fibrocyte:
- Less active form of fibroblast.
- fluid connective tissue:
- Specialized cells that circulate in a watery fluid containing salts, nutrients, and dissolved proteins.
- gap junction:
- Allows cytoplasmic communication to occur between cells.
- gland:
- Structure made up of one or more cells modified to synthesize and secrete chemical substances.
- goblet cell:
- Unicellular gland found in columnar epithelium that secretes mucous.
- ground substance:
- Fluid or semifluid portion of the matrix.
- hemidesmosomes:
- Type of cell junction that links cells to the extracellular matrix.
- holocrine secretion:
- Release of a substance caused by the rupture of a gland cell, which becomes part of the secretion.
- hyaline cartilage:
- Most common type of cartilage, smooth and made of short collagen fibres embedded in a chondroitin sulfate ground substance.
- lacunae (singular = lacuna):
- Small spaces in bone or cartilage tissue that cells occupy.
- lamina propria:
- Areolar connective tissue underlying a mucous membrane.
- loose connective tissue (also areolar tissue):
- Type of connective tissue proper that shows little specialization, with cells dispersed in the matrix.
- matrix:
- Extracellular material that is produced by the cells embedded in it, containing ground substance and fibres.
- merocrine secretion:
- Release of a substance from a gland via exocytosis.
- mesenchymal cell:
- Adult stem cell from which most connective tissue cells are derived.
- mesoderm:
- Middle embryonic germ layer from which connective tissue, muscle tissue, and some epithelial tissue derive.
- mesothelium:
- Simple squamous epithelial tissue that covers the major body cavities and is the epithelial portion of serous membranes.
- microglia:
- Glial cells that protect the nervous system against infection but are not nervous tissue because they are related to macrophages.
- mucous gland:
- Group of cells that secrete mucous, a thick, slippery substance that keeps tissues moist and acts as a lubricant.
- mucous membrane (mucosa):
- Tissue membrane that is covered by protective mucous and lines tissue exposed to the outside environment.
- muscle tissue:
- Type of tissue that is capable of contracting and generating tension in response to stimulation; produces movement.
- myelin:
- Layer of lipid inside some neuroglial cells that wraps around the axons of some neurons.
- myocyte:
- Muscle cells.
- nervous tissue:
- Type of tissue that is capable of sending and receiving impulses through electrochemical signals.
- neuroglia (glial cells):
- Supportive neural cells.
- neuron:
- Excitable neural cell that transfers nerve impulses.
- oligodendrocyte:
- Neuroglial cell that produces myelin in the brain.
- osteocyte:
- Bone cell located within lacunae typically arranged in concentric circles around a central canal.
- parenchyma:
- Functional cells of a gland or organ, in contrast with the supportive or connective tissue of a gland or organ.
- pseudostratified columnar epithelium:
- Tissue that consists of a single layer of irregularly shaped and sized cells that give the appearance of multiple layers; found in ducts of certain glands and the upper respiratory tract.
- reticular fibre:
- Fine fibrous protein, made of collagen subunits, which cross-link to form supporting “nets” within connective tissue.
- reticular lamina:
- Matrix containing collagen and elastin secreted by connective tissue; a component of the basement membrane.
- reticular tissue:
- Type of loose connective tissue that provides a supportive framework to soft organs, such as the lymphatic tissue, spleen, and liver.
- Schwann cell:
- Neuroglial cell that produces myelin in the peripheral nervous system.
- serous gland:
- Group of cells within the serous membrane that secrete a lubricating substance onto the surface.
- serous membrane:
- Type of tissue membrane that lines body cavities and lubricates them with serous fluid.
- simple columnar epithelium:
- Tissue that consists of a single layer of columnlike cells; promotes secretion and absorption in tissues and organs.
- simple cuboidal epithelium:
- Tissue that consists of a single layer of cube-shaped cells; promotes secretion and absorption in ducts and tubules.
- simple squamous epithelium:
- Tissue that consists of a single layer of flat, scalelike cells; promotes diffusion and filtration across surfaces.
- skeletal muscle:
- Usually attached to bone, under voluntary control, each cell is a fibre that is multinucleated and striated.
- smooth muscle:
- Under involuntary control, moves internal organs; cells contain a single nucleus, are spindle-shaped, and do not appear striated; each cell is a fibre.
- stratified columnar epithelium:
- Tissue that consists of two or more layers of columnlike cells, contains glands, and is found in some ducts.
- stratified cuboidal epithelium:
- Tissue that consists of two or more layers of cube-shaped cells, found in some ducts.
- stratified squamous epithelium:
- Tissue that consists of multiple layers of cells, with the most apical being flat scalelike cells; protects surfaces from abrasion.
- striation:
- Alignment of parallel actin and myosin filaments, which form a banded pattern.
- supportive connective tissue:
- Type of connective tissue that provides strength to the body and protects soft tissue.
- synovial membrane:
- Connective tissue membrane that lines the cavities of freely movable joints, producing synovial fluid for lubrication.
- tight junction:
- Forms an impermeable barrier between cells that blocks hydrophilic ions or molecules.
- tissue:
- Group of cells that are similar in form and perform related functions.
- tissue membrane:
- Thin layer or sheet of cells that covers the outside of the body, organs, internal passageways, and the lining of joint cavities.
- transitional epithelium:
- Form of stratified epithelium found in the urinary tract, characterized by an apical layer of cells that change shape in response to the presence of urine.
Chapter Review
4.1 Types of Tissues
The human body contains more than 200 types of cells that can all be classified into four types of tissues: epithelial, connective, muscle, and nervous. Epithelial tissues act as coverings controlling the movement of materials across the surface. Connective tissue integrates the various parts of the body and provides support and protection to organs. Muscle tissue allows the body to move. Nervous tissues propagate information.
The study of the shape and arrangement of cells in tissue is called histology. All cells and tissues in the body derive from three germ layers in the embryo: the ectoderm, mesoderm, and endoderm.
Different types of tissues form membranes that enclose organs, provide a friction-free interaction between organs, and keep organs together. Synovial membranes are connective tissue membranes that protect and line the joints. Epithelial membranes are formed from epithelial tissue attached to a layer of connective tissue. There are three types of epithelial membranes: mucous, which contain glands; serous, which secrete fluid; and cutaneous, which make up the skin.
4.2 Epithelial Tissue
In epithelial tissue, cells are closely packed with little or no extracellular matrix except for the basal lamina that separates the epithelium from the underlying tissue. The main functions of epithelia are protection from the environment, coverage, secretion and excretion, absorption, and filtration. Cells are bound together by tight junctions that form an impermeable barrier. They can also be connected by gap junctions, which allow free exchange of soluble molecules between cells, and anchoring junctions, which attach from cell to cell or from cell to matrix. The different types of epithelial tissues cover the outside of the body, organs, and internal cavities and are characterized by their cellular shapes and arrangements: squamous, cuboidal, or columnar epithelia. Single-cell layers form simple epithelia, whereas stacked cells form stratified epithelia. Very few capillaries penetrate these tissues.
Glands are secretory tissues and organs that are derived from epithelial tissues. Exocrine glands release their products through ducts. Endocrine glands secrete hormones directly into the interstitial fluid and bloodstream. Glands are classified both according to the type of secretion and by their structure. Merocrine glands secrete products as they are synthesized.
Apocrine glands release secretions by pinching off the apical portion of the cell, whereas holocrine gland cells store their secretions until they rupture and release their contents. In this case, the cell becomes part of the secretion.
4.3 Connective Tissue Supports and Protects
Connective tissue is a heterogeneous tissue with many cell shapes and tissue architectures. Structurally, all connective tissues contain cells that are embedded in an extracellular matrix stabilized by proteins. The chemical nature and physical layout of the extracellular matrix and proteins vary enormously among tissues, reflecting the variety of functions that connective tissue fulfils in the body. Connective tissues separate and cushion organs, protecting them from shifting or traumatic injury. Connective tissues provide support and assist movement, store and transport energy molecules, protect against infections, and contribute to temperature homeostasis.
Many different cells contribute to the formation of connective tissues. They originate in the mesodermal germ layer and differentiate from mesenchyme and hematopoietic tissue in the bone marrow. Fibroblasts are the most abundant and secrete many protein fibres, adipocytes specialize in fat storage, hematopoietic cells from the bone marrow give rise to all the blood cells, chondrocytes form cartilage, and osteocytes form bone. The extracellular matrix contains fluid, proteins, polysaccharide derivatives, and in the case of bone, mineral crystals. Protein fibres fall into three major groups: collagen fibres that are thick, strong, flexible, and resist stretch; reticular fibres that are thin and form a supportive mesh; and elastin fibres that are thin and elastic.
The major types of connective tissue are connective tissue proper, supportive tissue, and fluid tissue. Loose connective tissue proper includes adipose tissue, areolar tissue, and reticular tissue. These serve to hold organs and other tissues in place and, in the case of adipose tissue, isolate and store energy reserves. The matrix is the most abundant feature for loose tissue, although adipose tissue does not have much extracellular matrix. Dense connective tissue proper is richer in fibres and may be regular, with fibres oriented in parallel, as in ligaments and tendons, or irregular, with fibres oriented in several directions. Organ capsules (collagenous type) and walls of arteries (elastic type) contain dense irregular connective tissue. Cartilage and bone are supportive tissue. Cartilage contains chondrocytes and is somewhat flexible.
Hyaline cartilage is smooth and clear, covers joints, and is found in the growing portion of bones. Fibrocartilage is tough because of extra collagen fibres and forms, among other things, the intervertebral discs. Elastic cartilage can stretch and recoil to its original shape because of its high content of elastic fibres. The matrix contains very few blood vessels. Bones are made of a rigid, mineralized matrix containing calcium salts, crystals, and osteocytes lodged in lacunae. Bone tissue is highly vascularized. Cancellous bone is spongy and less solid than compact bone. Fluid tissue, such as blood and lymph, is characterized by a liquid matrix and no supporting fibres.
4.4 Muscle Tissue and Motion
The three types of muscle cells are skeletal, cardiac, and smooth. Their morphologies match their specific functions in the body. Skeletal muscle is voluntary and responds to conscious stimuli. The cells are striated and multinucleated, appearing as long, unbranched cylinders. Cardiac muscle is involuntary and found only in the heart. Each cell is striated with a single nucleus, and they attach to one another to form long fibres. Cells are attached to one another at intercalated disks. The cells are interconnected physically and electrochemically to act as a syncytium. Cardiac muscle cells contract autonomously and involuntarily. Smooth muscle is involuntary. Each cell is a spindle-shaped fibre and contains a single nucleus. No striations are evident because the actin and myosin filaments do not align in the cytoplasm.
4.5 Nervous Tissue Mediates Perception and Response
The most prominent cell of the nervous tissue, the neuron, is characterized mainly by its ability to receive stimuli and respond by generating an electrical signal, known as an action potential, which can travel rapidly over great distances in the body. A typical neuron displays a distinctive morphology: A large cell body branches out into short extensions called dendrites, which receive chemical signals from other neurons, and a long tail called an axon relays signals away from the cell to other neurons, muscles, or glands. Many axons are wrapped in a myelin sheath, a lipid derivative that acts as an insulator and speeds up the transmission of the action potential. Other cells in the nervous tissue, the neuroglia, include the astrocytes, microglia, oligodendrocytes, and Schwann cells.