Thus far, on our journey through the human body, we have made our way through the chemical and cellular levels of organization. The next stop along the way is tissues. A tissue (TISH-ū; woven). is a group of similar cells working together to perform a particular function. There are four major groups of tissues: epithelial, connective, muscle, and nervous. The study of tissues is histology (his-TOL- ō-jē; histo, tissue), and histologists are scientists who study tissues. Although all tissues are groups of cells, some tissues have cells packed closely together, while others have cells more widely scattered. Before looking at specific types of tissue, let’s look at some of the ways cells hold onto one another within a tissue.
CELL CONNECTIONS
Cells may be “stuck” together in a variety of ways, but the two most significant ways are: (1) via the “sticky” glycocalyx (sugar coats) on the surfaces of the cells; and (2) via specialized junctions, which include desmosomes, tight junctions, and gap junctions (Figure 7-1).
Desmosomes (DEZ-mo-sōmz; desmo, band; somes, bodies) are filamentous connections that not only hold adjacent cells together but also give internal structural support to the cells. Each desmosome contains a plaque (PLAK; plate). located on the cytoplasmic surface of the plasma membrane. The plaque consists of glycoproteins and resembles a tiny button. Externally, filamentous proteins called cadherins (kad-HAIR-enz) radiate out from the plaque and intertwine with cadherin filaments from the adjacent cell.
Internally, intermediate filaments of the cytoskeleton attach to the plaques and anchor them to plaques on the opposite side of the cytoplasm. As a result, a pulling force on one side of the cell exerts tension throughout the cell’s cytoskeleton and not just on the plasma membrane. This internal support system reduces the likelihood that cells will tear apart. Desmosomes are found in tissues that experience stretching, such as in the skin (in these cells, the intermediate filaments of the cytoskeleton are made of the protein keratin). Since desmosomes are filamentous (not densely packed globular structures) in the intercellular space, they do not prevent materials from passing in between the connected cells.
Tight junctions are continuous bands of protein that form an almost impenetrable barrier to prevent substances from passing through the intercellular space between connected cells. A tight junction consists of integral proteins called occludins (ō-KLŪ-denz) that extend out from the plasma membrane of adjoining cells and fuse together. In effect, these junctions function like thick belts around the cells. The fact that the “belt” of one cell fuses with a similar belt on an adjacent cell prevents materials from passing in between the cells. This is important in regions where it is important to keep fluids in one compartment from leaking freely into another compartment. In order for materials to get past the tight junction barrier, they must pass through the cytoplasm of the adjoining cells. In this way, the cells can modify the substances as they pass from one compartment to another. Tight junctions are found in cells that line the stomach and intestines, and in cells that line blood vessels in the brain.
Gap junctions are tiny channels that allow materials to pass from the cytoplasm of one cell into the cytoplasm of an adjacent cell. A gap junction forms when transmembrane proteins called connexons (ko-NEX-onz) in adjacent cells bind to one another. The result is a tiny tube between the joined cells. These connections are abundant in cardiac and smooth muscle cells.
Figure 7-1. Cell connections
EPITHELIAL TISSUE
Epithelial tissue (ep-i-THĒ-lē-al; epithe, added on), also called epithelium, forms thin membranous coverings around and inside various organs and it forms glands. For this reason, epithelial tissue is classified as either membranous or glandular.
Glandular Epithelium
Glandular epithelium forms specialized structures called glands that release substances beneficial to the body. The word secretion (sē-KRĒ- shun; secret, set apart) is a noun when it refers to the substance released from the gland, and it is a verb when referring to the process of releasing the substance. Histologists classify glands based on the destination of their secretions.
Endocrine glands (EN-dō-krin; endo, inside; crine, secretion) secrete chemicals called hormones into tissue fluid or the bloodstream. Hormones function as chemical messengers sent through the blood from endocrine glands to other organs in the body. The prefix endo implies that the secretions of these glands remain inside the body. You will learn about endocrine glands in a later chapter.
Exocrine glands (EX-ō-krin; exo, outside) expel secretions into a cavity or tube that has connections with the outside of the body. These glands are either unicellular or multicellular. The only example of a unicellular gland in the body is a goblet cell, which secretes mucus directly onto the surface of certain epithelial membranes. Multicellular glands expel their secretions into a tiny tube called a duct (literally, “to lead”). Histologists classify multicellular glands based on the structure of the gland and the method of secretion. Here we will consider only the method of secretion:
- Apocrine glands (AP-ō-krin; apo, off) make secretions consisting of portions of glandular cells that have broken off (notice the literal meaning of the name). Mammary glands that produce milk in the breasts are the only glands in humans that are apocrine type. You can remember apocrine glands by relating the first four letters of their name with their mode of secretion: A Piece of Cell is in the secretion.
- Holocrine glands (HŌ-lō-krin; “whole secretion”) make secretions consisting of entire cells that have disintegrated. An example of a holocrine gland is a sebaceous (oil) gland in the skin. Remember holocrine glands have “whole” cells disintegrating to enter the secretion.
- Merocrine glands (MER-ō-krin; mero, part) make secretions by exocytosis. Merocrine is the most common type of exocrine gland and includes sweat glands, salivary glands, and glands in the stomach and intestine.
Membranous Epithelium
Membranous epithelium (MEM-bruh-nus) exists on the free surfaces of various organs, including the skin where it forms the epidermis (the outer, visible part of the skin). It also covers the outside of visceral organs such as the stomach and intestines, and lines their internal cavities. Membranous epithelium functions in protection, absorption, filtration, and secretion. Unlike cell membranes, which are phospholipid bilayers, epithelial membranes are multicellular. Membranous epithelium has the following characteristics:
- Tightly packed cells: There is little extracellular space in membranous epithelium due to desmosomes and, sometimes, tight junctions that hold cells close together.
- Cells arranged in layers: Some epithelial tis- sues may consist of only one layer of cells, or it may have more than 30 layers, or strata; each layer is a stratum.
Avascular (ā-VAS-kū-lar; a, without; vascul, vessel): There are no blood vessels in membranous epithelium. Nutrition for membra- nous epithelial cells arrives by diffusion from blood vessels in nearby connective tissues.
- Nerve supply: Epithelial tissues contain nerve fibers from the nervous system; therefore, we say the tissues are innervated (EN-er-vā- ted). As a result, damage or irritation of epithelial tissue can cause pain.
- Polarity: All membranous epithelia exhibit anatomical and physiological differences between their cells located at the free surface (in contact with the outside of the body or a body cavity) and their cells in contact with underlying connective tissues. This characteristic is called polarity. An epithelium’s apical surface is its outermost layer of cells in contact with the “free” surface; the epithelial cells in this region are the apical cells. An epithelium’s basal surface is its deepest layer of cells; the epithelial cells in this region are basal cells.
- Basement membrane: A thin layer of glycoproteins and collagen fibers make up a basement membrane that separates membranous epithelium from the underlying connective tissues. This membrane supports the overlying epithelial tissue and serves as a passageway for the diffusion of nutrients from the underlying connective tissue. The glycoprotein component of the basement membrane is secreted from the basal cells of the epithelium and is called the basal lamina (LAM-i-na; thin plate). The collagen protein component of the basement membrane is secreted from connective tissue cells lying beneath the epithelium. The collagen fibers in this region intertwine to form a net-like pattern; for this reason, this part of the basement membrane is called the reticular lamina (re-TIK-ū-lar; reti, net).
- Some house receptors: Special structures called receptors exist in some epithelia and allow the body to detect changes in the internal or external environment. In the nasal cavity, some receptors within epithelia are sensitive to odors, while others are sensitive to touch.
CLASSIFICATION OF MEMBRANOUS EPITHELIUM
Histologists classify membranous epithelium by cell shape, cell arrangement, and location
Classified by Cell Shape (Figure 7-2)
- Squamous (SKWĀ-mus; thin plate) cells are flat and resemble scales on a fish. The outer part of your skin consists of dead squamous cells. In a side view, a squamous cell might resemble a fried egg with the nucleus domed upward like the yolk of the egg.
- Cuboidal (kū-BOY-dal; oid, resembles) cells are cube shaped, and are found lining tiny tubes in the kidneys.
- Columnar cells are column shaped, being taller than they are wide. These cells are found in various glands.
- Transitional cells can change shape, ranging from cuboidal to squamous, and are found in the urinary bladder. If the bladder is full of urine, hydrostatic pressure force on these cells compresses them, so they become squamous like. When the bladder is empty, the lack of pressure and compression allows the cells to “pop out” and become more cuboidal.
Figure 7-2. Shapes of epithelial cells
Classified by Cell Arrangement (Figure 7-3)
- Simple epithelium has only one layer of cells; includes simple squamous, cuboidal, and columnar epithelium.
Figure 7-3. Shapes of epithelial cells
- Stratified epithelium has two or more layers of cells. The name for a particular stratified epithelium depends on the shape of the apical cells. Examples include stratified squamous, stratified cuboidal and stratified columnar epithelium.
- Pseudostratified epithelium (SU-dō-STRA- ti-fīd) is actually a simple epithelium but it looks stratified (pseudo, false). The reason for this illusion is that although each cell in the epithelium rests on the basement membrane, not all of them reach the apical surface
Classified by Location
When the names of some membranous epithelia are used, it is understood where those membranes are located. Classifying membranous epithelia by location includes endothelium, serous membranes, mucous membranes, and the cutaneous membrane.
- Endothelium (en-dō-THĒ-lē-um) is simple squamous epithelium lining the inside of blood vessels and lymph vessels.
- Serous membranes (SĒR-us) are simple squamous epithelia covering visceral organs (internal organs of the torso) and lining major body cavities that do not have connections with the outside of the body. A serous membrane, also called mesothelium (mez-ō-THĒ-lē-um), secretes a watery liquid called serous fluid that reduces friction as the visceral organs move. The word serous comes from serum, which means “whey”, the liquid component of milk after the fat (curd) has been removed. Serous membranes include the peritoneal, pleural, and pericardial membranes.
- Mucous membranes (MŪ-kus) may be simple columnar, pseudostratified, or stratified squamous epithelia, but all line cavities that have a connection to the outside of the body. Examples include the linings of the nasal cavity, esophagus, trachea, stomach, intestines, and tubes associated with the urinary and reproductive systems. Mucous membranes contain goblet cells that secrete a viscous fluid called mucus (note the different spelling) onto the membrane’s free surface. A protein called mucin gives mucus its thick viscous nature.
- Cutaneous membrane (ku-TANE-e-us; “skin”) refers to the skin. In reality, the visible part of one’s skin is called the epidermis, which lies superficially to the “true” skin, which is the dermis. The dermis of a cow is used to make leather products. You will learn more about the cutaneous membrane in the next chapter.
SPECIFIC MEMBRANOUS EPITHELIA
We will now describe specific types of membranous epithelium. Learn to recognize these tissues and know their locations.
- Simple squamous epithelium consists of a single layer of flat cells and forms serous membranes, small sacs within the lungs, filtering devices within the kidneys, and endothelium. It protects underlying tissues by reducing friction (in the case of serous membranes) and allows diffusion of gases through the walls of capillaries and tiny sacs in the lungs (Figure 7-4a).
- Stratified squamous epithelium contains multiple layers of cuboidal or columnar cells, with flat apical cells covering the free surface. It exists on the surface of the skin and the inner linings of the mouth, esophagus, and vagina where it provides protection to underlying tissues (Figure 7-4b).
- Simple cuboidal epithelium is a single layer of cube-shaped cells and exists in tiny tubes of the kidney and certain glands, and on the surface of the ovary. It protects underlying tissues and allows passage of material between the blood and kidney fluid destined to become urine (Figure 7-4c).
- Stratified cuboidal epithelium consists of multiple layers of cube-shaped cells and exists in the ducts of certain glands. Its primary function is protection of underlying tissues (Figure 7-4d).
- Simple columnar epithelium is a single layer of columnar cells interspersed with goblet cells. A nonciliated type makes up the inner lining of the digestive tract where it allows absorption of digested food molecules. A ciliated type lines the oviducts and the small tubes in the lungs, where it moves mucus that traps dust and other particles (Figure 7-4e).
- Stratified columnar epithelium has multiple layers of column-shaped cells and lines the male’s urethra and ducts of some glands. It protects underlying tissues (Figure 7-4f).
- Pseudostratified ciliated columnar epithelium is a single layer of columnar cells in which some cells do not reach the free surface. It contains ciliated cells and goblet cells and lines the nasal cavity, trachea, and part of male’s urethra. It protects underlying tissues by secreting and moving mucus. (Figure 7-4g)
- Transitional epithelium is a stratified epithelium in which the apical cells change shape when compressed. It exists in the lining of the urinary bladder, ureters, and part of the urethra, where it allows these organs to expand(Figure 7-4h).
Figure 7-4. Specific types of membranous epithelium
CONNECTIVE TISSUE
Connective tissue is the most abundant tissue in the body and serves primarily to bind structures together. Other functions include support, protection, insulation, movement, shock absorption, transport of nutrients and metabolites, and storage. Histologists classify connective tissues according to the abundance of cells, amount of extracellular space, arrangement of extracellular fibers, and function. Major groups include connective tissue proper, cartilage, vascular tissue (blood), and osseous tissue (bone). All connective tissues arise from mesenchyme (MEZ-en-kīm), an undifferentiated, embryonic tissue. While most mesenchyme differentiates to form specific types of connective tissue, a mature body still possesses some mesenchyme that can replace damaged tissues when needed.
The major structural characteristics of connective tissue include vascularization and an extensive matrix. All connective tissues, except cartilage, are supplied with blood vessels, a condition known as vascularization (vas-kū-lar- ZĀ-shun). Loose connective tissues are the most vascularized, while vessels are sparsely scattered in dense connective tissues. Most connective tissues contain a significant amount of material in between the cells called extracellular matrix. The connective tissue matrix includes two components: ground substance and extracellular fibers.
Ground Substance
Ground substance is the fluid component of the matrix and contains water, ions, nutrients, metabolites, adhesion proteins, and proteoglycans. Adhesion proteins, the most common being fibronectin (fī-brō-NEK-tin), connect cells to extracellular fibers and the ground substance. Proteoglycans resemble bristle brushes used to clean bottles. The central “wire” of this proteoglycan “brush” consists of a filamentous protein, while the “bristles” include a variety of polysaccharides, collectively called glycosaminoglycans (GAGs; GLĪ-kōs-am-ē-nō- GLĪ-kanz); formerly called mucopolysaccharides.) Because they have the capacity to intertwine and trap water, GAGs contribute to the ground substance’s viscosity (gel-like nature).
Two common polysaccharides (GAGs) associated with proteoglycans are hyaluronic acid and chondroitin sulfate. Hyaluronic acid makes ground substance very slippery and aids in lubrication, especially in joints. Chondroitin sulfate makes the ground substance more viscous and helps hold the matrix together, especially in cartilage and bone tissue.
Extracellular Fibers
Extracellular fibers, as the name implies, are found outside the cells. Remember, however, that these fibers are filamentous proteins, and all proteins are made at ribosomes. Therefore, after synthesizing these proteins, the filaments are transported out of the cell via exocytosis and in the ECF they intertwine to form coiled strands. Extracellular fibers are abundant in most connective tissues and exist throughout the ground substance of the matrix. These fibers provide structural support and strength for the connective tissue. There are three major types of protein fibers in various connective tissues: collagenous, reticular, and elastic.
- Collagenous fibers (ko-LAJ-e-nus) are made of the protein collagen and appear as white fibers in the matrix. They offer the connective tissue a great amount of resistance to tension (pulling force). Collagen is the body’s most abundant protein, and it is the primary component of leather.
- Reticular fibers are very thin, highly branched collagenous fibers that give the matrix of certain connective tissues a net- like appearance (ret, net). Reticular fibers are abundant in the reticular lamina of the basement membrane that supports all epithelial tissues.
- Elastic fibers contain elastin protein and appear as yellow fibers. Their highly coiled structure allows certain connective tissues to recoil after stretching. Elastic fibers are abundant in stretchable organs, including the lungs, most blood vessels, and the skin.
CELLS IN CONNECTIVE TISSUES
Since all connective tissues arise from mesenchyme tissue, the mesenchyme cells must differentiate to give rise to the many unique connective tissues. The early developmental stage of any connective tissue involves highly active cells with names that end with blast (“to bud”). The blast cells of most connective tissues produce the matrix of the developing tissue (cells that give rise to blood cells are an exception). In many connective tissues, after the blasts surround themselves with matrix and become “mature” they become less active. The mature cells primarily associated with a particular connective tissue have names that end with –cyte (sīt; cell). Examples of blasts and cytes in connective tissues include fibroblasts, chondroblasts, osteoblasts, and hemocytoblasts.
- Fibroblasts (FĪ-brō-blasts) produce the matrix of most connective tissues. In most cases, fibroblasts become fibrocytes, but in adipose (fat) tissue they become adipocytes.
- Chondroblasts (KON-drō-blasts) produce the matrix of cartilage and eventually become chondrocytes.
- Osteoblasts (OS-tē-ō-blasts) produce the matrix of bone and eventually become osteocytes.
- Hemocytoblasts (hēm-ō-SĪ-tō-blasts) are cells that give rise to different blood cells (described shortly) but do not produce the liquid matrix, or plasma, of blood.
In addition to the major cell types, there are a number of other cells associated with various connective tissues. These include mast cells and macrophages. Mast cells congregate around blood vessels and when stimulated by a variety of chemicals release the chemical histamine (HIS-ta- mēn). Histamine causes the blood vessels to dilate (open up) and become “leaky.” As a result, fluid can leave the vessel and enter the surrounding tissues, causing them to swell. Macrophages (MAK-rō- fā-jez) are specialized white blood cells that once lived in the blood but squeezed out of the blood vessel into the surrounding tissues. Macrophages crawl around engulfing foreign particles and dead or damaged body cells.
Now we turn our attention to the major types of connective tissue in the body. These tissues can be grouped into four major classes: connective tissue proper, cartilage, osseous tissue, and blood.
CONNECTIVE TISSUE PROPER
Connective tissue proper includes all connective tissues except bone, cartilage, and blood, and it is subdivided into loose connective tissues and dense connective tissues.
Loose Connective Tissues
Loose connective tissues have loosely packed intercellular fibers and include areolar, adipose, and reticular tissue.
Areolar tissue (a-RĒ-ō-lar; area) is the most widely distributed connective tissue. It has a gel- like matrix and contains collagenous, reticular, and elastic fibers produced by fibroblasts (Figure 7-5a). Large phagocytic cells called macrophages are abundant in this tissue and defend the body against foreign particles such as bacteria. Areolar tissue also contains mast cells that secrete histamine (mentioned earlier). Allergic reactions in skin and mucous membranes result when mast cells react to various “foreign” chemicals. Lamina propria (LAM-i-na PRŌ-prē-a) is areolar tissue beneath a mucous membrane.
Adipose tissue (AD-i-pōs; adipo, fat) is a loose tissue containing adipocytes (fat cells). The cytoplasm of adipocytes contains a large droplet consisting of triglyceride molecules (Figure 7-5b). Adipocytes form early in life and do not divide, but they can swell and shrink depending on the amount of lipid molecules stored in the cytoplasm. The functions of adipose tissue include energy storage, insulation, protection, and heat generation. Adipose (fat) tissue exists in two forms, yellow and brown. Yellow fat gets its characteristic yellow color from pigments such as carotene (common in many vegetables). Yellow fat is the most common fat in an adult and is abundant just below the skin of the abdomen, buttocks, and thighs.
Reticular tissue gets its name from the thin, highly branched collagen fibers that produce a net-like appearance (Figure 7-5c). The fibers intertwine in a loose fashion, but link together to form the internal framework, or stroma, of the liver, spleen, lymph nodes, and bone marrow.
Dense Connective Tissues
Dense connective tissues have protein fibers packed together tightly so that fibroblasts appear squeezed in between them. Dense connective tissues include elastic, dense regular, and dense irregular tissue.
Elastic tissue has many extracellular, elastin protein fibers. This tissue is strong and stretchable, providing elasticity to blood vessels, lungs, and vocal cords.
Dense regular tissue has collagen fibers arranged in a parallel fashion with fibroblasts squeezed in between them. It has a limited ability to stretch but provides tremendous strength to tendons (tough fibrous bands that attach muscles to bones) and ligaments (similar bands that attach bones to other bones).
Dense irregular tissue gets its name because of its irregular arrangement of collagen fibers. This tissue offers structural strength and is more resilient (flexible) than dense regular tissue. Dense irregular tissue is abundant in the skin and coverings of the kidneys, bones, and testes.
Brown fat has a brown color due to pigments in its numerous mitochondria and an abundance of blood vessels coursing through the tissue. It is more abundant in infants than in adults. Brown fat is specialized to generate heat from fat catabolism. In adults, brown fat is found primarily in the armpits, neck, and kidney regions.
Figure 7-5. Loose connective tissues
Figure 7-6. Dense connective tissues
CARTILAGE
Cartilage (KAR-ti-lij; gristle) is a strong, yet somewhat flexible, connective tissue that functions primarily in support. Cartilage is the only avascular connective tissue, that is, it lacks blood vessels. During growth or repair of cartilage, cells called chondroblasts divide and secrete a matrix containing fine collagen fibers. The matrix traps the chondroblasts within tiny cavities called lacunae (la-KŪ-nā; bowl). After this, the trapped chondroblasts are chondrocytes. A fibrous membrane, the perichondrium (per-i-KON-drē-um) covers the surface of cartilage. Three types of cartilage are hyaline, elastic, and fibrocartilage.
Hyaline cartilage (HĪ-a-lin; glassy) has very fine collagenous fibers that are virtually invisible with a light microscope. It has a “glassy” appearance and is the tissue that most people call gristle (Figure 7-7a). Hyaline cartilage covers the ends of bones and reduces friction when the ends of bones rub against each other. This cartilage forms the “growth plates” of bones, allowing them to grow in length. It(pulling) strength and compression strength for intervertebral discs (pads between the vertebrae), pubic symphysis (connection between the two hipbones), and menisci (crescent-shaped pads in certain joints) also forms parts of the nose, larynx (voice box), and trachea (windpipe)
Elastic cartilage contains abundant elastic fibers, which make it very flexible (Figure 7-7b). This cartilage maintains the shape of the ear and epiglottis (a flap that covers the opening to the trachea when you swallow).
Fibrocartilage has a matrix packed with very thick collagen fibers. It provides tension (pulling) and compression strength for intervertebral discs (pads between vertebrae), pubic symphysis (connection between the two hip bones), and menisci (crescent-shaped pads in certain joints).
Figure 7-7. Types of cartilage
Figure 7-8. Osseous (bone) tissue
OSSEOUS TISSUE
Osseous tissue (bone) (OS-ē-us) is the connective tissue with the hardest matrix. Osteoblasts secrete an organic material, called osteoid (os-tē- OYD) that hardens when calcium salts precipitate within it. The hardened matrix eventually surrounds the osteoblasts, which then become osteocytes trapped within a lacuna (similar to a trapped chondroblast in cartilage) (Figure 7-8). Bones provide support for the body, attachment sites for muscles, storage sites for minerals and fat, and they contain marrow that manufactures blood cells.
Cartilage Type Comparison Tool
Explore the three types of cartilage: structure, properties, and locations
- Smooth, glass-like appearance
- Fine collagen fibers
- Provides smooth surfaces
- Found in joints, nose, trachea
- Contains elastic fibers
- Very flexible and resilient
- Returns to original shape
- Found in ear, epiglottis
- Dense collagen fiber bundles
- Great tensile strength
- Resists compression
- Found in discs, menisci
Figure 7-9. Blood
BLOOD
Blood is a unique connective tissue in that it flows throughout the body; therefore, in a sense, it “connects” all parts of the body. Blood consists of two components: (1) formed elements, which are the cellular components, and (2) plasma, the liquid extracellular matrix that surrounds the formed elements (Figure 7-9). While it is flowing through the body, plasma does not contain extracellular fibers. However, when blood forms a clot, the extracellular fibers develop from soluble proteins already present in the plasma. The plasma transports gases, nutrients, and cellular metabolites, including wastes, hormones, proteins, and other chemicals.
The blood’s formed elements include red blood cells, white blood cells and platelets
- Red blood cells (RBCs or erythrocytes: e-RITH-rō- sīts; erythro, red) are anucleate and transport oxygen from the lungs to the tissues throughout the body.
- White blood cells (WBCs or leukocytes: LŪ-kō-sīts; leuk, white) are part of the body’s defense system. Some of these leave the bloodstream, enter other tissues, and differentiate to become macrophages.
- Platelets (or thrombocytes: THROM-bō-sīts; thromb, clot) are small, anucleate cell fragments that release chemicals that promote blood clotting.
TISSUE HEALING
Shallow (superficial) wounds may damage only a membranous epithelium. In this case, healthy basal cells from nearby regions can proliferate and replace the lost or damaged cells. Shallow skin wounds, such as abrasions, non-bleeding cuts, and first-degree and second-degree burns, heal through division and migration of basal cells, which eventually fill in the lost tissue. When the two sides of the wound grow together, most cells stop migrating due to a phenomenon called contact inhibition. When wounds are deeper and more extensive, the repair process is more complex. Major phases in the wound healing process include hemostasis, inflammation, fibrosis, maturation, and remodeling.
1. Hemostasis
When blood vessels break it is important to stop the bleeding that would occur to prevent a loss of blood. Hemostasis, which literally translates “blood standing still,” refers to events involved in preventing a loss of blood from the body. That is, hemostasis is the body’s attempt to maintain a constant amount of blood. Hemostasis involves several different-but-related events, including vascular spasm, platelet plug formation, and coagulation.
The first stage of hemostasis is vascular spasm, which refers to spontaneous vasoconstriction in vessels that are broken. The damage to the vessel wall allows calcium ions to enter the vessel’s smooth muscle tissue resulting in contraction. This very rapid vasoconstriction can help slow down, and in some cases actually stop, the loss of blood from the damaged vessel.
The second phase of hemostasis is the formation of a platelet plug. The membranes of platelets (thrombocytes) in damaged tissues become sticky and cause them to adhere to other platelets. This aggregated group, or “plug,” of platelets can also help slow down the loss of blood from a broken vessel.
The final stage of hemostasis is coagulation (or clotting). This event is characterized by blood cells becoming trapped in a network of protein fibers that develop in the plasma at the site of a wound. Platelets and damaged blood vessels in the wound release chemicals that cause the conversion of a soluble protein, called fibrinogen, in the plasma to an insoluble form called fibrin (FĪ-brin). The fibrin fibers intertwine and form a netlike mesh in the broken vessel that can trap blood cells. This is like a fish net spread across a stream and trapping fish. The collection of fibrin strands and the blood cells is a blood clot. A stationary clot is a thrombus (THROM-bus) while a clot that has broken loose and is traveling in the blood is called an embolus (EM-buh-lus).
Not only can fibrin fibers help slow down the loss of blood they can also develop outside the blood vessel and help “wall off” the area, preventing bacteria from moving into surrounding tissues. During this time, mast cells release histamine that makes local blood vessels dilate and become “leaky.” As plasma leaks out of the vessel, the surrounding tissues experience edema (e-DĒ-ma; swelling).
2. Inflammation
If bleeding ceases, the next phase of healing is inflammation (in-fluh-MA-shun, “on fire”), so named because of the heat associated with it. Signs of inflammation include heat, redness, and swelling, all of which result when chemicals released from damaged tissues cause local blood vessels to dilate. This vasodilation increases blood flow and provides more nutrients and oxygen to the damaged tissue. However, it also causes the tissues to swell as fluid leaks out of the dilated vessels. A symptom of inflammation is pain in and around the damaged tissue resulting, in part, from edema exerting pressure on local pain receptors.
During inflammation, damaged tissues release a variety of chemicals that attract WBCs to the damaged site. The movement of a cell toward a chemical is chemotaxis. When WBCs detect these chemicals, known as chemotactic agents, they begin to cling to the blood vessel’s inner wall in a process called margination (mar-jeh-NĀ-shun). The WBCs then move along slowly downstream until the concentration of the chemotactic agent is sufficiently high enough to entice the cells to leave the vessel. The WBCs exit the vessel in a process called diapedesis (dī-a-pe-DĒ-sis; dia, through; pedesis, leaping), whereby it squeezes between the vessel’s endothelial cells. In the surrounding tissues, the WBCs can engulf foreign particles and remove dead cells via phagocytosis.
3. Fibrosis
After bleeding ceases, the clotted blood and tissue fluid harden to form a scab that protects the underlying tissue as it begins fibrosis, the second stage of wound healing. As its name implies, fibrosis involves production of numerous protein fibers. As a result, this stage is also referred to as the proliferation stage. Beneath the scab, fibroblasts produce fibers that help “bridge the gap” between the cut edges of the damaged tissue. At this time, capillaries, nerve endings, and other damaged structures begin to regenerate.
Near the end of the healing process, a scar forms due to excessive fibers being laid down in the wound site. Closing a large wound with sutures (stitches) limits the amount of fibrosis and can minimize scarring. Allowing excessive fibrous tissue production results in raised scars known as keloid scar (KE-loid, “tumor like”). During the fibrosis stage, blood platelets release platelet-derived growth factors (PDGFs) that stimulate fibroblasts to produce more extracellular fibers, thereby, accelerating tissue repair.
4. Maturation
As fibrosis causes the damaged tissue to return to a more normal state, evidence of tissue replacement is a pink sensitive tissue called granulation tissue, beneath the scab. As nerve tissue regenerates in the wounded area, they often send signals to the brain spontaneously, which causes the wound to itch. Eventually, scab will fall off, revealing the granulation tissue beneath.
5. Remodeling
Often, more replacement material is deposited during fibrosis and maturation than was originally present in the undamaged tissue. If this excess material is not needed for structural integrity, it may be removed slowly over time in a process called remodeling. During this stage, forces of tension, compression, and torsion will influence the alignment of collagen fibers, causing them to realign in the direction of the applied stresses.
TISSUE NECROSIS
Tissue death can occur in response to a number of factors, but one factor involves a lack of needed substances. Tissues deprived of oxygen or other nutrients for an extended time will die and decay. This process, called tissue necrosis (neh-KRŌ-sis; necro, corpse) is common during wound healing. However, when necrosis becomes extensive, a condition called gangrene (gan- GRĒN; “gnawing sore”) can develop and lead to septicemia (sep-ti-SĒ-mē-a; septi, putrefying; emia, blood), also known as blood poisoning. As bacteria decompose tissues, various chemicals form, including ptomaine (tō-MĀNE; “corpse”), which is a toxic alkaline material released when bacteria decompose certain amino acids. The foul smell of decaying tissue is most commonly due to a chemical called cadaverine (ka-da-ver-ĒNE; cadaver, to perish), which forms when bacteria decompose the amino acid lysine.
Cell Junction Matching Game
Match cell junctions with their functions and characteristics
Flashcards
Master the characteristics and types of body tissues.
Glossary
Comprehensive terminology for the Tissues chapter. Click on any term to see its definition.
