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What are the four primary types of tissue?
Epithelial, connective, muscle, and nervous
What is histology?
Histology is the study of the microscopic structures of tissues and organs.
List seven functions performed by epithelial cells.
Epithelial cells (1) protect, cover, and line other tissues; (2) filter biochemical substances; (3) absorb nutrients; (4) provide sensory input; (5) manufacture secretions; (6) manufacture excretions; and (7) act as an interface layer that separates and defines the beginning and ending of different types of tissues.
What four attributes characterize epithelial tissue in general?
- 1. Epithelial cells are polar, that is, they have a sense of direction relative to surrounding structures. Each epithelial cell has an apical surface and a basal surface, which are quite different from each other. The apical surface is the side of the cell that faces the lumen or body cavity, and the basal surface is the side of the cell that faces the underlying connective tissue.
- 2. Epithelial cells have lateral surfaces that are connected to neighboring cells by junctional complexes. These junctions bring the cells into close apposition to one another, leaving little room for extracellular matrix. The matrix that surrounds epithelia therefore exists in very small quantities, if at all.
- 3. All epithelial cells lack blood vessels or capillaries. They are avascular and rely on underlying connective tissue to provide oxygen and nutrients.
- 4. Although some epithelia lack nerves, for example, those in the stomach, intestines, and cervix, most epithelial cells are innervated and provide valuable sensory input.
List four types of cellular junctions. Can you describe them?
- 1. Tight junction: formed by the fusion of the outermost layers of the plasma membranes of adjoining cells. The matrix-filled space between cells is lost at the site of a tight junction. For centrally placed cells, the fusion occurs as a strip that wraps around the entire circumference of the cell like a belt. In this way, an impenetrable barrier is formed that prevents the passage of substances from the luminal end to the basal end of the cell and vice versa. Only by passing through the body of the cell can substances pass through the epithelial layer. Tight junctions are found in tissues in which there can be no leaks, for example, in the urinary bladder, where urine is held, or in the digestive tract, where tight junctions play a critical role in preventing the leakage of digestive enzymes into the bloodstream.
- 2. Desmosome: Strong, welded plaque that connects the plasma membranes of adjacent cells. The bond is a mechanical coupling formed by filaments that interlock with one another, just as plastic fibers do in Velcro. Tonofilaments, or intermediate filaments, may also extend from the desmosomic plaque into the cytoplasm of each cell like anchors, forming stabilizing bases for the membrane junction. In this way, desmosomes form tough bonds between cells and therefore are found most commonly in tissues that undergo repeated episodes of tension and stretching, such as the skin, heart, and uterus.
- 3. Hemidesmosome: Junctions that look like half-desmosomes and link epithelial cells to the basement membrane.
- 4. Gap junction: Made of tubular channel proteins called connexons and extend from the cytoplasm of one cell to the cytoplasm of another. These transmembrane proteins allow the exchange and passage of ions and nutrients (e.g., nucleotides, sugars, and amino acids) from one cell to another. Gap junctions are most commonly found in intestinal epithelial cells, the heart, and smooth muscle tissue. The function of gap junctions in epithelial cells is not yet fully understood, but their ability to quickly transport electrical signals from one cell to another explains their presence in cardiac and smooth muscle cells, where they help coordinate contraction.
How does the basement membrane act as a partial barrier between the epithelial cell and the underlying connective tissue?
Oxygen and nutrient molecules are supplied to the epithelial cells by diffusing through the basement membrane from capillaries in the underlying connective tissue. Similarly, nutrient substances that are absorbed and waste that is excreted by the epithelium diffuse across the basement membrane into the blood supply of the connective tissue.
Why do some epithelial cells have cilia and microvilli? What role do they play? Where are the cells with these specialized surfaces found in the body?
Microvilli increase the surface area of cells and allow more absorption and secretion. They are found on epithelial cells in the intestines and urinary tract. Cilia are found on the free surfaces of cells, usually in the respiratory and urogenital tracts. Ciliary movement occurs in coordinated “beats” which enable the efficient transport of material. In the trachea, cilia help propel mucus and debris up and away from the lungs toward the mouth. In the uterine tube, the beating motion of cilia encourages newly released ova into the oviduct, or infundibulum.
Epithelial tissue is characterized as simple, stratified, or pseudostratified. What does this mean?
If there is only a single layer of epithelial cells, the tissue is classified as simple. If there is more than one layer of cells, the tissue is called stratified. Pseudostratified columnar epithelium is an epithelial layer that is not truly stratified. The epithelial cells appear to be stratified because the nuclei are found at different levels across the length of the tissue layer. In addition, not all the cells reach the luminal surface; therefore the cells appear to be at different levels, as though stratified. In reality, each cell forms a distinct attachment, however subtle, with the basement membrane. In this way, pseudostratified columnar epithelium forms a single layer and therefore is considered a simple epithelium.
What are the three basic shapes of epithelial cells?
Squamous, cuboidal, and columnar
Draw a picture of each of the following types of epithelia and give an example of where each of them can be found in the body.
- 1. Simple squamous epithelium can be found in the inner lining of the lung (where oxygen is absorbed and carbon dioxide released) and in the filtration membranes of kidneys (where water and other small molecules are excreted as urine).
- 2. Simple cuboidal epithelium can be found on the surface of ovaries; in the secretory portions of glands, such as the thyroid; and in the lining of the ducts of the liver, pancreas, kidney, and salivary gland.
- 3. Simple columnar epithelium is found lining the length of the gastrointestinal tract from the stomach to the rectum.
- 4. Stratified squamous epithelium is found lining the mouth, esophagus, vagina, and rectum.
- 5. Pseudostratified columnar epithelium is found in the respiratory tract and in portions of the male reproductive tract.
- 6. Transitional epithelium is found in portions of the urinary tract where great changes in volume occur (urinary bladder, ureters, urethra, and calyxes of the kidney).
What is a gland?
A gland is a cell or group of cells that have the ability to manufacture and discharge a secretion.
How did glands develop embryologically?
Multicellular glands form during embryonic development from the infolding of a layer of epithelial cells. Initially, these "invaginations" form ducts and tubules that maintain contact with the surface epithelium. In the course of development, some of the glands lose the ducts and become separated from the parent epithelial sheet. In this way, glands are derived from epithelium.
What is the difference between endocrine and exocrine glands? Can you give examples of each?
- Endocrine glands do not have ducts or tubules, and their secretions are distributed throughout the body. They produce and secrete regulatory chemicals known as hormones into the bloodstream or the lymphatic system, where they are carried to many regions of the body. The pituitary gland in the brain and the adrenal gland near the kidney are examples of endocrine glands.
- Exocrine glands possess ducts. They are more common than endocrine glands and act by discharging secretions through their ducts directly into local areas, where they may
- cover cell surfaces or empty into body cavities. The secretions of exocrine glands act locally and do not normally enter the circulation. Examples include hepatoid, musk, sweat, and salivary glands. Exocrine glands in the liver secrete bile. The pancreas has both endocrine and exocrine glands.
Where are goblet cells found? What type of secretion do they produce?
The goblet cell is a modified columnar epithelial cell and is found interspersed among the columnar cells of the respiratory and digestive tracts and in the conjunctiva of the eye. Goblet cells secrete mucin, a thick, sticky mixture of glycoproteins and proteoglycans. When combined with water, mucin becomes mucus. The mucus functions in two ways: it helps protect the apical surface of the epithelial layer, and it assists with the entrapment of microorganisms and foreign particles.
In general, how are multicellular exocrine glands constructed?
Multicellular exocrine glands are made up of two distinct components, a secretory unit in which secretions are produced by secretory cells and a duct that carries the secretion to the deposition site. In most glands the secretory unit is surrounded by connective tissue rich in blood vessels and nerve fibers. It not only nourishes the secretory unit but also provides structural support and may extend into the gland to form distinct lobes. In some exocrine glands the secretory unit is surrounded by contractile cells called myoepithelial cells that assist with the discharge of secretions into the glandular duct.
Can you describe merocrine, apocrine, and holocrine glands? How do they differ from one another?
- Merocrine glands: The majority of glands package their secretions into granular units and release them via exocytosis as they are manufactured. In merocrine glands, the secretory cells remain undamaged during secretion. Secretion in apocrine glands involves the loss of the apex of the secretory cell.
- Apocrine glands: The secretory cells in apocrine glands do not release their granules as they are manufactured. Instead they store the granules until the apex of the cell is full, at which point the cell pinches in two and the top part (the apex) is released into the duct system. Later, the cell repairs the damage and repeats the process.
- Holocrine glands: Holocrine glands also store granules in the secretory cells until they are needed. However, in holocrine glands the entire secretory cell is destroyed in the act of releasing its secretory product. The subsequent degeneration of the cell allows the release of the granules.
How are serous and mucous secretions different?
Serous secretions are watery and contain a high concentration of enzymes, whereas mucous secretions are thick, viscous, and composed of glycoproteins. Secretory cells that manufacture both types of secretions are common in the digestive and respiratory tracts. Mixed exocrine glands contain both mucous and serous components.
How are connective tissue and epithelial tissue similar? How are they different?
Epithelial and connective tissue are similar in that they may be linked to form membranes in the body. Membranes are thin, protective layers that line body cavities, separate organs, and cover surfaces. They are composed of a multicellular epithelial sheet bound to an underlying layer of connective tissue. Unlike epithelial tissue, connective tissue is composed primarily of nonliving extracellular matrix. While epithelial tissue has no direct blood supply, connective tissue is vascularized, although the level of vascularity varies among different connective tissue types.
What are the three basic constituents of connective tissue?
Extracellular fibers, ground substance, and cells
List seven functions of connective tissue.
- 1. Forms metabolic and structural connections between other tissues
- 2. Forms a protective sheath around organs
- 3. Helps insulate the body
- 4. Acts as a reserve for energy
- 5. Provides the frame that supports the body
- 6. Composes the medium that transports substances from one region of the body to another
- 7. Plays a vital role in the healing process and in the control of invading microorganisms
What are GAGs and what role do they play in connective tissue? Why do you suppose animals with joint injuries are sometimes given dietary supplements of GAGs?
Glycosaminoglycans (GAGs) are the ground substance in soft connective tissue made of unbranched chains of glycoproteins. Animals with joint injuries are sometimes given GAGs because they may help with joint healing. Joints contain hyaluronic acid, which is the most commonly found GAG in connective tissue. GAGs are large molecules that help to orient the formation of fibers within the tissue during healing.
Compare and contrast collagenous, reticular, and elastic fibers.
- Collagenous fibers are strong, thick strands composed of the structural protein collagen. Collagen fibers are organized into discrete bundles of long, parallel fibrils, which in turn are composed of bundled microfibrils. Because they possess tremendous tensile strength enabling them to resist pulling forces, collagenous fibers are found in tendons and ligaments that are continually being pulled and stretched. When not under pressure, collagenous fibers look wavy. The fiber itself is white, and the tissue it forms when the fibers are packed closely together is also white. Therefore it is not surprising that collagenous fibers are sometimes known as the white fibers.
- Reticular fibers, like collagenous fibers, are composed of collagen, but they are not thick. Instead they are thin, delicate, and branched into complicated networks. Reticular fibers form a kind of "mist net" (rete is Latin for "net") that provides support for highly cellular organs such as endocrine glands, lymph nodes, spleen, bone marrow, and liver.
- Elastic fibers are composed primarily of the protein elastin. Like reticular fibers, elastic fibers are branched and form complex networks, but they lack the tensile strength of collagenous fibers. Elastic fibers are composed of bundles of microfibrils, and because they are coiled, they can stretch and contract like a rubber band. Therefore elastic fibers tend to occur in tissues that are commonly subjected to stretching (vocal cords, lungs, skin, and walls of blood vessels). Because of their color, elastic fibers are sometimes referred to as the yellow fibers.
What are fibroblasts and what role do they play in connective tissue?
Fibroblasts are fixed cells. They are large, irregularly shaped cells that manufacture and secrete both the fibers and the ground substance characteristic of their particular matrix. As the cells mature and the matrix is formed, fibroblasts adopt a less active role. If additional matrix is required later, the cells can convert back to the active form.
Can you give three examples of cells that are transient in connective tissue? Can you describe their form and function?
- Leucocytes, mast cells, and macrophages are transient cells in connective tissue. Leucocytes are relatively large and round, compared with red blood cells. They can squeeze through the walls of tiny blood vessels to enter the surrounding tissue. This process is called diapedesis. Leukocytes are important members of the defensive immune system.
- Mast cells are oval cells easily identified by the large number of dark-staining granules stored in the cytoplasm. These granules contain histamine and heparin, potent biochemicals that when released into the tissue initiate an inflammatory response. Histamine increases blood flow to the area by making the capillaries leaky, and heparin prevents blood from clotting and ensures that the pathways for increased blood flow remain open.
- Macrophages are massive, irregularly shaped phagocytizing scavengers that may be either fixed or transient in connective tissue. They engulf microbes, dead cells, and debris that are subsequently digested in the macrophage’s lysosomes. Mobile macrophages are drawn to sites of infection or inflammation, where they move aggressively through the affected area to engulf microinvaders. In this way they are an important part of the immune system and help tissues fight infection.
Connective tissue is divided into two broad categories. What are they?
The two broad categories of connective tissue are connective tissue proper and specialized connective tissue.
What are the components of areolar tissue?
Randomly placed fibers and cells suspended in a thick, translucent ground substance. The tissue appears relaxed with a myriad of round and star-shaped cells placed among crisscrossing fibers. The predominant cell is the fibroblast, a large spindle-shaped cell that manufactures the elastic, reticular, and collagenous fibers found throughout the tissue.
What is the common term for adipose tissue?
Both in terms of its form and function, how is brown fat different from white fat?
White adipose is found throughout the body, particularly in the deep layers of the skin. Initially, white adipocytes resemble fibroblasts, but as they fill with lipid, the organelles and nuclei are pushed to one side and the cells become large spheres with eccentrically placed nuclei. As the cells swell, the cytosol is compressed into a thin, barely visible rim that surrounds the lipid droplet. Despite the compact condition of the cytoplasm, it continues to house all the organelles normally found in cells. During tissue preparation for microscopic examination, the lipid content of the adipocyte is extracted, leaving a large unstained space in the center of the cell. This combined with the densely cellular nature of adipose tissue lends itself to the "chicken wire" appearance that is evident microscopically. Brown adipose fat is found in newborn animals and in animals that hibernate during the winter. It is a highly specialized form of adipose and plays an important part in temperature regulation because it is a site of heat production. In brown fat, as in white fat, the nucleus is eccentrically placed; however, the cytoplasm in brown fat is clearly visible, and lipid is stored in multiple small vesicles rather than in a single large droplet. In brown fat, the energy derived from the oxidation of lipids and the energy released from electron transport are dissipated as heat, not adenosine triphosphate (ATP). For this reason, brown fat contains an exceptionally high number of mitochondria (the site of electron transport), which become darkly stained in the cytoplasm. This dark coloration gives brown fat its name. Brown fat is also more vascular than white fat. Its rich vascular network helps distribute the heat produced to many areas of the body. In this way, neonatal animals and hibernating animals can generate enough body heat during vulnerable periods (after birth and during the winter) to survive. Histologically, brown fat looks glandular and therefore is sometimes called the hibernating gland.
What are three subtypes of dense connective tissue?
Three types of dense connective tissue are: dense regular, dense irregular, and elastic.
Give three examples of specialized connective tissue. How are they similar to connective tissue proper? How are they different?
- Three types of dense connective tissue are cartilage, bone, and blood.
- Cartilage is similar to connective tissue proper in that it is composed of cells, fibers, and matrix. It is different in that it is more rigid than dense connective tissue.
- Bone is similar to connective tissue proper in that it is also composed of cells, fibers, and matrix; however, bone is much more dense. In fact, it is the hardest, most rigid type of connective tissue.
- Blood is similar in that it has a matrix, plasma, a fibrous component that is visible when blood clots, and cells. It is different in that it is almost always fluid, but can clot when necessary.
Why is cartilage limited in thickness and slow to heal?
It is limited in thickness because nutrients diffuse from the surrounding perichondrium through the matrix to the chondrocytes. Therefore chondrocytes that are farthest away from the perichondrium are potentially less well nourished than cells close to it. Cartilage is slow to heal because it is avascular.
Describe three types of cartilage. What are their differences and similarities?
- Hyaline cartilage: Hyaline cartilage is the most common type of cartilage in the body. It is composed of closely packed collagen fibers that make it tough but more flexible than bone. Macroscopically, hyaline cartilage resembles a blue-white, frosted ground glass. It is found in the growth plates of long bones, where it supports continued bone development and the extension of bone length. Hyaline cartilage is the most rigid type of cartilage and is enclosed within a perichondrium.
- Elastic cartilage: Elastic cartilage is similar to hyaline cartilage but contains an abundance of elastic fibers, which form dense branching bundles that appear black microscopically. Elastic cartilage is found in the epiglottis of the larynx and in the pinnae ( external ears) of animals.
- Fibrocartilage: Fibrocartilage usually is found merged with hyaline cartilage and dense connective tissue. Like hyaline cartilage, it contains thick bundles of collagen fibers, but it has fewer chondrocytes and lacks a perichondrium. Fibrocartilage is found in the spaces between vertebrae of the spine, between bones in the pelvic girdle, and in the knee joint.
Even though blood and bone appear to be very different grossly, they both represent types of connective tissue. Why?
Blood and bone both contain cells, a matrix, and extracellular fibers.
Membranes are composed of what two tissue types?
Epithelial and connective tissue
Where are mucous membranes found? What functions do they perform?
Mucous membranes, or mucosae, are always found lining the organs with connections to the outside environment. These organs are part of the digestive, respiratory, urinary, and reproductive tracts and include the mouth, esophagus, stomach, intestines, colon, nasal passages, trachea, bladder, uterus, and others. With the exception of the mucosae of the urinary tract, mucosae in general can produce large quantities of protective and lubricating mucus. Goblet cells or multicellular glands may be found throughout the tissue. These structures are responsible for the production and secretion of mucus, which consists primarily of water, electrolytes, and a protein called mucin. Mucus is slippery and therefore can decrease friction and assist with the passage of food or waste. Because of its rich supply of antibodies and its viscous consistency, mucus is also helpful in the entrapment and disposal of invading pathogens and foreign particles. This is particularly apparent in the nasal passages, where microorganisms and debris are inhaled and trapped by mucus. Some mucosae can also absorb as well as secrete. For example, the epithelial layer in the intestine is specially designed for rapid and efficient transfer of nutrient molecules from the intestinal lumen to the underlying connective tissue and its blood supply. The mucosa therefore plays an important role in monitoring and controlling what enters the body, and mucous membranes form an important barrier between the outside environment and the delicate inner workings of the body. The secretory and absorptive qualities of mucosae make them particularly well suited for this role.
What portion of a serous membrane covers the outer surface of organs?
The visceral layer.
What is an effusion? What is ascites?
Effusion is the escape of fluid from its normal vessels into a body cavity, where it may accumulate in large amounts. Ascites is the presence of an effusion in the peritoneal space of the abdominopelvic cavity and can be caused by a wide range of pathological conditions (congestive heart failure, nephrosis, malignant neoplastic disease, and peritonitis among them).
What is another name for “cutaneous membrane”?
Where are synovial membranes found? How are they different from other membrane types?
Synovial membranes line the cavities of joints. They are different from other membrane types because they have no epithelium and are composed completely of connective tissue?
In what ways are muscle fibers uniquely adapted for contraction?
Muscle fibers are composed of specialized proteins called actin and myosin, which are arranged into microfilaments. Contraction, or shortening, of the muscle cell occurs when the microfilaments slide over one another like the bars in an old-fashioned slide ruler. In this way, the cells change shape and can be made shorter or longer.
List three types of muscle. How do they differ from one another?
- Three types of muscle are skeletal, smooth, and cardiac.
- Skeletal muscle cells are striated, or striped, because histologically they have alternating bands of light and dark. Unlike cardiac and smooth muscle, skeletal muscle is usually controlled through conscious effort and therefore is called voluntary muscle. (In other words, the animal can control its movement through conscious thought.) Thus skeletal muscle is striated voluntary muscle.
- Smooth muscle is composed of small, spindle-shaped cells that lack striations or bands and therefore appear “smooth.” Like skeletal muscle, smooth muscle may be stimulated to contract by the action of nerves, but unlike skeletal muscle,the contractions cannot be consciously controlled. Smooth muscle is therefore nonstriated involuntary muscle.
- Cardiac muscle exists only in the heart and possesses the remarkable ability to contract even when neural input has been altered. Specialized pacemaker cells within the heart muscle supply the signal for the heart to contract at regular intervals. This input is entirely involuntary and is responsible for initiating the pumping force which propels blood through blood vessels. Cardiac muscle is striated involuntary muscle.
What are the two basic cell types that make up neural tissue?
Two basic types of neural tissue are neurons and supporting neuroglial cells.
What is the most important function of neural tissue?
The most important function of neural tissue is to receive and transmit electrical and chemical signals throughout the body.
Describe the process of inflammation. What causes the clinical signs of heat, swelling, redness, and tenderness?
- Inflammation begins with a 5- to 10-minute period of vasoconstriction followed by a sustained period of vasodilation. The initial constriction occurs in the small vessels of the injured tissue and aids in the control of hemorrhaging. Histamine and heparin molecules released from mast cells stimulate vasodilation and increase permeability of capillaries. Blood flow to the area is increased, which in turn causes the clinical signs of heat and redness. Blood flow also increases the supplies of oxygen and nutrients to the active cells of the damaged tissue.
- Plasma fluid, composed of enzymes, antibodies, and proteins, pours into the affected area, causing swelling of the soft tissue structures. This swelling irritates delicate nerve endings and causes pain and tenderness in the affected area.
- Clot formation begins to take place, which slows bleeding. The clot also helps isolate the wound from the invasion of pathogens and helps prevent bacteria and toxins from spreading to surrounding soft tissue structures. A clot first forms when platelets become sticky and clump together. Fibrinogen, found in rich quantities in the swollen tissue, is converted to an insoluble protein called fibrin. Fibrin is woven into a netlike structure that surrounds the platelets and provides support and stability to the newly formed clot. It also forms a framework to support the movement of cells throughout the site. Clots that form on the skin eventually dry and become known as scabs.
- Large cells, such as macrophages and neutrophils (a type of white blood cell), move through blood vessels and can squeeze through dilated capillaries to assist in the removal of debris and microinvaders. The phagocytic cells are short lived, however, and can function for only a few hours before dying. Pus, which is an accumulation of dead and degenerated neutrophils and macrophages, may therefore collect in the injured area.
- With increased blood flow, histamine and heparin are dispersed, and their levels drop in the affected area. The decrease in these molecules causes the return of normal capillary size and permeability. When capillaries return to normal size, blood flow and fluid leakage into the affected area abate. Swelling, heat, and redness begin to subside.
- To sum up the sources of the clinical signs accompanying inflammation: Heat and redness are caused by increased blood flow to the area. Swelling is caused by fluid from plasma, composed of enzymes, antibodies, and proteins, pouring into the affected area. This swelling irritates delicate nerve endings and causes pain and tenderness in the affected area.
When does the healing process begin?
Healing begins with inflammation.
What is granulation tissue? Why is it important in the healing process?
Granulation tissue is a bright pink tissue that forms as macrophages work to clear debris from beneath the overlying blood clot or scab. Composed of a layer of collagen fibers manufactured by fibroblasts, granulation tissue is richly infiltrated with small permeable capillaries that have branched off existing capillaries in the deeper layers of the damaged tissue. These new tiny vessels push up into the bed of collagen fibers and provide rich supplies of nutrients and oxygen to hard-working cells such as fibroblasts, macrophages, and neutrophils. Macroscopically, the capillaries appear to be minute granules and therefore account for the name “granulation tissue.” Granulation tissue produces bacteria-inhibiting substances, making it highly resistant to infection.
Describe first, second, and third intention wound repair.
- First intention wound repair occurs with wounds whose edges are held in close apposition. These wounds may be superficial scratches or wounds that have been sutured or held closed with special bandages. The skin forms a primary union without the formation of granulation tissue or significant scarring.
- Second intention wound repair is also known as contraction and epithelialization. The edges of the wound are separated from each other, and the wound is allowed to heal without surgical closure. Granulation tissue forms to close the gap, resulting in scar formation.
- Third intention wound repair results when the wound is sutured at least three to five days after the injury. Granulation tissue is present in the wound by this time and helps to control infection and fill the tissue defect.