Tissues
1. Define tissue.
A tissue is a group of cells performing a specialized structural or functional role.
2. Name the four major types of tissue found in the human body.
The four major tissue types are epithelial, connective, muscle, and nervous.
3. Describe the general characteristics of epithelial tissues.
Epithelial tissues cover the body surfaces, cover and line internal organs, and compose glands. Because they cover the surfaces of all cavities and hollow organs, they always have a free surface (one exposed to the outside or having an open space). Epithelial tissues always anchor to connective tissue by a noncellular layer called the basement membrane. Generally epithelial tissues lack blood vessels. Epithelium reproduces readily and heals quickly. They are tightly packed with little intercellular material. Because of this, they serve as excellent barriers. Other functions include secretion, absorption, excretion, and sensory reception.
4. Distinguish between simple epithelium and stratified epithelium.
Simple epithelium occurs as a single cell or a single sheet of cells. Stratified epithelium consists of layers of cells.
5. Explain how the structure of simple squamous epithelium provides its function.
Simple squamous epithelium consists of a single layer of think, flattened cells. These fit together like floor tiles and the nuclei are broad and thin. Substances diffuse easily through this tissue. Because of this, simple squamous epithelium lines the alveoli of the lungs, forms the walls of capillaries, lines the insides of blood vessels, and covers the membranes that line body cavities. Because it is so thin, simple squamous epithelium is damaged easily.
6. Name an organ that includes each of the following tissues, and give the function of the tissue.
a. Simple squamous epithelium—Found in the walls of capillaries; it functions to allow the exchange of oxygen and waste products between the blood and the cells.
b. Simple cuboidal epithelium—Found in kidney tubules; it functions in secretion and absorption.
c. Simple columnar epithelium—Found in the intestinal tract; it functions in secretion of digestive fluids and absorption of nutrient molecules.
d. Psuedostratified columnar epithelium—Found in the passages of the respiratory system; the ciliated
free surface moves the mucous produced by goblet cells up the respiratory tract and out of the airways.
e. Stratified squamous epithelium—Forming the outer layer of the skin (epidermis); it becomes
hardened with keratin and makes a tough, dry, protective covering.
f. Stratified cuboidal epithelium—Found in the larger ducts of salivary glands; it provides extra protection.
g. Stratified columnar epithelium—Found in the male urethra; the goblet cells provide mucous for lubrication.
h. Transitional epithelium—Forming the inner lining of the urinary bladder; because of its stretchable
nature, it forms a barrier that prevents the contents of the urinary tract from diffusing back into the body fluids.
7. Define gland.
A gland is composed of cells specialized to produce and secrete substances. Most commonly these cells are columnar or cuboidal epithelium. One or more of these cells constitutes a gland.
8. Distinguish between an exocrine gland and an endocrine gland.
Exocrine glands secrete their products into ducts that open onto an internal or external surface. Endocrine
glands secrete directly into tissue fluid or blood.
9. Explain how glands are classified according to the structure of their ducts and the organization of their cells.
A single cell can make up an exocrine gland. This is called a unicellular gland. If it is made up of two or more cells, it is called a multicellular gland. Multicellular glands can be further subdivided into two groups based upon their duct structure. A simple gland has an unbranched duct. A compound gland has a branched duct.
These can be further classified into tubular glands (epithelial lined tubes), or acinar glands (saclike
dilatations).
10. Explain how glands are classified according to the function and the nature of their secretions.
Merocrine glands are glands that release fluid products through cell membranes without the loss of cytoplasm. Apocrine glands lose small portions of their glandular cell bodies during secretion. Holocrine
glands are glands that release entire cells filled with secretory products. They are also classed as secreting serous fluid or mucus.
11. Distinguish between a serous cell and a mucous cell.
Serous cells produce a watery fluid that has a high enzyme concentration. Mucous cells produce a thick mucus that is rich in the glycoprotein mucin.
12. Describe the general characteristics of connective tissue.
Connective tissue is found throughout the body and is the most abundant type by weight. It binds structures, provides support, serves as frameworks, fills spaces, stores fat, produces blood cells, protects against infection, and helps repair damage. These cells are not adjacent to each other like epithelial cells and have abundant intercellular material called matrix. This material consists of fibers and a ground substance whose consistency varies from fluid to solid. Connective tissue has a good blood supply and is well nourished. Bone and cartilage are quite rigid; however, loose connective tissue, adipose, and fibrous connective tissue are more flexible.
13. Define extracellular matrix and ground substance.
Matrix is intercellular material between the connective tissue cells. This matrix consists of a ground substance whose consistency varies from fluid to semisolid to solid. The ground substance binds, supports, and provides a medium through which substances may be transferred between the blood and cells within the tissue.
14. Describe the three major types of connective tissue cells.
Fibroblast—a fixed cell in connective tissues. It produces fibers by secreting protein into the matrix of connective tissues. Mast cells—another fixed cell releases histamine and heparin. Macrophages—wandering cells that can detach and move about. These are specialized to carry on phagocytosis.
15. Distinguish between collagen and elastin.
Collagen fibers are thick, threadlike, and made of the protein collagen. They are formed in long, parallel
bundles and are flexible, but not elastic. That is, they can bend, but they cannot stretch. They have great tensile strength and are important to structures such as tendons. Elastin is the protein that elastic fibers originate from.
These fibers are branched and form complex networks. They have low tensile strength, but are very elastic.
That is, they can be easily stretched and resume their original length and shape. They are the primary component of the vocal cords.
16. Explain the difference between loose connective tissue and dense connective tissue.
Dense connective tissue has abundant collagenous fibers that appear white. This is sometimes known as white fibrous connective tissue. Loose connective tissue or areolar tissue has sparse collagenous fibers.
17. Explain how the quantity of adipose tissue in the body reflects diet.
Individuals are born with a certain number of fat cells. Excess food calories are likely to be converted into fat and stored. This illustrates that the amount of adipose tissue in a human is reflective of the individual diet.
18. Distinguish between regular and irregular dense connective tissue.
Regular dense connective tissue has organized patterns of the fibers. It is very strong, enabling the tissue to withstand pulling forces. It often binds body parts together. Irregular dense connective tissue has thicker, interwoven, and more randomly organized patterns of fibers. This allows for the tissue to sustain tensions exerted from many different directions. It is found in the dermis of the skin.
19. Distinguish between elastic and reticular connective tissues.
Elastic connective tissue is made up of yellow elastic fibers in parallel strands or in branching networks. In the fibers of this tissue are collagen fibers and fibroblasts. This tissue is found in the walls of certain hollow internal organs. Reticular connective tissue is composed of thin, collagenous fibers arranged in a three-dimensional network. It supports walls of certain internal organs such as the liver, spleen, and lymphatic organs.
20. Explain why injured loose connective tissue and cartilage are usually slow to heal.
Because fibrous connective tissue and cartilage are so dense and so closely packed, they lack a direct blood supply. For this reason, nutrients diffusing from outside tissues take a long time to reach the cells. This makes injury repair a very slow process.
21. Name the major types of cartilage, and describe their differences and similarities.
a. Hyaline—the most common type of cartilage. It looks somewhat like white plastic. It is found at the ends of bones in many joints, in the soft part of the nose, and in the supporting rings of the respiratory passage.
It is also important in the development of bones.
b. Elastic—is very flexible and its matrix contains many elastic fibers. It is found in the external ears and in parts of the larynx.
c. Fibrocartilage—a very tough tissue, it contains many collagenous fibers. It is designed to function as a shock absorber. It forms the intervertebral disks and the protective cushions between bones in the knee and the pelvic girdle.
22. Describe how bone cells are organized in bone tissue.
The matrix for bone is laid down in thin layers called lamellae. The lamellae are arranged in concentric
patterns around tubes called osteonic canals. Between the layers of lamellae the osteocytes are placed in
depressions called lacunae. This pattern of concentric circles forms a cylinder-shaped unit called the osteon.
23. Explain how bone cells receive nutrients.
An osteon is a cylinder-shaped unit that the concentric circular pattern of bones cells form. Each osteonic canal contains blood vessels so that every cell is close to a nutrient supply. Bone cells also have cytoplasmic processes called canaliculi that extend outward and attach to the membranes of other cells. As a result, nutrients move rapidly between the bone cells.
24. Describe the composition of blood.
Red blood cells are the cells that carry oxygen to, and carbon dioxide from, cells. White blood cells function in immunity and infection control. Platelets are cellular fragments that function in blood clotting.
25. Describe the structure of epithelial membranes.
They are composed of epithelium and loose connective tissue.
26. Identify locations in the body of the four types of membranes.
Epithelial membranes cover body surfaces and line body cavities and organs.
Synovial membranes line joints.
Serous membranes line body cavities.
Mucous membranes line the cavities and tubes that open to the outside of the body.
27. Describe the general characteristics of muscle tissues.
Muscle tissues are contractile. Muscle fibers within the tissue change shape to become shorter and thicker. This causes muscle fibers to pull at the attached ends and move body parts.
28. Distinguish among skeletal, smooth, and cardiac muscle tissues, in terms of location, cell appearance, and control.
Skeletal muscle tissue is found in the muscles attached to bones and can be controlled by conscious effort.
Because of this, it is also called voluntary muscle tissue. The cells or muscle fibers are long and threadlike with alternating bands of dark and light cross-markings called striations. Each fiber has many nuclei located near the cell membrane. When the muscle is stimulated by nerve fibers, it contracts and relaxes.
Smooth muscle tissue is named for its lack of striations. It is found in the intestinal tract, urinary bladder, blood vessels, and other hollow organs. It is not consciously controlled, and is therefore called involuntary muscle. Smooth muscle cells are shorter than those of skeletal muscle, and have a single, centrally located nucleus.
Cardiac muscle is found only in the heart. Its cells, which are striated, are joined end to end by a specialized connection called an intercalated disk. The cardiac muscle fibers are branched and interconnected in a complex network. Although it is striated, it cannot be controlled voluntarily.
29. Describe the general characteristics of nervous tissue.
Nervous tissue is found in the brain, spinal cord, and peripheral nerves. It is composed of neurons (nerve cells) and supporting neuroglial cells.
30. Distinguish between neurons and neuroglial cells.
Nervous tissue is composed of two types of cells. Neurons, or nerve cells, are sensitive to changes in their
surroundings and respond to stimulation of impulses along nerve fibers. In addition to neurons, neuroglial cells serve to support the neurons and bind the nervous tissue together. Neuroglial cells carry on phagocytosis and bring nutrients to the neurons as well as remove waste from cells. They also serve to bind nervous tissue together. In many ways, these cells act as connective tissue found only in nervous tissue.
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