I. The Study of Tissues (p. 172)

A. The Primary Tissue Classes (p. 172; table 6.1)

1. All tissues consist of extracellular material (matrix) and cells.

2. Spaces between cells and fibers within a tissue are filled with tissue fluid (also known as extracellular or interstitial fluid).

B. Embryonic Tissues (p. 172)

1. All mature tissues arise from the three primary germ layers of the embryo.

a. Ectoderm gives rise to the epidermis and nervous system.

b. Endoderm gives rise to the lining of the digestive and respiratory tracts and to the digestive glands.

c. Mesoderm gives rise to muscle, bone, blood, and other tissues.

2. Early embryonic tissue gives rise to mesenchyme, which in turn produces most of the permanent connective tissue, as well as muscle.

C. Interpreting Tissue Sections (p. 172)

1. Knowledge of tissues is obtained from the proper interpretation of their observation. (fig. 6.1; TR 129, 130)

2. Specimens are first preserved in fixative, then cut into histological sections. Last, they are stained and observed.

3. Various angles of observation include longitudinal, cross, transverse, and oblique sections. (fig. 6.2; TR 131 )

II. Epithelial Tissue (p. 174)

A. Epithelial tissues are classified as simple or stratified.

1. Epithelial tissue covers the body surface, lines body cavities, and covers many organs.

2. It adheres to underlying tissue by means of a basement membrane made up of collagen and glycoproteins.

B. Simple Epithelia (p. 175; fig. 6.3; TR 132)

1. Simple epithelium is made up of one layer of cells, all adhering to the basement membrane.

a. Simple squamous epithelium is one cell layer thick and is well-adapted to transporting substances across it. It has a single, centrally located nucleus. (fig. 6.4; TR 133; table 6.2)

b. Simple squamous epithelium lines the alveoli. The walls of capillaries are made up of the same tissue, but it is called endothelium.

c. Moist simple squamous epithelium is called mesothelium and covers the external surfaces of many body organs and cavities.

2. Simple cuboidal epithelium is made up of a single layer of cube-shaped cells with a centrally located nucleus. (fig. 6.5; TR 134; table 6.2)

a. This type is common in glands, and lines kidney tubules and lung bronchioles (where it is ciliated).

3. Simple columnar epithelium is a single layer of vertically elongated, thin cells. (fig. 6.6; TR 135; table 6.2)

a. Nuclei are present in the basal one-third of the cell.

b. This tissue often has a secretory function, but absorbs nutrients in the small intestine. It is also found lining the uterine tubes and in the uterus, stomach, and large intestine.

4. In pseudostratified epithelium, not every cell reaches the basement membrane. (fig. 6.7; TR 136; table 6.2)

a. This type is abundant in the respiratory tract where it is ciliated and produces mucus.

b. It is also found within the male reproductive tract, and often contains goblet cells.

C. Stratified Epithelia (p. 175; see fig. 6.3, TR 132)

1. The surface cells of stratified squamous epithelium are flattened, while deeper cells take on a variety of shapes. (see figs. 6.10, 6.11; TR 139, 140; table 6.3)

a. It is the tissue of the epidermis of the skin.

b. Within the epidermis, older cells become keratinized.

c. Nonkeratinized stratified squamous epithelium can be found on the tongue, and in the esophagus, vagina, and anal canal. (figs. 6.8, 6.9; TR 137, 138)

2. Stratified cuboidal epithelium lines the follicles in the ovary and seminiferous tubules in the testes. Sweat ducts are also lined with this type of tissue. (fig. 6.12; table 6.3)

3. Stratified columnar epithelium is a rare tissue found in short transitional zones where one type of epithelium grades into another, such as in the pharynx, larynx, anal canal, and male urethra.

4. Transitional epithelium is found in the urinary system where it is capable of distention. (fig. 6.13; table 6.3)

a. When not stretched, it appears to have many cell layers; when distended, there are two to three layers of cells.

III. Connective Tissue (p. 178)

A. Overview (p. 178)

1. Subclasses of connective tissue include fibrous tissue, fat, cartilage bone, blood, and others.

2. Connective tissue is functionally diverse: it binds organs, provides support, facilitates movement, protects, provides immune defense, stores energy and minerals, helps to produce heat, and transports within the bloodstream.

B. Fibrous Connective Tissue (p. 181)

1. Components of fibrous connective tissue include fibroblasts, macrophages, leukocytes, plasma cells, mast cells, and adipocytes.

a. Fibroblasts are the most common cells of connective tissue. They are large, flat, branching cells that produce fibers and ground substance.

b. Macrophages wander through the connective tissues, destroying bacteria, foreign particles, and dead and dying cells and activating the immune system.

c. Leukocytes, especially neutrophils, reside in connective tissue and react against bacteria, toxins, and other foreign matter.

d. Plasma cells produce antibodies and are only found in inflamed tissue and the wall of the digestive tract.

e. Mast cells, found near blood vessels, produce heparin and histamine.

f. Adipocytes (fat cells) appear in some types of fibroconnective tissues.

2. Fibers are made of protein. Three types are found in connective tissue: collagenous, reticular, and elastic.

a. Collagenous fibers are tough, flexible, and resist stretching. (fig. 6.14)

i. Collagen constitutes 25% of the body's protein. ii. These are also called white fibers.

b. Reticular fibers are thin collagen fibers in reticular connective tissue.

c. Elastic fibers are made of the stretchy protein elastin. These are also called yellow fibers.

3. Amid the cells and fibers in some tissue sections is a lot of space occupied by ground substance.

a. The components of ground substance are tissue fluid, minerals, and proteoglycans, the especially large colloidal particles that form a viscous tissue gel. (fig. 6.15; TR 141)

b. In blood vessels and bones, tissue gel is made up of chondroitin sulfate; in fibrous connective tissue, hyaluronic acid or heparin comprises the gel tissue.

4. One of two broad types of fibroconnective tissue is loose connective tissue (areolar, reticular, and adipose).

a. Areolar tissue has loosely organized fibers and all six cell types. (fig. 6.16; TR 142; table 6.4)

i. Most of the space is fluid-filled and highly vascular.

ii. Areolar tissue surrounds blood vessels and nerves, supports epithelium, attaches skin to underlying organs, and forms the lamina propria.

b. Reticular tissue is a mesh of reticular fibers and fibroblasts in the walls of lymph nodes, spleen, thymus, and bone marrow. (fig. 6.17; TR 143; table 6.4)

c. Adipose tissue is areolar tissue with clusters of adipocytes.(fig. 6.18; TR 144; table 6.4)

i. Most fat is located between the skin and muscle (subcutaneous fat), as well as in bone marrow and around the eyes, heart, kidneys, and the spinal cord.

ii. Obesity is a significant health risk. Most adults have white fat, but infants may also have brown fat that generates heat.

5. The second type of fibroconnective tissue is dense connective tissue (dense regular and dense irregular).

a. Dense regular connective tissue is the tissue of tendons and ligaments. (fig. 6.19; TR 145; table 6.5)

i. All its fibers run parallel to each other, lending strength to tendons but allowing them to pull in one direction only.

ii. Yellow elastic tissue, a variant of dense regular connective tissue, is found in the vocal cords and ligaments of the spinal column and penis. This type of tissue also imparts elasticity to walls of arteries.

b. Dense irregular connective tissue has dense bundles of collagen fibers and little ground substance. (fig. 6.20; TR 146; table 6.5)

i. The bundles run in random directions, so this tissue can resist stretching in many directions.

ii. This type of tissue forms most of the dermis and makes a protective capsule around the spleen, kidneys, testes, as well as bones, cartilage, and nerves.

C. Cartilage (p. 184)

1. Cartilage cells are called chondroblasts while they are secreting the gel-like matrix that surrounds them, then turn into chondrocytes when they are trapped in depressions called lacunae.

2. Hyaline cartilage has a smooth, glassy appearance due to its thin collagen fibers. (fig. 6.21; TR 147; table 6.6)

a. It is found at the ends of bones, in movable joints, and in the fetal skeleton.

3. Elastic cartilage is yellow and is covered with a perichondrium. (fig. 6.22; TR 148; table 6.6)

a. It is found in the outer ear, epiglottis, and auditory tube.

4. Fibrocartilage is transitional between dense fibroconnective tissue and hyaline cartilage. (fig. 6.23; TR 149; table 6.6)

a. It has parallel fibers, but no perichondrium.

b. It is found in intervertebral discs and the pubic symphysis; pads of fibrocartilage are located in the knee.

D. Bone (p. 188)

1. Spongy bone fills the heads and shafts of long bones and has a spongy appearance.

2. Compact bone is dense tissue, forming the exterior of all bones. (fig. 6.24; TR 150; table 6.7)

a. It is arranged into microscopic osteons, with a central (haversian) canal.

b. Layers (lamellae) of cells (osteocytes) surround the canal.

3. Whole bones are surrounded by a tough, fibrous periosteum.

E. Blood (p. 188)

1. Blood consists of a ground substance (plasma) and formed elements, erythrocytes (red blood cells), leukocytes (white blood cells), and platelets. (fig. 6.25; TR 151; table 6.8)

IV. Nervous and Muscular Tissue—Excitable Tissues (p. 190)

A. Nervous Tissue (p. 190)

1. Nervous tissue consists of cells conducting impulses (neurons) and helper cells (neuroglia).

2. Neurons are made up of a soma (cell body), dendrites (fibers that receive the message), and one axon (nerve fiber) that conveys the message away from the soma. (fig. 6.26; TR 152; table 6.9)

B. Muscular Tissue (p. 190)

1. Muscular tissue is specialized for locomotion and movement of substances through the body and for speech, respiration, and heat production.

2. The three histological types of muscular tissue are skeletal muscle cardiac muscle, and smooth muscle.

3. Skeletal muscle consists of cylindrical fibers, usually attached to bones. (fig. 6.27; TR 153; table 6.10)

a. Some skeletal muscles form a ringlike sphincter.

b. Striations appear, thus this form of muscle is striated muscle, as well as voluntary and multinucleate.

4. Cardiac muscle comprises the heart and nearby major blood vessels. (fig. 6.28; TR 154; table 6.10)

a. Cardiac muscle is also striated but involuntary.

b. Cells of cardiac muscle are called myocytes; these cells branch and join each other by intercalated discs to allow electrical communication between cells.

5. Smooth muscle is involuntary. (fig. 6.29; TR 155; table 6.10)

a. Smooth muscle cells are short and fusiform, with one centrally located nucleus.

b. Smooth muscle is located within the viscera and the lining of the alimentary canal, blood vessels, and urinary tract.

V. Intercellular Junctions, Glands, and Membranes (p. 191)

A. Intercellular Junctions (p. 191; fig. 6.30; TR 156)

1. Intercellular junctions occur between adjacent cells and help maintain the structural integrity of a tissue as well as allowing communication between cells.

2. Tight junctions are formed by a zipperlike fusion of membrane proteins of adjacent cells.

a. In the intestine, tight junctions keep bacteria out and ensure that nutrients are absorbed into cells, rather than passing between them.

3. A desmosome is a "spot weld" between cells that holds them together to help the tissue resist mechanical stress.

4. Gap (communicating) junctions are formed by a ringlike structure made up of membrane proteins.

a. Chemical communication and nutrients can pass from one cell to the next through gap junctions.

B. Glands (p. 193)

1. Glands are made up of individual cells or organs that produce a substance and secrete it elsewhere.

a. Glands are made mostly of epithelium.

b. Goblet cells are unicellular glands found among nonsecretory epithelium.

2. Glands are broadly classified as endocrine and exocrine.

3. Endocrine glands secrete their products (hormones) into the bloodstream.

4. Exocrine glands have ducts that convey their secretions to their destinations. (fig. 6.31; TR 157)

a. Most exocrine glands are enclosed in a capsule that extends into the interior of the gland, dividing it into lobes. Lobes are further subdivided into lobules.

b. Cells that produce the secretions are called parenchyma cells.

c. Ducts leaving exocrine glands can be simple or compound, and glands are tubular or acinar. (fig. 6.32; TR 158)

5. Glands are classified not only by their structure but by the nature of their secretions.

a. Serous glands produce a thin, watery fluid (perspiration, milk, tears, digestive juices).

b. Mucous glands secrete a glycoprotein (mucin) that, combined with water, produces mucus.

c. Mixed glands produce a mixture of the other two types of secretions.

d. Cytogenic glands (testes and ovaries) produce cells.

6. Glands are classified as merocrine or holocrine depending on how they produce their secretions. (fig. 6.33; TR 159, 160)

a. Merocrine (eccrine) glands have secretory vesicles and release products by exocytosis; these include tear glands, the pancreas, gastric glands, and others.

b. Holocrine glands accumulate a product and secrete it as the gland cells rupture (e.g., oil glands).

c. Apocrine glands produce merocrine-type secretions (certain sweat glands, mammary glands) and were once mistakenly thought to represent a third type of gland.

C. Membranes (p. 195)

1. The comparatively dry cutaneous membrane, composed of stratified squamous epithelium with a layer of fibroconnective tissue, makes up the skin.

2. Mucous membranes lining passageways that open to the exterior are made up of epithelium, areolar connective tissue (lamina propria), and sometimes a muscle layer (muscularis mucosae). (fig. 6.34; TR 161)

3. Serous membranes are made up of simple squamous epithelium and areolar connective tissue. They produce serous fluid and line the insides of body cavities and cover some of the organs inside.

4. Certain joints are lined with synovial membranes that lack epithelium but secrete a lubricating synovial fluid.

VI. Tissue Growth, Development, Death and Repair (p. 197)

A. Changes in Tissue Type (p. 197)

1. Unspecialized tissues undergo differentiation during embryonic development.

2. Metaplasia is transformation of one tissue into another.

B. Tissue Growth (p. 198)

1. Hyperplasia is cell multiplication.

2. Hypertrophy is the enlargement of existing cells.

3. Neoplasia is the growth of abnormal, nonfunctional cells.

C. Tissue Shrinkage and Death (p. 198)

1. Atrophy is shrinkage of existing tissue.

2. Necrosis is the premature, pathological death of tissue.

3. Apoptosis is the normal death of cells that have completed their function.

D. Tissue Repair (p. 198)

1. Tissue is repaired by either regeneration or fibrosis. (fig. 6.35; TR 162)