Lecture 8. 9. 10
Objectives: 1. List several structural and functional characteristics of epithelial tissue.
2. Name, classify, and describe the various types of epithelia; also indicate their chief function(s) and location(s).
3. Define gland. Differentiate between exocrine and endocrine glands.
4. Indicate common characteristics of connective tissue, and list and describe its structural elements.
5. Describe the types of connective tissue found in the body, and indicate their characteristic functions.
6. Indicate the general characteristics of nervous tissue.
7. Compare and contrast the structures and body locations of the three types of muscle tissue.
Histology : the study of tissues
Tissue : a group of similar cells with a particular function
Types of Tissues
I. Epithelial—Special characteristics : close packed cells; little ECM (extracellular matrix), continuous sheets bound by tight junctions and desmososmes. Apical or free surface and basal or attached surface. Attached to a basement membrane of collagen fibers (often from areolar or loose connective tissue). Avascular but in contact with nerve endings. Very regenerative.
**Homeostatic imbalance: cancerous epithelial cells will penetrate the basement membrane to tissues underneath (metastasis).
A. Functions: covers body surfaces or lines body cavities or tracts, vessels, and comprises glandular tissue. Functions include: protection, absorption, filtration, excretion, and secretion.
B. Classification
1. Simple vs. Stratified : simple is a single layer of cells for absorption, exchange and filtration. Stratified is made of multiple layers of cells for protection, often in areas of abrasion
(skin, mouth)
2. Shape : squamous: flattened or scalelike
cuboidal: boxlike
columnar: tall and column-shaped
C. Types
1.
Simple squamous epithelium : cells are flattened, cytoplasm is sparse. Carries out filtration or exchange of materials by diffusion. Found in lungs, endothelium
(inner lining) of lymph, blood vessels, and heart. Capillaries are exclusively endothelium. Forms mesothelium in serous membranes.

2. Simple cuboidal epithelium : Kidney tubules are lined with simple cuboidal epithelium, also found in glandular epithelium and some small ducts. Carries out secretion and absorption.
3. Simple columnar epithelium : Lines digestive tract from stomach to rectum.
Ciliated form found in uterine tubes. Dense microvilli on apical surface present to increase surface area for absorption. Goblet cells secrete a protective, lubricating mucus.
4. Pseudostratified (ciliated) columnar epithelium : Found in trachea and bronchi of respiratory tract. All cells rest on the basement membrane but only the tallest cells reach the free surface of epithelium. Secretion and absorption, cilia propel mucus, trapping dust and moving it away from lungs.
5. Stratified squamous epithelium : Found in epidermis of skin, mouth, esophagus, vagina. For protection; free surface cells are squamous, lower layers are cuboidal. Subject to wear and tear; surface cells are rubbed away and replaced. Layers farther away from basement membrane atrophy, flatten, and die.
6. Transitional epithelium : Lines urinary organs. Basal cells are cuboidal, apical are rounded when bladder is empty, flattened when filled.
D. Glandular
1. Exocrine, endocrine, and holocrine : exocrine glands have ducts to the surface
(salivary, mammary, etc.) Holocrine exocrine glands rupture to release cell contents
(sebaceous). Endocrine glands do not have ducts but secrete their products into the surrounding interstitial fluid and then into the bloodstream.
II. Connective Tissue
A. Common characteristics : vascularized, except for cartilage; consists of an extracellular matrix (ECM) made of ground substance, fibers, and cells. Ground substance may be chondroitin sulfate or hyaluronic acid. Fibers can be collagen, that are tough with tensile (pulling) strength; elastic, that are stretchy and branching; and reticular, that are made of fine collagen to form at network (“rete” in Latin is “network”). Cells include fibroblasts (common in loose connective and in some dense connective), chondrocytes (cartilage), and osteocytes (bone).
B. Types of connective tissue
1. Mesenchyme (embryonic stem cells) : precursor to all connective tissues, formed from mesoderm (middle embryonic germ layer).

2. Loose connective i. Areolar : forms the packing material between other tissues (epithelium, mucous membranes, organs, capillaries). It is a loose arrangement of fibers (collagen, elastic, reticular), the rest is ground substance of hyaluronic acid and fibroblasts. It is a reservoir of water and salt and edema (swelling) occurs in this tissue.
Supports, binds, holds body fluids, defends against infection (with lymphocytes, mast cells, and phagocytes within it) and stores nutrients. ii. Adipose : “fat cells”. Very little ECM, mature cells are amitotic (do not divide once formed), richly vascularized; found under skin and over organs. Stores lipid in cells to insulate and act as a shock absorber, may also help make testosterone, estrogen. Miles of capillaries in every pound of fat. iii. Reticular : reticular fibers and fibroblasts, found in lymph nodes, spleen, and bone marrow to support free blood cells.
3. Dense connective i. Dense regular connective : consists of collagen fibers running in the same direction, parallel to the direction of pull. Produces a white, flexible structure with resistance to tension. Rows of fibroblasts are found between the fibers. It is poorly vascularized. Forms tendons, aponeuroses, and ligaments. ii. Dense irregular connective : collagen fibers are thicker and arranged irregularly in more than one plane, compared to dense regular. Found in dermis of skin, joint capsules, covers testes, kidneys, bone, cartilage, muscle, and nerves.
4. Cartilage : consists of chondroblasts in lacunae of growing cartilage; avascular, so it heals slowly. Its matrix is made of chondroitin sulfate and hyaluronic acid. It resists tension and compression because it is 80% water. i. Hyaline : “gristle”; most abundant cartilage type. Consists of uniform collagen fibers in matrix for a glossy or glass-like appearance. Found at the ends of long bones, tip of nose, costal cartilage (between sternum and ribs), respiratory passages, embryonic skeleton and epiphyseal plate. ii. Elastic : matrix has more elastic fibers than hyaline; found in external ear
(pinna) and epiglottis. iii. Fibrocartilage : consists of chondrocytes in rows that alternate with collagen. Designed to resist heavy pressure between vertebrae and forms meniscus of knee.
5. Bone : consists of osteocytes imbedded in a mineral matrix, in an orderly arrangement called osteons.

6. Blood : formed elements (cells called erythrocytes (RBC’s), leukocytes (WBC’s) and thrombocytes (platelets)) in a fluid matrix.
III. Nervous : composed of neurons that conduct impulses, and neuroglia that support and protect neurons.
IV. Muscle
A. Skeletal : Cylindrical cells that are multinucleated, parallel, striated and act voluntarily.
B. Cardiac : Branching cylindrical cells with a single nucleus, striated with intercalated discs and gap junctions, and involuntary.
C. Smooth : Spindle-shaped cells with a central nucleus, no striations, organized into sheets and involuntary.