I. Tissues

• Tissues are groups of similar cells that share structure and function.

  • Origin: Come from the same part of the embryo.

  • Function: Carry out specific functions for the body, typically as part of an organ.

• Four major tissue types:

  1. Epithelial tissue

  2. Connective tissue

  3. Muscle tissue

  4. Nervous tissue

• An organ is composed of at least two different tissue types, usually a mix of all four types. Tissues are organized into organs that carry out specific functions for the body.

II. Epithelial Tissues

A. General Locations

1. Lining body surfaces that open to the outside (e.g., oral cavity, nasal cavity).

2. Lining internal body cavities.

3. Glands (e.g., sweat, salivary, endocrine).

B. General Functions

1. Protection (e.g., skin).

2. Sensory Functions (e.g., tactile cells in the stratum basale).

3. Absorption and Filtration

   • Example: Epithelial tissue of the small intestine aids in nutrient absorption.

   • Example: Epithelial tissue of kidneys filters blood.

4. Secretion

   • Example: Epithelial tissue of the stomach secretes acid.

   • Endocrine glands secrete hormones.

5. Gas Exchange

   • Example: Epithelial surfaces in lungs and capillaries facilitate gas exchange.

C. General Structure

• Epithelial cells are tightly packed together.

• On the lower surface, epithelial cells are attached to a basement membrane.

• Capillaries in the underlying connective tissue allow blood flow but do not penetrate epithelial tissue.

D. Specializations of Structure

1. Microvilli (in small intestine) increase surface area for absorption.

2. Cilia (in airway of lungs) aid in movement of mucus.

3. Goblet Cells - Unicellular glands that produce mucus.

E. Classification of Epithelial Tissue

• Based on:

  1. Cell Shape: Squamous, Cuboidal, Columnar.

  2. Number of Layers: Simple, Stratified.

Glandular Epithelial Tissue

• Exocrine Glands

  • Have ducts through which their secretions travel (e.g., lacrimal, sweat, salivary glands).

• Endocrine Glands

  • Ductless; hormones diffuse into the extracellular fluid (e.g., pituitary, thyroid, ovaries).

III. Specific Types of Epithelial Tissue

A. Simple Squamous Epithelial

• Function: Facilitates rapid diffusion and reduces friction.

• Locations: Lungs, lining of blood vessels, body cavities.

B. Simple Cuboidal Epithelial

• Function: Absorption, secretion.

• Locations: Kidney tubules, certain glands (e.g., thyroid).

C. Simple Columnar Epithelial

• Function: Absorption and secretion (especially mucus).

• Locations: Lining of small intestines and oviducts, often contains goblet cells.

D. Stratified Squamous Epithelial

• Two Types:

  1. Keratinized - outer layers consist of dead cells packed with keratin.

  2. Non-keratinized - outer layers are living cells.

• Function: Protection from abrasion and pathogens.

• Locations: Skin, cornea, body cavities that open to the outside.

E. Stratified Cuboidal Epithelial

• Function: Secretion and some protection.

• Locations: Ducts of sweat and salivary glands.

F. Stratified Columnar Epithelial

• Function: Secretion and protection.

• Locations: Larynx, mammary gland ducts, parts of the male urethra.

G. Pseudostratified Columnar Epithelial

• Function: Secretion and movement of mucus.

• Locations: Lining of nasal cavity, trachea, and upper respiratory tract.

Effects of Smoking on Ciliated Epithelial Tissue

• Smoking damages cilia and mucus function.

• Without cilia, mucus builds up, causing symptoms known as “smokers cough.”

H. Urothelium (Transitional Epithelium)

• Function: Allows structures to stretch and provides protection from urine chemicals.

• Locations: Ureters, bladder, urethra.

IV. Epithelial Cell Junctions

A. Gap Junctions

• Hold cells together with hollow cylinders (connexons), allowing the passage of small molecules and ions.

B. Desmosomes

• Hold cells together with proteins and glue, resisting stretching and tearing.

• Common between superficial epithelial cells.

C. Tight Junctions

• Interlocking proteins connect cell membranes, blocking the passage of substances between cells.

• Common in superficial epithelial layers (e.g., the skin).

Multiple Junction Types

• Two cells can have a mixture of different junctions, providing a strong and functional framework.