3. Epithelial tissue

Epithelial Tissue (Classification According to Structure)

1. Trabecular Epithelium: Cells arranged in cords or plates (e.g., liver, endocrine glands).
2. Reticular Epithelium: Cells organized in a 3D network (e.g., stroma of thymus, epithelium of tonsillar crypts, stellate reticulum).
3. Planar Epithelium: Tightly cohesive layers of cells, further classified by:
   • Shapes:
     • Squamous: Flattened thin cells with a thin nucleus.
     • Cuboidal: Cells with roughly equal dimensions and a large spherical nucleus.
     • Columnar.
   • Layer Number:
     • Simple: One layer of cells.
       • Simple Squamous: For diffusion (e.g., lining of blood vessels, endothelium) or as serous lining of cavities (e.g., pericardium, pleural cavities).
       • Simple Cuboidal: Mainly for secretion (e.g., ovary, thyroid).
       • Simple Columnar: For secretion and absorption (e.g., lining of intestine, gall bladder).
       • Pseudostratified Columnar: Nuclei at different levels (lining of respiratory tract).
         • Ciliated Cells: Columnar epithelial cells with ciliary modification.
         • Goblet Cells: Shaped like a wine glass, secreting mucus.
         • Basal Cells: Small, nearly cuboidal cells.

4.    

1. • • Stratified: Cell shape at the top of the layers is used as a definition.
       • Stratified Squamous Keratinized: For protection (e.g., epidermis of the skin).

1. • • Simple Squamous Non-Keratinized: Covers wet linings (e.g., mouth, esophagus, larynx, vagina).

1. • • Stratified Cuboidal Cells: For protection and secretion (e.g., developing ovarian follicles, sweat glands).

1. • • Stratified Columnar: For protection (e.g., conjunctiva).
     • Transitional Epithelium: Lines urinary bladder, ureter, and upper part of the urethra; dome-like cells that protect.

Epithelial Tissue (Classification According to Function)

1. Protection: Physically covering and lining surfaces (e.g., skin, urinary bladder) or acting as a selective barrier.
2. Secretion: E.g., sweat glands, columnar epithelium of stomach and gastric glands.
3. Transportation: Transport materials (e.g., via motile cilia in the small intestine or transcytosis across epithelium).
4. Sensation: Receive and transduce external stimuli (e.g., olfactory epithelium, photoreceptors, mechanoreceptors).
   • Primary Sensory Cells: Have axons (e.g., rods, cones, olfactory cells).
   • Secondary Sensory Cells: Lack axons (e.g., hair and taste sensory cells).
5. Myoepithelial Cells: Contract due to actin and myosin filaments; surround glandular cells (eccrine/merocrine and apocrine glands).
6. Germinal Epithelium:
   • In Males: Innermost layer of testicles (wall of seminiferous tubules).
   • In Females: Ovarian surface epithelium (layer of simple squamous to cuboidal cells covering the ovary).
7. Respiratory Epithelium: Found in the respiratory tract, moistens and protects airways (ciliated pseudostratified columnar epithelium).