1. Epithelia tissue

2. Epithelial Tissue
-Tightly packed cells which form continuous sheets -Epithelial tissue are found lining the skin, airways, inside mouth, stomach, glands….. - Undergo continuous mitotic division

3. Seeley pg. 102

4. Functions of Epithelial Tissues
I protection
The coverings/ linings serve as selective barriers between the environment and the organ, prevent invasion of foreign bodies …
II Absorption
Cells often have microscopic projections on the apical surface of their plasma membranes called microvilli which increase surface area in order to facilitate absorption.

6. III Secretion
In glands, epithelial tissue is specialized to secrete specific chemical substances such as enzymes, hormones and lubricating fluids (glands)
IV Sensation
Converting signals from the environment into impulses which can be processed by the nervous system. (sense organs)
V Contractility
Contain the contractile proteins myosin and actin (glands)

7. Morphology of Epithelial tissue
- Epithelium compose of cells tightly-bounded
- They have free/apical surface that is not associated with other cells, a basal surface which is attached to a basement membrane and lateral surfaces attached to other cells with specialized cell to cell junctions

9. Cell to cell junctions
- Tight junction fuses the plasma membranes of cells on their lateral surfaces prevent molecules from moving between the cells and also serve as barriers to diffusion of molecules across epithelial surfaces. - Gap junction allows the exchange of nutrients, ions, signals between adjacent cells and are also able to communicate and coordinate their activity.

10. - Desmosome have Proteins embedded within the cell junctions, allowing individual cells to function as a solid sheet rather than as isolated cells to resist stretching forces. The intercellular space is very wide and the junctional complexes contain channels which permit ion flows and other passages - Hemidesome are very small complexes on the inner basal surface of cell connecting cell to basement membrane

11. Structure of Desmosome

12. Apical surface
- Specific structures which supports various functions are found on the apical surface of some epithelial tissues:
Microvilli
Cilia
Flagella.

13. Basement membrane
-Thin sheet of fibers that underlies the epithelium and separate epithelium from underlying connective tissues
- Composed of a special type of collagen and laminin
The fusion of two sub layers - basal lamina
(It consists of an electron-dense membrane) and reticular lamina an underlying network of reticular collagen

14. Function of Basement membrane -Serve as orientation platform in relation to other tissues and form basis upon which new cells attach themselves during cell division/healing. - The basement membrane acts as a mechanical barrier, preventing malignant cells from invading the deeper tissues

15. Classification of Epithelium Classified according to shapes, layers and by general functions.
I Simple epithelium Cells arranged in a single layer for covering/lining - Simple squamous epithelia Tissues composed of irregular, thin, flat cells with elongated nuclei ( alveoli, endothelium of vessels….)

17. - Centrally placed spherical nuclei.
Simple cuboidal
- Single layer of cube-shaped and hence similar in width & height.
- Centrally placed spherical nuclei.
- Kidney tubules, ovarian surface, Salivary/ thyroid gland

18. Simple columnar
- Tall, elongated, rectangular-like cells with the nucleus displaced toward the base of the cell.
- Found in the lining of the stomach, intestine, gallbladder and uterine tubes

20. II Stratified epithelium
Cells arranged in two or more layers.
Found primarily on the "wear and tear" surfaces of the body such that layers can be lost without exposing subepithelial layers.

21. Stratified Squamous - Multilayer of flat cells - keratinized (protein - keratin ) epidermis of the skin or nonkeratinized e.g mouth, vagina, and anal canal Stratified cuboidal -Sweat /salivary gland Stratified Columnar epithelium -Not very common. Pancreatic duct, conjunctiva ...

22. Stratified cuboidal

23. III Pseudostratified epithelium
- Intermediary between simple and stratified epithelium. - Consists of one layer of irregularly shaped and sized cells with nucleus at different levels, giving the appearance of more than one layer with misleading (hence "pseudo") impression that the epithelium is stratified - Trachea, bronchi, pharynx, nasal cavity, and urethra.

25. IV Transitional epithelium
- Unstable shape/size of cells - Found in elastic tissues - Can appear to be stratified cuboidal when the tissue is not stretched or stratified squamous when the organ is distended and the tissue stretches - Urinary bladder and Ureter