Epithelium (Epithelial Tissue)

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Four Basic Types of Epithelia:

• Epithelium.

• Connective (includes bone and blood).

• Muscle.

• Neural.

Lamina Propria:

Lamina propria is loose connective tissue immediately underlying an epithelium.

Epithelial cells form continuous and contiguous sheets that line Internal surfaces and cover external surfaces. Lack blood vessels and receive oxygen and nutrients via diffusion.

Epithelial Embryology:

Epithelium is derived from all 3 germ layers:

Ectoderm:

• Skin

Mesoderm:

• Coelom and blood vessels

Endoderm:

• Gut.

Functions of the Epithelium:

• Protective barrier.

• Transepithelial transport.

• Surface transport.

• Absorption and secretion.

• Sensory reception.

• Contractility.

Classification of Epithelia:

• Cell shape - squamous, cuboidal or columnar.

• Number of layers - simple or stratified.

• Keratinized or non-keratinized/mucosal.

• Cilia - with or without.

• Other, ex. Mucous.

• Pseudostratified and Transitional are exceptions.

Special Epithelia:

Consider the special cases: transitional and pseudostratified.

1. Name the number of cell layers.

2. Name the shape of the top layer.

3. Name specializations.

Basement Membrane:

Basement membrane is a LM term for the PAS+ structure at the base of epithelial cells. Basal lamina is a TEM term that refers to the composite structure consisting of: Lamina lucida (produced by epithelial tissue) Lamina densa (produced by epithelium) Lamina fibroreticularis (produced by connective tissue).

Basal Lamina:

Basal lamina is only the lamina densa and lamina lucida.

Type IV collagen:

• A mesh rather than fibrils, unique to BM.

Laminin:

• A sulphated glycoprotein, the major extracellular linker molecule between cells and ECM.

Type IV collagen Binding Sites:

• Heparin sulfate, entactin and cell surface.

• Heparin sulfate proteoglycan.

• Entactin sulfated glycoprotein.

Lamina externa:

Similar to BL but around smooth and striated muscle, Schwann cells of PNS and adipose cells.

Junctional Complex:

Historically, a "junctional complex" referred to the composite structures: tight junction, adhesion belt and spot desmosome At LM level is termed a "terminal bar."

Tight Junctions:

Tight junctions restrict movement across cells to different cell domains, apical, basal and lateral.

Hemidesmosome:

Intercalated disc of cardiac muscle. These are termed fascia adherens and resemble zonula adherens with actin.

Gap Junction:

Gap junction, communicating junction or nexus channels are gated.

Ions and small molecules 1,500 MW and lower can pass (amino acids, cyclic AMP, sugars and nucleotides) but larger molecules such as protein and nucleic acids are too large to pass.

Gap junctions in cardiac muscle Most cells in early embryos are coupled by gap junctions. This allows coordination of activities between cells. In embryogenesis, beginning in the 8 cell stage in mouse embryos, most cells are electrically coupled. As specific groups of cells begin to differentiate, they commonly uncouple. Gap junctions in embryos may provide positional information to control differentiation according to their location in the embryo.

Cadherins:

Cadherins (more than a dozen) mediate calcium dependent cell-cell adhesion. Main adhesion molecule in early embryonic tissues. E-cadherin is expressed in blastomeres at 8 cell stage. Cells then go on to form junctional complexes. The immunoglobulin family of proteins mediate calcium-independent cell-cell adhesion. Neural cell adhesion molecules (N-CAM) (at least 20 forms) are the best studied and are transiently expressed during Embryogenesis in other than neural cells. Nonjunctional contacts initiate tissue specific cell-cell adhesions that junctional complexes then orient and stabilize. Migrating embryonic cells rely on nonjunctional adhesions, then cells form junctional complexes when differentiating into tissues.

Additional Reading:

Basic Histology

1. Introduction to Histology
2. Basic Cell Physiology
3. Actin, Microtubules, and Intermediate Filaments
4. Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi
5. Epithelium (Epithelial Tissue)
6. Connective and Adipose Tissue
7. Types of Cartilage
8. Osteogenesis
9. Nervous Tissue
10. Muscle Tissue
11. Cardiovascular System
12. Blood and Hematopoiesis
13. Lymphoid Tissue
14. Digestive Tract I: Oral Cavity
15. Digestive Tract II: Esophagus through Intestines
16. Liver, Pancreas, and Gall Bladder
17. Respiratory System
18. Integument
19. Urinary System
20. Endocrine System
21. Male Reproductive System
22. Female Reproductive System
23. Eye and Ear

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