Histology
Histology of Epithelium
Definition and Characteristics of Epithelium
- Definition: Continuous layers of cells covering free surfaces.
- Common Characteristics:
- Tightly Packed: Cells are closely joined together.
- Little to No Extracellular Matrix (ECM): Minimal space between cells.
- Intercellular Glycosaminoglycans (GAGs): Small amounts allow cells to glide over each other and help in leucocyte and connective tissue cell migration.
- Junctional Complexes: Attachment points holding adjacent cell membranes together.
- Avascular: No direct blood supply; nutrients obtained via diffusion from the basement membrane.
- Rest on Basement Membrane: Separates epithelium from underlying connective tissue.
- High Cell Division Rates: Rapidly renewing tissue.
Functions of Epithelium
- Protection: Shields underlying tissues from injury, dehydration, and bacterial invasion.
- Example: Epidermis of the skin.
- Regeneration: High turnover allows for quick healing.
- Skin: Wound healing and epidermal replacement every 28 days.
- Gastrointestinal Tract: Cell replacement every 4 to 6 days.
- Secretion: Glandular epithelial cells produce substances.
- Into blood (e.g., thyroid hormones).
- Into ducts (e.g., pancreatic enzymes).
- Onto skin (e.g., sweat).
- Absorption: Acts in the intake of various substances.
- Example: Lipids in the small intestine; sodium in kidney tubules.
- Detection of Sensations: Specialized cells detect stimuli.
- Lubrication: Mucus secretion along the gastrointestinal tract (GIT).
- Example: Mesothelial cells secrete serous fluid.
- Excretion: Removal of waste products (kidney tubules, sweat).
- Diffusion of Gases: Function of endothelium in capillaries for gas exchange in lungs.
Classification of Epithelium
- Mesothelium: Lines closed cavities (pleural, pericardial, peritoneal).
- Endothelium: Lines cardiovascular and lymph vessels.
- Membranous (Lining/Covering) vs. Glandular Epithelium.
- Shape Classification:
- Stratified: Named by the shape of the uppermost layer.
Glandular Epithelium Classification
- Classified by duct branching and shape of acini (secretory part).
- Secretory portions often stain lighter; ductal portions stain darker:
- Remember: "Double D – Ducts are Dark!"
Types of Glandular Secretion
- Merocrine (Eccrine): Secretions stored in vesicles released by exocytosis.
- Apocrine: Secretion involves pinching off of part of the cell.
- Holocrine: Entire mature cell dies and becomes the secretion.
Summary Table: Epithelium Classification
| Number of Cell Layers | Cell Form/Structure | Main Function | Examples of Distribution |
|---|---|---|---|
| Simple | Squamous: Flat cells with elongated nucleus | Facilitates movement, secretion, protection | Blood vessel linings, alveoli, Bowman’s capsule |
| Simple | Cuboidal: Cube-shaped cells | Covering, secretion, absorption | Thyroid follicles, kidney tubules, ovary covering |
| Simple | Columnar: Long cells | Protection, absorption, lubrication | Stomach, intestine lining, gall bladder |
| Pseudostratified | Columnar with nuclei at varying levels | Protection, secretion | Lining of trachea, bronchi, epididymis |
| Stratified (>2 layers) | Squamous (Keratinized): Flattened | Protection from abrasion | Epidermis |
| Stratified (>2 layers) | Squamous (Non-Keratinized): Moist | Protection, secretion, prevents water loss | Mouth, esophagus, vagina |
| Stratified (>2 layers) | Cuboidal | Protection, secretion | Sweat glands, gland ducts |
| Stratified (>2 layers) | Columnar | Protection | Salivary glands |
| Stratified (>2 layers) | Transitional | Protection, distensibility | Bladder, ureter |
Skin and Associated Structures
- Layers of Skin:
- Epidermis: Outer layer for protection.
- Stratum Basale: Lowest layer with cuboidal cells; site of mitotic division.
- Contains melanocytes and Merkel cells.
- Stratum Spinosum: Contains spiky keratinocytes filled with tonofilaments; largest layer.
- Stratum Granulosum: Flatter keratinocytes secrete lipids, forming a barrier.
- Stratum Lucidum: Clear layer (only in thick skin), contains dead cells.
- Stratum Corneum: Topmost layer of dead, keratinized cells.
- Dermis:
- Contains blood vessels, nerves, and skin appendages.
- Papillary Layer: Loose connective tissue with capillary loops.
- Reticular Layer: Dense irregular connective tissue for support.
- Hypodermis: Subcutaneous layer with adipose tissue, insulation, and cushioning.
Notable Skin Appendages
- Glands: Eccrine (thermoregulation) and Apocrine (pheromones).
- Hair: Developed from follicles; contains melanocytes.
- Nerve Endings: Specialized for touch, pressure, pain, and temperature.