Epithelium: Function and Types

• Function and types of epithelium: Epithelium can cover and protect surfaces, aid in diffusion and absorption, excretion, secretion, sensation, and contractility.
• Covering epithelium is involved in covering and protecting surfaces, diffusion, and absorption.
• Glandular epithelium is involved in excretion and secretion.
• Sensory epithelium is involved in sensation.
• Myoepithelium is involved in contractility.

Glandular Epithelium

• Epithelial cells are specialized to produce secretions.
• Secretion is the exocytotic release of products and does not include metabolic wastes.
• Molecules to be secreted are stored in membrane-bound secretory granules.

Classification of Glands

• Glands are classified as endocrine and exocrine, depending on their route of secretion.
• Exocrine and endocrine gland formation begins with the proliferation of cells in the epithelium.
• This is followed by the downgrowth into the subjacent connective tissue.
• A basal lamina and connective tissue underlie the epithelium.
• Exocrine gland formation involves invagination from the epithelial layer whereas endocrine gland formation involves cords of cells forming the endocrine gland.
• Disappearance of duct cells occurs in endocrine gland formation
• Both exocrine and endocrine gland formations have secretory portions and capillaries nearby.

Endocrine Glands

• Endocrine glands secrete hormones directly into the bloodstream.
• Endocrine glands do not have ducts.

Exocrine Glands

• Exocrine glands can be classified according to: the number of cells, the mode of secretion, the shape, and produced secretory products.
• Regarding the number of cells, exocrine glands can be unicellular or multicellular.
• Regarding the mode of secretion, exocrine glands can be holocrine, apocrine, or merocrine.
• Regarding to shape, exocrine glands can be tubular, alveolar (acinar), or tubuloalveolar.
• Regarding secretory products, exocrine glands can be mucous and serous.

Classification by Cell Number

• Unicellular glands consist of single cells, whereas multicellular glands consist of multiple cells.
• Multicellular glands can be simple with one unbranched duct or compound with branched ducts.
• A simple multicellular gland example: sweat gland.
• A compound multicellular gland example: parotid gland.

Unicellular Glands

• Unicellular glands are scattered among other non-secretory epithelial cells.
• They lack ducts and secrete their products directly onto the free surface.
• Goblet cells, which secrete mucus, are the most common type of unicellular exocrine glands.
• Goblet cells are single cells distributed among non-secretory cells.
• Goblet cells are found in the trachea and small intestine.

Multicellular Glands

• Multicellular glands are formed by invagination of the epithelial layer in deeper tissue, forming a diverticulum.
• The proximal part forms the duct, while the distal part forms the secretory element.

Exocrine vs. Endocrine

• Exocrine glands pour their secretions onto an epithelial surface, either directly or through ducts.
• Endocrine glands (ductless) pour their secretions directly into the blood.
• Exocrine glands release secreted substances through a duct to a digestive tract or outside of the body whereas endocrine glands release hormones into the extracellular fluid that flow into the capillaries.

Classification by Duct Shape and Branching

• Classification is based on the shape of secretory units (tubular or alveolar) and branching patterns.
• There are two types: simple and compound.
• Examples of simple glands include simple tubular, simple coiled tubular, simple branched tubular, and simple branched alveolar.
• Examples of compound glands include compound tubular, compound alveolar, and compound tubulo-alveolar.
• Examples of simple glands are alveolar (acinar), tubular, and tubulo-alveolar.

Examples of Glands

• Simple tubular glands include crypt of Lieberkuhn.
• Simple coiled tubulars include sweat glands.
• Simple branched tubular glands include fundic glands of the stomach.
• Simple branched acinar include meibomian glands.
• Compound tubulars include Brunner glands.
• Compound tubuloacinar include submandibular glands.
• Compound acinar glands include mammary glands.

Modes of Secretion

• Modes of secretion describe how products leave the cell.
• There are three primary modes of secretion: merocrine, apocrine, and holocrine.
• Merocrine secretion does not affect the well-being of the cell. A few examples are sweat glands, salivary glands, and pancreas.
• Apocrine secretion involves a small part of the cell cytoplasm being lost with the secretion; an example is mammary glands.
• Holocrine secretion involves a great deal of cytoplasm being lost with the secretion; the cell dies, as seen in sebaceous glands.
• Merocrine secretion is secretion by exocytosis. Vesicles fuse with the plasma membrane and release their contents.

Apocrine Secretion

• An illustration of apocrine secretion shows pinched-off portions of cells released as the secretion in the mammary gland.

Holocrine Secretion

• In holocrine secretion, disintegrating cells and their contents are released as secretions, as seen in sebaceous glands.

Classification by Nature of Secretion

• Glands can be classified based on the nature of their secretion: serous, mucous, and mixed.

Serous Glands

• Serous glands secrete a thin, watery substance rich in enzymes and protein.
• The cytoplasm of serous glands is granular, staining darkly from pink to dark purple with H/E stain, due to being rich in rough endoplasmic reticulum (rER).
• Serous glands have rounded and basally placed nuclei.

Mucous Glands

• Mucous glands secrete mucopolysaccharides, which accumulate in the apical part of the cell.
• The cytoplasm of mucous glands stains very lightly with H/E stain, giving an empty look.
• Mucous glands have basally placed and flattened nuclei due to the mucoid pushing the nuclei.

Mixed Glands

• Mixed glands contain both serous and mucous secretory units.
• Serous cells sometimes form crescentic caps, or serous demilunes, on mucous acini.