Epithelial Tissue: Secretory Epithelia & Exocrine Glands

• Unit 2 covers epithelial tissue.

Secretory Epithelia

• Epithelial cells that produce and secrete macromolecules may be part of epithelia with other functions
• Alternatively they can comprise specialized organs called glands.

Function of Secretory Cells

• Secretory cells synthesize, store, and release macromolecules.
• Secretory cells also secrete water and electrolytes.
• Scattered secretory cells (unicellular glands) are common in simple cuboidal, simple columnar, and pseudostratified epithelium.
• Mucus-secreting goblet cells are scattered among other epithelial cells

Exocrine Glands

• Exocrine glands form from covering epithelia
• Exocrine glands are categorized as simple or compound

Simple Exocrine Glands

• Simple glands do not have branched ducts
• Simple tubular glands have elongated secretory portions with short or absent ducts; examples include mucous glands of the colon and intestinal glands.
• Simple branched tubular glands have several long secretory parts joining to drain into one duct; Glands in the uterus and stomach are examples of this
• Simple coiled tubular glands feature very long, coiled secretory portions; Sweat glands are an example.
• Simple acinar glands have rounded, sac-like secretory portions; an example is small mucous glands along the urethra
• Branched acinar glands have multiple saclike secretory parts entering the same duct, like sebaceous glands of the skin

Compound Exocrine Glands

• Compound glands include ducts from several secretory units converging into larger ducts.
• Compound tubular glands consists of several elongated, coiled secretory units converge to form larger ducts; Submucosal mucous glands (of Brunner) in the duodenum are examples of this.
• Compound acinar glands consists of several sac-like secretory units with small ducts converging at a larger duct; the exocrine pancreas is an example
• Tubuloacinar Exocrine Glands consist of both tubular and acinar secretory units converging at larger ducts; Salivary glands are an example.

Exocrine vs Endocrine

• A key check is to differentiate between exocrine and endocrine glands

Exocrine Secretion Methods

• Exocrine glands secretion modes are merocrine, holocrine, and apocrine.
• Merocrine secretion involves contents transferring from inside the cell to outside the cell via exocytosis; the cell remains unharmed in this process.
• Holocrine secretion involves disintegration of cells to becoming the secretion
• Apocrine secretion involves pinching off of apical portion of the cell to secrete

Exocrine Secretion Types

• Exocrine secretions can be mucous
• Exocrine secretions can be serous
• Exocrine secretions can be seromucous or mixed

Myoepithelial Cells

• Exocrine glands contain myoepithelial cells

Endocrine Glands

• Endocrine glands lack myoepithelial cells
• Endocrine glands are specialized for steroid or protein hormone synthesis.
• Proteins are released by exocytosis
• Steroids are released via diffusion through the cell membrane.

Transport Across Epithelia

• Involves ion and water absorption and secretion
• Can be receptor mediated, receptor independent or transcytosis

Epithelial Renewal

• Most epithelial cells are continuously renewing cell populations.
• In other epithelia, such as complex glands and the liver, epithelial cells belong to a stable cell population.
• Stable cell populations exhibit little mitotic activity and typically only divide when stimulated in response to injury.
• Replacement cells are produced by mitotic activity of self-maintaining adult stem cells.

This section discusses secretory epithelia, which produce and secrete macromolecules, either as part of other epithelia or as specialized glands. It covers the function of secretory cells, including synthesis, storage, and release of macromolecules, as well as water and electrolytes. It also introduces exocrine glands, categorized as simple or compound based on their duct structure.