Secretory Epithelia and Glands

• The lecture covers secretory epithelia, transport across epithelia and the renewal of epithelial cells.
• At the end of the unit, students should be able to differentiate the different types of secretory epithelia based on structure and the products they secrete.

Secretory Epithelia

• These produce and secrete macromolecules.
• They may occur in epithelia with other major functions or in specialized organs called glands.
• Secretory cells synthesize, store, and release macromolecules.
• Secretory cells also secrete water and electrolytes.

Unicellular Gland: Goblet Cells

• Scattered secretory cells, also known as unicellular glands are common in simple cuboidal, simple columnar and pseudostratified epithelium.
• It is important for respiratory tract epithelium to have goblet cells.

Formation of Glands

• Glands form from covering epithelia.
• Epithelial cells proliferate and grow into the subjacent connective tissue.
• Exocrine glands maintain a duct; the cells form a secretory portion.
• Endocrine glands lose their ducts and are associated with capillaries.

Exocrine Glands

• The general structure of these includes the stroma with a septum, a capsule, and parenchyma with lobules.
• Exocrine glands are classified based on their structure
• Glands are either simple, where the ducts do not branch, or compound, where ducts from several secretory units converge.
• Simple exocrine glands have differing structures -Simple tubular glands consist of an elongated secretory portion and a short or absent duct. -Branched tubular glands boast several long secretory parts draining into one duct. -Coiled tubular glands possess a very long, coiled secretory portion. -Acinar or alveolar glands have a rounded saclike secretory portion. -Branched acinar glands feature multiple saclike secretory parts entering the same duct.
• Compound glands also have structural differences
• Tubular glands consist of several elongated coiled secretory units with ducts that join to form larger ducts.
• Acinar glands have several saclike secretory units with small ducts converging at a larger duct.
• Tubuloacinar glands feature both tubular & acinar secretory units that converge into larger ducts.

Acne Vulgaris

• Acne vulgaris often comes from blocked ducts in sebaceous glands.
• It shows excessive sebum and keratin production triggered by puberty's hormonal changes.

Exocrine Glands

• They are classified based on the manner of secretion, specifically Merocrine, Holocrine, and Apocrine secretion.
• Merocrine secretion involves exocytosis, with contents transferring from inside to the outside of the cell while the cell remains unharmed.
• Holocrine secretion involves disintegration where cells disintegrate and contents become the secretion.
• Apocrine secretion involves pinching off of the apical portion of secretory cells.

Exocrine Glands

• They are also classified by the type of secretions they produce.
• Mucous secretions are viscous.
• Serous secretions are watery.
• Seromucous or mixed glands produce a mixture of both.

Endocrine Glands

• Unlike exocrine, endocrine glands lack myoepithelial cells.
• They're specialized for steroid or protein hormone synthesis.
• Hormones synthesized from proteins are released by exocytosis.
• Hormones made of steroids are released through the cell membrane via diffusion.

Transport

• Transcytosis involves transport across epithelia.
• This can happen with receptor-mediated or receptor-independent transport.

Epithelial Cell Renewal

• Most epithelial cells are continuously renewing their cell population.
• Other epithelia present in complex glands and the liver belong to a stable cell population with little mitotic activity, and where cells divide when stimulated in response to injury only
• Replacement cells are produced by the mitotic activity of self-maintaining adult stem cells.

This lecture discusses secretory epithelia, types of glands, and the processes involved in secretion. It covers unicellular glands like goblet cells, the formation of glands from covering epithelia, and the structure and function of different secretory cells.