GLANDULAR EPITHELIUM

What defines a gland?

One or more epithelial cells highly specialized for secretion — forming a structure that synthesizes, stores, and releases specific products.

What are the four phases of the secretory process?

Assimilation (uptake of precursors)

Synthesis (production of secretory material)

Storage (in granules or vesicles)

Secretion (release of product)

What are the two main categories of glands based on how they release their products?

Exocrine glands: discharge onto epithelial surfaces via ducts

Endocrine glands: release directly into the bloodstream (no ducts)

How are endocrine glands connected to epithelial surfaces?

They have no connection to the surface — completely surrounded by connective tissue.

How do endocrine glands release their secretions?

Directly into the bloodstream or interstitial fluid.

What are the typical structural organizations of endocrine glands?

Anastomosing cords (interlaced with capillaries)

Cell nests Follicles (e.g., thyroid)

Islets (e.g., pancreas)

Isolated cells (Diffuse Neuroendocrine System, DNES)

What are examples of endocrine glands with epithelioid organization?

Leydig cells (testis),

lutein cells (ovary),

islets of Langerhans,

adrenal cortex,

anterior pituitary.

What are the chemical types of endocrine secretory products?

• Small polypeptides: e.g., DNES hormones

• Proteins/large polypeptides: e.g., parathyroid hormone

• Cholesterol-derived: e.g., steroid hormones (adrenal cortex)

How can endocrine glands be classified by timing of secretion release?

• Immediate release: e.g., adrenal cortex

• After storage: intracellular (granules) or extracellular (thyroid follicles)

How can endocrine glands be classified by range of action?

Autocrine: act on the same cell

Paracrine: act on neighboring cells

Endocrine: act at distant sites via circulation

What distinguishes exocrine from endocrine glands?

Exocrine glands maintain connection to epithelial surfaces via excretory ducts.

What are the classification criteria for exocrine glands?

• Number of cells: unicellular or multicellular

• Duct structure: simple (unbranched) or compound (branched)

• Shape of secretory unit: tubular, acinar, or tubuloacinar

• Nature of secretion: serous, mucous, or mixed

• Mechanism of secretion: merocrine, apocrine, or holocrine

What is the only example of a unicellular gland?

Goblet cell — secretes mucus in intestinal and respiratory epithelia.

What is a multicellular gland?

Gland composed of multiple secretory and duct cells forming a defined structure (e.g., salivary glands, pancreas).

What is a simple gland?

An exocrine gland with an unbranched excretory duct.

What is a compound gland?

An exocrine gland with a branched excretory duct system.

What are the different secretory portion shapes in exocrine glands?

Tubular (straight, coiled, or branched)

Acinar (alveolar)

Tubuloacinar

Give examples of each type of secretory unit.

Tubular: intestinal glands

Coiled tubular: sweat glands

Branched tubular: stomach glands

Acinar: pancreas

Branched acinar: sebaceous glands

What characterizes serous glands?

Secrete protein-rich watery fluid; cells have round nuclei and basophilic cytoplasm (e.g., pancreas, parotid).

What characterizes mucous glands?

Secrete viscous glycoprotein (mucus); cells have pale-staining cytoplasm and flattened basal nuclei (e.g., sublingual glands).

What are mixed glands?

Contain both serous and mucous cells (e.g., submandibular glands).

What are the three main mechanisms of secretion in glands?

Merocrine: exocytosis without cell damage (e.g., pancreas)

Apocrine: apical cytoplasm is pinched off with product (e.g., mammary gland)

Holocrine: entire cell disintegrates releasing secretion (e.g., sebaceous gland)

Give examples for each secretion type.

Merocrine: salivary, pancreas

Apocrine: mammary gland

Holocrine: sebaceous gland

What is the function of ion-transporting epithelial cells?

Actively transport ions (e.g., Na^+, Cl^−) and water; form active barriers for concentration and absorption (e.g., kidney tubule cells).

What are the structural adaptations of ion-transporting cells?

• Apical microvilli

• Basal and lateral infoldings (basal labyrinth)

• Abundant mitochondria near basal membrane

• Tight junctions apically to prevent ion backflow

What are the features of protein-secretin

• Basal RER and Golgi above the nucleus

• Apical zymogen granules

• Polarized structure (e.g., pancreas, salivary glands

What is the sequence of the secretory cycle in protein-secretin

• Amino acid uptake (basal)

• Protein synthesis in RER

• Processing in Golgi

• Packaging into zymogen granules

• Transport to apical surface via cytoskeleton

• Exocytosis

What are the features of mucus-secretin

• Basally located nuclei

• Apical mucin granules

• Well-developed Golgi

• Found in goblet cells, gastric mucous cells, and mucous salivary acini

What is mucus chemically composed of?

Hydrated mucins (high molecular weight glycoproteins).

How is mucus formed and released?

Glycoproteins synthesized in RER → modified in Golgi (glycosylation/sulfation) → packed into granules → hydrated upon exocytosis → mucus.

Where are lipid-secretin

• Exocrine: sebaceous glands

• Endocrine: Leydig cells (testis), follicular cells (ovary), adrenal cortex

What do lipid-secretin

Steroid hormones or lipid-based secretions.

What are myoepithelial cells and where are they located?

Star-shaped contractile epithelial cells surrounding secretory units of

glands: salivary, lacrimal, sweat, and mammary glands.

What are the structural features of myoepithelial cells?

• Contain cytokeratin (epithelial) and actin/myosin (contractile) filaments

• Connected by desmosomes (to each other), hemidesmosomes (to BM), and gap junctions (for synchronization)

What is the function of myoepithelial cells?

Contract to aid in expulsion of glandular secretions.

What is the Diffuse Neuroendocrine System (DNES)?

Scattered endocrine cells in epithelial linings (digestive & respiratory tracts) secreting peptide hormones and amines.

What are examples of enteroendocrine cells and their secretions?

• D cells → somatostatin

• D₁ cells → VIP

• EC cells → serotonin

• I cells → cholecystokinin

• K cells → GIP

• L cells → enteroglucagon

• Mo cells → motilin

• N cells → neurotensin

• S cells → secretin