Epithelial Tissue: Glandular Epithelia

• Epithelial Tissue 2 focuses on glandular epithelia.
• Renáta Mikušová from the Institute of Histology and Embryology, Faculty of Medicine CU Bratislava, created this presentation.

Development of Glandular Epithelial Tissue

• Glandular epithelial tissue development involves cell proliferation and downgrowth into subjacent connective tissue.
• The process can form exocrine or endocrine glands.
• Exocrine glands maintain a duct, while duct cells disappear in endocrine glands.

Classification of Glandular Epithelia According to Secretion Method

• Exocrine glands secrete products onto the body surface, into body cavities, or hollow organs. They release products through ducts.
• Endocrine glands secrete hormones directly into the bloodstream and lack a duct system.

Exocrine Glandular Epithelia

• Specialized cells produce secretory products.
• Glandular cells release secretory products into the extracellular space. These products benefit other tissues and organs.
• Secretory products vary in composition and include proteins, lipids, and complexes of carbohydrates with proteins or lipids.
• Secretory products release either continuously or upon stimulus.
• Control of glandular cell activity is genetic or neuro-endocrine. An example includes secretin and cholecystokinin for the exocrine part of the pancreas, with salivary glands under nerve control.
• Exocrine glands have a secretory part (A) and a duct (B)

Classification of Exocrine Glandular Epithelia

• Exocrine glands can be classified by cell type and secretory products.
• Exocrine glands can be classified by method of secretion.
• Exocrine glands can be classified by the number of secretory cells.

Serous Cells

• Serous cells are pyramidal shaped.
• Serous cells have a large, rounded nucleus.
• Serous cells contain numerous spherical secretory granules in the apical cytoplasm.
• Serous cells have large amounts of rough endoplasmic reticulum (rER).
• Serous cells secrete a watery secretion (serous) rich in proteins, due to high proteosynthetic activity.

Mucous Cells

• Mucous cells have a flattened nucleus in basal cytoplasm.
• Mucous cells contain large numbers of mucinogen granules in the apical cytoplasm, appearing empty in H&E stains.
• Mucous cells produce gel-like mucinogen/mucus, glycoproteins with high viscosity, binding water. They have protective and lubricating functions, appearing pale in HE, with PAS-positivity
• Intraepithelial mucous cells include goblet cells in the trachea and lining of the stomach.
• Extraepithelial mucous cells form secretory tubular compartments in the sublingual gland.

Unicellular Gland - Goblet Cell

• Goblet cells are unicellular glands that produce mucin.
• Goblet cells are commonly found in the lining epithelia of the trachea.

Myoepithelial Cells

• Myoepithelial cells are a contractile type of cell.
• Myoepithelial cells are found in salivary glands, mammary glands, and sweat glands.

Method of Secretion

• Merocrine secretion releases secretory vesicles via exocytosis. The cell remains unchanged. An example includes the parotid salivary gland.
• Apocrine secretion involves pinching off the apical portion of the secretory cell containing secretions and some cytoplasm. An example includes the mammary gland.
• Holocrine secretion involves disintegration of cells with contents becoming the secretion, and the cell is part of the secretion product. An example includes the sebaceous gland of skin.

Classification by Number of Cells

• Unicellular glands consist of a single secretory cell. An example includes goblet cells.
• Multicellular glands have multiple secretory cells and can be tubular, alveolar/acinar, or tubulo-alveolar/acinar.

Multicellular Exocrine Glands

• Multicellular exocrine glands maintain a connection with the epithelium from which they originated.
• They possess a conducting portion (duct/ducts) lined with lining epithelium.
• Multicellular exocrine glands have a secretory part (alveoli/acini or tubuli) with serous or mucous secretory cells.

Ducts of Multicellular Exocrine Glands

• Ducts are primarily for the discharge of secretory products.
• Ducts can also modify the ionic composition of saliva like Na+, Cl-, and HCO3- in striated ducts (salivary glands), leading to primary and definitive saliva.

Duct Lining Epithelia

• Intestine ducts are sometimes indistinct or absent.
• Ducts can be simple (straight or coiled) or simple branched.
• Ducts can be compound and are classified as:
  • Intercalated intralobular (simple low cuboidal epithelium)
  • Larger intralobular (sometimes striated, with simple cuboidal or low columnar epithelium)
  • Interlobular (in septa)
  • Lobar
  • Main

Exocrine Gland Specimens

• Unicellular glands and goblet cells can be found in the small intestine (PAS-H staining).
• Simple tubular glands are in the large intestine (HE staining).
• Simple branched alveolar glands with holocrine secretion are in the labium minus (HE staining).
• Compound tubulo-alveolar glands with apocrine and merocrine secretion are in the mammary gland (HE staining).
• Compound acinar glands with merocrine secretion are in the parotid gland (HE staining).
• Compound tubulo-acinar glands can be found in the submandibular and sublingual glands (Azan staining).

Endocrine Glandular Epithelia

• Endocrine glandular epithelia lack ducts.
• Endocrine glandular epithelia have many capillaries.
• Endocrine glandular epithelia are secretory.

Products of Endocrine Glandular Epithelia

• Endocrine glandular epithelia produce hormones.
• Hormones can be classic, paracrine (acting nearby), or autocrine (affecting the producing cell).
• Hormones are stored in cell granules and released by exocytosis in hydrophilic forms such as polypeptides (insulin), glycoproteins (gonadotropins), and biogenic amines (adrenaline).
• Steroid hormones are lipophilic and stored in lipid droplets (adrenal cortex, gonads) or in a closed compartment (thyroid gland).

Endocrine Glands

• Endocrine glands lose connection with surface epithelium; thus, lack a duct system.
• They release hormones into a dense network of fenestrated or sinusoidal capillaries.
• Endocrine glandular epithelia are arranged in two types: glandular cells forming follicles or arranged in cords.

Follicular Type

• Follicular arrangement found in the thyroid gland.

Cord Arrangement

• Arrangements of cells into cords found in the cortex of the suprarenal gland.