Glandular and Secretory Epithelium

Glandular Epithelium

• Epithelial cells function to produce and secrete various macromolecules
• They may occur in epithelia with other major functions or create specialized organs called glands.
• Products for secretion are stored in the cells within membrane-bound vesicles, called secretory granules.

Secretory Epithelial Cells

• Secretory epithelial cells synthesize, store, and release proteins, lipids or carbohydrate and protein complexes
• Epithelia of mammary glands secrete all three substances
• Some glands, like sweat glands, have little synthetic activity and secrete water and electrolytes mainly from blood

Exocrine Glands

• Exocrine glands release secretory product via a duct system onto a body surface in contact with the external world
• These include salivary glands, tear glands, anal glands, and stomach pits

Endocrine Glands

• Endocrine glands release secretory products (typically hormones) into spaces between the secretory cells
• Hormones enter the bloodstream from these spaces
• Endocrine glands are ductless
• These include the pituitary gland, pineal gland, thyroid gland, adrenal gland, pancreas, and testes/ovaries

Classification of Exocrine Glands

• Exocrine glands are classified based on cell number, shape, and branching pattern of their secretory portions and ducts.

Unicellular Glands

• Consist of a single secretory cell
• In mammals, goblet cells are the only example of unicellular exocrine glands
• Goblet cells occur in the epithelium of many mucous membranes.
• Goblet cells secrete the glycoprotein mucin
• Scattered secretory cells, sometimes called unicellular glands, are common in simple cuboidal, simple columnar, and pseudostratified epithelia
• Goblet cells, abundant in the lining of the small intestine and respiratory tract, secrete lubricating mucus

Multicellular Glands

• The simplest form is a secretory epithelial sheath, which is a surface epithelium of secretory cells
• Other multicellular glands have secretory portions embedded in the connective tissue underlying the epithelium
• Glands can be simple, with unbranched ducts, or compound, with two or more branches
• Secretory portions can be tubular (short, long, or coiled) or acinar (rounded and saclike)
• Compound glands have branching ducts and multiple tubular, acinar, or tubuloacinar secretory portions

Simple Glands (Ducts Do Not Branch)

• Simple tubular glands have elongated secretory portions and short/absent ducts
  • Mucous glands of the colon and intestinal glands/crypts of Lieberkühn are examples
• Branched tubular glands have several long secretory parts that joining to drain into 1 duct
  • Glands in the uterus and stomach are examples
• Coiled tubular glands have very long and coiled secretory portions
  • Sweat glands are examples
• Acinar glands have rounded, saclike secretory portions
  • Small mucous glands along the urethra are examples
• Branched acinar glands have multiple saclike secretory parts entering the same duct
  • Sebaceous glands of the skin are examples

Compound Glands (Ducts from Several Secretory Units Converge into Larger Ducts)

• Tubular glands have several elongated, coiled secretory units with ducts converging to form larger ducts
  • Submucosal mucous glands (of Brunner) in the duodenum are examples
• Acinar glands have several saclike secretory units with short ducts converging at a larger duct
  • The exocrine pancreas is an example
• Tubuloacinar glands have both tubular and acinar secretory units that converge at larger ducts
  • Salivary glands are an example

Combinations

• The pancreas is a tubulo-acinar gland
• Tubular cells secrete the alkaline bicarbonate to neutralize stomach's acidic contents
• Acinar cells creates digestive enzyme precursors
• Straight tubular glands are sections of the stomach (principal glands) or colon (intestinal glands)

Examples of Glands

• Simple alveolar glands are found in the penile urethra
• Simple branched alveolar glands can be found as sebaceous glands
• Compound alveolar glands are found in the pancreas, parotid gland, mammary gland and glands of the respiratory tract
• Simple tubular glands are Crypts of Leiberkuhn
• Simple branched tubular glands are uterine glands, pyloric and fundic glands
• Compound tubular glands are Brunner’s Gland and cardiac glands
• Simple coiled tubular glands are sweat glands
• Compound tubulo-alveolar glands are submandibular and sublingual salivary glands

Coiled Tubular Glands

• Coiled Tubular glands of the skin are sweat glands
• Simple tubular glands, these have two types of sweat glands in humans
• Distinguished by their secretory mechanism into merocrine (~eccrine) and apocrine sweat glands
• The secretory epithelium is cuboidal or low columnar
• Has two cell types: a light type secreting watery eccrine sweat, and a dark type, may produce a mucin-like secretion
• Apocrine sweat glands are stimulated by sexual hormones and are not fully developed/functional before puberty
• Apocrine sweat is a milky, proteinaceous, odourless secretion

Alveolar Glands

• These are lactating mammary glands, which are modified skin glands and exocrine glands
• Consist of 15-25 compound branched alveolar glands lobes, separated by dense interlobar connective tissue
• Contain a protein-rich secretion (large amounts of immunoglobulins) that is eosinophilic called the colostrum or foremilk
• Secretion of milk proteins proceeds by exocytosis for merocrine secretion
• Whereas lipids proceed by apocrine secretion
• Prolactin stimulates secretion

Myoepithelial Cells

• Contractile cells in the epithelia of exocrine glands such as sweat, lachrymal, salivary, & mammary glands
• They are located inside the basal lamina around the basal ends of secretory/duct cells
• Long processes of myoepithelial cells embrace an acinus similar to how an octopus embraces a boulder
• In ducts, the longitudinal arrangement of myoepithelial cells is more apparent
• They are connected to each other/other epithelial cells via gap junctions and desmosomes containing actin filaments and myosins
• Contractions help proper secretory product expulsion through the duct system

Histological Structure of Exocrine Glands

• Secretory and supporting connective tissue (parenchyma) is similar in most glands
• A dense connective tissue layer, capsule, surrounds gland externally
• Connective tissue sheets, known as septs, subdivide gland internally into lobes
• Within lobes, thinner connective tissues, known as lobules, are septs subdivide the lobes to create lobules
• Reticular connective tissue surrounds and supports secretory units/excretory ducts as they are being made

Secretion Types

• Epithelial cells in multicellular glands release product via three mechanisms; distinguished histologically

Merocrine Secretion

• Most common method of protein secretion
• Involves typical exocytosis of proteins or glycoproteins from membrane-bound vesicles

Holocrine Secretion

• Cells accumulate product as they mature and undergo terminal cell differentiation, culminating in complete cell disruption
• Contents and debris is released to the gland's lumen
• Common in sebaceous glands of skin

Apocrine secretion

• Product accumulates at the cells’ apical ends
• Portions released by extrusion together with a bit of cytoplasm and plasma membrane
• How droplets of lipid secrete in the mammary gland

Exocrine Glands based on Merocrine Secretion

• They can be further categorized as either serous or mucous based on products, giving different staining properties
• Serous cells synthesize mostly non-glycosylated proteins, like digestive enzymes
• Serous cells have well-developed RER and Golgi complexes filled apically with secretory granules in stages of maturation
• Serous cells stain intensely with basophilic or acidophilic stains
• Acini of the pancreas and parotid salivary glands consist of serous cells

Types of Multicellular Exocrine Glands

• Serous glands: Secretion is watery and protein-rich
• Mucous glands: Secretion is mucous and protein-poor
• Seromucous glands: Intermediate

Goblet Cells

• Have RER and Golgi complexes filled with apical secretory granules
• Contain heavily glycosylated proteins called mucins
• When mucins are released, they hydrate and form mucus
• Hydrophilic mucins are washed from cells during common histological preparations and mucinogen granules stain poorly with eosin
• Sufficient oligosaccharides remain and allow mucous cells to be stained by the periodic acid-Schiff (PAS) method

Mixed Seromucous Glands

• Some salivary glands are mixed seromucous glands with serous acini and mucous tubules capped by serous cells

Endocrine Glands

• Hypothalamus, pituitary, thyroid, pancreas, adrenal glands, gonads or sex organs, and pineal gland
• Endocrine action acts at a distant site
• Paracrine action acts on nearby cells
• Autocrine action acts on hormone-producing cell itself
• Some endocrine/paracrine cells also occur singly or in small groups in the digestive and respiratory epithelium
• Hormones also secreted from certain specialized cells, such as cardiac muscle cells or fat cells
• Some glands, like the pancreas, contain cells that support both endocrine and exocrine functions

Liver Cells

• Liver cells exert both functions in the same cells
• They secrete bile components within a duct system and release other bloodstream products

Renewal of Epithelial Cells

• Epithelial tissues have relatively labile structures of continuously renewed cells because of mitotic and stem cell activity
• The rate of renewal varies widely. Fast- intestinal epithelium for example is weekly. SLow-large glands
• In stratified epithelial tissues, stem cells and mitosis only occurs within the basal layer contacting basal lamina
• Stem cells located in restricted niches, some distance away from transit-amplifying/differentiating cells
• Epithelium lining of the small intestine stems completely from stem cells found in simple glands existing in the simple glands between the intertestinal Villi

Development of Glands

• Developmentally derived from epithelia, both endocrine and exocrine ones originate as glands from covering epithelia in growing fetus
• They originate as result of cell proliferation and cell growth into connective tissue for further specialization
• Exocrine glans retain connection to the surface epithelium to leave the tissue
• On the other hand,Endocrine glands lose surface epithelium attachment as result of missing any ducts