Epithelium and Glands - Vocabulary Flashcards

Epithelium forms and basic concepts

• Epithelium exists in two major forms:
  • Surface epithelium: forms sheets of cells that cover all external surfaces and line internal surfaces of vessels and body cavities, enabling transport of substances and fluids; examples include endothelium (lining vessels) and mesothelium (lining body cavities) (transport epithelium).
  • Glandular epithelium: forms groups of secretory cells into glands; secretes various chemical substances into ducts that lead to the exterior (exocrine) or into the blood (endocrine) (secretory epithelium); examples include the pancreas (exocrine) and the thyroid gland (endocrine).

Formation of glands from covering epithelia

• During fetal development, epithelial cells proliferate and penetrate the underlying connective tissue.
• These cells may—or may not—maintain a connection with the surface epithelium.
• The connection is maintained to form a duct in exocrine glands; it is lost as endocrine glands develop without ducts.

Glandular epithelium: classification

• Glands are a collection of secretory epithelial cells.
• Glands are classified as either endocrine or exocrine.

Endocrine glands

• Endocrine cells release hormones into the intercellular spaces, from which they diffuse rapidly into surrounding capillaries and are transported by the bloodstream to their target organ.
• Visualization cue: secretory cells and capillaries are often shown with secretory cells releasing into intercellular spaces and surrounding capillaries.

Follicular endocrine gland: Thyroid

• The thyroid is unusual among endocrine glands in that it stores hormone (thyroxine, T4) within roughly spherical cavities enclosed by the secretory cells; these units are called follicles.
• Structures involved: capillary networks surrounding follicles; follicles are the storage units.

Exocrine glands

• Classified based on:
  • nature of the secretory product
  • mode of secretion
  • morphology
• Unicellular glands: goblet cells – isolated mucus-secreting cells scattered within a surface epithelium; found in the epithelial lining of the intestines and respiratory tract.
• Multicellular glands: most common form of gland.

Exocrine glands: secretory products

• Mucous cells produce a thick, viscous product; mucous cell nuclei are flattened to the periphery of the cell; mucus doesn’t stain well with H&E.
• Serous cells produce a thin, watery product containing protein; serous cell nuclei are round and easily visible.
• Sebaceous cells produce sebum (lipids); cytoplasm is pale and ‘foamy’ due to lipid droplets (lipids often wash out in routine processing).

Exocrine glands: unicellular gland – the goblet cell

• Example: goblet cells in the duodenum (rat), shown in H&E with mucus occupying part of the cell and a conspicuous nucleus.
• Key features: columnar epithelial cell, goblet cell, nucleus location, association with lamina propria and basement membrane.

Exocrine glands: multicellular glands – secretory cell types

• Multicellular glands can be described as serous, mucous, or a serous-mucous mix.
• Serous glands secrete serous fluid (water and enzymes).
• Mucous glands secrete mucus (mucin glycoproteins and water).
• Secretory unit shapes: serous acini (spherical clusters with round nuclei) and mucous acini (spherical clusters with flat nuclei).
• Some glands contain mixed mucous and serous cells.

Serous vs Mucous glands examples

• Serous gland: serous acini produce watery secretions containing enzymes.
• Mucous gland: mucous acini produce mucins and water; nuclei flattened at the periphery.
• Mixed glands: contain both serous and mucous cells.

Pancreas: Exocrine and Endocrine portions

• Exocrine portion: consists of pancreatic acini (composed of serous cells that secrete digestive enzymes).
• Endocrine portion: consists of islets of Langerhans (clusters of hormone-producing cells).

The exocrine pancreas and endocrine pancreas organization

• Exocrine pancreas (3): numerous secretory acini deliver their secretory material into the excretory duct (1);
  • duct is lined by simple cuboidal epithelium and surrounded by connective tissue.
• Endocrine pancreas (5): the pancreatic islet or islets of Langerhans (5) are separated from exocrine tissue by a thin connective tissue capsule (4).
• The endocrine islet does not contain excretory ducts; it is highly vascularized, and all secretory products exit via blood vessels to reach the bloodstream.

Exocrine glands: examples of gland types

• Parotid salivary gland: composed of serous acini.
• Mandibular salivary gland: composed of mucous and serous acini.
• Terminology in images:
  • SA: Serous acini
  • MA: Mucous acini
  • SD: Serous demilune
  • StD: Striated duct
  • MC: Myoepithelial cells

Mandibular salivary gland: histology notes

• Mandibular gland shows serous demilune and mucous acinus with peripheral flattened nuclei.
• Dense irregular connective tissue present.
• Mucous secretions appear as mucous regions; basal details include basement membrane and brush border in adjacent epithelium.

Exocrine glands: Shape/morphology of the secretory portion/unit

• Duct: the duct is a defining feature separating secretory unit from the exterior.
• Secretory portion can be:
  • Tubular: secretory cells line the entire duct.
  • Acinar/Alveolar: secretory units form rounded sacs.
• Glands can be classified as Simple (unbranched duct) or Compound (ducts from several secretory units converge into larger ducts).
• Possible combinations include Tubular, Alveolar, or Brached variants like Tubuloacinar.

Simple shape/morphology of the secretory portion/unit

• Simple tubular
• Simple branched tubular
• Simple coiled tubular
• Simple acinar
• Simple branched acinar

Compound glands: overall organization

• Compound glands have ducts that originate from multiple secretory units and converge into a branched network of ducts.
• Examples:
  • Brunner’s glands of the duodenum: the duct system is branched; secretory portions are tubular.
  • Other compound glands may have acinar or tubuloacinar secretory units.

Secretory unit types in glands

• Acinar (alveolar): rounded secretory units.
• Tubular: elongated secretory units.
• Tubuloacinar: combination of tubular and acinar components.
• Compound glands can have multiple secretory unit types within the same gland.

Exocrine glands: Morphology and terminology table (summary)

• Simple glands: tubular or acinar; unbranched duct.
• Compound glands: multiple secretory units leading to branched ducts.
• Secretory unit shapes include tubular, acinar, alveolar, or mixed (tubuloacinar).
• Gland types include mucous glands (mucins), serous glands (enzymes and watery secretions), and mixed glands.
• Duct and secretory portions define secretion delivery into the ductal system.
• Modes of secretion (secretory processes) include merocrine, apocrine, holocrine.

Exocrine glands: Mode of secretion

• Merocrine (eccrine): entire secretory cell remains intact; secretion released by exocytosis.
• Apocrine: a portion of the secretory cell is lost with the secretory product.
• Holocrine: the secretory cell disintegrates, releasing its contents with cell debris.

Exocrine glands: clinical and histological notes

• Intestinal glands in the mucosa of the intestinal wall are tubular and use merocrine secretion.
• Sebaceous glands are simple branched acinar glands that open into hair follicles and release sebum by holocrine secretion.
• Apocrine sweat glands are coiled tubular glands; the apical portion of secreting cells is released with the secretory product.

Myoepithelial cells

• Contractile cells associated with secretory units of salivary, mammary, and sweat glands.
• Rich in actin filaments and myosins.
• Endocrine glands lack myoepithelial cells.

Renewal of epithelial cells

• Epithelial cells are constantly lost and replaced.
• Renewal occurs via mitotic activity and stem cell populations.
• The renewal rate varies widely depending on epithelial type and function.

Stratified squamous keratinized epithelium

• Stem cells are in the basal cell layer.
• Cells move upward, differentiate, and are shed from the surface.
• This tissue organization is summarized by the term EPIdermis (epidermis).

Exocrine glands: summary of key features

• Can be unicellular or multicellular.
• Multicellular glands contain secretory portions and ductal portions; secretions enter the ductal system.
• Simple glands exhibit unbranched ducts; examples include intestinal glands.
• Coiled tubular glands are seen in sweat glands.
• Compound glands have repeated ductal branching with either acinar (alveolar) or tubular secretory portions.
• Compound acinar glands are seen in mammary glands.
• Compound tubuloacinar glands are seen in salivary glands.
• Mucous glands lubricate and protect inner linings of organs; serous glands produce watery secretions with enzymes; mixed glands contain both serous and mucous cells.
• Merocrine glands (e.g., pancreas) release secretions without cell loss; holocrine glands (e.g., sebaceous glands) release secretions with cell components.
• Endocrine cells, tissues, and glands:
  • Diffuse neuroendocrine system (DNS): individual cells acting as endocrine glands in digestive and respiratory systems.
  • Endocrine tissues: isolated endocrine cells mixed with exocrine glands (as in pancreas and reproductive organs).
  • Major endocrine organs include pituitary, thyroid, and adrenal glands.
  • Do not have excretory ducts and are highly vascularized; secretory products enter bloodstream (capillaries) for systemic distribution.

Type of epithelium and sites (summary table style)

• Type of epithelium: Squamous; Subclassification: Simple, Stratified, Stratified keratinising; Cuboidal; Simple, Stratified; Columnar; Simple; Pseudostratified, ciliated; Simple ciliated; Transitional; Glandular; Simple; Compound; Glandular-solid organs; Exocrine; Endocrine
• Sites (examples):
  • Lining blood vessels (endothelium), lining body cavities (mesothelium), alveoli of lungs, Bowman's capsule and loop of Henle of kidney
  • Lining oral cavity, epiglottis, oesophagus, anus, cervix, vagina, vulva, glans penis, cornea
  • Skin (epidermis)
  • Collecting tubules of kidney, rete testis, small ducts of exocrine glands, surface of ovary
  • Larger ducts of exocrine glands
  • Gallbladder, collecting ducts of kidney, endocervix
  • Respiratory tract including nose and sinuses
  • Fallopian tubes
  • Lower urinary tract (renal pelvis, ureters, bladder and urethra)
  • Colon, stomach, eccrine sweat glands
  • Sebaceous glands, Brunner's glands of duodenum, small salivary glands, breast, prostate
  • Major salivary glands, liver, pancreas (acinar tissue)
  • Thyroid, anterior pituitary, adrenal, pancreas (islets of Langerhans)