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Epithelial Tissues and Glands — Quick Review

  • Nonciliated Simple Columnar Epithelium

    Lines most of the gastrointestinal (GI) tract from the stomach to the upper portion of the anal canal.

    • Cell type: simple columnar; microvilli form a brush border (collectively bright and fuzzy under light microscopy).

    • Goblet cells scattered; secrete mucin (glycoprotein) which, when hydrated with water, forms mucus.

    Ciliated Simple Columnar Epithelium

    • Has cilia on the apical surface; mucus is produced by goblet cells and moved along by ciliary beating.

    • Locations: bronchioles in the lungs; uterine tubes (helps move an oocyte from the ovary to the uterus).

    • Goblet cells typically dispersed throughout; apical surface covered by mucus.

    Pseudostratified Columnar Epithelium

    • Appears multi-layered but all cells contact the basement membrane; nuclei at different distances from the basal surface; not all reach the apical surface.

    • Two forms:

      • Pseudostratified Ciliated Columnar Epithelium (with cilia on the apical surface): contains goblet cells secreting mucin that hydrates to mucus; mucus traps foreign particles and is moved by cilia.

      • Pseudostratified Nonciliated Columnar Epithelium (lacks cilia): rare; protective function.

  • Located in larger air passageways of the respiratory tract (nasal cavity, part of pharynx, part of larynx, trachea, bronchi).

Stratified Squamous Epithelium

  • Multiple cell layers; deepest layer in contact with the basement membrane.

  • Basal cells are cuboidal or polyhedral; apical cells become flattened (squamous).

  • Function: protection from abrasion and friction; stem cells in the basal layer continuously divide.

  • Forms two forms:

    • Keratinized: superficial cells are dead, lack nuclei, filled with keratin; water resistant; epidermis is keratinized stratified squamous epithelium.

    • Nonkeratinized: surface cells remain alive with nuclei (not detailed in this summary but included as form).

Stratified Cuboidal Epithelium

  • Two or more cell layers; superficial cells tend to be cuboidal.

  • Functions in protection and secretion.

  • Forms walls of some ducts of exocrine glands and certain reproductive structures (e.g., sweat glands ducts).

Stratified Columnar Epithelium

  • Relatively rare; two or more layers with apical surface that is columnar.

  • Functions in protection and secretion.

  • Found in the large ducts of salivary glands, conjunctiva covering the eye, and a segment of the male urethra (membranous urethra).

Transitional Epithelium

  • Located in the urinary tract (urinary bladder, ureters, part of the urethra).

  • Must stretch as urine volumes vary; appearance changes with relaxation vs distention.

  • Relaxed state: basal cells appear cuboidal/polyhedral; apical cells large and rounded.

  • Distended state: apical cells flatten and become almost squamous.

  • Distinguishing feature: binucleated cells.

  • Function: allows stretching to accommodate urine volume and prevents seepage into underlying tissues.

Glands: Overview

  • Glands are either single cells or multicellular organs composed mainly of epithelial tissue.

  • Secretions may include mucin, ions, hormones, enzymes, and nitrogenous wastes (e.g., urea).

Endocrine vs Exocrine Glands

  • Endocrine glands:

    • Lack ducts; secrete hormones into the blood.

    • Hormones act as chemical messengers throughout the body.

    • Examples include thyroid and adrenal glands.

  • Exocrine glands:

    • Typically originate from an invagination of epithelium and maintain a duct to the epithelial surface.

    • Secretions discharged onto the epithelial surface via the duct.

    • Examples: sweat glands, mammary glands, salivary glands.

Unicellular vs Multicellular Exocrine Glands

  • Unicellular exocrine glands:

    • Do not contain a duct; located close to the epithelial surface.

    • Most common example: goblet cell (secretes mucin).

  • Multicellular exocrine glands:

    • Contain many cells working together to produce a secretion.

    • Gland structure typically includes acini (secretory clusters) and one or more ducts forming larger ducts.

    • Surrounded by a fibrous capsule; extensions called septa partition the gland into lobes.

Classification of Exocrine Glands

  • By Anatomic Form:

    • Simple glands: unbranched duct.

    • Compound glands: branched ducts.

    • Secretory portions shapes:

    • Tubular: secretory portion and duct have the same diameter.

    • Acinar: secretory portion forms saclike acini.

    • Tubuloacinar: both tubular and acinar components.

  • By Method of Secretion:

    • Merocrine glands: secretion released by exocytosis; cells remain intact.

    • Apocrine glands: apical portion pinches off; cell repairs and repeats secretion.

    • Holocrine glands: entire secretory cell disintegrates; basal cells divide to replace.

Examples of Exocrine Glands

  • Merocrine: lacrimal (tears), salivary glands, sweat glands, pancreas (exocrine portion), gastric glands.

  • Apocrine: mammary glands, ceruminous (ear) glands.

  • Holocrine: sebaceous glands.