Overview
This lecture introduces histology—the study of tissues—focusing on epithelial tissues, their classifications, structures, functions, and the types and mechanisms of glandular secretions.
Introduction to Tissues and Histology
- Histology is the study of tissues, groups of similar cells performing related functions.
- Tissue slides are prepared by fixing, sectioning (longitudinal, cross, oblique), and staining tissues for microscopic observation.
- Four primary tissue types: epithelial, connective, muscle, and nervous.
Epithelial Tissue: Structure and Functions
- Epithelial tissue covers surfaces, lines cavities, and forms glands.
- Main functions: protection, absorption, filtration, excretion, secretion, and sensory reception.
- Epithelia exhibit polarity (distinct apical and basal surfaces), specialized contacts (tight junctions), and always rest on connective tissue.
- Epithelia are avascular (no direct blood supply) but highly innervated and can regenerate.
Epithelial Surface Features and Support
- Apical surface is exposed to a cavity or exterior, may have microvilli (for absorption) or cilia (for movement).
- Basal surface attaches to the basal lamina (glycoprotein and collagen-rich glue).
- Supported by reticular lamina; together form the basement membrane for strength and repair.
Classification of Epithelia
- By cell shape: squamous (flat), cuboidal (cube-shaped), columnar (tall).
- By layers: simple (one layer), pseudostratified (looks multilayered, but isn't), stratified (multiple layers).
- Regeneration occurs from basal cells upward.
Types of Epithelial Tissue and Locations
- Simple squamous: thin, for diffusion/absorption (e.g., kidney, lungs, endothelium, mesothelium).
- Simple cuboidal: secretion/absorption (e.g., kidney tubules, small ducts).
- Simple columnar: absorption/secretion (e.g., digestive tract, may have goblet cells and microvilli).
- Pseudostratified columnar: ciliated, appears layered (e.g., trachea, respiratory tract).
- Stratified squamous: protection, surface cells flattened (e.g., skin, mouth).
- Keratinized (waterproof, dead surface cells, no nuclei—skin)
- Non-keratinized (moist linings, nuclei visible—mouth)
- Stratified cuboidal/columnar: rare, found in sweat glands, mammary glands, male urethra.
- Transitional epithelium: stretches, found in urinary system.
Glandular Epithelia
- Glands are classified by product release: endocrine (into tissue/hormones) or exocrine (onto surface).
- Unicellular glands: Goblet cells produce mucin (becomes mucus in water).
- Multicellular glands: classified as serous (watery), mucous, mixed, or cytogenic (release whole cells).
Exocrine Gland Secretion Methods
- Merocrine: release via exocytosis, cell remains intact.
- Apocrine: release part of cell with secretion, cell survives.
- Holocrine: whole cell ruptures and is lost with secretion (e.g., oil glands).
Key Terms & Definitions
- Histology — Study of tissues.
- Epithelial tissue — Covers surfaces and lines cavities.
- Polarity — Having distinct apical (top) and basal (bottom) surfaces.
- Basal lamina — Adhesive sheet under epithelia, rich in glycoproteins and collagen.
- Stratified — Comprising multiple cell layers.
- Keratinized — Containing keratin protein; surface cells are dead and waterproof.
- Goblet cell — Unicellular gland secreting mucus.
- Merocrine gland — Secretes products via exocytosis.
- Holocrine gland — Secretes by cell rupture.
Action Items / Next Steps
- Review epithelial tissue types and their characteristics.
- Practice distinguishing types under microscope images by shape and layering.
- Prepare for next class on connective tissues.