Overview
This lecture introduces the basics of histology (the study of tissues), focusing on the four primary tissue types, with an in-depth look at epithelial tissue, its structure, classification, and functions.
Introduction to Tissues and Histology
- Tissues are groups of similar cells performing related functions.
- The four primary tissue types: epithelial, connective, muscle, and nervous tissue.
- Histology studies these tissues by examining preserved, sectioned, and stained specimens under microscopes.
Epithelial Tissue: Structure and Functions
- Epithelial tissue (epithelium) is a sheet of cells covering surfaces or lining cavities.
- Two general forms: covering/lining epithelium and glandular epithelium.
- Main functions: protection, absorption, filtration, excretion, secretion, and sensory reception.
Special Characteristics of Epithelial Tissue
- Polarity: cells have apical (top/free) and basal (bottom/attached) surfaces.
- Apical surface may have microvilli (increase surface area) or cilia (move substances).
- Basal surface rests on the basal lamina, part of the selective-filter basement membrane.
- Supported by connective tissue via reticular lamina; together form the basement membrane.
- Avascular (no blood vessels), but innervated (has nerves); receives nutrients via diffusion.
- High regenerative capacity.
Classification of Epithelial Tissue
- Classification by number of layers: simple (one layer) or stratified (multiple layers).
- Classification by cell shape: squamous (flat), cuboidal (box-like), columnar (tall/column-shaped).
Types and Functions of Epithelial Tissue
- Simple squamous: single layer, flat cells for diffusion and filtration (lungs, kidneys, blood vessels).
- Endothelium: simple squamous lining blood/lymph vessels and heart.
- Mesothelium: simple squamous lining serous membranes of ventral body cavity.
- Simple cuboidal: single layer, cube-shaped cells for secretion/absorption (kidney tubules, small glands, ovary surface).
- Simple columnar: single layer, tall cells (may have goblet cells/cilia), for absorption/secretion (digestive tract, bronchi, uterus).
- Pseudostratified columnar: appears layered but isn't; nuclei at different levels, functions in secretion/propulsion (trachea, respiratory tract).
- Stratified squamous: multiple layers, surface cells squamous, protective (skin, mouth, esophagus, vagina).
- Stratified cuboidal/columnar: rare, in certain glands.
- Transitional epithelium: resembles stratified squamous/cuboidal, stretches (bladder, ureters, urethra).
Glandular Epithelium and Glands
- Glands consist of cells that make and secrete products.
- Classified by where they secrete: endocrine (into blood/interstitial fluid) and exocrine (onto surfaces/into cavities).
- Unicellular glands (e.g., goblet cells) and multicellular glands (with ducts).
- Ducts: simple (unbranched) or compound (branched).
- Secretory units: tubular, alveolar, or tubuloalveolar.
- Modes of secretion: merocrine (exocytosis), holocrine (cell rupture), apocrine (bud off portions of cell).
Tissue Growth, Repair, and Death
- Hyperplasia: increased cell number.
- Hypertrophy: increased cell size.
- Neoplasia: abnormal growth/tumor.
- Differentiation: cells become specialized.
- Metaplasia: tissue type changes.
- Regeneration: replacement of dead cells.
- Fibrosis: scar tissue formation.
- Atrophy: shrinkage due to size/number reduction.
- Necrosis: pathological cell death (e.g., infarction, gangrene).
- Apoptosis: programmed cell death.
Key Terms & Definitions
- Histology — the study of tissues.
- Epithelium — sheet of cells covering/lining surfaces/cavities.
- Apical surface — free/top surface of epithelial cell.
- Basal surface — attached/bottom surface of epithelial cell.
- Basement membrane — structure anchoring epithelium to connective tissue.
- Microvilli — fingerlike projections for absorption.
- Cilia — hairlike projections for movement.
- Simple/Stratified — one/multiple layers of cells.
- Squamous/Cuboidal/Columnar — flat/box-like/tall cells.
- Endocrine/Exocrine gland — internal/external secretion.
- Goblet cell — unicellular mucus-secreting gland.
- Merocrine/Holocrine/Apocrine — types of glandular secretion.
- Hyperplasia/Hypertrophy/Neoplasia/Atrophy — cell growth/death terms.
Action Items / Next Steps
- Review and memorize the four primary tissue types and their characteristics.
- Study and differentiate epithelial tissue classifications and their specific locations/functions.
- Learn key terms and definitions for histology.
- Prepare for lab identification of epithelial tissues and practice distinguishing cell shapes and layers.