Overview
This lecture provides an overview of human tissue types, with a detailed focus on epithelial tissue, its structure, classification, and functions.
Major Tissue Types
- Tissues are groups of similar cells performing a common function in multicellular organisms.
- The four main tissue types are epithelial, connective, nervous, and muscle tissue.
Epithelial Tissue: Functions and Structure
- Epithelial tissue covers body surfaces, lines cavities and organs, and forms glands.
- Covering and lining epithelium acts as a barrier; glandular epithelium forms glands for secretion.
- Epithelial cells have an apical (exposed) surface and a basal (attached) surface.
- The apical surface may have microvilli (increase surface area) or cilia (move substances).
- Cells show polarity (differences between apical and basal surfaces).
- The basal surface attaches to the basal lamina, which, along with the reticular lamina, forms the basement membrane.
- Epithelial cells are tightly packed in sheets and connected by desmosomes, tight junctions, and gap junctions.
- Epithelial tissues are avascular (no blood vessels) but innervated (have nerves).
- They regenerate quickly due to exposure to friction and injury.
- Functions include protection, absorption, filtration, excretion, secretion, and sensory reception.
Classification of Epithelial Tissue
- Classified by number of layers: simple (one layer) or stratified (multiple layers).
- Classified by cell shape: squamous (flat), cuboidal (boxy), columnar (tall).
- Simple squamous epithelium: single layer of flat cells, ideal for rapid diffusion.
- Simple cuboidal epithelium: single layer of cube-shaped cells, for secretion/absorption.
- Simple columnar epithelium: single layer of tall cells, for secretion/absorption in digestion.
- Pseudostratified columnar epithelium: appears layered due to staggered nuclei, but is actually one layer.
- Stratified squamous epithelium: multiple layers; surface cells are flat and protective (e.g., skin).
- Stratified cuboidal and columnar epithelia are rare and found in some glands.
- Transitional epithelium can stretch and change shape, important in the urinary system.
Glandular Epithelium
- Glandular epithelium forms glands that secrete substances.
- Exocrine glands release products through ducts to surfaces.
- Endocrine glands secrete products like hormones directly into the blood.
Key Terms & Definitions
- Tissue — a group of similar cells performing a common function.
- Epithelial tissue — tissue that covers surfaces, lines cavities, and forms glands.
- Apical surface — the exposed surface of an epithelial cell.
- Basal surface — the side attached to underlying connective tissue.
- Basement membrane — structure supporting epithelial cells, formed by basal and reticular lamina.
- Avascular — lacking blood vessels.
- Desmosome — junction that binds adjacent cells together.
- Simple epithelium — one cell layer thick.
- Stratified epithelium — multiple cell layers thick.
- Squamous, cuboidal, columnar — flat, boxy, and tall cell shapes, respectively.
- Gland — structure of one or more cells that secrete a product.
- Exocrine gland — secretes products via ducts onto surfaces.
- Endocrine gland — secretes products directly into the bloodstream.
Action Items / Next Steps
- Review the main characteristics and classifications of epithelial tissue.
- Be prepared to learn about connective tissue in the next lecture.