Overview
Today's lecture focused on the structure, classification, and functions of epithelial tissue, one of the four main tissue types in the human body.
Types of Basic Tissues
- The four basic tissues are epithelial, connective, muscle, and nervous (neuronal) tissue.
- Muscle tissue includes cardiac, smooth, and skeletal muscles.
Structure and Characteristics of Epithelium
- Epithelium consists of tightly packed cells with little intercellular material and no blood vessels (avascular).
- It rests on a basement membrane that separates it from underlying connective tissue.
- Epithelial cells are arranged as sheets (membranous) or glands (glandular).
- Membranous epithelium covers external surfaces, lines internal cavities (like pericardial, pleural, and peritoneal), and lines body tubes connecting to the exterior.
Functions of Epithelial Tissue
- Protection: forms barriers (e.g., skin) against injury.
- Secretion: produces substances (e.g., glands, GI tract).
- Absorption: takes up nutrients (e.g., intestines).
- Sensory Detection: specialized epithelium detects stimuli (e.g., taste buds, olfactory epithelium, inner ear).
Classification of Epithelial Tissue
- Based on cell shape: squamous (flat), cuboidal (cube-like), columnar (tall).
- Based on layers: simple (single layer, all cells touch basement membrane) or stratified (multiple layers, only basal cells touch membrane).
- Stratified epithelium is named after the cell shape of the top layer.
Types of Epithelia and Examples
- Simple squamous: lines cardiovascular system (endothelium), alveoli (gas exchange), kidney nephrons (filtration).
- Simple cuboidal: found in kidney tubules, glandular ducts, and ovary surface.
- Simple columnar: lines GI tract and gallbladder.
- Stratified squamous: keratinized (skin, protective), non-keratinized (oral cavity, esophagus, vagina).
- Stratified cuboidal: ducts of some glands.
- Stratified columnar: large ducts of glands.
- Transitional: found in urinary system (bladder, ureter), adapts to stretching, urine-proof.
- Pseudostratified: appears layered but all cells touch basement membrane, found in respiratory tract and male epididymis.
Specialization and Polarity of Epithelial Cells
- Epithelial cells have an apical (free), lateral (cell-to-cell), and basal (attached to basement membrane) domain.
- Each domain has different structural, functional, and biochemical specializations (cell polarity).
Key Terms & Definitions
- Epithelium — Tissue made of tightly packed, avascular cells resting on a basement membrane.
- Basement membrane — Thin layer separating epithelium from connective tissue.
- Stratified epithelium — Multiple layers of epithelial cells; only basal cells contact the basement membrane.
- Keratinized — Cells filled with keratin protein, found in tough, dry surfaces like skin.
- Pseudostratified epithelium — Single layer appearing multilayered due to nuclei at different levels.
- Transitional epithelium — Multi-layered, stretchable epithelium in the urinary system.
Action Items / Next Steps
- Review diagrams of epithelial tissue types and their locations.
- Prepare examples of where each epithelial type is found for discussion.
- Read next section on surface specializations of epithelial cells.