Tissues: Epithelium Summer Scholars 2007

Definition of tissues Tissue = a group of cells (and their extracellular products) that share a similar structure or function.

Basic Tissue Types All 200+ cell types can be grouped into these main tissue types. Each organ is made up of several tissue types. Epithelial tissues Epithelial tissues Connective tissues Connective tissues Muscle tissues Muscle tissues Nervous tissues Nervous tissues Blood and lymphatic tissues (also considered CT) Blood and lymphatic tissues (also considered CT)

Epithelial Tissues Cover surfaces of the body. Cover surfaces of the body. Line tubes within the body. Line tubes within the body. Form various glands. Form various glands.

Common characteristics shared by all epithelia 1. Form continuous sheets or tubes, 1 to many cells thick. 2. Consist of closely packed cells with very little intercellular matrix.

Common characteristics shared by all epithelia 3. Extensively interconnected by cell-cell junctions.

Terminal Bars represent cell-to-cell attachment sites

Common characteristics continued 4. Cells are polar with structurally and functionally distinct surface domains: apical, basal, lateral. 5. Supported by a basal lamina, a basement membrane that cells use for structural support and attachment.

apical portion of a cell basal portion of a cell lumen

Common characteristics of epithelium 6. Avascular 7. Dependent on diffusion of dissolved gases and metabolites. 8. Continually renewed by mitosis. –Intestinal epithelium half-life = 4-6 days –Skin epidermis half-life = 2-4 weeks

Classification of Epithelia must be in this order: 1. Number of cell layers –Simple – one cell layer resting on the basal lamina –Stratified – >1 cell layer thick, only basal layer rests on basal lamina –Pseudostratified – looks stratified, but all cells are actually in contact with BL. 2. Shape of the most superficial layer of cells –Squamous – flattened in shape –Cuboidal – cube-shaped or square –Columnar – rectangular or elongated with respect to the basal lamina 3. Surface specializations –Ciliated or non-ciliated –Keratinized or non-keratinized –Brush border (microvilli) or stereocilia –Goblet cells

Simple squamous epithelium of capillaries

kidney tubules

Human kidney

Duodenum

Esophagus classify this epithelium

keratin Thick skin Stratified Squamous Epithelium

Trachea

Epididymis

Bladder

Functions of Epithelial Tissues Protection Protection Containment Containment Transport Transport Absorption Absorption Secretion Secretion Sensation Sensation