Tissues: EpitheliumTissues: Epithelium

Summer Scholars 2007Summer Scholars 2007

Definition of tissuesDefinition of tissues

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

Basic Tissue TypesBasic Tissue Types

All 200+ cell types can be grouped into these All 200+ cell types can be grouped into these main tissue types.main tissue types.

Each organ is made up of several tissue types.Each organ is made up of several tissue types.• Epithelial tissuesEpithelial tissues• Connective tissuesConnective tissues• Muscle tissuesMuscle tissues• Nervous tissuesNervous tissues• Blood and lymphatic tissues (also considered Blood and lymphatic tissues (also considered

CT)CT)

Epithelial TissuesEpithelial 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 Common characteristics shared by all epitheliashared by all epithelia

1. Form continuous sheets or tubes, 1 1. Form continuous sheets or tubes, 1 to many cells thick.to many cells thick.

2. Consist of closely packed cells with 2. Consist of closely packed cells with very little intercellular matrix.very little intercellular matrix.

Common characteristics Common characteristics shared by all epitheliashared by all epithelia

3. Extensively interconnected by cell-3. Extensively interconnected by cell-cell junctions.cell junctions.

Terminal Bars represent cell-to-cell attachment sites

Common characteristics Common characteristics continuedcontinued

4. Cells are polar with structurally and 4. Cells are polar with structurally and functionally distinct surface domains: functionally distinct surface domains: apical, basal, lateral.apical, basal, lateral.

5. Supported by a basal lamina, a 5. Supported by a basal lamina, a basement membrane that cells use basement membrane that cells use for structural support and for structural support and attachment.attachment.

apical portionof a cell

basal portionof a cell

lumen

Common characteristics of Common characteristics of epitheliumepithelium

6. Avascular6. Avascular

7. Dependent on diffusion of dissolved 7. Dependent on diffusion of dissolved gases and metabolites.gases and metabolites.

8. Continually renewed by mitosis.8. Continually renewed by mitosis.– Intestinal epithelium half-life = 4-6 daysIntestinal epithelium half-life = 4-6 days– Skin epidermis half-life = 2-4 weeksSkin epidermis half-life = 2-4 weeks

Classification of Epithelia must be in this Classification of Epithelia must be in this order:order:

1. Number of cell layers1. Number of cell layers– Simple – one cell layer resting on the basal laminaSimple – one cell layer resting on the basal lamina– Stratified – >1 cell layer thick, only basal layer rests on Stratified – >1 cell layer thick, only basal layer rests on

basal laminabasal lamina– Pseudostratified – looks stratified, but all cells are Pseudostratified – looks stratified, but all cells are

actually in contact with BL.actually in contact with BL.

2. Shape of the most superficial layer of cells2. Shape of the most superficial layer of cells– Squamous – flattened in shapeSquamous – flattened in shape– Cuboidal – cube-shaped or squareCuboidal – cube-shaped or square– Columnar – rectangular or elongated with respect to the Columnar – rectangular or elongated with respect to the

basal laminabasal lamina

3. Surface specializations3. Surface specializations– Ciliated or non-ciliatedCiliated or non-ciliated– Keratinized or non-keratinizedKeratinized or non-keratinized– Brush border (microvilli) or stereociliaBrush border (microvilli) or stereocilia– Goblet cellsGoblet cells

Simple squamous epithelium of capillaries

kidney tubuleskidney tubules

Human kidneyHuman kidney

DuodenumDuodenum

EsophagusEsophagus

classify thisepithelium

keratin

Thick skin

StratifiedSquamousEpithelium

TracheaTrachea

EpididymisEpididymis

BladderBladder

Functions of Epithelial Functions of Epithelial TissuesTissues

ProtectionProtection ContainmentContainment TransportTransport AbsorptionAbsorption SecretionSecretion SensationSensation