I. General Features and Considerations Epithelial Tissue I. General Features and Considerations

A. Characteristics Lines internal and external surfaces Single or multiple layers of cells Little or no intercellular space Avascular Polarization Keratin Cell junctions Basement membrane

1. Protection

2. Absorption

2. Absorption

3. Secretion

3. Secretion

4. Excretion

5. Sensation

6. Contraction

II. Classification of Epithelia Epithelial Tissue II. Classification of Epithelia

A. Terminology Classification by number of layers: - simple - stratified Classification by shape of surface cells: - squamous, cuboidal, or columnar Classification by structural specializations: - pseudostratified - transitional (urinary) - surface structures

B. Simple epithelia Simple squamous Simple cuboidal Simple columnar

Simple Squamous Epithelium

Simple Squamous Epithelium

Endothelium

Mesothelium

Simple Cuboidal Epithelium

Simple Cuboidal Epithelium

3. Simple columnar epithelium in profile are rectangular with nuclei usually approximately the same level most likely to show polarity often function in absorption, secretion or both may show extensive surface specializations such as cilia and microvilli “pseudostratified” columnar - all cells touch the basement membrane (so it’s “simple) but all do not reach the free surface so it looks like it is multilayered

Simple Columnar Epithelium

B. Stratified epithelia Stratified squamous epithelium - Non-keratinized - Keratinized Stratified columnar (or cuboidal) epithelium

1. Stratified squamous epithelium usually 5-25 cell layers thick cuboidal cells on the basement membrane and squamous cells at free surface found on surfaces subject to injury, wear & tear non-keratinized - lining inside surfaces - all cells including the surface cells viable - surface cells possess functional nuclei keratinized - surfaces exposed to external world - surface cells non-viable and do not possess nuclei - surface cells contain almost only keratin (eosinophilic)

Non-Keratinized Stratified Squamous

Keratinized Stratified Squamous

2. Stratified columnar (or cuboidal) epithelium deep cells small, irregularly polyhedral while superficial cells cuboidal or columnar located at sites of transition from one type of epithelium to another provides more robust lining than a simple type of epithelium

Stratified Columnar Epithelium

D. Specialized epithelia Pseudostratified columnar epithelium Transitional (urinary epithelium)

1. Pseudostratified Columnar all cells contact the basement membrane but not all cells reach the free surface nuclei aligned at two or more levels

Pseudostratified Epithelium

2. Transitional Epithelium form of stratified epithelium that changes in thickness due to the stretch of the organ it lines when relaxed, the surface cells cuboidal and when distended, the surface cell become more squamous in shape found in organs of the urinary system

Transitional Epithelium

IV. Surface Specializations Epithelial Tissue IV. Surface Specializations

A. Microvilli Large numbers on a cell surface constitute a brush or striated border by light microscopy 1.0 mm X 0.1 mm evaginations of the luminal plasmalemma

A. Microvilli Composed of actin filaments, terminal web extends into cytoplasm Usually covered with a glycocalyx (sugar coat) Functions - increase surface area for absorption

Microvilli

Stereocilia

B. Cilia Actively motile evaginations of luminal plasmalemma, 2-10 mm long

B. Cilia Core of longitudinal microtubules called an axoneme (9+2) Basal bodies at base of cilia nine triplet microtubules Function in transport

Cilia

Epithelial Tissue V. Basal Lamina

A. Components Acellular supportive structure that can be up to 100 nm thick Composed mainly of type IV collagen, laminin, and proteoglycans

B. Basement membrane vs. basal lamina

Basement Membrane vs. Basal Lamina

C. Functions Barrier and support Contains recognition and regulatory factors Carries a positive charge - thromobogenic