Epithelial Tissues

Epithelial tissues Introduction They cover the outer body surface and some body organs. They line cavities of organs and the internal body cavity (peritoneal cavity). Common characteristics They rest on a basement membrane. With very little intercellular substance or matrix. They are never penetrated by blood vessels. They are able to divide and form new cells.

Epithelial tissues Classification of epithelial tissues Epithelia tissues are classified according to three structural characteristics: The number of cell layers: A single layer of epithelial cells is termed simple epithelium. Epithelium with more than one layer is called stratified epithelium.

Epithelial tissues Classification of epithelial tissues Epithelia tissues are classified according to three structural characteristics: The shape of the component cells (the shape of the outermost layer in case of stratified epithelia): Squamous (spindle shape). Cuboidal. Columnar.

Epithelial tissues Classification of epithelial tissues Epithelia tissues are classified according to three structural characteristics: The presence of surface specialization: Cilia. Keratin.

Epithelial tissues Classification of epithelial tissues Simple epithelia Simple squamous epithelium: Cells are arranged in one layer. Cells are flattened with condensed nucleus. They are almost found on absorptive or secretory surfaces. Location: Found in lung alveoli and wall of Bowman's capsules in kidney. Form lining of blood vessels (endothelium) and peritoneal cavity (mesothelium).

Epithelial tissues Classification of epithelial tissues Simple epithelia Simple squamous epithelium: Simple squamous epithelium in a Bowman's capsule in kidney. Simple squamous epithelium in a blood capillary.

Epithelial tissues Classification of epithelial tissues Simple epithelia Simple cuboidal epithelium: Cells are arranged in one layer. Cells are square in shape with rounded central nucleus. The epithelium usually lines small ducts of exocrine glands and small kidney tubules which have secretory and excretory functions, respectively. Location Pancreatic ducts Small collecting tubules of kidney.

Epithelial tissues Classification of epithelial tissues Simple epithelia Simple cuboidal epithelium: Simple cuboidal epithelium in small collecting tubules of kidney. Simple cuboidal epithelium in a pancreatic duct.

Epithelial tissues Classification of epithelial tissues Simple epithelia Simple columnar epithelium: Cells are arranged in one layer. Cells are tall with ovoid and basally located nucleus. The epithelium usually lines intestine and large kidney tubules which have absorptive and excretory functions, respectively. Location Gastrointestinal tract, from stomach to rectum. Large collecting tubules of kidney.

Epithelial tissues Classification of epithelial tissues Simple epithelia Simple columnar epithelium: Simple columnar epithelium of large collecting tubule in kidney. Simple columnar epithelium in small intestine.

Epithelial tissues Classification of epithelial tissues Simple epithelia Simple ciliated columnar epithelium: This epithelium is similar to the previous type, but the free border of the cells have cilia. Location Female reproductive tract (Fallopian tubes).

Epithelial tissues Classification of epithelial tissues Simple epithelia Pseudostratified ciliated columnar epithelium: Another type of simple epithelium (all cells rest on the basement membrane). Not all cells extend to the surface. The nuclei of cells lie at different levels giving the epithelium a false stratified appearance. Location Lining of trachea and nasal cavities. Pseudostratified ciliated columnar epithelium in trachea

Epithelial tissues Classification of epithelial tissues Stratified epithelia: Consisting of two or more layers of cells. Have a protective function. The cells of the basal layer are usually cuboidal or short columnar. The cells of the surface layer vary. The intermediate layers are polygonal. The classification of epithelia is related to the structure of the surface layer.

Epithelial tissues Classification of epithelial tissues Stratified epithelia: Non-keratinized stratified squamous epithelium: The cells at the free surface are squamous. The epithelium is adapted to withstand moderate abrasion (friction). Location Lining of the oral cavity, pharynx and esophagus. Non-keratinized stratified squamous epithelium

Epithelial tissues Classification of epithelial tissues Stratified epithelia: Keratinized stratified squamous epithelium: The cells at the free surface are squamous. During maturation, the epithelial cells undergo a process called keratinization forming a non-cellular surface layer known as keratin. The epithelium is adapted to withstand abrasion (friction) and desiccation. Location Skin Keratinized stratified squamous epithelium

Epithelial tissues Classification of epithelial tissues Stratified epithelia: Transitional epithelium: It is highly specialized to accommodate a great degree of stretch and to withstand the toxicity of urine. In the relaxed state (bladder is empty), transitional epithelium appears formed of 4-8 layers of cells. In the stretched state, transitional epithelium appears formed of 2 or 3 layers of cells, and the intermediate and surface layers are extremely flattened. Location Lining of urinary bladder, ureters and urethra. Transitional epithelium (relaxed)

Epithelial tissues Exocrine glands: They are glands which discharge their secretory product via (through) a duct. Exocrine glands can be divided morphologically into: Simple glands (with a single unbranched excretory duct). Compound glands (with a branched duct system). The secretory portion of both simple and compound glands have two main forms: tubular (the secretory part is tubular). acinar or alveolar (the secretory part is flask-shaped).

Epithelial tissues Exocrine glands: