General Biology/Tissues and Systems/Epithelial Tissue

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Epithelial tissue

Comes from various sources, ectodermal or endodermal material. Cell sheet lines a surface or body cavity. One side, called freesurface or Apical, is exposed to

• animal interior (forming the lumen) or
• exterior of its body cavity.

The other side rests on the basal layer.

Epithelial tissue is not penetrated by blood vessels.

Two categories:

• sheets
• glands

Classified on two features:

• simple, (a single layer of cells),
• stratified, (more than one cell layer.)

Cell shape at free surface:

• squamous (broad and flat)
• cuboidal (spherish)
• columnar (tall and thin)

Simple squamous epithelium

usually lines body cavities and vessels,alveoli, glomeruli of kidney; in blood and lymph vessels called endothelium; in body cavities called mesothelium (serosae): parietal serous membranes line body wall, visceral serous membranes cover organ

Simple cuboidal epithelium

in ducts like kidney and salivary glands.

Simple columnar epithelium

nonciliated type lines digestive tract, ciliated type lines some regions of uterine tubes and lungs

Stratified squamous epithelium

(important) lines mouth, esophagus,and vagina. Cells sometimes dead, flat and keratinized, making them resistant to abrasion. Stratified squamous epithelium changes to columnar squamous epithelium progressively down esophagus to the stomach.

Epidermis

from epithelium. Below this is dermis, thicker and with blood vessels.

Two specialized epithelia:

• pseudostratified
• transitional

Pseudostratified epithelia

lines the trachea (where it is ciliated)and the male urethra (where it is non ciliated), looks stratified but not.

Transitional epithelia

found only in bladder and urinary system. As it stretches it appears to go from 6 to 3 cell layers deep.

Glandular epithelia

(gland: group of cells that excretes something.. mostly derived from epithelium. Glands are classified into endocrine and exocrine by where they excrete.

Endocrine glands

secrete hormones into the blood without use of ducts.

Exocrine glands

secrete onto the body surface or into a cavity, thru a duct. Exocrine substances include sweat, mucous, oil, and saliva. An exocrine gland is the liver, which secretes bile.