Introduction to Basic Tissues

Study Notes

There are four basic tissue types in the human body: epithelial tissue, connective tissue, muscular tissue, and nervous tissue.
Tissues are composed of cells, which are organized structures containing a nucleus and cytoplasm, and extracellular matrix, which is the non-cellular material surrounding the cells.

Epithelial tissue covers surfaces and lines cavities (covering epithelium) or forms glands (glandular epithelium).
Composed primarily of cells with little intercellular substance.
Epithelial cells rest on a basement membrane, which separates the epithelium from the underlying connective tissue.
Epithelium lacks blood vessels (avascular) but can be innervated by nerves.
Nourishment for epithelial tissue comes through diffusion from blood vessels in the underlying connective tissue.

There are three main categories of epithelium: covering and lining epithelium, glandular epithelium, and specific epithelium.
Covering and lining epithelium is further categorized as simple (single layer) or stratified (multiple layers).
Glandular epithelium includes both exocrine glands (secrete products into ducts) and endocrine glands (secrete products directly into the bloodstream).
Specific epithelium includes myoepithelial cells (contractile cells associated with glands), specialized sensory epithelium (involved in sensing), and neuroepithelium (epithelial cells that also function as neurons).

Simple epithelium is further categorized based on cell shape.
Simple squamous epithelium is a single layer of flat cells with flattened nuclei, commonly found lining blood vessels (endothelium) and in the alveoli of the lungs.
Simple cuboidal epithelium is a single layer of cube-shaped cells with central rounded nuclei, found in thyroid follicles and lining the convoluted tubules of the kidney.
Simple columnar epithelium is a single layer of tall columnar cells with basal oval nuclei. It can be ciliated (lining the uterus and fallopian tubes for movement) or non-ciliated (lining the stomach and intestine for secretion and absorption).
Pseudostratified columnar epithelium appears stratified but is actually a single layer of cells with varying heights, all resting on the basement membrane. The appearance of multiple layers is due to the nuclei being at different levels. It is often ciliated (in the trachea) and contains goblet cells (mucus-producing cells).

Goblet cells are modified simple columnar cells that secrete mucus.
They are present in the small intestine, large intestine, and respiratory tract.
Goblet cells have a cup-like expanded apical part filled with mucus, and a basal cylindrical part containing a flat nucleus.
They appear as vacuoles or pale areas in the epithelial membrane due to the mucus dissolving during preparation.

Stratified epithelium is characterized by multiple layers of cells. The shape of the cells in the superficial layer is used for classification.
Stratified squamous epithelium has several layers of cells with basal layers being cuboidal to columnar, intermediate layers being polyhedral, and surface layers being flattened (squamous). It serves a protective function.
- Keratinized stratified squamous epithelium: has a dead superficial layer of cells and is found on dry surfaces like the epidermis of the skin.
- Non-keratinized stratified squamous epithelium: has a living superficial layer of cells, and is found on wet surfaces like the esophagus, mouth, and vagina.
Stratified cuboidal epithelium is composed of multiple layers of cube-shaped cells, primarily found in larger ducts of sweat glands.
Stratified columnar epithelium is composed of multiple layers of columnar cells, found in the male urethra and the lining of the larger ducts of glands.
Transitional epithelium is found in the urinary system (urinary bladder and ureters). It can stretch and change shape depending on the fullness of the organ.