Tissue level of organization

Epithelial Tissue contains…?

Epithelia and glands

Epithelia is…?

Layers of cells that cover internal and external surfaces.

Glands are…?

Fluid secreting cells derived from epithelia.

Important characteristics of epithelia

Cells are close together. A free surface exposed to the environment or to internal chambers/passageways. Attached to underlying connective tissue by a basement membrane. No blood cells (avascular), gets nutrients from deeper tissues. Continous replacement/regeneration of epithelial cells.

Functions of epithelia

Provides physical protections, controls permeability, provides sensations and produces specialized secretions.

Exocrine secretions

Discharged onto the surface of the epithelium.

Endocrine secretions

Discharged into surrounding tissue fluid and blood. Hormones are chemical messengers.

Extracellular connections

Epithelial cells stay firmly connected to eachother and to basement membrane

Cell adhesion molecules (CAMs)

Transmembrane proteins that interconnect large areas of opposing plasma membranes. These proteins bind together to extracellular materials by a thin layer of proteoglycans.

Cell junctions

Specialized attachment sites that attach cell to cell or cell to extracellular materials.

Tight junctions

Lipid layers of adjacent plasma membranes tightly bound together by interlocking membrane proteins. Under tight junctions, a continuous adhesion belt forms that encircles cells and binds them to each other. Prevents water and solutes from passing cell to cell, prevents damage to underlying tissue.

Gap junctions

Two cells held together by embedded membrane proteins called connexons. Connexons form narrow passageways that let small molecules and ions pass cell to cell. Abundant in cardiac muscle and smooth muscle tissue, essential to coordination of muscle contractions.

Desmosome

The plasma membranes of two cells are locked together by CAMs and proteoglycans. This area is linked to cytoskeleton by a network of intermediate filaments, helps cells withstand mechanical stresses such as stretching, bending, twisting, or compression.

Spot desmosomes

Desmosomes that form a small disc.

Hemidesmosomes

Resemble half of a spot desmosome, attaches cell to the basement membrane.

Epithelial surface

Epithelia that lines the internal passageways have microvilli on their exposed surfaces, especially abundant where absorption and secretion takes place.

Ciliated epithelium

Moves materials.

Transport specialists

Epithelial cells in kidneys and digestive system are transport specialists.

Basement membrane

A network of protein fibers that connect epithelial cells with underlying connective tissue. Epithelial cells are attached to the protein fibers by hemidesmosomes.

Stem cells (germinative cells)

Unspecialized cells found in the deepest layers of the epithelium near the basement membrane.

Simple epithelium

Single layer of cells covering the basement membrane. They're thin, only found in protective areas, and reduce the time for materials to cross epithelial barrier.

Stratified epithelium

Provides greater protection since it has several layers of cells above basement membrane. Usually found in areas exposed to mechanical or chemical stresses, and surface of skin and lining of the mouth.

Squamous cells

Thin and flat, the nucleus is the thickest part of the cell.

Cuboidal cells

Little hexagonal boxes, like square, round nuclei in the center of the cell.

Columnar cells

Hexagonal but taller and slender, and kind of resembles rectangles.

Simple squamous epithelium

Found in protective areas where absorption happens or slippery surface reduces friction. Kidney tubules, gas exchange, surface of the lungs, lining of cavities and blood vessels.

Simple cuboidal epithelium

Provides limited protection in the pancreas and salivary glands, secrete enzymes, also lines portions of the kidney tubules.

Simple columnar epithelium

Provides some protection, may occur in areas of absorption or secretion, lines the stomach, intestinal tract, and excretory ducts.

Stratified squamous epithelium

Where chemical stresses are severe, surface of the skin and lining of the mouth, tongue, esophagus, and anus.

Stratified cuboidal epithelium

Relatively rare, occurs along the ducts of sweat glands and in larger ducts of the mammary glands.

Stratified columnar epithelium

Relatively rare, found in long portions of the pharynx, epiglottis, anus, urethra, and a few large excretory ducts. If more than two layers are present, only the top cells are columnar.

Pseudostratified Epithelium

Looks like it has multiple layers because the nuclei are at different heights, but all cells touch the basement membrane, so it is not truly stratified. It usually has cilia and lines much of the respiratory tract (nasal cavity, trachea, and bronchi).

Transitional epithelium

A stratified epithelium that stretches and recoils, changing appearance as it stretches. It lines the ureters and urinary bladder; cells are plump/cuboidal when relaxed and become flattened when stretched.

Unicellular exocrine glands (mucous or goblet cells)

Single cells that secretes substances.

Multicellular exocrine glands

Multiple cells that secretes substances.

Secretory sheet

Simplest multicellular exocrine gland. Epithelium of mucin secreting cells that lines the stomach and protects it from acids.