Module 2.2: Epithelial Tissue

Five Defining Characteristics

Polarity & unique surfaces, avascular, basal surface connects to basement membrane, highly regenerative, always connected to one another because of specialized cell junctions (cellularity)

Polarity

Cells have unique surfaces with specialized features

Apical

The surface facing the external environment or inside of a lumen (opening) allowing the cells to secrete or absorb materials from the lumen

Microvilli 

Finger-like extensions on the apical surface of most epithelial cells, adding extra surface area to cells and allowing for additional secretions and absorption

Cilia

Longer cytoplasmic extensions with an internal structure that allows for their movement. Found in places such as the trachea and bronchi, arranged in neat rows.

Avascular

Cells that do not have a direct blood supply. Nutrients must enter, and wastes must leave by diffusion or absorption from underlying tissues because no blood vessels cross the basement membrane.

Basal

Surface of epithelia connecting to the basement membrane

Basement Membrane (basal lamina)

Noncellular and protein-rich, allowing for cells to anchor to it for support. Separates epithelium from other tissue, produced by the basal surface of epithelial cells, as well as connective tissue cells deep to it.

Gap Junctions (Communicating Junctions)

Unique cellular structures (embedded protein pores) that permit the free diffusion of ions and small molecules between cells.

Tight Junctions (Occluding Junctions)

Interlocking proteins fuse the outer layers of the plasma membrane forming an impermeable layer

Adhesion Belt

Forms deep to the tight junctions, forming a continuous bandlike structure between the cell

Spot Desmosomes

Link adjacent cells together via CAMS to give strength for mechanical stress

Hemidesmosomes 

Junctions that attach epithelial cells to the basement membrane in stratified or complex epithelia

Stem Cells

Regenerative cells found near the basement membrane that replenish lost cells

Terminal Web

Meshwork of microfilaments at the base of the microvilli on the apical side of the cell

CAMs

Cell Adhesion Molecules 

Simple Epithelial Tissue

Cells that are only one layer thick

Stratified Epithelial Tissue

When the cells have two or more layers

Squamous

When the cells are wider than the height (thin/flat).

Cuboidal (square/cube)

When the cells have approximately the same height, width, and depth.

Columnar (tall, thin, rectangular)

When the height of the cells is greater than width.

Simple Squamous Epithelium (Function)

One layer thick, allows for materials to be exchanged via diffusion through the cells. Act as a thin barrier or allow for lubrication/secretion to help reduce friction between two structures.

Simple Squamous Epithelium (Location)

Vascular system, respiratory spaces in the lungs, and lining of the body cavities

Endothelium (Simple Squamous)

Lining of the blood and lymphatic vessels

Endocardium (Simple Squamous)

Lining of the ventricles and atria of the heart

Mesothelium (Simple Squamous)

Lining the walls and closed cavities of the body (such as the abdominal, pericardial, and pleural cavities)

Stratified Squamous Epithelium (Functions)

Provide physical protection against abrasion, pathogens, and chemical attacks. Forms the epidermis.

Stratified Squamous Epithelium (Locations)

Surface of skin; lining of mouth, throat, esophagus, rectum, anus, and vagina

Simple Cuboidal Epithelium (Function)

Limited protection, secretion, absorption

Simple Cuboidal Epithelium (Location)

Glands; ducts, portions of kidney tubules, thyroid gland

Exocrine Glands

Glands that secrete a substance through a duct onto an epithelium rather than into the bloodstream

Stratified Cuboidal Epithelium (Function)

Protection, secretion, absorption (Stratified Cuboidal)

Stratified Cuboidal Epithelium (Location)

Lining of some ducts (rare), sweat glands

Simple Columnar Epithelium (Function)

Protection, secretion, absorption

Simple Columnar Epithelium (Location)

Lining of stomach, intestine, gallbladder, uterine tubes, and collecting ducts of kidneys

Stratified Columnar Epithelium (Function)

Protection

Stratified Columnar Epithelium (Location)

Small areas of pharynx, epiglottis, anus, mammary glands, salivary gland ducts

Pseudostratified Epithelium

When the cells appear to be stratified and have more than one layer, but there is actually just one layer of cells made up of different sizes

Pseudostratified Epithelium (Function)

Protection, secretion, move mucus with cilia

Pseudostratified Epithelium (Location)

Lining of nasal cavity, trachea, and bronchi; portions of male reproductive tract 

Transitional Epithelium

Specialized cells in the urinary tract

Transitional Epithelium (Function)

Permits repeated cycles of stretching without damage

Transitional Epithelium (Location)

Urinary bladder; renal pelvis; ureters

Endocrine Gland

Do not have a duct system and instead secrete their products directly into the connective tissue

Hormones

The products of an endocrine gland, which enter the bloodstream in order to reach their target cells

Paracrine Glands 

Secrete substances that do not reach the bloodstream but only affect other cells nearby in the same epithelium

Merocrine Secretion

When a product is delivered via membrane-bound vesicles to the apical surface, vesicle must fuse with the plasma membrane and release via exocytosis, found in salivary glands

Apocrine Secretion

Secretion involves loss of apical cytoplasm; inclusions, secretory vesicles, and other cytoplasmic components are shed in the process, then the cell grows and repairs itself; found in mammary glands

Holocrine Secretion

Occurs as superficial gland cells burst, cells have to be replaced, found in sebaceous glands

Apoptosis

Programmed Cell Death

Unicellular Gland

One single secretory cell among other non-secretory cells

Goblet Cells

Found in ciliated columnar epithelium in the lining of the intestines and trachea, function to secrete mucous to provide a protective layer.

Multicellular Glands 

A structure of more than one cell

Simple Gland

Duct is unbranched

Compound Gland

Branched Duct

Simple Tubular (Simple Gland)

Shaped like a tube

Simple Coiled Tubular (Simple Gland)

Shaped like a coil

Simple Branched Tubular (Simple Gland)

Multiple secretory branches

Simple Alveolar (Acinar, Simple Gland)

Outpouching of secretory portion

Simple Branched Alveolar (Simple Gland)

Branches of multiple acinar-shaped secretory portions

Compound Tubular (Compound Gland)

More than one branch with tube-like secretory portions

Compound Alveolar (Acinar, Compound Gland)

More than one branch with outpouching of secretory portions

Compound Tubulo-Alveolar (Compound Gland)

More than one branch with a combination of tubelike and alveolar type secretory portions