Epithelium and Cell Junctions

Dr. Cathleen Pettepher

January 14, 2002

Four Basic Tissue Types

4 ~200 distinctly different types of cells that compose the human body

4 Arranged and organized into basic tissues:

– epithelium

– connective tissue

– muscle

– nervous tissue

4 Tissues are assembled into organs that carry out functions of the body

Epithelial Tissue

4 Present in two forms:

– sheets of contiguous cells -- epithelia

• cover the body on its external surface

• line the body on its internal surface

– glands which originate from invaginated epithelial cells

4 Derived from all 3 germ layers but mostly from endoderm and ectoderm

4 Ectoderm derivatives:

– oral & nasal mucosa, cornea, mammary glands

– epidermis and glands of skin

4 Endoderm derivatives:

– lining of respiratory & GI tract

– liver and pancreas

4 Mesoderm derivatives:

– lining of male & female reproductive tract

– lining of cardiovascular system

4 Mesothelium

– membranous lining of the body cavities

4 Endothelium

– lining of heart, blood vessels and lymphatics

4 Both linings are continuous sheets composed of contiguous cells that represent typical epithelium

– They just have a special name!!

Functions of Epithelium

4 Protection of underlying tissues from abrasion and injury

4 Transcellular support of molecules across epithelial layers

4 Secretion of mucus, hormones and enzymes

4 Absorption of material from the lumen

4 Selective permeablility of intercellular junctions which control movement of materials between body compartments

4 Detection of sensations

Characteristics of Epithelial Tissue

4 Cells are contiguous

– Close cell-to-cell contact and tightly bound together by junctional complexes

4 Avascular

4 Cells have cytoskeletal components composed of cytokeratin

4 Cells exhibit apical-basal polarity

– organelles and membrane specializations differ within the apical and basal regions

4 Separated from the underlying CT tissue by basal lamina

– secreted by epithelial cells

Classification of Epithelium

4 Number of cell layers between the basal lamina and the free surface

– composed of a single layer -- “simple”

– composed of multiple layers -- “stratified”

4 Morphology of the cells when viewed in sections taken perpendicular to basement membrane

– flat -- “squamous”

– cuboidal

– columnar

4 Two additional distinct types:

– Pseudostratified epithelium

• single layer of cells with some cells shorter than others and not reaching the cell surface

– Transitional epithelium

• changes shape as the membrane is stretched

Simple Epithelium

4 Composed of a single layer of cells

4 Always found at interfaces involved in selective diffusion, absorption or secretion

4 Provide little protection against mechanical abrasion and therefore are not found on surfaces subject to stress

4 Range in shape from extremely flattened to tall columnar, depending on their function

– flattened cells are suited to diffusion

• air sacs of lung

• lining of blood vessels and body cavities

– tall cells are highly active

• accommodate the appropriate organelles

• lining of small intestine

4 May exhibit surface specializations

Simple Squamous Epithelium

4 Squamous means “scale-like”

4 flattened, irregularly shaped cells that are tightly packed

4 Viewed from the surface, tissue looks like a “tile floor” with a centrally placed bulging nucleus

4 Viewed in section, only some cells display nucleus because of plane of section

4 delicate lining supported by basement membrane (BM)

– rarely thick enough to be detected by LM

4 specialized surface receptors that control secretion of locally acting chemical messengers

Simple Cuboidal Epithelium

4 Single layer of polygonal-shaped cells

4 Viewed in sections perpendicular to the basement membrane, cells present a square profile with a centrally placed round nucleus

Simple Columnar Epithelium

4 Columnar-shaped cells

4 height varies depending upon degree of functional activity

4 nuclei are elongated and may be located near the base, center or apex -- polarity

4 Luminal plasma membranes of highly absorptive cells exhibit finger-like projections

– microvilli

– greatly increase surface area

– too small to see by LM

– collectively form “brush border”

Simple Columnar “Ciliated” Epithelium

4 Presence of surface specializations -- Cilia

– larger than microvilli

– visible with LM

– beat in a “wave-like” manner to propel fluid or particles over the surface

4 Not Common

– female reproductive tract

Pseudostratified Columnar Epithelium

4 Most, but not all cells extend up to the free surface

4 All lie in contact with the basement membrane

4 Nuclei are visible at more than one level

– makes it appear stratified

4 May contain cilia on the surface of cells

4 May contain goblet cells

– individual mucus producing cells

– nucleus in stem

– free surface may bear some microvilli

– appear pale by H&E

4 Distinguishing Characteristics:

– individual cells exhibit polarity, with nuclei being mainly confined to the basal two-thirds of the epithelium

– Cilia are never present on stratified epithelium!!

4 Locations:

– exclusively confined to the larger airways of the respiratory system -- respiratory epithelium

Stratified Epithelia

4 Epithelia consisting of two or more layers

4 Serve mainly as a protective function

4 Degree and nature of stratification are related to kinds of physical stresses it is exposed to.

4 Classification is based on shape and structure of surface cells

– basal layer are usually cuboidal in shape

Stratified Squamous Epithelium

4 Variable number of cell layers which exhibit a transition

– cuboidal basal layer to a flattened surface layer

– basal layer divides continuously, with offspring being pushed to the top

4 during this process, cells undergo:

– maturation

– degeneration (pyknotic nuclei of surface cells)

– disintegration

4 may be “keratinized”

– scales that adhere to underlying layers

4 during maturation

– cross-linked cytoskeletal proteins accumulate in the epithelial cells

– forms “non-living” surface

Transitional Epithelium

4 Located exclusively in the urinary tract

4 Many layers:

– basal -- low columnar

– middle -- polyhedral cells

– surface -- rounded domes, binucleated

• flattened when bladder is distended

Transitional Epithelium Basement Membrane

4 Layer of variable thickness beneath the basal surfaces

4 Requires special stains (PAS) to visualize

4 Stain reacts with sugar moieties of proteoglycans and accumulates

Basal Lamina

4 Lamina densa

4 electron-dense matrix 50-100nm thick

– contains 3-4nm filaments

4 structural attachment sites for anchoring fibrils

– extend to reticular fibers of CT

Lamina Lucida

4 between basal lamina and the cell

4 relatively clear or electron-lucent area

4 contains fine filaments that join the plasma membrane to basal lamina

Functions of Basal Laminae

4 Compartmentalization

– separates CT from epithelia, nerve or muscle tissues

4 Filtration

– regulates movement of substances to and from the CT

– occurs largely through ionic charges

4 Polarity induction

– attributes specific properties to the basal membrane surface

4 Tissue scaffolding

– serves as a guide or scaffold during regeneration

Composition of Basal Laminae

4 Type IV collagen

– short filaments that provide structural integrity

– secreted by epithelial cells

4 Proteoglycans

– hydrated molecules that form bulk volume of lamina

– high negative charge density so regulate passage of ions

4 Laminin

– cross-shaped glycoprotein molecule secreted by epithelial cells

– domains that bind to Type IV collagen, heparin sulfate & integrins

– bridges lamina lucida to link basal lamina to plasma membrane

4 Entactin & Fibronectin

– glycoproteins that act as an adhesive substance

– binds plasma membrane to proteoglycans

4 Anchoring filaments

– type VII collagen that link basal lamina to reticular lamina

• 2° layer of fibrils of Type III collagen

• thickness varies with amount of frictional forces

Terminal Bars

4 contact point between adjacent epithelial cells

4 intercellular cement

4 continuous around entire circumference of each cell

4 composed of junctional complexes

Junctional Complexes

4 Occluding junctions

– form impermeable barrier

– prevent material from taking an intercellular route in passing across epithelial sheath

4 Anchoring junctions

– maintaining cell-to-cell adherence

4 Communication junctions

– permit movement of ions or signaling molecules between cells

Zonulae Occludentes (Tight junctions)

4 most apically located

4 encircles the cell

4 series of focal fusions of the outer leaflets of plasma membrane

– transmembrane junctional proteins come in contact to seal off intercellular space

Zonulae Occludentes

4 Prevent movement of membrane proteins from the apical to basolateral domain

4 Prohibit water-soluble molecules from passing between cells.

4 May be ‘tight’ or ‘leaky’

Zonulae Adherentes

4 band or belt that reinforces the lateral plasma membrane

4 15-20nm space between opposing membranes

– electron-lucent

– cadherins -- proteins that bind to actin

4 calcium dependent

Macula Adherentes (desmosomes)

4 spot weld-like junctions that are randomly distributed

4 disk-shaped attachment plaque on cytoplasmic side

– anchors intermediate filaments of cytokeratin which disperse shearing forces on the cell

4 wider intercellular space

4 contains a dense medial band

– contains desmoglein

– provides attachment of the two cells through interaction of extracellular linker filaments

Hemidesmosomes

4 attach basal membrane to the basal lamina

4 attachment plaques on cytoplasmic side

4 found where abrasion and mechanical shearing forces would tend to separate epithelium from underlying CT

Gap Junctions

4 Circular intercellular contact areas

4 contain tiny pores

4 permit passage of small molecules between adjacent cells

– + changed ions

– nutrients

– signaling agents

4 Each pore consists of a minute tubular structure -- connexon -- which traverses gap

– pair of grommet-like cylinder

• six transmembrane proteins

• Ca+ dependent

– penetrating each of the opposing cell membranes