Cell junctions and epithelia

Question 1

Epithelial tissue

Answer 1

one of 4 types of tissues, comprimises 60% of all body cells
the other three are- connective tissue, muscular tissue, nerve tissue

Epithelial tissue is :
Polarized, completely cellular, avascular, contacts the basal lamina/basement membrane

Question 2

Functions of epithelial tissue

Answer 2

Protection: Provides a barrier against mechanical injury, chemical injury, pathogenic invasion, water loss

Diffusion: enables transport of gasses, liquids, and nutrients from high to low concentration

Absorption: uptake of materials from lumen

Secretion: release of substances such as mucus, hormones, and enzymes

Excretion: release of waste products for elimination

Sensation: detects sensory stimuli

Question 3

Organization and Classification of Epithelial tissues

Answer 3

Number of cell layers:
Simple: 1 layer (diffusion, absorption, secretion, excretion)
Stratified: >1 layers (strength and protection)
Transitional Epithelium: relaxed/stretched (urinary system)
Pseudostratifed: All cells contact basement membrane (mainly in lungs)

Cell Shaped:
Squamous: flat cells (diffusion)
Cuboidal: cube shaped
Columnar: rectangular shaped cells highly metabolic cells

Presence of cell surface specializations:
Keratinization: outermost cell layer replaced with keratin (skin)
Glycocalyx: carb rich complex on apical surface (GI)
Ciliated: cilia on apical cells (respiratory and oviducts)
Microvilli: fingerlike projections

Question 4

Lateral Cell membrane specializations

Answer 4

Maintin coherence, distribute stress, used for comminication

4 types of specializations occuring between cells along their lateral domains:

1. Zonula occludens (tight juctions)- occluding junctions
2. Zonula adherens- anchoring juctions
3. Macula adherens (desmosomes)- anchoring
4. Gap junctions- communicating

Question 5

Tight junctions

Answer 5

Seal adjacent cell membranes, electron microscopy shows the characteristic pentalaminar apearance (DLDLD)

function as a selectively permeable barrier in epithelia, block tracer molecules

Proteins involved: Claudins/occludins bind to ZO1/2/3 proteins which bind to actin, and the JAM-1 proteins dimerize

Tight junctions drive TRANScellular transport as opposed to paracellular

Question 6

Zonula adherens (Adherens Junctions)

Answer 6

Cadherens are Calcium dependent homotypic interacting proteins that bind to Catenins, Vinculin, VASP and F actin

Different types of Cadherins:
E- epithelium
N: neurons, heart, muscle
P: placenta epidermis, breast
VE: endothelium

Epithelial adherens juctions form a continuous adhesion belt consisting of a contractile bundle of actin and an adhesion belt. The actin bundles are parallel to the plasma membrane linked into an extensive transcellular network

Question 7

Macula adherens (Desmosomes)

Answer 7

Anchoring junctions that spot wield and DO NOT bind to actin but BIND TO IM FILAMENTS
the intermediate filaments bind to desmoplakin which bbinds to anchorproteins (plakoglobin and plakophilin) which bind to cadherin family adhesion proteins (desmoglein and collin)

these proteins provide mechanical strenght and are plentiful in epidermis

Question 8

Gap junctions

Answer 8

couple cells electrically and metabolically
Comminicating junctions are transmembrane proteins

Connexins: six individual connexin subunits make up a connexon

Inorganic ions, small water soluble molecules pass directly between cytoplasms of adjacent cells via channels, couples cells electrically

there is no linkage to the cytoskeleton

Question 9

Basal Cell membrane Specializations

Answer 9

attach epithelial cells to the ECM of the underlying basment membrane

Hemidesomosomes and Adhesion plaques:

Hemidesmosomes: half desmosomes
Transmembrane linker protein: integrin
Attach to Laminin, collagen and fibronectin (not cells)
linked to intermediate filaments (keratin)

Adhesion Plaques- focal adhesions
Transmembrane linker protein: Integrins
Attach to ECM instead of cells
Plaques contain a-actinin and vinculin and attach to actin filament

Question 10

Epithelium and Cancer

Answer 10

Many epithelial cells are constantly self renewing (skin replaces every 28 days), absorptive epithelium turns over every 5 days

Carcinoma: malignant tumors of epithelial origin
Adenocarcinoma: malignancy of specific epithelial glandular cells

Epithelial tumors are 85% of cancer
loss of cell adhesion leads to pentration of bad cells through basement membrane (metastisis)

E-Cadherins protein is absent in cancer cells
Immuno histochemistry is used to distinguish cancer type

intermediate filament cell subtypes:
epithelial: keratin
glial: GFAP
muscle: desmin
CT: vimentin