Epithelium, Glands, & Connective Tissue

epithelium:

adhere strongly dt lamina propria (a thin layer of ECM which usually contains blood vessels. is separated from connective tissue by basal lamina and is avascular

types- dry/wet/mesothelium (lines serous body cavities)/endothelium (line blood vessels)

epithelial cell functions:

Barriers – covering or lining/Absorption/Secretion/Contractile – myoepithelium/Special sense organs – hearing, vision, smell

polarity:

apical surface- free surface. specialized free cell surface/ion channels

lateral surface- intercellular junctions

basal surface- basal infoldings/hemidesmosomes/basal lamina

basal surface:

infoldings- absorption

hemidesmosomes- attachment

hemidesmosomes:

anchored to tonofibrils in the cytoplasm to laminin and type IV collagen in basal lamina portion of basement membrane. use integrins that link tonofilaments to laminin molecules

basement membrane: basal lamina

Epithelial derived extracellular matrix

Laminin (Anchoring filaments) – large glycoprotein attached to integrins

Lamina lucida- Houses laminin, laminin receptors and integrins, Anchoring filaments

Lamina densa Type IV collagen coated with laminin and fibronectin

basement membrane: reticular lamina

Connective tissue derived.

Type VII collagen called anchoring fibrils- Bind anchoring plaques on Type IV collagen of connective tissue to basal lamina

basement membrane: structure summary

Intermediate filaments in the cytoplasm are attached to hemidesmosomes 2. Hemidesmosomes are attached to transmembrane proteins – integrins 3. Integrins are attached to glycoproteins – laminin 4. Laminin (Anchoring filaments) in the lamina lucida of the basal lamina are attached to Type IV collagen in the lamina densa 5. Type VII collagen (Anchoring fibrils) in the lamina densa connect the Type IV collagen to anchoring plaques (Type IV collagen) in the connective tissue

basement membrane:

functions-

Attach epithelium to lamina propria (connective tissue under epithelium)

Mediate cell-to-cell interactions

Serve as a scaffold for rapid cell repairs and regeneration

lateral surfaces: intercellular junctions

cell junctions: tight junctions

tightest junction is zonula occludens. closest to apex to cell. only one that create a fusion of fused strand

block all paracellular (intercellular) pathways

Allow epithelia to function as a barrier

Substances must be actively transported transcellularly

cell junctions: anchoring junctions

zonula adherens band around cell usually below tight junctions

has space between the cell, so not completely closed

Transmembrane glycoproteins: E-Cadherins (Cell Adhesion Molecules) bind to cadherins in other cells in the presence of calcium

Cytoplasmic side: cadherins bind to catenins that link to actin through actin binding proteins

cell junctions: anchoring junctions

macula adherens- desmosome. spot welding

anchoring junction

Transmembrane glycoproteins: Large cadherins (desmogleins, desmocollins) bind intermediate filaments (rather than actin) Intermediate filaments of cytokeratin called tonofilaments

hemidesmosomes:

Protects against abrasion and mechanical shearing forces Provides adhesion of epithelial cells to the underlying basement membrane (basal lamina)

Use integrins (transmembrane linker protein) instead of cadherins to link intermediate filaments (keratin) to laminin molecules in the basal lamina

gap junction:

communication connection

Intercellular communication

Aggregation of transmembrane protein complexes called connexins Connexins form heteromeric complexes called connexons Connexons have a central hydrophilic pore (1.5 mm in diameter)

specialized apical cell surface:

microvilli- Enhance absorption – increase surface area, Contain actin filaments bound to each other and to the surrounding plasma membrane by binding proteins

specialized cell surfaces- apical

stereocilla- elongated microvilli/less mobile

cilia- Arise from a basal body* Compared to microvilli** Longer (5-10 µm) and wider (0.2 µm) Have microtubules instead of actin filaments Exhibit rapid beating patterns that move content in one direction – sliding of microtubules*. Actively motile – propel fluid and particles. Core of microtubules arising from a basal body.

cell surface specialization for absorptive cells:

cell junctions: lateral infoldings- increase surface area

microvilli- increase surface area

basal infoldings- ion transporting/compartmentalize mitochondria

simple squamous epithelium:

mesothelium- simple squamous. lining pleura, pericardium, mediastinum, peritoneum

endothelium

simple cubodial epithelium:

often cilliated

idneys/thyroid/ducts

simple columnar:

gi tract/gallbladder/cilliated

statified epithelium:

2 or more cell layers

keratinized and non keratinized

provides protection

pseudostatified epithelium:

all cells touch basement membrane but dont reach surface. upper respiratory tract

transitional epithelium or urothelium:

urinary tract

structure vs function:

simple- secretion and absorption

stratified- protection/impermeability

glands:

secretory epithelium- cells or glands

secretory- synthesize intracellularly

unicellular- goblet cells. release mucin/lubricates and protects the surface

formation of glands (multicellular):

exocrine glands:

Secretion moves through ducts to surface

myoepithelium epithelium:

Epithelial cells in certain glands (acini) and ducts that take on the characteristics of smooth muscle cells. actin and myosin

found in the iris of the eye

Exocrine Glands Classified by Mechanism of Content Release** (ducts)

Merocrine Exocrine Glands Classified by Type of Secretion*

Serous – watery, apical granules of non- glycosylated large enzymes Deep basophilic cytoplasm - rER

Mucin – viscous, apical granules with heavy glycosylated proteins Cytoplasm with washed-out appearance

Mixed (Seromucus) Some salivary glands

holocrine secretion:

Sebaceous glands and meibomian glands of the eyelid

endocrine glands:

No ducts – secretions enter blood vessels or tissue directly Can be unicellular or multicellular

no myoepithelium

mixed glands- panceas/liver

paracrine: Cell-to-cell communication by local dispersion of a signaling factor

juxtacrine: Requires cell-to-cell contact: signaling molecules remain on secreting cell surface (gap junction)

autocrine: Cell signaling in which a cell secretes a hormone or chemical messenger that binds to a receptor on the same cell