Epithelia and Cell-to-Cell Junctions

junctions

help epithelial tissues (surfaces and linings) cohere, maintain integrity, and function

types of tissues

• epithelia

• connective

• muscular

• neural

cell

fundamental functional unit of life

tissue

a collection of (similar) cells working together for a common purpose

organ

a group of tissues (often different types) carrying out a specific type of function

organ system

a system of related organs executing a common task

cellularity of epithelial tissues

• compose almost entirely of cells in layers (little extracellular matrix)

• connected by intercellular junctions

polarity of epithelial tissues

defined apical (top) and basal (bottom) surfaces

attachment of epithetial tissues

to basement membrane which also has the basal and reticular lamina

basement membrane

acellular surface produced by epithelial and connective cells

basal lamina

epithelial secretions: collagen, laminin, proteoglycans

reticular lamina

connective secretions: reticular fibers, fibronectin, glycoproteins

What does it mean to say epithelial tissue is avascular

no blood vessels (gets nutrients through diffusions)

regeneration of epithelial tissue

readily divide mitotically and replace old cells

functions of epithelia tissues

• physical protection

• selectively permeability

• movement/” clearing”

• secretion

• sensation

physical protection

• protects from dehydration, abrasion, and physical, chemical, and biological agents

• ex: skin which has multiple layers

selectively permeable

• regulates the passage of molecules in and out of a certain region of the body

  • exchange - passive flow between compartments

  • transport - active or facilitated movement of solutes and water between two compartments

movement/”clearing”

pushing of fluid or other material along a surface

secretion

release of materials (i.e. sweat, saliva)

sensation

receptors connected to nerve endings that can detect light, chemical mechanical stimuli, cold or heat

exchange epithelia

• have leaky exchange

  • allows movement through gaps between cells

transport epithelia

• use tight junctions

  • prevent movement between adjacent cells

  • substances must instead pass through the epithelial cells, crossing two cell membranes as they do

    • carriers and pumps heavily involved

ciliated epithelium

• for clearing or movement

• ex: lining in airways or fallopian tubes

cilia

propel substances over surfaces

microvilli

higher exposed surface area, often in columnar epithelia

Kartagener’s Syndrome

• disease involving ciliated epithelium

• primary ciliary dyskinesis (PCD)

sensory epithelia examples

• neuroepithelial membrane of retina

• basilar membrane of the cochlea

examples of secretory epithelia

• intestinal gland goblet cells

  • secrete mucus into the lumen of hollow organs like the intestine

functions of glands

secretions (mucus, hormones, enzymes, and waste

structure of glands at the tissue level

ducts or vessels envelopments by gland

structure of glands at the cellular level

often long shape, lots of secretory granules and endoplasmic reticulum

exocrine glands

• secrete products through ducts

• products enter '“exterior”

• ex: intestinal lumen, skin surface

endocrine glands

• ductless

• products enter body “interior”

• direct secretion through the bloodstream or interstitial fluid

• no connecting cells

modes of secretion

• merocrine

• apocrine

• holocrine

merocrine

• product secreted by vesicular exocytosis

• most common form of secretion

• often continuous

• ex: pancreatic cells, sweat, saliva

apocrine

• partial cellular breakdown (fragmentation)

• cytoplasm and vesicular contents secreted

• ex: milk, under-arm perspiration

holocrine

• entire cell lost with secretion

• stem cells divide to replace lost cells

• ex: sebaceous (oil) glands

Hyperhidrosis

• excessive sweating beyond biological need

• thought to be due to over-activity of the sympathetic nervous system (part of automatic nervous system)

intercellular junctions and types

• membrane specializations epithelial cells share on their lateral surfaces to strongly bind to each other

• types:

  • tight junctions

  • adhering junctions

  • desmosomes

  • gap junctions

tight junctions

• prevent paracellular movement (ziplock bag)

• encircle cells near apical surface

• “gatekeeper” role

• structure: occludin to ZO-1, claudin to ZO-2, and Junctional Adhesion Molecule (JAM) to ZO-3)

What is the general structure of junctions

• extracellular proteins with transmembrane components anchoring it to the cytoskeleton

adhering junctions

• actin filament-based systems (zonula adherens) (zipper)

• encircle cell immediately adjacent to all its neighbors

• actins (primary component of microfilaments, intracellular) and cadherins (extracellular protein) play pivotal role

desmosomes

• macula adherens (button)

• two types:

  • button desmosomes

  • hemidesmosomes

button desmosomes

• tie cells together

• supported internally by intermediate filaments (made of keratin)

• allows for bending and twisting

• don’t encircle the cells

• provide resistance to stress at a single point

hemidesmosomes

• tie cell to basal lamina

gap junctions

• protein channels that allow rapid communication between cells

• specific functions

  • tissue homeostasis

  • cell growth regulation

  • embryonic development

  • electrical/metabolic coupling

• each pore has 2 hemi-channel connexons

• 6 connexons make up a connexin

2 criteria for classifying epithelia

1. number of layers

   • one layer

   • many layers

2. shape of cells

   • squamous - flat

   • cuboidal - square

   • columnar - elongated