Epithelia 1 Flashcards
- State the structural arrangements, classifications, and functions of epithelial tissues, and state their general structural relationships (orientation) to connective tissue, blood vessels, muscle, and neurons (peripheral nervous tissue).
-Epithelia = tissue that line body surfaces and face the ‘outside’ world.
Properties:
-Epithelial cells are adherent to each other, arranged in one or many layers/sheets, are polarized (apical = outer, basolateral = connected to underlying CT), have a basal lamina (extracellular material), undergoes a lot of turn-over, are replaced by stem cells, are avascular (nutrients diffuse from underlying CT through basal lamina), and highly diverse.
Functions:
- Barrier that protects internal tissues,
- Select absorption and transport,
- Secretion,
- Movement of particles though passage ways,
- Biochemical modifications of some molecules,
- Communication between tissues and organs, a
- Reception of sensory stimuli.
- Epithelia does not equal endothelial cells (faces blood/lymph) or mesothelium (cells that line enclosed internal spaces).
- Derived from all 3 germ layers –> embryonic epithelia migrates to different parts of the body into mesenchymal tissue = EMT transition.
- Apical side faces space/outside world and basal side is attached to basal lamina, which is attached to underlying CT.
- Blood vessels/nerves run through CT.
- Epithelia = avascular, so get O2 and nutrients from blood vessels.
- CT underlying the epithelial cells house capillaries and lymph vessels.
- Muscles can be imbedded in connective tissue layers. -Other tissues (muscle, blood vessels, etc.) have their own basal lamina.
- So epithelia are directly attached to CT, but are separated from but attached to blood vessels, muscles, and nerves.
- Exceptions = sensory nerve cells may be in direct contact with epithelia (taste buds); immune system cells (dendritic cells) can infiltrate epithelia.
- Mucosa = most internal linings separating out from in –> 2 layers = outer epithelium and CT underneath (lamina propria CT).
- Deeper CT tissue = submucosa.
- Skin = epidermis (epithelia), dermis (underlying CT), and hypodermis (deeper CT).
- Overall relationship = lumen –> epithelia –> basal lamina –> CT –> deeper CT-embedded tissues (vessels, muscles, nerves).
- Epithelia can be simple (single layer/sheet) or stratified (more than 1 layer in which outer layers don’t touch basal lamina). Pseudostratified = cells seem stratified, but all the ‘layers’ do directly touch the basal lamina (generally happens with columnar cells).
- Squamous = flattened, cuboidal = cubed, and columnar = taller than they are wide. Name stratified epithelia based out outermost layer.
- Describe the epithelial to mesenchymal transition during development.
-During development, embryonic epithelia disassemble and move into the mesenchymal/connective tissues –> migrate to other locations to form new epithelia or give rise to new tissues = EMT transition.
Cancers can acquire this.
- Describe the cellular basis for apical-basal polarity of epithelial cells and describe the functions of epithelial polarity.
- Polarization = outer domain/apical domain that faces free surface versus inner basolateral domain that faces the basal lamina.
- Plasma membrane composition is segregated into domains –> apical domain contains distinct membrane proteins and a distinct phospholipid content compared to the membrane in the basal domain.
- Different membrane proteins found in the both domains (transporter enzymes, ion channels, receptors, etc.).
- Tight junction complexes generally near apical surface.
- Cytoplasm is also polarized –>organelles distributed in specific polarized pattern and vesicles move from one end to the other in the cell.
- Polarity allows unidirectional secretion and/or absorption of molecules to or from one side of epithelium.
- Trans-epithelial transport across cell –> endocytosis of substances from one membrane region to their exocytosis on the other side = transcytosis.
- State the different cell junctions that connect epithelial cells to one another and to the basal lamina, and describe their key components and functions.
Most epithelial cells are tightly adherent.
1) Tight junctions = highly selective barrier, limits diffusion between cells. Key proteins = occludins and claudins. Tight barriers force cells to use specific transport pathways when absorbing/secreting material.
2) Adherence junctions = promote attachment and polarity, morphological organization, and stem cell behavior. Use specific cadherins that link to actin filaments. Cadherins = transmembrane proteins with extracellular domains that interact with other cadherins; cytoplasmic tails bind adapters and actin.
3) Desmosomes = promote mechanical strength and resist shearing; promote structural organization. Use a different class of cadherins that link to intermediate filaments.
4) Gap junctions = allow rapid communication between epithelial cells via diffusion of ions and small molecules between cells
