Transcript Epithelial Tissues

BCH 443
Biochemistry of
Specialized Tissues
4. Epithelial Tissues
Epithelia
• Epithelial tissues arise from any of the 3 primary
germ layers of the embryo.
• Epithelium
– Skin (ectoderm)
– Digestive tract (endoderm)
• Mesothelium
– Peritoneal cavity (mesoderm)
• Endothelium
– Blood vessels, lymphatic vessels, heart
(mesoderm)
Functions of Epithelia
• Protection
– Against wear and tear
– Keratin and mucus prevent drying
• Absorption
– Microvilli in kidney and intestine
• Surface transport
– Via cilia on cell surface
• Secretion (all glands are epithelia)
– Hormones, digestive enzymes, mucus
• Sensory reception
How to classify epithelia
(1) Shape
(2) Number of layers
(3) Specializations
(4) Covering and lining vs.
glandular
All Epithelia are Avascular
• Blood vessels supplying
epithelia are found in the
underlying connective tissue.
• Beneath the basement
membrane
Basement Membrane
• Three layers
• Lies between epithelium and
underlying connective tissue
• It is the external lamina (coat)
for muscle and nervous tissue
– Top two from epithelia,
also called basal
lamina
• Lamina rara (lucida)
• Lamina densa
– Lowest from
connective tissue
• Reticular lamina
(lamina
fibroreticularis)
Basement membrane components
lumen
Basal
lamina
Lamina rara
Lamina densa
Basement membrane
Can be seen with light microscopy
Reticular lamina
Type II collagen
(reticular fibers)
Parts of the basement membrane
• Basal lamina
– type IV collagen
– Heparan sulfate
– Fibronectin and
laminin
• Reticular lamina
– Type II collagen
– Also called reticular
fibers
Functions of the basement
membrane
•
•
•
•
Structural support via cell-matrix adhesions
Allow nutrients and waste to diffuse
Filter for macromolecules (kidneys)
Zone for differentiation and polarization of
cells
• Plays a role in regeneration by acting as a
“highway” for cell migration
Simple Vs. Stratified Epithelia
• Simple
– One layer of cells
– All cell touch basement membrane
• Stratified
– Two or more layers
– Only bottom layer of cells touch basement
membrane
Simple Squamous Epithelia
• Flattened, scale-like, disc shaped nucleus
• Exchange simple gases, protection
• Kidney tubules, blood vessels, alveoli,
lining major body cavities
Simple Cuboidal Epithelia
• Cuboidal shape, spherical nucleus
• Secretion and absorption
• Ovary, renal medulla, ducts
Simple Columnar Epithelia
•
•
•
•
•
•
Column shaped
Organelles near lumenal surface
Nucleus near basement membrane,
Height varies by functional activity
Absorption in small intestine
Secretion in stomach
Cell surface specializations
• Microvilli
– Contain actin filament
core
– Atop most absorptive
columnar epithelia
– Called striated or brush
border
• Kidney, small intestine
• Cilia
– Contain 9x2
arrangement of
microtubules
– On pseudostratified
epithelia
• trachea
– Some columnar
epithelia
• fallopian tube
One more specialization
• Stereocilia
– Modified microvilli,
longer
– do NOT contain
microtubules
– May help facilitate
absorption
– Epididymis, hair cells
of ear
Pseudostratified Ciliated Columnar
Epithelia
• All touch basement membrane
• Columnar cells, ciliated, some secrete
mucus, nuclei at varying levels
• Traps and moves dirt out
• Trachea and bronchi
Stratified Squamous Non-keratinizing
Epithelia
• Several cell layers deep
• Cells toward basement membrane cuboidal,
near lumen, flattened
• Protects against abrasion in moist areas
• Mouth, esophagus, vagina and anus
lumen
Esophageal-gastric junction
esophagus
stomach
Stratified squamous non-keratinizing epithelia
lumen
Stratified Squamous Keratinizing
• Same as nonkeratinizing but
with layers of
keratin and dead
cells on surface
• Protects against
abrasion in nonmoist areas
• Skin
– thicker in areas
prone to more
abrasion
Thick skin
keratin
Epithelial cells
Connective
tissue
Stratified Cuboidal Epithelia
•
•
•
•
Rarely found
Usually only two or three layers thick
Stronger than simple epithelium
Found lining larger ducts
– salivary, pancreas, sweat ducts
Transitional Epithelia
• Able to stretch and relax
to accommodate urine in
bladder,
• Morphology changes
depending on distention.
– Rounded, scalloped
edges when bladder is
empty
– Stretched-out, stratified
squamous appearance
when bladder is full
Types of Junctions
• Tight junctions
=occluding junctions
• Desmosomes
– belt desmosomes
=zonula adherens
– spot desmosome
=macula adherens
• Hemidesmosomes
• Gap junctions
Tight junctions
• Extracellular surfaces of two adjacent
plasma membranes are joined together
so there is no extracellular space
between them
• Occurs in a band around the entire cell
Sertoli cells
• In the seminiferous
tubule
• Sit on basal lamina
• form zonula
occludentes
• protect sperm cells
from immune system
• Artificial pancreas?
Tight junctions (Con’t)
• Restrict the movement
of most organic
molecules between
cells, but may leak
small ions and water
• Not associated with
any cytoskeletal
components
Belt desmosome
• Zonula adherens
• Another belt around
the cell
• Below the tight
junctions
• An anchorage
junction
• Associated with
actin filaments
• Space between
membranes can be
seen
Spot Desmosomes
• A region between two cells where
membranes are separated by 20nm
• Dense accumulation of protein at the
cytoplasmic surface of the membrane
Desmosomes (Con’t)
• Keratin fibers extend from
the cytoplasmic surface to
other side of cell to next
desmosome
• Holds adjacent cells
together in areas of
stretching
– skin, cardiac muscle, bladder
Gap junctions
• Protein channels link the cytosols of cells
– Passage of small molecules and ions (Na+, K+)
– Excludes large molecules
– Transmits electrical activity between cardiac
and smooth muscle cells
– Allows chemical messengers to cross from one
cell to another
– Coordinates activities between cells
Gap junctions
Freeze fracture freeze etch
TEM
Gap junction connexons
• A connexon is a cylinder with a central
open pore
• One gap junction connexon is made up of
six connexins
• The pore is a hydrophilic channel between
two cytoplasms
• Plasma membranes come within 2-4nm of
each other
Cellular Junctions
Occluding jxns
zonula adherens
macula adherens
Hemidesmosome
• Assymetrical structures
• A plate anchors the basal part of cell to the
basal lamina
• This plate contains IFs called keratins or
tonofilaments
• Membrane plaque linking hd to bl via
anchoring filaments
• Contributes to overall stability of epithelia
Hemidesmosomes
Melanin Formation
HO
Tyrosinase
NH3 +
CH2CHCO2-
Tyrosine
HO
NH3 +
CH2CHCO2-
HO
DOPA
Tyrosinase
O
Melanin
(Black polymer)
Highly colored
polymeric
intermediates
O
Melanin formed in skin (melanocytes), eyes, and hair
In skin, protects against sunlight
Albinism: genetic deficiency of tyrosinase
www.albinisim.org
CH2CHCO2+ NH3
Dopaquinone