Epithelial Tissue Flashcards
Parenchyma
cells responsible for the main functions
typical of the organ
Stroma,
Supporting tissue
principal functions of epithelial
- Covering, lining, and protecting surfaces (eg, skin)
- Absorption (eg, the intestines)
- Secretion (eg, the epithelial cells of glands)
- Contractility (eg, myoepithelial cells).
Lamina propria is a
- Thin layer of loose connective tissue
- Or dense irregular connective tissue,
- Lies beneath the epithelium
- Together with the epithelium constitutes the mucosa
The area of contact between epithelium and lamina propria is increased by
irregularities in the connective tissue surface in the form of small
evaginations called papillae
Papillae occur most frequently
in epithelial tissues subject to friction, such as the
covering of the skin or tongue
The region of the cell that faces the
connective tissue is called
the basal pole
Basement Membranes =
basal lamina + Reticular Lamina
macromolecules components of basal laminae
- Laminin: These are large glycoprotein molecules that self-assemble to form a lace-like sheet immediately below the cells’ basal poles where they are held in place by the transmembrane integrins.
- Type IV collagen: Monomers of type IV collagen contain three polypeptide chains and self-assemble further to form a felt-like sheet associated with the laminin layer.
- Entactin (nidogen), a glycoprotein
- Perlecan, a proteoglycan with heparan sulfate side chains: these glycosylated proteins and others serve to link together the laminin and type IV collagen sheets.
reticular fibers are made of
type III collagen
Basal laminae have many functions
- simple structural and filtering functions
- influence cell polarity
- regulate cell proliferation an differentiation by binding and concentrating growth factors
- influence cell metabolism and survival;
- organize the proteins in the adjacent plasma membrane (affecting signal transduction)
- serve as pathways for cell migration
lateral membranes of epithelial cells exhibit several specialized intercellular junctions:
- Seals to prevent the flow of materials between the cells (occluding junctions)
- Sites of adhesion (adhesive or anchoring junctions)
- Channels for communication between adjacent cells (gap junctions)
The most apical junctions are:
Tight junctions, or zonulae occludens
The seal between the Tight junctions are by:
claudin + occludin
An example of epithelia with one or very few fusion sites of zonulae occludens
proximal renal tubule
Principal function of the tight junction
To ensure that molecules crossing an epithelium ,in either direction,do so by going through the cell (transcelluar pathway) rather that between them (Paracelluar pathway)
zonulae occludens in sheets of epithelial cells help form two functional compartments:
- an apical compartment that is composed of an organ cavity (such as the lumen of a secretory unit or the gut)
- a basal compartment that begins at the junctions and encompasses the underlying tissue.
Zonula Adherens cell adhesion by:
Cadherins–>Catenin—>Actin binding proteins—->Actin
Cadherins are..
Transmembrane glycoproteins that interact in presence of Ca2+
Desmosome or Macula Adherens is a:
Single spot weld
Desmosomes contain larger members of cadherin family called
Desmoglein an Desmocollin
Desmoglein an Desmocollin bind to:
Plakoglobin and desmoplakin that bind to intermediate filaments of cytoplasm
Gap junctions mediate:
Communication
Gap junctions proteins called connexins form:
hexameric complexes called connexons
Connexons have
a central hydrophilic pore about 1.5 nm in
diameter.
Gap junctions permit:
the rapid exchange between cells of molecules with small (<1.5 nm) diameters
Substances that move readily through gap junctions
molecules mediating signal transduction, such as cyclic AMP, cyclic GMP
When two cells attach, connexins in the adjacent cell membranes move laterally and align to form connexons between the two cells with each
gap junction having
dozens or hundreds of aligned pairs of connexons
Bind the cell to the basal lamina
hemidesmosomes
In desmosomes the attachment plaques contain
Cadherins,
In hemidesmosomes the
plaques contain abundant
integrins, transmembrane proteins that are receptor sites for the extracellular macromolecules laminin and collagen type IV.
nutrients for the epithelial cells must pass out of the capillaries in
the underlying lamina propria.
Receptors for chemical messengers (eg, hormones,
neurotransmitters) that influence the activity of epithelial cells are localized
in the basolateral membranes.
In absorptive epithelial cells, the apical cell membrane contains, as integral membrane proteins:
enzymes such as disaccharidases and peptidases, which complete the digestion of molecules to be absorbed.
microvillus dimensions are
(size)
1 um high and 0.08um wide
The complex of microvilli and glycocalyx is easily seen in the light microscope and is called
the brush or
striated border.
Stereocilia are long apical processes of cells in other absorptive epithelia such as
that lining the epididymis and ductus
deferens.
Stereocilia
(vs mmicrovilli)
are much longer and less motile than
microvilli, are branched
Epithelia can be divided into two main groups according to their
structure and function
- Covering (or lining) epithelia
- Glandular epithelia.
Simple epithelia
contain only one layer of cells
stratified epithelia
contain more than one layer of cells
Stratified columnar epithelium can be found
in the conjunctiva lining
the eyelids
Stratified cuboidal epithelium is
restricted to large excretory ducts of sweat and
salivary glands, where it apparently provides a lining more robust than that of a simple epithelium.
Transitional epithelium found in
urinary bladder, the ureter, and the upper part of the urethra, is characterized by a superficial layer of domelike cells that are neither squamous nor columnar. These cells, sometimes called umbrella cells,
are essentially protective against the hypertonic and potentially cytotoxic effects of urine.
pseudostratified columnar epithelium found in
lining the passages of the upper respiratory tract
The classic unicellular gland is
the goblet cell in the lining of the small intestine
Glands that retain their connection with the surface epithelium
Exocrine glands
Glands that have lost their connection to the surface from which they originated during
development.
Endocrine glands
Ducts can be simple ie.
unbranched
Ducts can compound ie.
with two or
more branches
Tubular secretory portion can be:
either short or long and
coiled
Acinar Secretory portions can be:
round or globular
Merocrine secretion
involves typical exocytosis of proteins or glycoproteins. This is the most common mode of secretion.
Holocrine secretion
involves the cell filling with secretory
product and then the whole cell being disrupted and shed. This is best seen in the sebaceous glands of skin.
Apocrine secretion
secretory product is typically a large lipid droplet and is discharged together with some of the apical cytoplasm and plasmalemm
Exocrine glands with merocrine secretion can be further categorized as either:
serous or mucous
Serous Cells synthesize
proteins that are mostly non glycycosylated, such as digestive enzymes.
serous cells basal ends have
well-developed RER
and Golgi complexes
serous cells have in apical end:
secretory granules in different stages of maturation
Serous cells therefore stain intensely with
any basophilic or acidophilic stain.
Mucous cells eg.
Goblet cells
Mucous cells are filled apically
with secretory granules containing
strongly hydrophilic glycoproteins called mucins.
When mucins are
released from the cell
they become hydrated and form mucus, a viscous, elastic, protective lubricant material.
Mucin-containing granules stain well with
the periodic acid-Schiff (PAS) method for
glycoproteins but are not intensely acidophilic like
zymogen granules of serous cells
salivary glands are
mixed seromucous
myoepithelial cells are located
between the basal lamina and the basal pole of secretory or duct cells
Long processes of myoepithelial cells
embrace an acinus as an octopus might embrace a rounded boulder
Myoepithelial cells are connected to each other and to the epithelial cells by
both gap junctions and desmosomes.
Myoepithelial cells are specialized for.
contraction and contain myosin and a large number of actin filaments
myoepithelial cells major function is
to contract around the secretory or conducting portion of the gland and thus help propel secretory products into the duct
Endocrine glands are the producers
of hormones, which are generally
polypeptide or lipid-derived factors that are released into the
interstitial fluid.
paracrine
receptors may also be on cells very close to the hormone-secreting cells
Autocrine,
receptors on the secreting cell itself
Intestinal epithelium,
which is replaced
every week
In stratified epithelial tissues, mitosis only occurs
within the basal layer in contact with the basal lamina.
In some functionally complex
epithelia, stem cells have been identified only in
restricted niches some distance from the transit amplifying cells and differentiating
cells.
Epithelium lining the small intestine is derived
completely from stem cells found
in the simple glands between the intestinal villi.
In the epidermis, stem cells are located
at a characteristic position along the wall of hair follicles.
Malignant tumors
derived from glandular epithelial tissue are usually called
adenocarcinomas
Metaplasia is the
reversible replacement of one differentiated cell type with another mature differentiated cell type.
Metaplasia from cigarette smoke
mucus-secreting ciliated pseudostratified columnar respiratory epithelial cells that line the airways to be replaced by stratified squamous epithelium
Metaplasia in vitamin A deficiency
Epithelial tissue of bronchi and urinary bladder are replaced with stratified squamous epithelium
Immotile Cilia Syndrome
Kartagner Syndrome
Clostridium Perfringens enterotoxin binds to
Claudin molecules of intestinal cells…—> loss of water by paracellular pathway.
Blistering (bullous) diseases eg. Pemphigus vulgaris are due to
abnormal desmosome function caused by autoimmune reactions to specific desmogleins
Examples Of Simple Squamous Epithelia
- Endothelium
- Serous lining of cavities
- Pericardium
- Pleura
- Peritoneum
Examples of Simple Cuboidal epithelia:
- Covering the ovary
- Thyroid
Examples Of Simple Columnar epithelia:
- Lining of intestine
- Gallbladder.
Examples Of Pseudostratified Epithelia
- Lining of trachea
- Bronchi
- Nasal cavity
Examples of Stratified Squamous keratinized
Epidermis
Examples Of Stratified Squamous Non-keratinized
- Mouth
- Esophagus
- Larynx
- Vagina
- Anal canal
Example of stratified Cuboidal
- Sweat glands
- Developing ovarian follicles
Examples Of Stratified Transitional Epithelia
- Bladder,
- ureters
- renal calyces.
Examples Of Stratified Columnar Epithelia
Conjunctiva
