MA3 - Epithelia Flashcards
What is an epithelium?
sheet of cells that covers a surface, often enclosing a lumen
What are the two primary functions of an epithelium?
semipermeable barrier; protection
Define parenchyma.
cells that mediate functional aspect of a tissue (i.e. hepatocyte of liver)
Define stroma.
cells that serve as support structures of a tissue
What are the three distinct surfaces of plasma membranes?
apical; lateral; basal (+ basolateral, but it’s not considered a distinct surface)
Describe the function/location of the apical membrane.
commonly faces a lumen
Describe the function/location of the lateral membrane.
faces and binds to adjacent epithelial cells
Describe the function/location of the basal membrane.
commonly faces underlying connective tissue
At which end/surface is the nucleus of a cell found?
basal end of the cell
What structure connects the basal surface to the underlying connective tissue?
basement membrane
What are the two components of the basement membrane?
basal lamina; reticular lamina
The basal lamina is synthesized by
epithelial cells
The reticular lamina is synthesized by
fibroblasts of lamina propria
The basement membrane is best visualized with what stain?
PAS, since basement membrane (specifically, the reticular lamina) is rich in proteoglycans
(T/F) The basal lamina is completely acellular.
True.
What are the further subdivisions of the basal lamina? (2)
lamina lucida; lamina densa
Describe the lamina lucida.
rich in laminins which bind to junctions on cell surface
Describe the lamina densa. (4)
rich in type IV collagen, fibronectin, proteoglycans, and linking fibers
Through which structures do cells bind to the basal lamina? (2)
through hemidesmosomes and focal adhesions
What is the external lamina?
variant of basal lamina that covers skeletal/smooth muscle and transmits force generated by these muscles
Compare the reticular and basal lamina in terms of thickness.
reticular lamina is much thicker than the basal lamina
(T/F) The reticular lamina is not acellular.
False, the reticular lamina (like the basal lamina) is acellular.
What structures do hemidesmosomes use to connect epithelial cells to the basal lamina?
use integrins to connect intermediate filaments (keratins, cytokeratins, tonofilaments) of epithelial cells to basal lamina
What are the two types of basal membrane junctions?
hemidesmosomes; focal adhesions
Focal adhesions use integrins to connect what structures together?
use integrins to connect actin filaments of epithelial cells to the basal lamina
What are the four types of lateral membrane junctions?
zonula occludens (tight junction); zonula adherens (adherens junction); desmosomes (macula adherens); gap junctions (macula communicans)
Where are tight junctions located?
encircles epithelial cells at the top of lateral membrane
Tight junctions are connected to
actin filaments of the terminal web
Which junction is responsible for apical/basolateral polarity?
tight junctions
What is the function of tight junctions?
provides a barrier that blocks or restricts passage of solutes between epithelial cells
Where are adherens junctions located?
encircle epithelial cells just basal to zonula occludens
Adherens junctions are connected to
actin filaments of the terminal web
Contrast tight and adherens junctions.
actin linkages are more extensive at adherens junctions than at tight junctions.
What is the function of adherens junctions?
provide most of mechanical strength necessary to seal epithelium
Where are desmosomes located? (2)
present in small foci that are scattered along lateral membrane; common just basal to adherens junctions
Desmosomes connect
intermediate filaments (cytokeratins) of adjoining cells
What are the functions of desmosomes?
provide mechanical strength; major mechanism holding adjacent cells together in epithelia, particularly epithelia subject to abrasion
What makes up the tripartite junction?
tight junction + adherens junction + desmosome
Where are gap junctions found?
present in foci along lateral membrane
Describe the linkage function of gap junctions.
do NOT have any cytoskeletal linkages and do NOT provide structural strength
What is the function of gap junctions?
provide contiguous portal between cells and help to coordinate actions of epithelia
(T/F) Gap junctions play an important role in providing structural strength to cells.
False
What are the four possible morphologies of the apical membrane?
primary cilia; motile cilia; microvilli; stereocilia
Describe the composition of motile cilia.
core of microtubules (9xdoublet+2 axoneme) that inserts into basal body
What causes cilia to beat?
action of dyneins on axonemal microtubules
Motile cilia are most common on
pseudostratified and simple columnar epithelia
Describe the composition of primary cilia.
9+0 axoneme that inserts into centriole
Contrast motile and primary cilia.
primary cilia lack motile structures and are longer than motile cilia
What is the function of primary cilia?
play signaling/sensing roles
Primary cilia are found on
most non-dividing epithelial cells
(T/F) A cell can have more than one primary cilium.
False
Describe the composition of microvilli.
core of actin filaments that insert into the terminal web
Are microvilli motile?
No.
What is the function of microvilli?
serve to increase apical membrane surface area
Microvilli are present on
most epithelial cells
What is a brush border?
when highly abundant, microvilli form a “brush border”
Describe the composition of stereocilia.
core of actin filaments that insert into the terminal web
Contrast stereocilia and microvilli.
stereocilia are much longer than microvilli and can be branched
Where are stereocilia found?
since they have specialized functions, present at only a few locations, like the testes or inner ear
What does it mean to have a simple epithelium?
all cells touch the basement membrane
What does it mean to have a stratified epithelium?
multiple layers but only the most basal cells touch the basement membrane
Epithelia are classified according to
the shape of the most superficial layer
What are the two types of special morphologies of cells?
pseudostratified epithelia; transitional epithelia
What is the composition of pseudostratified epithelia?
both short cuboidal and tall columnar cells
Describe the location of pseudostratified epithelia.
all cells sit on basement membrane but not all cells reach the lumen
Where is pseudostratified epithelium prevalent?
in the respiratory system
Describe the appearance of transitional epithelia.
appearance transitions from stratified cuboidal (relaxed state) to stratified squamous (distended state)
Keratinization occurs in what kind of epithelium?
normally only occurs in stratified epithelia exposed to dry conditions or subject to abrasion (i.e. skin or tongue)
An epithelium is termed “keratinized” when
the most superficial layer fills with keratins and cells lose their nuclei
Give an example of a keratinized epithelium.
epidermis
What is metaplasia?
conversion of epithelium into abnormal form in response to stress
How does metaplasia differ from neoplasia or dysplasia?
cells in metaplastic state are NOT transformed, unlike dysplasia or neoplasia
Describe how smoking causes metaplasia.
smoking causes pseudostratified epithelium of trachea and bronchi to adopt stratified squamous morphology, which provides better abrasion resistance
Describe how acid reflux causes metaplasia.
acid reflux causes stratified squamous epithelium of esophagus to adopt simple columnar morphology, which has better resistance to acid because of more mucus production
Describe how puberty causes metaplasia.
puberty causes simple columnar epithelium of endocervix into stratified squamous morphology similar to surrounding ectocervix and vagina
Endothelium refers to
simple squamous epithelium that lines blood vessels
Mesothelium refers to
simple squamous epithelium on surface of most organs and lines body cavities such as the peritoneum
Urothelium refers to
transitional epithelium in urinary tract
Explain how glands form.
epithelial sheet invaginates and grows into underlying connective tissue
Define exocrine.
secretory products leave cell via apical surface, where they enter environment directly or via duct
Define endocrine.
secretory products leave cell via basal surface, where they diffuse into bloodstream
Define paracrine.
like endocrine, but products act on nearby cells rather than via bloodstream
What are the three mechanisms of exocrine secretion?
merocrine; holocrine; apocrine
Define merocrine.
secretory vesicles/granules fuse with plasma membrane and expel products into lumen
Define holocrine.
cell fills with secretory products and then undergoes apoptosis, rupturing its plasma membrane and releasing secretory products into lumen
Define apocrine.
exocrine mechanism in which cytosolic secretory products are packaged and budded off from apical surface of cell
What are the three possible shapes of the secretory portion of the exocrine gland?
tubular; acinar; alveolar
Define tubular [shape of secretory portion of exocrine gland].
secretory elements line a tubular lumen
Define acinar [shape of secretory portion of exocrine gland].
secretory elements line a small spherical lumen (i.e. the lumen is smaller than the secretory epithelial cells)
Define alveolar [shape of secretory portion of exocrine gland].
secretory elements line a large spherical lumen (i.e. lumen is wide and is usually bigger than the secretory epithelial cells)
What are the three types of secretory content of exocrine glands?
serous; mixed; mucous
Define serous [content of secretory portion of exocrine gland].
secretory elements secrete watery protein products
Define mucous [content of secretory portion of exocrine gland].
secretory elements secrete mucin, a heavily glycosylated protein that is the principal component of mucus
Define mixed [content of secretory portion of exocrine gland].
both serous and mucous secretory elements that combine in a common duct
Differentiate between simple and compound ducts.
simple = single non-branching duct; compound = has a branching ductal system
