Publication: Villous trophoblast of human placenta:
a coherent view of its turnover, repair and
contributions to villous development and maturation
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Date
2001
Authors
Mayhew, T. M.
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Publisher
F. Hernández y Juan F. Madrid. Universidad de Murcia: Departamento de Biología Celular e Histología
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DOI
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info:eu-repo/semantics/article
Description
Abstract
J\ coh e re nt vicw o f hum a n v illou s
trophoblast as a continuously renewing epithelium is
presented. Epithelia undergoing continuous rcnewal (e.g.
intestinal mucosa, epidermis) display clonogenic ce lls
which pass throug h sevc ral transit di visions be fore
migrating out of proliferation zones and into zones of
maturati on/differenti ati on. Quantitative relations (e.g.
re lati ve numbers of cells) betwee n proliferati on and
diffe rentiation zones help to define the steady state and
this may va ry in res po nse to ph ysi o log ic al and
pathological circumstances. From the differenti ati on
compartment, ce lls or ce ll fr agments arc eve ntu all y
extruded by mechanisms which may involve apoptosis.
All these features are seen in trophoblastic epithelium.
Cy totrophobl ast ce lls (CT, proliferation zone) divide
continuously throughout gestation and post-mitotic cells
are recruitcd into syncytiotrophoblast (ST, diffe rentiation
zone) aft cr membrane fusion. Evidence of fu sion events
includes localised confluence of CT and ST cytoplasms,
and intrasy ncyti al plasma membrane segments bearing
desmosomal remn ants. During diffe renti ati on, nu clei
undergo changes in shape, chromatin condensation and
packing densit y. Densely-clustered nuclei are associated
with cy tokeratin intermed iate fil aments and annul ate
lamellae . Both clustered and non-clustered nuclei show
ultrastructural fea tures of pre-apoptosis and apoptosis.
Normall y, apoptosis is triggered only when nuclei are in
the syncytium. Some (pre-)apoptotic nuclear aggregates
are se qu este red in sy nc yti a l knots, extrud ed as
troph obl ast fr agments into the intervill ous space and
th e n depo rt ed int o th e mate rn a l c irc ul ati o n to be
ph agocytosed at extrapl acental sites. During gestation,
there is some constancy in the numerical ratios between
CT and ST nuclei pointing to a normal steady state. The
steady state may be perturbed when the epithelium is
damaged loca ll y. Whe re the epithelium is denud ed,
fibrin-type fibrinoid from the intervillous space plugs the discontinuity and , with CT proliferation, facilitates reepitheli alisation. Features of normal villous development
(e.g. sprouting, int ervillous bridge formati on, bridge
abrupt ion, sy ncytial knot formation) arc explicable in the
co nt ex t of tr o ph obl ast turn ove r with ea rl y CT
pro li fe rati o n be in g ma inl y fo r g row th a nd la te r
proliferation for renewa l and repair. Adaptive re-settings
of the epithelial steady state may also occur in abnormal
pregnancies.
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