Regulation of endometrial regeneration; mechanisms contributing to repair and restoration of tissue integrity following menses
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Date
28/11/2014Author
Cousins, Fiona Lyndsay
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Abstract
The human endometrium is a dynamic, multi-cellular tissue that lines the inside of the
uterine cavity. During a woman’s reproductive lifespan the endometrium is subjected to
cyclical episodes of proliferation, angiogenesis, differentiation/decidualisation, shedding
(menstruation), repair and regeneration in response to fluctuating levels of oestrogen and
progesterone secreted by the ovaries. The endometrium displays unparalleled, tightly
regulated, tissue remodelling resulting in a healed, scar-free tissue following menses or
parturition. Mechanisms responsible for initiation of menses have been well documented:
following progesterone withdrawal there is an increase in inflammatory mediators, focal
hypoxia and induction and activation of matrix-degrading enzymes. In contrast, the
molecular and cellular changes responsible for rapid, regulated, tissue repair at a time when
oestrogen and progesterone are low are poorly understood.
Histological studies using human menstrual phase endometrium have revealed that tissue
destruction and shedding occur in close proximity to re-epithelialisation/repair. It has been
proposed that re-epithelialisation involves proliferation of glandular epithelial cells in the
remaining basal compartment; there is also evidence for a contribution from the underlying
stroma. A role for androgens in the regulation of apoptosis of endometrial stromal cells has
been proposed but the impact of androgens on tissue repair has not been investigated. Studies
using human xenografts and primates have been used to model some aspects of the impact of
progesterone withdrawal but simultaneous shedding (menses) and repair have not been
modelled in mice; the species of choice for translational biomedical research.
In the course of the studies described in this thesis, the following aims have been addressed:
1. To establish a model of menses in the mouse which mimics menses in women,
namely; simultaneous breakdown and repair, overt menstruation, immune cell
influx, tissue necrosis and re-epithelialisation.
2. To use this model to determine if the stromal cell compartment contributes to
endometrial repair.
3. To examine the impact of androgens on the regulation of menses (shedding) and
repair.
An informative mouse model of endometrial breakdown that was characterised by overt
menses, as well as rapid repair, was developed. Immunohistological evidence for extensive
tissue remodelling including active angiogenesis, transient hypoxia, epithelial cell-specific
proliferation and re-epithelialisation were obtained by examining uterine tissues recovered
during an “early window of breakdown and repair” (4 to 24 hours after progesterone
withdrawal). Novel data included identification of stromal cells that expressed epithelial cell
markers, close to the luminal surface following endometrial shedding, suggesting a role for
mesenchymal to epithelial transition (MET) in re-epithelialisation of the endometrium. In
support of this idea, array and qRTPCR analyses revealed dynamic changes in expression of
mRNAs encoded by genes known to be involved in MET during the window of breakdown
and repair. Roles for hypoxia and tissue-resident macrophages in breakdown and tissue
remodelling were identified.
Treatment of mice with dihydrotestosterone to mimic concentrations of androgens circulated
in women at the time of menses had an impact on the timing and duration of endometrial
breakdown. Array analysis revealed altered expression of genes implicated in MET and
angiogenesis/inflammation highlighting a potential, previously unrecognised role for
androgens in regulation of tissue turnover during menstruation.
In summary, using a newly refined mouse model new insights were obtained, implicating
androgens and stromal MET in restoration of endometrial tissue homeostasis during
menstruation. These findings may inform development of new treatments for disorders
associated with aberrant repair such as heavy menstrual bleeding and endometriosis.