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Pregnancy after diapause in the tammar wallaby, Macropus eugenii

Shaw, Geoffrey (1983) Pregnancy after diapause in the tammar wallaby, Macropus eugenii. PhD thesis, Murdoch University.

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Abstract

Female tammars ovulate and mate post-partum. The ovulated follicle develops into a corpus luteum which remains quiescent during lactation, and the blastocyst which develops enters diapause. When the pouch young is removed, the corpus luteum grows and secretes more progesterone, and embryonic diapause terminates leading to birth about 27 days later.

Blastocysts at day 5 and later stages after removing the pouch young, incorporate uridine at a much greater rate in vitro than diapausing blastocysts. This "metabolic reactivation" of the blastocyst occurs well before the first visible expansion at about day 8, and before any obvious hypertrophy of the corpus luteum.

Since the embryo lies free in the uterine lumen until about day 18, the change which triggers embryonic reactivation must come via the uterine fluid. Endometrial protein synthesis and secretion rates in the gravid uterus increase markedly through gestation. This increase begins before day 4, preceding the earliest measured increase in blastocyst activity and before the previously reported peak in plasma progesterone concentrations. This suggests that another factor may act with progesterone to increase endometrial secretion at reactivation. Oestradiol 17β and progesterone both stimulate endometrial protein synthesis and secretion in ovariectomized tammars, and oestradiol concentrations in the peripheral plasma increase to a peak at day 5. Thus, oestradiol may act with progesterone to stimulate uterine reactivation. In tammars which mate post-partum, plasma oestradiol concentrations are highest around the time of oestrus, but no rise is seen in females which do not mate, suggesting that oestradiol probably triggers oestrous behaviour.

The changes in endometrial protein synthesis and secretion serve as a useful index of many changes in the uterine fluids, which will occur together as the endometrium is stimulated to reactivate, but the particular changes in uterine fluid which result in embryo reactivation are not clear. No qualitative changes are seen in the electrophoretic pattern of uterine secretory proteins, and there are no marked changes in the concentration of Na, K, Ca, or Mg in uterine flushings up to day 7. However, Na and K concentrations are greater on day 9, suggesting that embryonic expansion may be accompanied by an increased volume of uterine secretion.

Although tammars are monotocous they have two completely separate uteri, so a comparison of the two uteri differentiates local effects of pregnancy from systemic influences. Endometrial protein secretion, histology and the concentration of ions in the uterine flushings are similar when the two uteri of each female are compared up to about day 12. However, in the second half of gestation the gravid uterus becomes more secretory, and the non-gravid uterus regresses, so that at day 26 the two uteri are strikingly different in appearance and secretory capacity.

Similarly, there are differences between gravid and non-gravid myometria in late pregnancy. Gravid myometria contain a lower concentration of collagen than the non-gravid myometria and are more responsive to oxytocin, although little spontaneous contractile activity is seen in either uterus at any stage. Myometria respond to prostaglandin F2α at all stages of gestation, and the concentration of PGF2α and PGE2 in uteri is greater at day 27 (full term) than day 25.
Although PGF2α may be a major uterine stimulant at parturition, plasma concentrations of PGF2α the main circulating metabolite of PGF2α are only briefly elevated around the time of parturition, if at all.

The results of this study suggest that both progesterone and oestradiol are involved in the increase in endometrial secretion that triggers the reactivation of the blastocyst from diapause. The precise temporal relationships of the hormonal changes, increasing endometrial secretion and blastocyst reactivation are not yet clear, but the results of this study suggest that progesterone and oestradiol are not the only luteal factors involved, and have confirmed that there is a unilateral stimulation of the gravid uteri. which probably reflects a feto-placental influence. The hormonal interactions leading to parturition are still unclear. Neither a fall in progesterone nor a rise in oestradiol are apparently necessary, but PGF2α may be briefly involved in stimulating uterine contractions.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Environmental and Life Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: repository@murdoch.edu.au. Thank you.
Supervisor(s): Renfree, Marilyn
URI: http://researchrepository.murdoch.edu.au/id/eprint/52093
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