Uterine remodelling during pregnancy and pseudopregnancy in the brushtail possum (Trichosurus vulpecula; Phalangeridae)

Abstract

The formation of a placenta is critical for successful mammalian pregnancy and requires remodelling of the uterine epithelium. In eutherian mammals, remodelling involves specific morphological changes that often correlate with the mode of embryonic attachment. Given the differences between marsupial and eutherian placentae, formation of a marsupial placenta may involve patterns of uterine remodelling that are different from those in eutherians. Here we present a detailed morphological study of the uterus of the brushtail possum (Trichosurus vulpecula; Phalangeridae) throughout pregnancy, using both scanning and transmission electron microscopy, to identify whether uterine changes in marsupials correlate with mode of embryonic attachment as they do in eutherian mammals. The uterine remodelling of T. vulpecula is similar to that of eutherian mammals with the same mode of embryonic attachment (non-invasive, epitheliochorial placentation). The morphological similarities include development of large apical projections, and a decrease in the diffusion distance for haemotrophes around the period of embryonic attachment. Importantly, remodelling of the uterus in T. vulpecula during pregnancy differs from that of a marsupial species with non-invasive attachment (Macropus eugenii; Macropodidae) but is similar to that of a marsupial with invasive attachment (Monodelphis domestica; Didelphidae). We conclude that modes of embryonic attachment may not be typified by a particular suite of uterine changes in marsupials, as is the case for eutherian mammals, and that uterine remodelling may instead reflect phylogenetic relationships between marsupial lineages.

Uterine molecular changes for non‐invasive embryonic attachment in the marsupials Macropus eugenii (Macropodidae) and Trichosurus vulpecula (Phalangeridae)

“…Indeed, the specific, basal recruitment of Talin in M. eugenii , T. vulpecula , and S. crassicaudata suggests that this pattern may also be a uterine defence mechanism that prevents removal of the uterine epithelium, irrespective of placentation mode in marsupials. This hypothesis is supported by cellular alterations of both M. eugenii and T. vulpecula that likely compensate for restricted resource access by facilitating haemotrophic nutrient transfer from maternal blood across the uterine epithelium, including apical migration of maternal blood vessels and folding of the base of uterine epithelial cells to increase surface area (Freyer, Zeller, & Renfree, ; Laird, McShea, McAllan, Murphy, & Thompson, ). Thus, maintenance of the uterine epithelium in M. eugenii and T. vulpecula likely affords maternal protection without compromising embryonic development, and thus plays a critical role in balancing maternal and embryonic requirements in marsupial pregnancy, irrespective of placentation mode.…”

“…Brush‐tail possums in New Zealand breed from February to April (Crawley, ; Tyndale‐Biscoe, ), so a complete set of reproductive‐stage tissues from 13 females was collected over two seasons (April 2014 and March 2015). We divided normal gestation in T. vulpecula into four time periods (approximate number of days post‐oestrus) relating to reproductive events and based on uterine and ovarian morphology following Laird, McShea, et al () (Figure ): Stage 1 (0–6d post‐oestrus; n = 6), Stage 2 (7–11d post‐oestrus; n = 2), Stage 3 (12–14d post‐oestrus; n = 4), Stage 4 (15–17.5d post‐oestrus; n = 1).…”

“…to April (Crawley, 1973;Tyndale-Biscoe, 1955), so a complete set of reproductive-stage tissues from 13 females was collected over two seasons (April 2014 andMarch 2015). We divided normal gestation in T. vulpecula into four time periods (approximate number of days postoestrus) relating to reproductive events and based on uterine and ovarian morphology following Laird, McShea, et al (2017) (Figure 7):…”

Non‐invasive placentation in the marsupials Macropus eugenii (Macropodidae) and Trichosurus vulpecula (Phalangeridae) involves redistribution of uterine Desmoglein‐2

“…The unusual localization patterns of Desmoglein‐2, in addition to the apical redistribution in the uterine epithelium, suggest that Desmoglein‐2 plays additional cellular roles that are independent of desmosomes during pregnancy in M. eugenii and T. vulpecula (Ebert et al, ; Hartlieb et al, ; Nava et al, ) and differ between marsupial lineages of non‐invasive placentation (Bininda‐Emonds et al, ; Mess & Ferner, ). Hence, different additional patterns of Desmoglein‐2 localization in M. eugenii and T. vulpecula , as well as differences in uterine cell morphology following remodeling (Laird, McShea, et al, ), suggest that the evolutionary transition from invasive to non‐invasive placentation likely involved lineage‐specific selective pressures (Martin, ), resulting in diverse molecular patterns at the maternal‐embryonic interface.…”

“…The brushtail possum T. vulpecula has a 28‐day oestrous cycle (Pilton & Sharman, ; Tyndale‐Biscoe, ) and a 17.5‐day gestation period (Pilton & Sharman, ; Sizemore, Hurst, & McLeod, ). Like M. eugenii , T. vulpecula is monovular, with ovulation occurring 1–2 days after oestrus (see Laird, Dargan, et al, ; Laird, McShea, McAllan, Murphy, & Thompson, for summary timelines); however, embryos of T. vulpecula do not undergo developmental arrest. The embryo attaches non‐invasively approximately 14 days after conception, with birth 3–4 day later (Pilton & Sharman, ; Tyndale‐Biscoe, ).…”