Australia: The Land Where Time Began

A biography of the Australian continent 

Embryonic Diapause

It has been found that in a wide variety of marsupials, as well as placentals, the blastocyst has the capacity to enter a dormant state, embryonic diapause, cell division and growth either stopping or continuing at a much reduced rate until the mother's body sends a very specific signal. This signal may be in the form of an ovarian hormone or a substance that is specific for growth promotion that is secreted by the uterus.

Embryonic diapause is common among the Macropodidae, occurring in all kangaroos and wallabies and rat kangaroos, with the exception of the dusky rat kangaroo, Hypsiprymnodon moschatus, western grey kangaroo, Macropus fuliginosus, Lumholtz's tree kangaroo, Dendrolagus lumholtzi, and it occurs in the pygmy possums, the feathered glider, Acrobates pygmaeus, and the honey possum. In the daysurids, though they don't undergo embryonic diapause, an important change occurs during blastocyst formation: though embryos can be grown in culture through the early cleavage divisions, it has been found to be not possible to grow embryos through to the blastocyst stage at this stage of research, blastocysts recovered after this critical period grow on readily inn culture (Selwood & Young, 1983). A block that is similar to the this has been found to exist in the mouse, the author suggesting that it may be found that all mammals there is a metabolic pathway change when the cleavage divisions are complete and cell growth has begun. The method of diapause control in marsupials has been fully investigated only in the tammar wallaby.

Initiation and control

In the tammar wallaby the females come into oestrus a few hours after giving birth and providing the newborn attaches to a teat and begins suckling, the new pregnancy is delayed unto lactation ceases. If the pouch young dies or is removed from the pouch, the corpus luteum begins growing, the pregnancy resumes, birth occurring 26-27 days later. The determine when diapause begins, the progress of early pregnancy in females carrying new young in the pouch is compared to females with no pouch young.

It has been fund that up to day 8 there is no difference in cleavage that can be detected, by which time the blastocyst, consisting of 80-100 cells, has formed. No further change in the blastocyst occurs if the female is lactating, the corpus luteum doesn't grow, there is no early pulse of progesterone, the uterine glands remain small. In non-lactating females there is growth of the new corpus luteum, the transient early pulse of progesterone occurs on the 7 th day, the uterine glands become secretory having enlarged, and cell division in the blastocyst continues, the blastocyst increasing in size, being 1 mm diameter by day 12 and continues to grow rapidly.

It has been found that fertilised eggs of the tammar wallaby can reach the blastocyst stage with no input from the corpus luteum, but it needs that products of the corpus luteum to continue growing.

The blastocyst remains in diapause for the entire time of lactation in the Tammar Wallaby, as well as for a number of months after lactation ceases, resuming its development after the summer solstice, at which time the corpus luteum grows and the serum levels of progesterone increase. It has been found that blastocysts can survive for several months in females that have had their ovaries removed, the author suggesting this indicates embryonic diapause is a passive state in which the blastocyst requires a special signal before it resumes growth.

The first steps in reactivation of the blastocyst

If pouch young are removed from the pouch a train of events in the ovaries, the uterus and the dormant blastocyst is begun that culminates in birth in 26-27 days. If the young is returned to the pouch within the first 3 days the process can be reversed if it starts sucking: if it is returned to the pouch after the 3 rd day the blastocyst reactivates whether or not a pouch young is sucking, this reactivation is irreversible, the embryo either continuing on to full term or dying after a short period of growth (Gordon, K., et al., 1988). Days 1-3 covers the time it takes the corpus luteum to resume its own development, becoming evident on the 4 th day as the luteal cells begin to divide and enlarge. Progesterone production increases, the transient pulse occurring on day 5, 6 or 7.

At the beginning of diapause the metabolism of the embryo, days 1,2, and 3 is anaerobic glycolysis, days 4 and 5 becoming glucose oxidation on reactivation, days 6 and 7 the uptake of glucose increasing.

During diapause there is no development of the embryo, the cells resuming division on reactivation, increased protein synthesis, blastocyst expansion begins, shell membrane breaks down, embryo begins production of cortisol, birth.

 

Sources & Further reading

  1. Tyndale-Biscoe, Hugh, 2005, Life of Marsupials, CSIRO Publishing.
  2. Tyndale-Biscoe, Hugh & Renfree, Marilyn, 1987, Reproductive Physiology of Marsupials, Cambridge University Press.
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 21/05/2012


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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading