Saturday, 11 January 2014

What Triggers Birth in Mammals?

I was reminded of my post from 31 August 2013 on the length of pregnancy and on animals which synchronise the onset of parturition when I read the Biographical Memoir written by Peter Gluckman and Tatjana Buklijas on ‘Mont’ Liggins*. Liggins set off a whole train of research from the 1960s onwards on the control of the onset of parturition by finding that, in sheep, the fetus provides the trigger for parturition through activation of the hypophysio-adrenocortical axis and the production of glucocorticoid. Gluckman and Buklijas describe Liggins’s discoveries very well in the context of all that was happening in research on the fetus and to who was doing what, where and why.

I kept a close eye on research on the control of the onset of parturition throughout the 1970s and 80s, in view of the link between parturition and the onset of copious milk secretion which we termed ‘lactogenesis stageII’†. It was soon obvious that, and quoting from Liggins and Buklijas, …there was no shared mechanism controlling the onset of labour across the entire eutherian clade. In some species, such as mice, goats and rabbits, the maintenance of pregnancy depended on the persistence of corpus luteum to the end, and a decrease in progesterone levels signalled the onset of labour. In other species—sheep, cows and primates including humans—the placenta took over the role of the corpus luteum, so these species were termed ‘placentally dependent’, yet even this group was far from homogeneous, with sheep and cows showing much clearer evidence of a marked hormonal change preceding parturition than did primates.

A number of large teams throughout the world, including a number of personal friends, have been working on the control of parturition in the decades following Liggins discoveries, both in collaboration and competition. However, I was surprised to find that there is a stark truth. Returning to the Biographical Memoir:

Writing in the 1990s, Liggins summed up the results of the decades of research into the physiology of human parturition: first, rather than either the fetus or the mother occupying the leading role, the mechanism involved an interaction between the two participants, fetus (chorion) and mother (decidua); and second, the synthesis and release of prostaglandin F2α from decidua was a key event in the onset of parturition. Little has changed since that time, although we now know much more about the complex paracrine interactions between the amnion, decidua and chorion in driving this process and of the possible role of placental corticotrophin-releasing factor.

In short, we still do not know the trigger for parturition in any species or indeed whether there is a single trigger in a particular species. Those animals that synchronise parturition, like the wildebeest and banded mongoose, provide an intriguing glimpse into control mechanisms that, as far as I am aware, has not yet been exploited.

I end on a large ‘phew’. The link between parturition and lactogenesis stage II is much simpler.

Wildebeest (Connochaetes taurinus). Masai Mara, Kenya, 1991

*Gluckman, P., Buklijas, T. 2013. Sir Graham Collingwood (Mont) Liggins. 24 June 1926 — 24 August 2010. Biographical Memoirs of Fellows of the Royal Society 59, 195-214.

†Fleet, I.R., Goode, J.A., Hamon, M.H., Laurie, M.S., Linzell, J.L., Peaker, M. 1975. Secretory activity of goat mammary glands during pregnancy and the onset of lactation. Journal of Physiology 252, 763-773.