The Corpus Luteum
By Dr Roger Lambert
Department of Zoology, University of Aberdeen
The name is based in Latin, Corpus meaning the ‘body of man and animals’ and Luteum which is derived from the Latin word for ‘yellow’. The definition of what is a corpus luteum, plural corpora lutea is perhaps not easy to understand. It is defined in the Oxford Dictionary of Biology as ‘The yellowish mass of tissue that forms from the granulosa cells in the cavity of a Graafian follicle in the ovary of a mammals after release of the egg cell. It secretes the hormone progesterone’. The photograph (courtesy of Deer-UK) shows a dissected roe deer ovary with the corpora lutea clearly visible. As you can see, to describe then as yellow is not entirely accurate. So ‘yellowish’ is perhaps a better description. Though the ovaries of the roe deer are small, clearly, if dissected with reasonable accuracy the corpora lutea are easy to see. Do not make the mistake of dissecting through the ovary and cutting a corpus luteum in half and then think there are two! In fact they should be visible without the need to actually dissect the ovary.
So what actually is the corpus luteum? Well there has been many thousands of scientific papers written over several hundred years about it and they are still being written today as biologists find out even more about this very complex bit of tissue.
From the very start of mammalian life in the womb even the developing fetus has a pool of follicles. They mature and regress throughout the life of the animal (apart from humans and some primates who actually run out of these follicles and reach what we refer to as the menopause). When the animal reaches puberty she starts her first reproductive cycle, which in roe deer is a seasonal monoestrus cycle (she has a single oestrus cycle once a year in either late July or early August). The anterior pituitary gland releases a hormone called FSH (follicle stimulating hormone) and this does exactly what it says, it stimulates the follicle to release an egg from each mature follicle, which then may or may not be fertilised by the buck. After the egg is released from the follicle what remains behind in the ovary is called the corpus luteum. In the diagram the egg is referred to as an ovulated oocyte.
In most mammals the fertilised egg develops quickly into an embryo and implants, forming a placental link with the mother, around about 14 days. During this 14 day period, prior to implantation, the embryo needs ‘nutrition’ in order to develop and survive. This is provided by the corpus luteum in the form of the release of the hormone progesterone.
The European roe deer is different, because she has the reproductive strategy of embryonic diapause (delayed implantation). The embryo at the start of diapause is called a blastocyst and has reached approximately the 20-30-cell stage of development. It is tiny, about 1mm in size. Too small to be identified by the human eye without a microscope. Because roe deer are monoestrus and have embryonic diapause there is no need for an embryonic signal for maternal recognition of pregnancy at the time of fertilisation, as in other mammals. Also, unlike any other mammal with delayed implantation she is missing a part of what is called the ‘feedback loop’. This is a hormonal mechanism that confirms pregnancy to the mother. Hence the roe doe, during diapause does not know whether she is actually pregnant. Eggs may not be fertilised or may not be viable and are re-absorbed by the doe but the corpora lutea from these eggs remain active in the ovaries because there has been no signal to ‘turn them off’. In fact these corpora lutea release progesterone at the same concentrations as if she had fertilised eggs, right up to the end of the period coincident with the end of diapause. The roe doe is the only mammal with delayed implantation to do this. In all other species with delayed implantation, during the period of diapause the corpora lutea are inactive.
At the end of the period of diapause the corpora lutea which released eggs that were fertilised and viable remain active and continue to exist in the ovaries right up to the birth of the kids, even though the fetus now has a placental attachment from which it now receives much of its ‘nutrition’. The corpora lutea that are not supporting an embryo regress and are reabsorbed into the ovaries at the time coincident with the end of the period of embryonic diapause.
So if you are still with me, herein lies the problem for deer managers. During the period of embryonic diapause the number of corpora lutea present in the ovaries is no indication of how many kids will subsequently be born. I have examined does during embryonic diapause who have had two corpora lutea and no embryos, four corpora lutea and two embryos, three corpora lutea and one embryo, two corpora lutea and two embryos and so on. The doe may even loose a viable fertilised embryo during the period between reactivation from diapause and implantation if she is not well or in poor body condition. She is managing her resources to ensure survival.
The doe does not get a signal for maternal recognition of pregnancy from the embryo until it starts to reactivate from diapause. So in a sense, during delayed implantation there is an assumption of pregnancy even if she is not actually pregnant. You may think that with an excess of active corpora lutea over the number of blastocysts that the embryos would be provided with extra ‘nutrition’ from the corpora lutea not supporting an embryo. Not so, our studies of progesterone release from corpora lutea during diapause shows that the levels of progesterone remain constant regardless of the number of corpora lutea present.
It goes without saying that the reproductive strategies of the roe deer are complex and much studied, because here is an animal that does it like no other. I hope this article will assist all those with an interest in roe deer to help understand this unique reproductive process a bit better.