Book: Life's vital link
Life's Vital Link: The Astonishing Role of the Placenta
Y. W. Loke, Oxford University Press (2013), RRP $32.95
The Old Testament refers to the placenta as “The Seat of the Soul”. Today it is still revered in some parts of Africa, because where your mother buries the placenta is your home, and that is where you will be buried when your time comes. What a lovely way of linking our life to Planet Earth, which sustains us.
Professor Charlie Loke has written an amazing book on the human placenta; no sooner had I finished reading it than I felt compelled to read it a second time.
Loke has spent a lifetime working on the placenta at the University of Cambridge, where he helped to set up and fund the Centre for Trophoblast Research – an amazing achievement. This book is an eminently readable account of his life’s work, with an excellent glossary, index and endnotes.
Today we have medicalised the placenta, belittling it by dismissing it as “the afterbirth”, and throwing it out with the hospital rubbish. But as Loke’s book makes clear, the placenta is an incredible organ. It is the go-between that plays a vital role in connecting the generations. It must express both the mother’s and father’s genes, thus making it impossible for an egg to accidentally start dividing and forming an embryo absent a sperm – a process called parthenogenesis (vigin birth) which takes place in some plants and lizards. On the other hand, eggs that somehow end up carrying sperm but losing the maternal genes, end up forming uterine cancers that that look like a bunch of grapes known as Hydatidiform moles.
Many of the genes that are active in the placenta are “imprinted”. That means the gene has a different activity depending on whether it came from the mother or father. But we don’t know how this occurs or why. One of the reasons why reproductive cloning, as in the case of Dolly the sheep, is so difficult to achieve is because of failure to develop a placenta. Dolly was the first successful clone after 277 failed attempts.
There is an enormous diversity of placental types across the species, from the simplest placenta of the horse to the elaborate placenta of humans. Surprisingly, it is the more complex human model that seems to be the ancestral form for mammals. The placenta also seems to be at war within itself, with paternal genes promoting placental growth and maternal genes restricting it.
Although many biologists think that marsupials don’t even have a placenta, that is completely wrong; a placenta is essential for intrauterine embryonic growth, even though the teat becomes the equivalent of an umbilical cord for several months after birth.
The elephant holds the record for the longest-lived placenta; it does the job for 22 months. The placenta itself may play a role in deciding when pregnancy should end. In some species, such as cows, it certainly stops producing the pregnancy-maintaining hormone progesterone but this does not seem to be the case in women.
Loke also gives an interesting account of possible problems associated with Assisted Reproductive Technology; in vitro fertilisation of an egg in the test tube followed by culture in an artificial medium before transfer back to the uterus may result in failure of imprinting, so it may not be as simple as it seems.
The only thing that I found missing in this book was any mention of the dreaded word abortion. What is the best way of terminating an unwanted pregnancy? The 1958 discovery by two Chinese doctors, Yuantai Wu and Xianzhen Wu, of vacuum aspiration abortion has brought relief to many millions of women around the world, and deserves our praise.