Y.W. Loke, Life’s Vital Link: The Astonishing Role of the Placenta

This fascinating book about the placenta tells the story of this underappreciated yet truly remarkable organ. In the nine chapters of this book (the number is not a coincidence), Y.W. Loke touches on how the placenta evolved, describes its development from conception to birth and details the myriad of complex functions it performs. I highly recommend this book for the layperson (non-scientists and nonreproductive biologists), for its many fascinating facts, up-to-date science and clear explanations. To explain how and why the human placenta works the way it does, Loke rightfully begins this volume by describing the strategies used by amniotes, non-placental mammals and non-human placental mammals to nourish their young offspring. This all lays the groundwork for understanding how the human placenta evolved and functions. For example, even though the platypus, a monotreme mammal, lays eggs like birds and lizards, the egg spends several weeks in the uterus during which time it is nourished by maternal secretions that diffuse across the eggshell. The production and uptake of these secretions was an important development in the evolution of the placenta, an organ that allows the embryo to obtain nutrients directly from the mother instead of the egg yolk. Loke nicely describes the mammalian placenta in all its diversity, from the superficial epitheliochorial placenta found in ruminants to the deeply invasive hemochorial placenta found in humans. His discussion of placental phylogeny is thorough, up-to-date and clear. The question of which type of placenta evolved first in mammals (superficial or invasive) has been hotly debated over the years, and Loke explains why scientists now think it was invasive, based on the current distribution of placental types among mammals. Later in the book, Loke discusses an exciting hypothesis on how the invasive placenta first evolved: by retroviral infections in our early mammalian ancestors. There is now a great deal of evidence that syncytin genes (and other genes/ regulatory regions) of retroviruses were co-opted independently in different placental mammals for their immunosuppressive and fusogenic properties. Thus, it is likely that a retroviral infection in the stem lineage of placental mammals was involved in the emergence of this structure as well. As with other topics in this book, Loke’s discussion of the role of retroviruses in placental evolution is compelling and up-to-date. In subsequent chapters, Loke provides all the details necessary to understand the form and function of the human placenta. He uses terms such as ‘mop of bucket in blood’ to describe the villous tree of the placenta. This and other analogies help illustrate a complex morphology to non-experts. He describes the intimate and complex relationship between maternal and fetal tissues during pregnancy, and devotes a whole chapter to the ‘tugof-war’ that occurs at the maternal–fetal interface. Loke clearly explains some aspects of maternal–fetal conflict theory as originally outlined out by David Haig; genomic imprinting in the placenta and brain, and the great diversity of placental types, are manifestations of this conflict at different organizational levels. However, he glosses over other aspects such as the large amount of placental hormones produced during pregnancy. His alternative explanation, that they reflect the health of the placenta, is insufficient. His coverage of reproductive immunology in the chapter ‘Nature’s Transplant’, on the other hand, is thorough, clear and sufficiently detailed to explain the ‘immunological paradox’ of pregnancy and its solution. While the biological terminology and concepts in this book are advanced, the author does a good job making the material accessible to non-experts. Moreover, there are fascinating details provided in the book that will be of great interest to many. For example, in Loke’s description of antibody transport book review 30