The Making of You
Author: Katharina Vestre
This relatively small book is a graphic depiction of the phenomenal process that is unleashed when a sperm and an egg fuse at conception, and the remarkable metamorphosis that follows as the formless embryo transforms into a concrete living creature. The book explores the chemical and genetic underpinnings of foetal development, delving into interesting concepts such as how the sonic hedgehog gene directs cells to their appropriate locations in the body, and guides their development. But beyond the core processes of embryology, the narrative also delves into the history of our understanding of this phenomenon, with notable contributions such as of Anton van Leeuwenhoek who was the first to observe sperm cells under the microscope, and of Friedrich Miescher who first noted the high concentration of nucleic acids in sperm cells. The book is also peppered with fascinating historical anecdotes such as how the pregnancy test has evolved from its very cumbersome origins when it involved injecting the urine of women into mice and rabbits (pages 9-23 and 70-72).
One of the most fascinating themes of the book is its chronological description of the events that transpire at fertilisation, an epic story that starts with several hundred million sperm cells swimming against the tide, competing for the prize – an egg which itself survived a competition of its own to be the one released from the ovary during the menstrual cycle. Attracted by the egg’s warm temperature and odour, the author depicted how only one sperm cell is enabled to burrow through the egg’s cell wall. In demonstrating the remarkable events that ensue from this point, the author portrayed an astonishingly rapid cell replication which is followed about a week later by ‘a brutal invasion‘ of the uterine wall by the embryonic cells. Depicting a spectacle resembling ‘a gory horror film‘, the author painted a savage scene replete with bursting blood vessels, and with cells dying ‘in their masses’ as the ‘ravenous‘ embryo executes its plan to fulfil its habitation and nutrition needs. The dramatic spectacle culminates in the establishment of the placenta which the author portrays as a veritable hormone-producing factory which, among other things, drives the mother to eat more, and keeps her blood vessels open (pages 1-5 and 18-23).
Just as compelling as the story of the formation of the embryo is the account of its subsequent evolution. In narrating this, the book describes how the foetus shape-shifts from ‘a little round plate‘ between the foetal sac and the yolk sac, to an oval-shaped structure consisting of three parts; a top layer which will become the skin, the nerves, and the brain; a middle layer which will develop into the muscles, the bones, and the heart; and a bottom layer which will form the intestines, the liver, and the lungs. In a similar way, the author details the formation of the different organ systems, for example demonstrating how the heart changes from a simple midline tube into ‘a compressed S‘ which ends up between the lungs and ‘pointing to the left’. The book expanded on this important concept of loss of symmetry in its description of the formation of many other organs; for example, in explaining how the spleen and liver end up taking ‘permanent residence on different sides of the body’, she referred to the influence of hairs on the back of the embryo called cilia which rotate in one direction thereby setting up a stream of fluid that sweeps towards the left, with the result that ‘the two sides of the body received slightly different commands‘. She also stressed the critical importance of this mechanism by highlighting the serious disorders that may result when it fails; an example of this is Kartagener’s syndrome – a disorder in which the asymmetrical organs develop on the wrong side of the body, and which may be complicated by such dysfunctions as infertility because the sperm cells lose the ability to swim effectively (pages 33-37 and 47-48).
The phenomenal facts about reproduction which the author packs in the book went a long way to emphasise the truly impressive nature of human development. Regarding the nervous system for example, the author depicted how, by the fourth month of gestation, ‘around 20,000 new nerve cells are popping up every single minute’ in the brain, but ‘only those that make the best connections survive’. It is interesting that this type of nerve cell attrition carries on beyond birth, the author maintaining that the reason newborns spend a lot of time in REM sleep is to enable the brain to remove ‘unnecessary connections between nerve cells’. The book makes equally compelling observations about the development of the other organs, such as how the heart starts to beat as early as twenty-two days after conception, and how it takes three attempts before the final version of the kidneys is established in the fifth week of conception. Other interesting facts about foetal development include how embryonic cells may cross the placenta and embed themselves in the mother’s organs – sometimes living there for decades; and how foetuses learn familiar sounds such as their mothers’ voices and the melodies of their mothers’ favourite television shows (pages 94, 128, 39, 88, 25-26 and 109).
Amongst the diverse themes the book explored, one of the most enlightening was that of twinning, a very intriguing concept which the author approached by elucidating its mechanism and its unique associated phenomena. For example, the author traced the origin of most identical twins to the stem cell stage after the embryo has attached to the womb, both twins ending up sharing the same placenta; she contrasted this to identical twins which have individual placentas if they are formed before they embed in the uterus. Equally informative are the differences that exist between identical twins, for example in their fingerprints, the author attributing this to the different environments the twins experience in the womb. On the other hand, in her discussion of non-identical twins, the author emphasised the similarities they share, such as how their common blood supply enables them to each possess two different blood groups. Just as fascinating is the book’s depiction of what happens when the twinning process goes wrong, a prominent example being when twins fuse and develop as a chimera – a single individual made of two sets of genetic elements; this situation has real-life consequences such as when it leads to inaccurate DNA tests in the determination of the maternity of a child – as illustrated by the celebrated case of Lydia Fairchild, a chimera who was charged with fraud for legitimately claiming child support (pages 26-29).
A theme that ran throughout the book, and which greatly amplified its value, is comparative embryology – a feature that was evident in the frequent, and refreshing, references the author made to other animals. For example, she highlighted how a lot of what is known about human embryology is derived from the study of non-human organisms. She illustrated this with the knowledge, derived from the study of the DNA of flies, that the segmentation of the body into somites during the third week of human conception is determined by the HOX genes. However, the subject that probably best symbolises the striking differences that exist between species is the development of sexual organs. For example, the author contrasted the determination of human gender, by the X and Y chromosomes, to what transpires in alligators in whom environmental temperature plays the equivalent function – males developing ‘if it gets warmer than thirty-four degrees’, and females emerging when the temperature is less than this. Perhaps even more astonishing is sex determination in the fish, Thalassoma bifasciatum, which can change gender ‘to suit their environment’ – becoming female if there is a dominant male, and becoming male to replace a dominant male that dies. Other interesting reproductive comparisons are to the painless delivery of kangaroos, and the excruciatingly painful birth of spotted hyenas (pages 52-55, 82-84 and 133).
This is a very simplified chronological account of human development which is written in beautiful prose and supported by interesting historical anecdotes. Perhaps to make the subject very accessible, the author avoided using many technical terms; whilst this achieved its objective, it made the book less challenging in terms of transmitting the scientific dimension of the subject. It also implied that the lay reader missed out on an opportunity to learn simple technical terms such as ectoderm, mesoderm, and endoderm. This however does not detract from the key goal of the book which is to transmit the fascination events of embryology.
This is a very simple but informative primer to the early development of the human. It highlights the major developmental landmarks, the guiding genetic and chemical principles, and the key clinical consequences when things go wrong. Although it ran the risk of oversimplification for the more specialist reader, this is not a major detriment to the book’s objectives. It is a well-written and educative account of embryology which highlights important clinical correlations for which I recommend it to all doctors.
Publisher, Place, Year: Profile Books, London, 2019
Number of chapters: 17
Number of pages: 161
Star rating: 5