Studies of the fetus, da Vinci
Embryology, the study of developing animals, is arguably the most complex of all branches of biology. Whilst some broad principles of early embryos can be gleaned from studying invertebrate (Drosophila) larvae, the further up the phylogenetic ladder one climbs, the more accurate become the extrapolations to the fate of tissues, organs and the organism itself.
When I first studied embryology at university and later in medical school in the 1970s, it was a black box: our professors were able to tell us, in excruciating detail, the events that preceded and followed the moment that one lucky sperm penetrated the egg’s zona pellucida. Once the ovum (egg) was fertilized at the fimbriae of the fallopian tube, it then divided into 2 cells, then 4, then many more to become a multicelled morula, then a blastula/blastocyst, at which point the business end of the early embryo is known as the embryoblast.
As the embryoblast/embryo grows, it divides into the so-called primitive germ layers:
• Ectoderm—which gives rise to skin and the nervous system
• Mesoderm—which gives rise to muscle, bone, vascular system, kidneys and other organs, excluding the tissue lining them, and
• Endoderm—which gives rise to the tissue lining the respiratory, urogenital and GI tracts
Fetal age and tissue development
Our professors could teach us the morphologic changes (see table, bottom) that occurred in a developing organism, the “what” that occurred from fertilization to birth, but not the critical “how”. Decades were to pass before we began to understand the how of embryology, which in large part we owe to the tsunami of information that followed sequencing of the human genome in 2003.
We now know that for an embryo to develop properly, certain genes are turned on and off at key moments, and that limbs or other structures sprout and develop in a region if one or more specific morphogens accumulate and reach a critical concentration, but won’t if they don’t (see French flag model).
Armed with an understanding of the complex interplay of gene products (proteins) in human development, perhaps the greatest surprise is that structural defects–e.g., congenital heart disease, neural tube defects, cleft lip and palate and other congenital defects are frankly rare: the vast majority of babies are morphologically normal.