Introduction

Abstract

These verses from Ecclesiastes and the Psalms articulate clearly the profound mystery of embryonic and fetal development, which has fascinated humankind since earliest times. Indeed, as astronomy and mathematics are to the physical sciences, the study of embryology and reproduction is one of the earliest of the biological sciences. One of the most mysterious and beautiful phenomenon is the development of a single cell, the fertilized egg, into a living, breathing, sentient human being with several trillion cells of about 200 distinct types. In frogs, fish, and other creatures, this transpires in full view of the observer. Within humans and other mammals however, this occurs hidden away within the dark, moist folds of the uterine cavity. In both instances, the events are similar, commencing with the totipotent fertilized ovum, followed by the first few cell divisions that give rise to the 16 cell morula, each cell of which remains totipotent. A few more cell divisions result in the structured blastocyst of 50 to 150 cells in the form of a shell surrounding a hollow space. From its inception, the blastocyst contains an outer covering of the placental-forming trophectoderm, or trophoblast, an inner cell mass of 20 to 30 pluripotent cells, the embryonic stem cells, and a fluid-filled cavity, the blastocoele. The embryonic stem cells have the ability to multiply indefinitely and to differentiate into any of about 200 cell types in the various tissues and organs. During the embryonic stages succeeding the blastocyst, the plasticity of developmental potency gradually diminishes until, near the end, most of the cells in mature tissues and organs are committed to their biological roles, and are unable to multiply or develop further. A fundamental “shift” in our understanding of cell commitment came with somatic cell nuclear transfer, the first successful application of which resulted in “Dolly” the sheep (Campbell 2002) and subsequently with the advent of “induced pluripotent stem [adult] stem cells” (Okita and Yamanaka 2011).