Syncytiotrophoblast source: Gilbert, 2000.
mass of a blastocyst (as, for instance, in preimplantation diagnosis), the blastocyst is still able to produce a complete late embryo and fetus. This illustrates a fundamental principle called regulation, or regulative development. Within the early embryo, cell fates are not definitely fixed but largely depend on interactions with neighboring cells, so that development adjusts to the presence or absence of specific environmental cues. The molecular basis and the genes responsible for these cues are increasingly well known.
At the blastocyst stage, the inner mass cells are pluripotent (i.e., they have developmental plasticity) and are able to participate in the formation of most cell types of the adult organism, as shown for instance by experiments with cultured immortalized blastomeres, called embryonic stem cells. Recent research does suggest that individual blastomeres acquire some degree of molecular specificity quite early. However, this inherent "bias" that tends to drive every blastomere towards a specific cellular fate can easily be overridden at this stage.
Around day 6, the blastocyst has hatched from the surrounding zona pellucida (the outer envelope of the ovum) and is ready for implantation. As it attaches to the endometrium, two distinctive layers appear in the inner cell mass. The ventral layer (hypoblast) contributes to the primitive yolk sac. The dorsal layer soon differentiates between the embryonic epiblast that will contribute to the embryo—to— be, and the amniotic ectoderm lining the newly appearing amniotic cavity (day 7—8). This two—layered structure is called the embryonic disk. All this happens as the blastocyst burrows deeper into the uterus wall and the trophoblast comes into close contact with maternal blood vessels. The trophoblast also produces human chorionic gonadotropin (hCG), which is the substance detected in pregnancy tests and is essential to the maintenance of pregnancy. Abnormal conceptuses are very common until that stage and are eliminated, usually without detectable signs of pregnancy. Inversely, fertilization occasionally results in a hydatidiform mole. This structure consists of trophoblastic tissue and therefore mimics the early events of pregnancy (hCG is produced), without their being any actual embryonic tissue present.
The term pre—embryo was often used to mark the embryonic stages described so far. This term is sometimes shunned in contemporary discourse, as it has been suspected to be a semantic trick to downgrade the standing of the very early embryo. Yet even writers like Richard A. McCormick belonging to the Catholic tradition, sets great store by the moral standing of the earliest forms of prenatal development, have expressed doubts about the validity of this suspicion (1991). More importantly, doing away with the term "pre—embryo" does not solve the two underlying conceptual problems that this term addresses. The first ensues from the cellular genealogy linking the zygote to the later stage embryo and fetus. Only a small part of the very early embryo is an actual precursor to the late embryo, fetus, and born child. Whatever terminology one wishes to use, no account of early development can avoid sentences such as this, written by Thomas W. Sadler in 2000, "[t]he inner cell mass gives rise to tissues of the embryo proper," or terms such as the embryo—to—be. This is an inescapable consequence of the fact that the late embryo includes only a small subset of all the cells that originate with the zygote and blastocyst (Figure 1 shows the complex genealogy of embryonic and extraembryonic tissues in human development). The second problem arises from the fact that the early embryo has a degree of freedom as regards its final numerical identity. Until about 12 days after fertilization, twinning can occur. In other words, until that stage, a single embryo still has the potential to divide in two embryos, ultimately developing into two separate persons. Therefore there is no intrinsic one—to—one relationship between the zygote and the late embryo, as there is between the late embryo, the fetus, and the born human.
GASTRULATION. Gastrulation begins with a wave of cellular movements that start at the tail end of the embryo and extend progressively forward. Future endoderm and mesoderm cells slip inside the embryonic disk through a groove called the primitive streak (day 14). The anterior end of the streak is called the node. Of the cells that migrate inside the streak, some form the endoderm and others will lie atop the endoderm and form the mesoderm. Finally, those cells that remain in their initial position on the surface of the embryonic disk become the ectoderm. Gastrulation sets the overall organization of the embryo in a definitive way. The main axes (anterior—posterior, left—right) are defined under the control of two central signaling centers: the node (which is the equivalent of the organizer discovered by embryologists working on frog and chick embryos) and the anterior visceral endoderm.
Recent data from molecular genetics have partially uncovered the molecular basis of axis determination. The determination of the anterior—posterior axis involves the HOX genes, a set of four gene complexes. Since HOX genes located at the "front end" of a HOX complex are expressed at the "front end" of the embryo, the arrangement of the various genes within each complex remarkably reflects the place at which they are expressed in the embryo along the anterior—posterior axis. The four HOX complexes thus provide four "genetic images" of the lengthwise arrangement of embryonic structures. The left—right asymmetry of the embryo (and thus of the future body plan) is thought to originate with specific cells in the node. In a way that is not fully understood, these cells induce a cascade of protein signals that is different on the left and right side of the embryo. This results in the synthesis of controlling factors that are laterally restricted. It is supposed that these controlling factors and other factors direct the development of asymmetric organs accordingly.
Through gastrulation, the embryo arises as a defined entity endowed with a much higher level of organic unity than at any stage before. The laying down of the head—to— tail axis and other defined spatial features, as well as the loss of pluripotentiality in many cell lineages, mark the beginning of a single individual human organism and thus provide one of the first important dimensions of the onto-logical continuity typical of the born human.
LATER DEVELOPMENTAL STEPS. In the initial step in organogenesis, the midline axial section of mesoderm—the notochord—instructs the overlying ectoderm to turn into the neural plaque. This structure soon wraps around to form the primitive neural tube, out of which the central nervous system will eventually grow. By the beginning of the fetal period (eighth week), the rudiments of the heart, blood and blood vessels, the major segments of the skeleton and associated muscle groups, the limbs, and many other structures are in place. It is noteworthy that although the primordial nervous system is one of the earliest organ systems to emerge in development, it takes the longest time to mature. Synaptogenesis (the formation of —contacts between nerve cells) starts on a grand scale only late in pregnancy and continues well after birth. This is important to keep in mind when interpreting early movements of the fetus, visualized more and more accurately by ultrasonography. These movements reflect the maturation of local neuromus-cular structures and are not due to significant brain function, since there is no "brain" in the sense of the later, much more developed anatomic and functional structure called by that name. This is different later in pregnancy, when fetal movement is more reactive to the environment and when it becomes arguably legitimate to interpret it as "behavior," insofar as it reflects the increased functional capabilities of the central nervous system. Finally, the concept of viability basically reflects the ability of fetal lungs and kidneys to support extrauterine life, which is impossible before the twenty-second week.
As mentioned before, the differentiation and migration of early gametes also occurs during the embryonic phase. This separation of the germ cell lineage from all other cell lineages marks a bifurcation in the life cycle. Unlike somatic cells, gamete precursors have a chance of becoming gametes and participating in fertilization, thus contributing to the next generation. In a way, the germ cell lineage is eternal through successive turns of the life cycle, whereas the rest of the embryo, the sum total of somatic cells, is inherently mortal.
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