The ethical relevance of studying human development appears when one asks which stages of the human life cycle embody significant ethical concerns. Between birth and death, the human organism is a person, equipped with the full measure of basic human rights. This much is not really controversial, and the debate primarily concerns the prenatal phase of development. Do human rights accrue to the unborn all at once, for instance at fertilization? Do they instead arise in a gradual manner, based on the various progressive steps through which the prenatal human organism acquires significant person—like properties? Besides personal rights, are there other ethically—significant values and properties that would justify a respectful treatment of embryos and fetuses? An understanding of prenatal development is a necessary, albeit in no way sufficient, condition for addressing these issues successfully.
To understand the basic biology of any sexually reproducing organism, one needs to grasp the primary concept of the life cycle. The life cycle of humans includes fertilization, cleavage, gastrulation, organogenesis, fetal development, birth, child development and puberty, gametogenesis and again fertilization. It is through the germ-line that the life cycle persists from generation to generation. On the other hand, the somatic cells (which comprise all the cells of the fetus, child, and adult that are not directly involved in reproduction) belong to an inherently mortal entity, the human organism, whose fate is senescence and death. One turn of the life cycle defines one generation. Fertilization and birth define the beginning and end of the prenatal phase of development, which is comprised of two stages: embryonic and fetal.
The embryonic phase initiates with fertilization, the meeting of the male (sperm) and female (oocyte) gametes, giving rise to the zygote. At fertilization, a new, diploid genome arises from the combination of the two haploid genomes included in the gametes. The zygote divides several times (cleavage stage) to form a blastocyst. The cells of the blastocyst, called blastomeres, are separated into two parts: an outer layer, called the trophoblast, that eventually contributes to the placenta; and an inner cell mass that contributes to the future embryo. About six days after fertilization, the blastocyst attaches to the endometrium (the epithelial lining of the uterus). This marks the beginning of pregnancy and further development depends on intricate biochemical exchanges with the woman's body. While the trophoblast invades the uterine wall, the inner cell mass undergoes further stepwise differentiation processes that lead to the formation of the embryonic epiblast (the precursor of the actual human individual) and several extraembryonic structures (Figure 1). The embryo then undergoes gastrulation, the process that starts with the formation of the primitive streak. This is the crucial developmental step, common to all animals but the most primitive invertebrates, by which the three basic germ layers of the embryo are formed. These are called ectoderm, mesoderm, and endoderm.
From the third to the eighth week, the process of organogenesis involves the differentiation of the three germ-layers into specific tissues and primordial organs. The earliest stage in organogenesis is called neurulation and starts when a specific area of ectoderm turns into the primordium of the nervous system. During organogenesis, many genes that are crucial to development are activated, and complex cell—to—cell signals insure the proper differentiation of various cell types, as well as the movement and migration of cells to their proper places in the developing embryo. For some cell types, this involves long—range navigation. For instance, the gamete precursors must travel from their initial position near the yolk sac to the primordial gonads.
At the end of the embryonic phase, many important organ systems are in place, at least in rudimentary form. The fetal phase is characterized by further differentiation and maturation of tissues and organs, as well as considerable growth, especially towards the end of pregnancy. In the late fetal phase, the nervous system undergoes an acceleration of synapse formation and maturation of the brain, which is increasingly sensitive to outside cues. This process continues well after birth.
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