Specific Developmental Stages in Detail

Especially in early development, specific developmental processes seem more meaningful than others in the ethical debate about the moral status of human prenatal life. These are described in more detail.

GAMETOGENESIS AND FERTILIZATION. The embryo is usually defined as coming into existence at fertilization and becoming a fetus when organogenesis is completed (eight weeks after fertilization). These borders are not sharply defined. The definition of an embryo thus cannot avoid being operational and context—dependent. The term conceptus is useful to denote any entity resulting from fertilization, when no reference to a more specific stage is intended. An additional complication results from the significant overlap between the final stages of female gametogenesis, fertilization, and initial cleavage.

Gametogenesis involves a special type of cell division called meiosis. When primordial germ cells (which are diploid—i.e., they have two complete sets of chromosomes) enter meiosis, their DNA is duplicated so that there are now four copies of each type of chromosome (a condition called tetraploidy). In the first meiotic division, there are genetic exchanges within each group of homologous chromosomes, which then separate into diploid daughter cells. In the second meiotic division, there is no further round of DNA duplication. Each chromosome in a pair is allotted to a separate daughter cell, now haploid. Each primordial germ cell thus gives rise to four daughter haploid cells.

In the male, all four cells resulting from meiosis ultimately become functional spermatozoa. In contrast, in the female, only one of the daughter cells becomes an oocyte, the other three cells are discarded as polar bodies. In addition, female meiosis is not completed until after fertilization has occurred. During each ovarian cycle of the sexually mature female, one oocyte progresses partially through meiosis but is arrested in the middle of the second meiotic division at the time it is discharged from the mature ovarian follicle into the oviduct. If the oocyte is fertilized, meiosis is completed. Within the newly fertilized egg, the male and female pronuclei undergo a protracted migration towards each other, while DNA is duplicated within both. Thereafter, both nuclear envelopes disappear and the chromosomes derived from the male and female gamete are involved in the first cleavage division. Thus the first genuine diploid nucleus is observed at the two—cell stage only (30 hours after initial contact of sperm and oocyte). While fertilization usually occurs close to the ovary, the conceptus is gently nudged towards the uterus, a voyage lasting about five days.

Both through recombination of gene segments during the first meiotic division, and through random assortment of homologous chromosomes in gametes, genetic novelty is generated. In other words, gametes are genetically distinctive in relation to their diploid progenitors and do not simply reflect the genetic structure of their parent organism. In a sense, gametes are distinctive "individuals" in relation to the organism that produces them. Fertilization creates genetic novelty of a different sort, by combining two independent paternal genomes. The zygote is genetically distinctive because it represents the meeting of two independent parental lineages. Thus genetic novelty appears twice per turn of the human life cycle.

CLEAVAGE, PLURIPOTENTIALITY, AND TWINNING. During cleavage, the zygote divides into smaller embryonic cells. At the 16—cell stage, the embryo is called a morula and a first differentiation into two cell types is initiated. The trophoblast is the cell layer that will soon connect with the uterine wall, whereas the inner cell mass includes the cells of the later stage embryo. At the blastocyst stage, a central cavity (blastocoel) is formed. If a blastomere is removed from the inner cell

Anxiety and Depression 101

Anxiety and Depression 101

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