The President's Council on Bioethics
Monitoring Stem Cell Research
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Monitoring Stem Cell Research

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Notes on Early Human Development



The term “embryo” refers to an organism in the early stages of its development. In humans, the term is traditionally reserved for the first two months of development. After that point, the term “embryo” is replaced by the term “fetus,” which then applies until birth. Some authors further reserve the term “embryo” for the organism only after it has implanted and established its placental connection to the pregnant woman. Similarly many also reserve the term “pregnancy” for the state of the woman only after implantation. At the beginning of the individual’s development, the entity is a single cell. After two months, it has limbs, distinct fingers and toes, internal development, and countless cells. So the term “embryo” applies to an individual throughout a vast range of developmental change. This document is a description of early human development, with emphasis on those events or structures that have figured most prominently in recent discussions of research using human embryos or their parts, especially for stem cell research.1

Development has fascinated centuries of observers, as they pursued deeper understanding of the stability of species characteristics at least from one generation to the next, as well as the uniqueness of each offspring. Uniqueness is especially marked in sexually reproducing organisms, that is, organisms where the genetic make-up of the offspring comes from a combination of maternal and paternal DNA, because a new genome is formed in each instance of conception. The stability reflects inheritance connecting one generation with the past and future members of its line.

Organisms and the processes of their development have evolved. As a result, the development of any organism has a species-specific pattern, but also shares many of the same developmental processes with other species related from its evolutionary origins. Many of the processes discussed here are common not just to all humans, or to all mammals, but to all vertebrates. In some cases they are shared even with invertebrates as well.

The process whereby a new individual of the species comes into being has been at the center of too many deep inquiries to list here, let alone discuss in the depth they deserve. But even in this short document it is important to note one question that is related to the connection of one generation to the next and previous generations. That is, how are we to understand the apparent directedness of development, following a complex network of pathways from a single cell to a multi-system, free-living, and even conscious being? This process occurs in a reliable pattern time after time, but also is sufficiently resilient to perturbations that developing entities can recover from significant disturbances. For example, at early stages of development an embryo may divide (or be cut) completely in half, and then each half recovers to form an entire offspring, resulting in identical twins.

Different notions of purposive directedness, functional explanation, and even vital forces have been invoked to explain development. One of the insights, from the relation of development to evolution, is that the development of an individual reflects the fact that it is descended from individuals that reproduced successfully and, like its forebears whose DNA it inherited, its development reflects their past survival with their particular characteristics. This legacy of ancestral success at survival is manifested in the new organism’s apparent directedness toward development along lines that enhance its own survival. Even very early embryos follow patterns of differentiation in the progeny of different cells. These patterns, in embryology, are called the fate of the progeny of a cell. The fate of the progeny of the newest single cell embryo is maximally broad—if it survives it will give rise to every type of cell of the species. But as the embryo becomes multicellular, its cells specialize and, in the absence of artificial perturbation, their progeny have increasingly specialized fates as well.

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The President's Council on Bioethics
Washington, D.C.
January 2004
http://www.bioethics.gov

National Center for Health Statistics, “Births: Final Data for 2001.” National Vital Statistics Reports 51(2) (2002), available at http://www.cdc.gov/nchs/data/nvsr/ nvsr51/nvsr51_02.pdf.