The «gene architecture» of its chromosomes is responsible for the control of embryonic development. Since for obvious ethical reasons experiments cannot be performed on humans, the results that have been obtained up till now are derived from the study of a few model organisms such as caenorhabditis elegans (C. elegans, nematodes), drosophila (fruit flies) and laboratory mice.
Embryonic development depends on genetic as well as environmental influences that are temporally and locationally adjusted to one another. The factors that, for example, determine the interactions between the tissues, the migration and differentiation of the cells, the proliferation of the cell colonies, as well as the apoptosis (programmed death of cells) are numerous.
Embryonic development is a process of growth and differentiation in which the embryo becomes increasingly complex and is more and more enhanced with structures and functions.
The growth depends on the somatic multiplication of the cells through mitosis. In order to control the growth, certain restriction mechanisms are needed which are able to stop cell divisions at the right moment. The complexity of the structures is connected with morphogenesis and differentiation.
One of the most fascinating aspects of embryonic development is the fact that out of a simple zygote (fertilized oocyte) an organism arises that consists of billions of cells. In the following chapter only a few of the many important factors for prenatal development are discussed.
Control factors of embryonic and fetal origin
Numerous molecules (hormones, growth factors and enzymes) play a role in the growth and differentiation of the embryo. Only a few are mentioned here since a complete study lies outside the bounds of this module.
Control factors of maternal origin
The maternal hormone and growth factors normally do not pass through the placenta. If it nevertheless happens, an altered placenta metabolism is present.
If the mother has consumed drugs (alcohol, tobacco) or is herself diseased (e.g., diabetes), this can influence embryonic and fetal growth.