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Early Development

We have already seen how sperm and eggs are produced, and noted the physical processes that allow them to meet. When a sperm contacts an egg (in the oviduct), a chemical reaction takes place between molecules on the surfaces of the sperm and egg. This binding signals the sperm to release enzymes that begin to dissolve the outer layers of the egg’s protective coating, and ultimately allows the fusion of the sperm and egg plasma membranes. The sperm nucleus now enters the cytoplasm of the egg, and several changes rapidly ensue:
  • Electrical and physical changes in the egg cause the plasma membrane to become unable to bind with any other sperm; these changes are called blocks to “polyspermy”, or the fertilization of an egg by multiple sperm cells. It should be clear why this is necessary, as any cell with more than a diploid set of chromosomes would be inviable.
  • The egg becomes metabolically active, and completes meiosis II. Protein synthesis increases dramatically.
  • When the nuclei of the sperm and egg fuse, a diploid cell is created called a zygote. Now the cell has the potential to continue along its developmental pathway.
  • The zygote begins to prepare for mitotic division.
After fertilization occurs, the egg continues its journey towards the uterus, and more changes take place along the way (see Figure 18.6).
  • Cleavage, a modified form of mitosis, begins. During cleavage, mitotic cell divisions are not accompanied by cell growth. Thus, the number of cells is increasing, but the overall volume is not increasing. This is because no nutrients are yet available to provide energy for growth processes.
  • A morula, or solid ball of cells, has been produced by the time the dividing cells reach the uterus. This usually takes about three days

Early developmental events


A blastocyst  is formed as the cells of the morula are pushed towards the periphery, and consists of a hollow ball of cells. The blastocyst forms after about six days, and is often called the blastula in other vertebrate species.
Implantation of the blastocyst into the uterine wall (the endometrium) occurs towards the end of the first week.
After implantation, the cell mass that has implanted itself can be properly referred to as the embryo. The embryonic stage lasts until the beginning of the third month of development. This is the most crucial period for proper formation of major structures, and many important changes occur during embryonic development.
The placenta forms between the embryo and mother, and allows transport of nutrients and oxygen to the embryo for energy production. The placental connection also allows the transport of embryonic wastes from the embryo to the blood stream of the mother. It is important to note that while small substances such as those mentioned above freely diffuse across the placenta, it is not a direct connection, and the circulation of mother and embryo remain separate.
Gastrulation changes the overall shape of the embryo so that it resembles a three-layered disc, the gastrula. Three primary germ layers are formed by this process, each of which will eventually give rise to different structures in the adult organism:
  • Endoderm, the inner layer, will give rise to the epithelium lining all inner cavities, including the linings of the digestive, respiratory, and urogenital tracts.
  • Mesoderm, the middle layer, will give rise to internal organs, muscle and connective tissues, and the majority of internal structures.
  • Ectoderm, the outer layer, will give rise to the nervous system and the skin.
  • The embryo becomes enclosed in a fluid filled sac, the amnion, surrounded by an amniotic membrane and several other membranes.
By the end of the embryonic period, roughly the first trimester, organogenesis and morphogenesis (the development of major organs and body shape, respectively) are basically complete. From the third month forward until birth (the second and third trimesters), the developing organism is referred to as the fetus. While the fetal stage lasts the longest, the overall body plan has already been laid out, and most of the time spent as a fetus involves the growth and maturation of existing structures. After a total of approximately nine months, labor begins as the uterus begins to contract and the amnionic membrane ruptures. The baby passes through the dilated cervix and vagina, and enters the world as a new individual.

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