What is Embryology?
Embryology focuses on the first eight weeks of human embryo development. This 56 to 60 day period is an intensely active time for the embryo. It grows from an almost invisible fertilized oocyte into a fetus with limbs, head, eyes, fingers, toes, brain activity and a heartbeat. This evolution of a single cell organism into a multi-cellular baby is an incredible scientific journey.
What Affects the Development of the Embryo?
A number of factors contribute to the development of the embryo. These include:
• Genetics of parents or sperm and/or egg donors
• Chromosomal mutations
• Environmental factors
The smallest change or event in the mother's uterine environment including toxicants or trauma can create significant alterations in the embryo's development. Before most women are even aware of the pregnancy, the process of human development is well underway.
Fertilization of the Embryo
With the fertilization of the woman's egg or ovum by the male partner's sperm or gamete, the embryo begins to develop. The two entities each carry 23 chromosomes encoded with parental genetic characteristics and unite to form a single-celled zygote. The creation of the zygote through fertilization takes about 24 hours and the result is a 46 chromosome one-celled zygote with all of the genetic information to create physical characteristics and, to come degree, mental capacity and personality traits. The two sets of 23 chromosomes combine to form a completely unique set of 46 chromosomes and a completely unique human being.
Becoming a Multi-Cellular Organism
The first action of a zygote post-fertilization is the process of mitotic division into a multi-cellular organism. The zygote divides into two blastomeres and then into smaller blastomeres, eventually compacting to create a morula made up of 12 to 16 small blastomeres. It is the morula that enters the fallopian tubes and travels to the uterus within three days of fertilization.
Implantation
Uterine fluid fills the blastocele or the center of the compacted blastomeres. The technical term for the embryo at this point is 'blastocyst,' whose embryoblast or the cell mass at its center will eventually become the embryo. The outer cells are collectively termed the 'trophoblast' and it is these cells that attach to the uterine lining and begin placenta formation within seven days of fertilization.
Placenta Formation
The placenta is the first life line of the embryo and—in conjunction with the umbilical cord which attaches it to the embryo and develops in the fifth week—will provide an influx of nutrients and an outflow of waste. It is after implantation takes place and the placenta begins to form that the embryblast begins its development into an embryo.