From the time sperm meets egg until the moment a fully formed baby human is delivered into the world, a fantastically complicated series of events unfolds. In addition to the first single cell of a new life--the fertilized egg--having to divide into the millions of cells that will make up a full-term fetus, the cells must also segregate and specialize into tissues, organs, and organ systems. This seemingly Herculean feat is completed within a mere nine months.
Zygote
The first stage of development is called the zygote stage, and lasts from the moment of conception until implantation, which takes place approximately two weeks after fertilization. The zygote stage begins when a male gamete, or sperm cell, meets a female gamete, or egg cell. The sperm and egg cells differ from most human cells in that each has only half the genetic information contained within a typical cell, meaning that neither alone could develop into a new person. Put together, however, their genetic information forms a complete set that represents the combination of mother and father. When the gametes meet, they fuse together into a single fertilized egg. Over the next several days, this single cell divides many times, forming a cell ball, explains Lauralee Sherwood, in her text "Human Physiology." As this division takes place, the zygote travels through the mother's fallopian tubes toward the uterus, where it will eventually implant.
Blastocyst
Eventually, the cells forming the zygote divide sufficiently to form a hollow ball of cells called a blastocyst. Around seven to 10 days following fertilization, according to the American Pregnancy Association website, the blastocyst embeds itself in the lining of the uterine wall. The blastocyst resembles a water balloon in many ways--it has thin walls and a fluid-filled cavity, but unlike a water balloon, it also has a very small, solid ball of cells attached to the interior of the wall. These cells will become the embryo, and eventually form every part of the baby, while the cells forming the blastocyst walls will become the placenta and membranes.
Embryo
Following implantation, the baby enters the embryonic developmental stage. During this phase, the internal organs form, though not all are completely functional by the end of the embryonic period. The Merck Manual online notes that the brain and spinal tube, which eventually becomes the spinal cord, form first, followed shortly by the heart. At the end of the embryonic phase, which lasts until the end of the eighth week of pregnancy, all major organ systems are in place. Further, the baby gains a distinctly human appearance during this phase of development--early embryos have a series of bumps near the head that will become the jaw and structures of the throat, as well as an elongated tail. Late embryos have recognizably human faces, have lost their tails, and appear quite similar to, albeit smaller than, full-term infants.
Fetus
The Merck Manual describes the fetal stage of development as a time of growth rather than of proliferation of new tissue types. With all systems in place by the end of the eighth week, the major task of a fetus between weeks nine and 40 is simply to grow. Muscles develop and begin to flex, producing palpable movements by the 18th week. The fetus' lungs develop, and it lengthens so dramatically that it no longer floats freely in the uterus, but is instead curled tightly in order to fit in its increasingly cramped quarters. In the last weeks of gestation, the fetus gains body fat--the primary difference between a baby delivered at forty weeks and one a week or two early is a matter of fat weight, which can accumulate at the rate of nearly 1/4 lb. a day.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- American Pregnancy Association: Blastocysts
- Merck Manual Online: Human Development
