What is Human Embryo Development?

The conception of life and how it grows is a miracle that continues to baffle the world of science. Human embryo development is a complex process in which all the intricacies of a living being are designed. This article will cover the developmental stages of a fetus, and address possible complications that can occur along the way.

Identification

The life of an embryo begins when a male's spermatozoa makes contact with a woman's egg. A zygote cell, the very first representation of the fetus, is the result of this fertilization process. Contained within this one cell is the DNA of both the male and female, as well as the blueprint from which the fetus will develop.
Within the first three to five days of conception, the zygote cell travels from the woman's ovary, through the Fallopian tube and on into the uterus. During this time the cell multiplies and becomes two groups of cells which form an inner group of cells, and an outer shell. This is the second stage of embryo development, called a blastocyst. The two cell groups have now become the actual embryo, surrounded by its protective membrane.

Features

Once the blastocyst reaches the uterus, the gestation period has begun. It immediately attaches itself to the uterine wall, and from there receives nourishment from the mother via the bloodstream. This is considered the first trimester, and it's during this time that a fetus is most susceptible to harm, and miscarriage. X-rays, or any type of radiation therapy, infection, alcohol, drugs or nutritional deficiencies can all have adverse effects on a fetus during this time of development. The length of the first trimester is 12 weeks as of the time of conception.
By the start of the second trimester, the embryo is now a fetus weighing about 0.5 to 1.0 ounce. This time period runs from 14 to16 weeks. The umbilical cord, placenta and amniotic sac grow along with the fetus and become its life support system. Situated between the amniotic sac and umbilical cord, the placenta transfers nutrients from the mother's body to the fetus, while distilling waste discharge from the fetus to the mother's body.
By week 28, the fetus is fully developed at the start of this third trimester, and has a 60 percent chance of survival if born prematurely. This trimester period can last up until the 37th to the 40th week. Measurements at this time range between 14 to 17 inches long, with a weight of around two pounds. A thin, wrinkled layer of skin cells is present at first, then gradually becomes smooth as fat deposits form under the skin. The size of the fetus nearly doubles within this trimester, with an average birth size of 20 to 22 inches long, weighing in at seven to eight pounds.

Function

Embryonic development happens in layers throughout the course of gestation. The first structures to take shape are the spinal cord, brain, heart and gastrointestinal tract. By the end of the first trimester all the essential organs have begun to form, with distinguishable limbs, and tissue formations about the vertebra. Five distinct areas are developed within the brain, and a rudimentary circulatory system runs through main blood vessels, so there's a steady heart rhythm in place as well.
At around the 12th week, fetal development has begun. Facial features become more distinguished, arms and legs grow longer, and the genitals are recognizable. At this point, the head makes up half the size of the fetus. By the end of the second trimester, fetal movements begin. Bone and muscle tissues are stronger and more developed, with bone marrows producing red blood cells. Airwaves are present within the bottom portion of the lungs, though not enough to allow for gas, or air exchange to take place. Fingerprints and footprints become visible through a nearly transparent skin-covering.

Time Frame

The third trimester marks the full maturation process for the fetus. Many bodily functions are now controlled by a maturing nervous system, as the brain undergoes rapid development at this time. The fetus is able to store needed nutrients, like phosphorous, calcium and iron. Air sacs have formed in the lungs to allow for rhythmic breathing patterns and air exchange is possible. Bone and muscle structures are fully developed, as body fat forms and increases in density. Once the fingernails and hair grow in, the fetus has reached full term.

Considerations

Embryonic development proceeds according to genetic markers that are predetermined by the DNA-chromosome materials present at conception. If one parent carries a defective gene, or if the parents' DNA combination is incompatible in some way, genetic problems can become a factor. Genetic abnormalities are present when an extra, or missing chromosome is present in the fetus. Down Syndrome, Klinefelter's Syndrome and sex-chromosome abnormalities are known conditions in which the fetus has taken on an abnormal number of chromosome pairs.
The presence of genetic abnormalities does not automatically warrant ending a pregnancy. Depending on the circumstances at hand, many parents opt to carry to full term and provide whatever care may be needed once the baby is born.

Resources

• Possible Drug Effects on Embryonic Development
• Embryonic Development Resource