Term Paper on "Human Reproduction: Conception, Embryonic and Fetal Development"

Term Paper 7 pages (2413 words) Sources: 3

[EXCERPT] . . . .

Human Reproduction:

Conception, embryonic and fetal development and birth

Humans have many unique features as sexually reproductive beings. Unlike single-celled organisms, humans do not divide into identical clones. Instead, the species relies upon the ability of two sets of genetic material to be combined and passed onto the resulting offspring. As in reptiles and all mammals fertilization takes place inside the body of the female. "This technique increases the chances of successful sexual reproduction" (Freudenrich 2010, p.1). It is also a far more complex process than mitosis, or simple cell division.

Despite the protections provided by human internal reproduction, there are many inefficient aspects of sexual distinction. Sexual reproduction, from an evolutionary standpoint, has disadvantages because of its inherent instability. If the goal is maximizing the spread of one's offspring in the interests of the survival of the species it is 'costly' in terms of time and energy. "Special reproductive cells and structures must be constructed" and our sexual organs are specialized for this purpose in a manner that leaves the male sexual organs vulnerable to injury, and the female vulnerable throughout her pregnancy during a long gestation period (Human reproduction and development, 2010, IUPUI).

However, sexual reproduction, it could be argued, has generated a rich human culture centering upon the process of love and courtship. In all sexually reproducing organisms, "hormones, colors, scents must be developed. Reproductive timing, finding and recognizing mates, sperm and egg available at the same time must occur," and "the developing embryo and young must be cared for, often for years or decades" (Human reproduction and development, 2010, IUPUI). "From the child's viewpoint he or she is about as dependent as any in the placental mammals. This dependency is extraordinarily prolonged and the child requires parental care for many years" (Johnson 2010). And that is assuming that reproduction takes place: males and females may reject one another and fail to reproduce or, the sperm and egg may not 'connect.' The embryo and developing child is highly vulnerable to external and internal threats throughout the mother's pregnancy.

So why has sexual reproduction continued to survive in multi-cellular organisms? The genetic diversity that results from sexual reproduction can be highly beneficial for the hardiness of a particular species. Having a greater variety of genetic combinations circulating in the gene supply allows more potentially adaptive responses to the environment to manifest. If there is a sudden change in the environment, genetically mutated offspring may survive better: for example, during the Industrial Revolution, a particular species of light-colored moth with dark mutations was able to survive because of the presence of dark-colored mutated moths within its gene pool which were able to camouflage themselves from predators due to the soot of industrialization (Miller 1999). This shows how genetic mutations can give certain species a comparative survival advantage over others within the same area. Sexual reproduction also mitigates the ability of potentially negative traits to be passed onto offspring. Of course, mutations can also be negative for the species, which yet another downside of sexual reproduction.

Most humans think of sexual intercourse as beginning with a mysterious emotion like love or lust that has nothing to do with the hard facts of science. But from a biological point-of-view, this sensation has a very practical purpose, to ensure that the male uses his exterior sex organs to impregnate the female. "From the outside, the male has two visible sex organs, the testes in a sac called the scrotum and penis. When engorged with blood during sexual excitation and intercourse, the spongy tissue stiffens and causes the penis to become erect, which is important for the penis's main function -- to place the sperm inside the female" (Freudenrich 2010, p.2). The sperm and egg meet to create a fertilized embryo. "Eggs develop inside the ovary and are released upon ovulation into a tube (the oviduct or Fallopian tube) lined with fingerlike projections. The egg travels through the Fallopian tube, where fertilization can take place, to a muscular chamber called the uterus" (Freudenrich 2010, p.3). If the egg is not fertilized, the uterus will gradually 'shed' its lining through the process of menstruation. "Reproduction is not a continuous activity and is subject to certain patterns and cycles. Oftentimes these patterns and cycles may be linked to environmental conditions which allow organisms to reproduce effectively" (Bailey, 2010, Sexual reproduction).

The male's sperm are far more numerous than the female's eggs, and far smaller. The number of sperm is designed to improve the chances that fertilization will occur. When "a single sperm penetrates the mother's egg cell, and the resulting cell is called a zygote. The zygote contains all of the genetic information (DNA) necessary to become a child. Half of the genetic information comes from the mother's egg and half from the father's sperm. The zygote spends the next few days traveling down the fallopian tube and divides to form a ball of cells" (Vorvick 2009). The zygote resembles both the mother and the father's genetic profile, but is a combination of both organisms, and thus exhibits unique features, unlike an organism that is the result of cell division or a clone.

As the zygote divides, it creates an inner group of cells with an outer shell. "This stage is called a blastocyst. The inner group of cells will become the embryo, while the outer group of cells will become the membranes that nourish and protect it. The blastocyst reaches the womb (uterus) around day 5, and implants into the uterine wall on about day 6. At this point in the mother's menstrual cycle, the lining of the uterus has grown and is ready to support a baby. The blastocyst sticks tightly to the lining, where it receives nourishment via the mother's bloodstream" (Vorvick 2009). The blastocyst is distinct from an embryo because it "represents such an early stage of embryonic development that the cells it contains have not yet differentiated, or taken on the properties of particular organs or tissues…kidneys, muscles, spinal cord, and so on. This is why the stem cells that are extracted from the blastocyst hold the promise of developing, with proper coaxing in the lab, into any kind of cell the researcher wants to study or repair" (Sandel 2009). The blastocyst is a blank slate, although it possesses a full copy of the resulting organism's potential DNA.

The next three weeks are some of the most critical weeks of the developing embryo's life. The process of differentiation occurs at a rapid pace, including the organism's blood cells, kidney cells, and cells that will eventually form the body's nervous system. The first trimester is the time when the development of the baby is most likely to be negatively impacted by maternal nutritional deficiencies, alcohol or drug use, or infections. The organs most essential for human life are taking shape. Then, during week 3 of gestation or week 5 of pregnancy, the brain, spinal cord, and heart begin to develop along with the gastrointestinal tract (Human reproduction and development, 2010, IUPUI). The embryo develops rapidly -- this is one reason why it is so important for the woman to find out as early as possible if she is pregnant, to alter her diet and lifestyle accordingly.

During weeks 4 to 5 of gestation (week 6-7 of pregnancy), arm, leg, eye and ear structures begin to form, the brain develops into five distinct areas, and heart and blood circulation develops. By week 7, all essential organs have formed and by 8 of gestation (week 10 of pregnancy) external features are evident such as facial features (Human reproduction and development, 2010, IUPUI). Around this time in the pregnancy, the parents will likely be wondering if they are having a boy or a girl. While "the sex of a baby is genetically determined at conception, during the early development of embryos, they [embryos] all look the same…All embryos have a small bud or swelling. it's known as the genital nub or protuberance. If you are having a boy, testosterone starts being produced when you are about seven weeks pregnant, prompting the bud to grow and develop into a penis and scrotum. In a girl the genital nub will become the clitoris and labia" (Hammonds 2009). Sexual differences are only observable from eleven weeks through an ultrasound, and even then mistakes are possible.

The reason for sexual differences is due to the fact that "the male gametes or sperm cells in humans and other mammals are heterogametic and contain one of two types of sex chromosomes. They are either X or Y. The female gametes or eggs however, contain only the X sex chromosome and are homogametic. The sperm cell determines the sex of an individual in this case. If a sperm cell containing an X chromosome fertilizes an egg, the resulting zygote will be XX or female. If the sperm cell contains a Y chromosome, then the resulting zygote will be XY or male" (Bailey, 2010, Sex chromosome abnormalities).

The second trimester, after week… READ MORE

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