Earth Science: Humans – Reproduction
When humans reproduce, two differentiated germ cells (egg and sperm) merge. A new person develops from the fertilized egg-a process that takes approximately nine months.
Before the germ cells merge, the genetic material-which in normal cells occurs as a set of two—is reduced to a single set of chromosomes. This reduction happens during meiosis.
During fertilization, one haploid germ cell from the mother combines with one from the father. Subsequently, the now fertilized egg cell (zygote) has a double set of genetic material: one set from the mother and one from the father.
Production of sperm
Males normally produce millions of sperm cells every day. But factors such as stress, illness, or toxins, for example nicotine, have a negative effect on this process. Male sperm cells develop in the seminiferous tubule of the testicles. They are then channeled into the epididymis for maturation and storage.
Sperm cells measure about 60 micrometers and consist of a small head with a nucleus and a tail, or flagella, which propels the sperm forward. Sperm cells are stored in a white and sticky liquid that is produced in the epididymis and take about three months to fully develop and mature. They are then ready to be released by muscle contractions leading to an ejaculation during an orgasm.
Afterward, the sperm cells actively make their way through the uterus and into the fallopian tubes of the woman. Initially, there are several million sperm cells, but this number de- creases rapidly and, finally, only one single sperm cell will be able to fertilize the female egg cell, unless nonidentical twins are conceived (see in focus).
Production of egg cells
Every four weeks, several egg cells mature within the female ovary. Usually one of them is released into the fallopian tubes during ovulation while the others degenerate. Egg cells are much larger than sperm cells. This is because they not only contain genetic material, but they also supply the nutrients necessary for an embryo to develop, if the egg is fertilized.
Before fertilization, the mobile sperm cells are drawn to the egg cell by chemical attractants at increasing concentrations the closer to the egg cell they get. After fertilization, a fertilization membrane is formed and surrounds the egg to prevent further sperm cells from entry while the female and male nuclei merge.
This concludes fertilization and begins the stage of embryonic development.
IN VITRO FERTILIZATION
It couples cannot conceive a child naturally, they may choose in vitro fertilization as an alternative. This process involves fertilizing a mature egg cell in a glass container.
After a few days, the developing embryo is implanted into the uterus of the woman.
TWINS
If two eggs are released from the ovaries and fertilized by two different sperm cells, nonidentical twins (also called fraternal, or dizygotic, twins) develop. Just like siblings born from separate pregnancies, they each carry different genetic material.
Sometimes, a fertilized egg divides into two embryos, which then develop separately. This results in mono- zygotic, or identical, twins. Each twin carries exactly the same genetic material.
Around one of every 85 births is a twin pregnancy. Higher multiple births are even rarer.