Learning Objectives : At the end of this lecture, you should be able to 1. However, within a matter of minutes, one or two spermatozoa begin to make an indentation in the line separating the cervical mucus from the ejaculate. The miracle of this separation of chromosomes is the most complicated event in the whole reproductive process; it determines the genetic makeup of the child and results in the genetic variability of the offspring. Diploid Saccharomyces cerevisiae, for example, undergo meiosis and produce spores when faced with unfavorable environmental conditions. During meiosis I, homologous chromosomes first pair with one another and then segregate to different daughter cells. Meiosis and Formation of Eggs and Sperm Meiosis and Formation of Eggs and Sperm February 16, 2000 Readings: Starr text: Ch 9 cover page, 9.
At the moment when fertilization is possible, near the time of ovulation, the mucus can be stretched out into a very thin strand; at other times in the cycle it is more sticky, and if stretched it will break. How Do the Egg and the Sperm Reach the Fallopian Tube? Bleeding usually ceases by day four or five and in most cases resumes after day twenty-eight of the cycle. Chromosome segregation in meiosis I. The resultant inactivation of leads to completion of the second meiotic division, with asymmetric as in meiosis I giving rise to a second small polar body. Of some 200 million sperm deposited into the vagina near the cervix in a typical ejaculation, only 100,000 ever get into the womb. Ejaculation into the Vagina Most of the spermatozoa in the ejaculate are contained in the very first portion of fluid that squirts out of the penis and enters the vagina.
Formation of the Follicle Each month, from the time of sexual maturity on, about one thousand undeveloped eggs, or oocytes, leave their prolonged resting phase and start to mature. In a menstrual cycle the buildup of the lining of the womb is so lush, and the drop in hormone level supporting that lining so abrupt, that at the end of the cycle the lining actually sheds and the woman bleeds for four to five days in what is commonly known as her period. Formation of human gametes eggs and sperm: V. Telophase I not shown : 2 new cells form. Completion of then gives rise to two embryonic cells, each containing a new genome.
Cervical mucus resembles a thick, clear liquid that can be poured from one container into another. To accomplish this, the cervix and the cervical mucus act as a reservoir from which spermatozoa are slowly released into the uterus and up to the fallopian tubes over a period of several days. Occasionally women actually feel several hours of discomfort in their lower abdomen during ovulation; this discomfort is called Mittelschmerz. Penetration of the Egg by a Sperm For a sperm to enter and fertilize the egg, it must dig its way through several layers of protective shields surrounding the egg. Therefore, multiple births are common in women who require medical treatment to help them ovulate. Esta aceleración se explica principalmente por una recuperación en las capturas de anchoveta en América del Sur tras el fin de El Niño, mientras que la producción acuícola mundial continúa creciendo a un ritmo de 4-5 por ciento anual.
As soon as the ruptured follicle begins to produce progesterone, the cervical mucus which had become maximally receptive to sperm invasion just prior to ovulation is suddenly caused to become sticky and totally impermeable to the invasion of sperm. Unicellular eukaryotes, such as , can undergo as well as reproducing by. It used to be thought that unless sperm resided for a certain period of time outside the male reproductive tract and in the specific fluids of the female reproductive tract, they would not be capable of fertilization, even though in every other respect they looked normal. In this sense, the canal of the cervix can be looked upon as a receptacle through which platoons of spermatozoa migrate and in which some are detained in order to ensure a continuous supply of smaller numbers, over a prolonged period of time, to the deeper recesses of the female where fertilization takes place. . If the egg is not penetrated by sperm soon after ovulation, it becomes overripe and dies. But in the process of sperm production, the chromosomes are reduced to half the normal number by a process called meiosis.
These outer walls safeguarding the inner confines of the egg represent an impressive barrier to sperm penetration, and a sperm cannot dig its way through these membranes without the aid of chemicals released from its warhead, the acrosome. Sperm then continue to invade the cervical mucus at that point much like a single-file line of army ants. Generally, the most fertile eggs are released earlier in life, and of her limited supply of four hundred thousand, about one thousand eggs will die inexorably every month. This injection of egg cytoplasm caused the embryonic cell to arrest at metaphase, indicating that metaphase arrest was induced by a cytoplasmic factor present in the egg. Spermatogonia 2n are the cells in the testes that will undergo meiosis.
If it is positive, i. The otherwise impenetrable mesh opens up and allows a successful invasion of sperm. Penetration of the egg membrane by the sperm head also sets in motion the second meiotic division of the egg with the release of the second polar body. The uterus is a hard, muscular, pear-shaped structure with a narrow, triangular cavity inside, so small that it would barely hold a teaspoonful of fluid. In some animals, oocytes remain arrested at the stage until they are fertilized, only then proceeding to complete meiosis. Only humans and the apes have menstrual cycles. Anaphase I is initiated by disruption of the at which homologous chromosomes are joined.
This complex keeps the homologous chromosomes closely associated and aligned with one another through the stage, which can persist for several days. Occasionally two follicles successfully reach maturity and are both ovulated. The disappears at the stage and the homologous chromosomes separate along their length. The sperm must be able to invade the cervix via the cervical mucus by virtue of their own swimming ability. The fallopian tubes are four inches long and hang freely in the abdomen.
The cilia work this magic by digging into the sticky gel, called the cumulus oophorus, that surrounds the egg, and they transport this whole sticky, gooey mass. The journey of the egg, or ovum, through the fallopian tube and finally into the uterus after fertilization is extraordinarily hazardous. However, has demonstrated that capacitation of sperm once considered one of the greatest problems in successfully achieving test-tube babies can occur in relatively simple, nonspecific fluids available in any laboratory. Because it is impossible for the follicle to make progesterone before ovulation, the production of progesterone implies that ovulation has occurred. In all other animals, however, this shedding does not occur, and the thick lining of the womb merely returns to the thinned-out condition, marking the beginning of the next cycle.