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Oocyte maturation within a follicle, followed by ovulation follicle rupture. The follicle becomes a corpus luteum after ovulation and degenerates if the egg is not fertilized. The oviducts , or fallopian tubes , extend from the uterus to the ovaries, but they are not in direct physical contact with the ovaries.
The ends of the oviducts flare out into a trumpet-like structure and have a fringe of finger-like projections called fimbriae. When an egg is released at ovulation, the fimbrae help the egg enter into the tube and passage to the uterus.
Fertilization the union of sperm and egg usually takes place within the oviducts and the developing embryo is moved toward the uterus for development. It usually takes the egg or embryo a week to travel through the oviduct. Oogenesis, the process of producing an egg cell, occurs in the the ovaries.
Egg stem cells, called oogonia , divide by mitosis to produce up to 2 million oocytes a precursor to the egg. The process of oogenesis begins while the female is still an embryo undergoing development: the oocytes start the process of meiosis and then pause during meiotic prophase I.
Because this process occurs during embryonic development, this means that a female mammal is born with every single egg she will be able produce during her lifetime already present in an immature form in her ovaries. This situation is very different from males, whose spermatogonia the sperm equivalent to oogonia do not begin producing spermatocytes the sperm equivalent to oocytes until puberty.
The oocyes remain in meiotic prophase I until the onset of puberty, when a series of events can lead to egg maturation:. Oogenesis begins when the 2n oogonium undergoes mitosis, producing a primary oocyte.
The primary oocytes arrest in prophase I before birth. After puberty, meiosis of one oocyte per menstrual cycle continues, resulting in a 1n secondary oocyte that arrests in metaphase II and a polar body. Upon ovulation and sperm entry, meiosis is completed and fertilization occurs, resulting in a polar body and a fertilized egg. Image credit: OpenStax Biology. One final point: when an oocyte undergoes meiosis, it produces only a single egg again, this is different from spermatogenesis, which produces four sperm from each spermatocyte.
The oocyte divides unequally, so that almost all of the cytoplasm goes into only one daughter cell rather than evenly distributed into both. The smaller cell is called a polar body, and normally dies. These hormones together regulate the ovarian and menstrual cycles. The ovarian cycle governs the preparation of endocrine tissues and release of eggs, while the menstrual cycle governs the preparation and maintenance of the uterine lining.
These cycles occur concurrently and are coordinated over a 22—32 day cycle, with an average length of 28 days:. The figure below visually compares the ovarian and uterine cycles as well as the hormone levels controlling these cycles. Rising and falling hormone levels result in progression of the ovarian and menstrual cycles. This video provides a great overview of the human female reproductive system, emphasizing many of the points described above:. In the male reproductive system, the scrotum houses the testicles or testes singular: testis , which produce sperm and some reproductive hormones.
Sperm become are immobile when kept at body temperature; therefore, the scrotum and penis are external to the body, as illustrated below, so that a proper temperature is maintained for motility. Infertility can occur in land mammals when the testes do not descend through the abdominal cavity during fetal development. Though sperm must be produced and stored at temperatures lower than body temperature in the testes, sperm are warmed to body temperature when deposited in the female reproductive tract.
The immediate warming of sperm causes them to experience a burst of swimming activity, but then they begin to lose motility after several hours at body temperature. Sperm are produced in the seminiferous tubules inside the testes. When the sperm have developed flagella and are nearly mature, they leave the testicles and enter the epididymis , where sperm mature.
During ejaculation, the sperm leave the epididymis and enter the vas deferens , which carries the sperm, behind the bladder, and forms the ejaculatory duct with the duct from the seminal vesicles. The bulk of the semen comes from the accessory glands associated with the male reproductive system.
These are the seminal vesicles , the prostate gland , and the bulbourethral gland , all of which are illustrated above. This table briefly summarizes the major organs, locations, and functions of mammalian male reproductive anatomy:. Spermatogenesis , illustrated below, occurs in the seminiferous tubules in the testes. Sperm stem cells called spermatogonia are present at birth but are inactive until puberty, when hormones from the anterior pituitary cause the activation of these cells and the continuous production of sperm.
First, he worked with human sperm and female chimpanzees, though he ultimately failed to create a sustainable pregnancy. Then he tried a more controversial method by introducing chimpanzee sperm to human females. Ultimately, the experiments were halted due to the death of his last chimp. A uniflagellar sperm cell that is motile is referred to as a spermatozoon, whereas a non-motile sperm cell is referred to as a spermatium. Sperm cells cannot divide and have a limited life span, but after fusion with egg cells during fertilization, a new organism begins developing, starting as a totipotent zygote.
Motile sperm cells. But what is the function of the syncytial arrangement? Cytoplasmic bridges in developing sperm cells and their precursors. The progeny of a single maturing spermatogonium remain connected to one another by cytoplasmic bridges throughout their differentiation into mature sperm. For the sake of simplicity, more Unlike oocytes, sperm undergo most of their differentiation after their nuclei have completed meiosis to become haploid.
The presence of cytoplasmic bridges between them, however, means that each developing haploid sperm shares a common cytoplasm with its neighbors. In this way, it can be supplied with all the products of a complete diploid genome.
Developing sperm that carry a Y chromosome , for example, can be supplied with essential proteins encoded by genes on the X chromosome. Thus, the diploid genome directs sperm differentiation just as it directs egg differentiation. Some of the genes that regulate spermatogenesis have been conserved in evolution from flies to humans.
The DAZ gene , for example, which encodes an RNA -binding protein and is located on the Y chromosome , is deleted in many infertile men, many of whom cannot make sperm. Two Drosophila genes that are homologous to DAZ are essential for spermatogenesis in the fly. RNA-binding proteins are especially important in spermatogenesis, because many of the genes expressed in the sperm lineage are regulated at the level of RNA translation. A sperm is usually a small, compact cell, highly specialized for the task of fertilizing an egg.
Whereas in human females the total pool of oocytes is produced before birth, in males new germ cells enter meiosis continually from the time of sexual maturation, with each diploid primary spermatocyte giving rise to four haploid mature sperm. The process of sperm differentiation occurs after meiosis is complete, requiring five weeks in humans.
Because the maturing spermatogonia and spermatocytes fail to complete cytokinesis , however, the progeny of a single spermatogonium develop as a large syncytium. Sperm differentiation is therefore directed by the products from both parental chromosomes, even though each nucleus is haploid. By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.
Turn recording back on. National Center for Biotechnology Information , U. New York: Garland Science ; Search term. Figure A human sperm. It is shown in longitudinal section. Figure Drawing of the midpiece of a mammalian sperm as seen in cross section in an electron microscope. Sperm Are Produced Continuously in Most Mammals In mammals, there are major differences in the way in which eggs are produced oogenesis and the way in which sperm are produced spermatogenesis.
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