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128 8 T he human reproductive system consists of internal and external organs that help identify one’s phenotype or degree of maleness or femaleness. The reproductive system is a complex organ system that begins to develop and differentiate early after conception. There are both physical and physiological differences between male and female genders. Males tend to have thick facial hair while women tend to develop breasts. But they are only subtly different or even the same at times in human development and maturation. For example, in the first several weeks postfertilization, the gonads are not yet sexually determined and can potentially develop as either male or female. There are also aspects of reproductive physiology that are much more complex. Answers to questions such as why women experience menopause midway through life while men are able to produce sperm cells to the end of normal life have evaded scientists since the phenomena were first discovered. While reproduction includes physiological concepts such as fertilization, implantation, placentation, parturition, and lactation , and the advanced student and clinician should be familiar with them, only the dynamic physiology of gametogenesis will be discussed here. Gametogenesis, Genetic, Gonadal, and Phenotypic Sex Gametes are germ or reproductive cells. The male gamete is the sperm cell and the female gamete is the egg or ovum. Gametogenesis is the process of differentiation , development, and maturation of the germ cells. The dynamic changes both in structure and function of the developing and differentiating gametes is called spermatogenesis in males and oogenesis in females. The mature end products, in males a spermatozoan and in females a graafian follicle, bear little resemblance to their primitive progenitor cells called spermatogonia in males and oogonia in females. Gametogenesis can be divided into the study of male gametes (spermatogenesis) and female gametes (oogenesis). The Reproductive System Determination and differentiation of the genders can be divided into three components: the genetic sex (genotype), the gonadal sex (whether the indifferent embryo develops testes or ovaries), and the phenotypic sex (the physiological/ morphological gender, or phenotype). When a sperm cell penetrates and fertilizes an ovum, the new cell is called a zygote. Maleness is determined by the presence of a Y chromosome from the paternal sperm cell. An X chromosome contributed by the father will yield two X chromosomes in the zygote or a genetic female organism. Thus, genetic sex is determined upon fertilization. For the first several weeks postfertilization, the developing embryo is bipotent , that is, it can become either female or male. Once the mechanisms that determine which gonadal system will develop have been initiated, the gonadal sex of the embryo is set. This happens at about five to eight weeks in the genetic male and several weeks later in the genetic female. During puberty, properly functioning gonads will assist in the development of internal reproductive systems including auxiliary organs such as the prostate gland in males and the endometrium in females. The gonads are also critical to the development of external genitalia. The combination of internal reproductive organs and external genitalia coupled with body morphology set the phenotype or physiological sex of the individual. In order for a zygote to be produced, copulation and ovulation must be coincidental. There is a relatively narrow window of opportunity lasting approximately twenty-four hours during which a viable sperm and egg must meet. Fertilization of the ovum occurs in the fallopian tube or oviduct where the sperm and egg meet during this window of opportunity. At about this time, commingling of nuclear chromosomes from the sperm and the egg define the end of fertilization and the beginning of embryogenesis (development and differentiation of the embryo). The fertilization process is complex and is divided into at least eight stages all of which must appear on time and in sequence for a viable zygote to be produced. None of the several million sperm that are released in the male ejaculate are capable of fertilizing an egg as ejaculated. They must undergo further preparation inside the fallopian tube and, as they come in contact with the ovum, a process called capacitation occurs. Under normal conditions, only one of these cells will actually penetrate the egg even though many will reach it. This is indeed a case where the race and prize go to the strong. It is survival and success for the fittest. After the sperm cell penetrates and once the male and female pronuclei (nuclear chromosomes) fuse in stage eight of the fertilization process, a new cell called the zygote...

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