Humans reproduce sexually to produce young that are born live. Sexual reproduction occurs when two gametes, sperm from males and eggs from females, each carrying a single set of chromosomes, unite in a process called fertilization. The fertilized egg, called a zygote, contains two sets of chromosomes, one from each of the gametes. The zygote begins the process of division and cell differentiation that results in an individual capable of living outside the womb at birth. The whole process is rather complicated and can be affected in many stages by toxic substances. Therefore, one of the primary concerns in toxicology is the influence of toxicants on the reproductive system.
The glands that produce sperm are the testes. Prior to copulation, the sperm are stored and undergo further development in the epididymis, located on the testicles. For delivery, sperm are incorporated into seminal fluid produced by seminal vesicles, the prostate gland, and the bulboure-thral gland, and ejaculated through the urethra of the penis. The process of forming sperm and other male sexual functions and characteristics are promoted by testosterone, the male sex hormone.
Whereas healthy males can produce sperm at any time, the production of fertilizable eggs by females is rather complicated. It involves the ovarian cycle of normally around 28 days. During the first half of the cycle usually one egg is produced and expelled from the ovary in a process called ovulation. If the egg is not fertilized and implanted in the uterus, the endometrium lining the uterus breaks down and is expelled through the vagina, a process called menstruation that occurs during the second half of the ovarian cycle. The egg produced by the ovary moves slowly toward the uterus through oviducts (fallopian tubes). The uterus is connected to the vagina, through which sperm enters by an opening called the cervix. Fertilization occurs in the upper region of the oviducts.
In successful pregnancies, the zygote formed by the merging of the egg and sperm begins to divide in the oviduct, forming a blastocyst that becomes implanted in the endometrium wall of the uterus.
Several hormones are involved in the ovulation process. These include gonadotropin-releasing hormone from the hypothalamus, lutenizing hormone and follicle-stimulating hormone from the anterior pituitary, and estrogen from the ovaries (see endocrine glands in Section 9.7). If the blastocyst becomes implanted in the endometrium, a layer of cells covering it begin to secrete human chorionic gonadotropin, produced only by pregnant females and used as the basis for pregnancy testing. These tissues also produce high levels of estrogen and progesterone that prevent the pituitary from generating gonadotropins, thus stopping ovulation and menstruation during pregnancy. Synthetic analogs of estrogen and progesterone in oral contraceptives act to prevent ovulation but not the uterine cycle of menstruation.
Various toxicants are toxic to sperm or adversely affect semen quality. Common parameters for detecting damage to sperm include sperm production, numbers, transit time, and mobility. The ultimate measure of sperm quality is the ability to produce pregnancy resulting in normal offspring. Toxicants may interfere with the process of sperm development. In rodents, these include heavy metals (cadmium), hormones (estrogen), herbicides (linuron), industrial chemicals (dimethyl form-amide), and pharmaceuticals, such as antihypertensive reserpine. Prominent among pharmaceuticals that are spermatotoxic are antimetabolites, such as cyclophosphamide, used in cancer chemotherapy. The cottonseed pigment gossypol adversely affects sperm, as does antifungal benomyl.
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Our internal organs, the colon, liver and intestines, help our bodies eliminate toxic and harmful matter from our bloodstreams and tissues. Often, our systems become overloaded with waste. The very air we breathe, and all of its pollutants, build up in our bodies. Today’s over processed foods and environmental pollutants can easily overwhelm our delicate systems and cause toxic matter to build up in our bodies.