Unit A Reproduction

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The Menstrual Cycle The menstrual cycle is the female reproductive cycle. It begins at the onset of puberty. The menstrual cycle lasts approximately 28 days, although it can vary in length from 20 to 40 days in different women. The length of the cycle can change throughout a woman’s reproductive life. In each cycle, usually a single egg matures and is released. The menstrual cycle keeps repeating until menopause, when it stops. Menopause most often occurs in women between 40 and 50 years of age. Several follicles develop during each reproductive cycle, but usually only a single follicle and egg reach maturity in either ovary. Occasionally, both ovaries release an egg, which can result in twins if both eggs are fertilized. As the follicle matures, nutrient-producing cells develop around the egg cell (Figure 1). The cells will divide and form a nutrient-rich cavity. Once the egg matures, it will burst through the wall of the ovary. This release from the ovary is called ovulation. The empty follicle is called the corpus luteum. The corpus luteum matures and produces the hormones estrogen and progesterone. LEARNING TIP Make connections to your prior knowledge. Ask yourself, “What do I already know about the menstrual cycle?” Consider the information you have read, observed, and/or experienced. The Path of the Egg After ovulation, the egg is propelled by finger-like projections that surround the opening to the oviduct. The oviduct (also called the fallopian tube) is a tube that transports the egg to the uterus (Figure 2). Peristalsis, which is wave-like muscular contractions, and tiny cilia move the egg through the oviduct. Figure 2 shows the two ovaries and the two oviducts. The uterus, also called the womb, is the organ that will receive the embryo if the egg is fertilized. The lining of the uterus is called the endometrium. It is rich in nutrients, blood vessels, and mucus, and provides the nourishment for the embryo. The cervix is the muscular opening between the uterus and the vagina. The vagina receives the male penis and sperm during sexual intercourse, and it is the birth canal through which a baby comes out. The opening to the vagina is directly behind the opening to the urethra. Unlike males, whose urethra transports both urine and semen, the female urethra only transports urine. uterus oviduct (fallopian tube) egg ovary endometrium uterine wall cervix vagina Figure 2 The female reproductive system NEL 4.2 The Female Reproductive System 113
Depending on whether the egg is fertilized, one of two events will happen: menstruation or pregnancy. Figure 3 This graph summarizes the events of the menstrual cycle in the ovary and the uterus. Menstruation If no sperm are present in the oviduct, then the egg continues to the uterus and is discharged out of the vagina. Since no zygote has implanted in the uterus, the endometrium stops developing and is shed. Menstruation is the shedding of the endometrium. This marks the first stage of each menstrual cycle (Figure 3), the flow phase. Because the endometrium is so rich in blood vessels, menstruation looks like blood is being released. Since no zygote has implanted, a new follicle starts to develop in the second phase, the follicular phase. Once the follicle has developed, the third phase, ovulation, occurs. In ovulation, the egg leaves the ovary and travels toward the uterus. Once the egg is released, the final stage, the luteal phase, begins, and the empty follicle develops into the corpus luteum. The corpus luteum releases the hormone progesterone, which stimulates the endometrium to develop in preparation for the fertilized egg. During pregnancy, progesterone will also prevent any other eggs from maturing. Table 1 summarizes the events in the menstrual cycle. follicle development ovulation corpus luteum thickness of the endometrium flow phase follicular phase luteal phase 0 5 14 28 days ovulation Table 1 A summary of the female menstrual cycle LEARNING TIP Tables help you identify specific information quickly. As you study Table 1, look at the headings. The headings will help you focus on what is important in the table. Day Phase Description of events Hormones produced (approximate) flow phase Endometrium is shed none 1–5 (menstruation). follicular phase Follicles in ovaries develop. Estrogen is produced 6–13 Endometrium re-forms. by follicles. ovulation Egg is released from ovary. 14 luteal phase Corpus luteum forms. Estrogen and progesterone 15–28 Endometrium thickens. are produced by the corpus luteum. 114 <strong>Unit</strong> A <strong>Reproduction</strong> NEL
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CHAPTER 2 Cell Growth and Reproduct
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2.1 The Importance of Cell Division
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Figure 2 Plant cell structures cyto
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Did You KNOW? The Human Genome The
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USING MITOCHONDRIAL DNA TO SOLVE MY
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centrioles nucleus chromosome spind
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Binary fission allows for rapid pop
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