In this section, we will examine the physiological processes that occur within the male and female reproductive systems. These processes are male sperm production (spermatogenesis), and the female ovarian and uterine cycles.
Spermatogenesis
Males create 1,500 sperm per second, and the process continues throughout the adult life span. This process, known as spermatogenesis occurs in the seminiferous tubules within the testes. The Sertoli cells within the seminiferous tubules support sperm development. Once the sperm are fully mature, they enter the seminiferous tubules and pass their way to the epididymis, where they travel through the epididymis and mature into fully functional sperm. This process takes about 20 days, and then the sperm are stored in the tail of the epididymis until ejaculation.
Ruiz Villarreal, M. (2006). Complete diagram of a human spermatozoa [Ilustration]. Wikimedia Commons. https://en.wikipedia.org/wiki/Sperm#/media/File:Complete_diagram_of_a_human_spermatozoa_en.svg
Sperm Anatomy
The head of the sperm contains the nucleus which houses the DNA or chromosomes. Capping the head of the sperm is a structure called the acrosome that contains the enzymes to help penetrate the egg during fertilization. The midpiece of the sperm contains the mitochondria that supplies the sperm with ATP to be able to propel the tail to move through the female reproductive tract. Finally, the tail of the sperm is the flagellum that beats like a whip to propel and move the sperm forward and enables it to move through the reproductive tract of the female. We will examine the physiology of fertilization in our next module.
Ovarian and Uterine Cycles
Female reproductive physiology is more complex, as it involves coordinating two cycles: the ovarian cycle and the uterine cycle. The ovarian cycle regulates egg development. The uterine cycle regulates endometrium development within the uterus. The cycles occur at the same time in a 21-40-day range; the most typical range is 28 days.
Females are born with approximately 1 million oocytes, of which only 300-500 will develop to the point of ovulation. The ovarian cycle occurs in the ovaries, and consists of follicle and oocyte maturation, ovulation, and development of the corpus luteum. The ovarian cycle consists of two phases, the follicular phase and the luteal phase.
The Follicular Phase
The follicular phase lasts from day 1-14 of the 28-day cycle; during this phase, a dominant follicle matures and secretes large amounts of estrogen. Ovulation of this follicle’s mature oocyte will occur on day 14. Ovulation is the process by which the egg leaves an ovary to enter into the fallopian tubes. At this transition point, remaining granulosa cells form the corpus luteum as the cycle transitions to the luteal phase. This luteal phase lasts from day 14-28 and involves the activity of the corpus luteum. The corpus luteum secretes progesterone, which influences endometrial lining development in the uterus.
This ovarian cycle is coordinated with the uterine cycle, which pertains to the events that occur within the uterus. Most of the uterine changes involve thickening of the endometrium to prepare to receive a potentially fertilized egg, and shedding of this lining through menstruation as the alternative. The uterine cycle is divided into three phases: menstrual phase, proliferative phase, and secretory phase.
The Menstrual Phase
Days 1-5 of the 28-day cycle are termed the menstrual phase, as the endometrium is discharged through the vagina. At this point, the controlling hormones of estrogen and progesterone are at their lowest. After this period, estrogen begins to rise, and the endometrium rebuilds to prepare to receive a potentially fertilized egg. At day 14, progesterone from the corpus luteum begins to rise, which leads to thickening of the endometrium. If the egg is not fertilized, the corpus luteum breakdown (as part of the ovarian cycle) leads to endometrial decomposition as the uterine cycle re-enters into the menstrual phase.
Menstrual cycle [Illustration]. (2009). Wikimedia commons. https://commons.wikimedia.org/wiki/File:MenstrualCycle2_en.svg
The Endocrine System’s Hormonal Regulation
Both of these cycles are under the endocrine system’s hormonal regulation. The hypothalamus secretes gonadotropin releasing hormone, which signals the anterior pituitary’s release of follicle stimulating hormone (FSH) and luteinizing hormone (LH). These regulate the ovaries’ estrogen production, which sends a feedback loop to the hypothalamus to maintain homeostasis.
Encyclopedia Brittanica. (2016). Female reproductive system [Illustration]. Britannica ImageQuest. https://go.openathens.net/redirector/rasmussen.edu?url=httpsquest-eb-com.eu1.proxy.openathens.netsearchfemale-reproductive-system1309_1156355Female-reproductive-system
Now that we have examined the physiology of the reproductive systems, check your understanding with the cases and activities that follow.
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