Chapter 27: The Reproductive System

The Reproductive System on The Reproductive System compares the general organization of male and female systems, detailing the anatomy, histology, cycles, and age-related changes of both. The male reproductive system centers on the testes, which are housed within the scrotum, an external pouch whose temperature is regulated by the dartos and cremaster muscles. The testes descend during fetal development, creating the spermatic cords, which consist of layers of fascia, connective tissue, blood vessels (like the testicular artery and pampiniform plexus), nerves, and the ductus deferens. Within the testicular lobules, seminiferous tubules are the sites of spermatogenesis, where stem cells called spermatogonia divide, eventually producing haploid spermatids that mature into spermatozoa via spermiogenesis. Nurse (Sertoli) cells are critical, as they maintain the blood–testis barrier and support developing sperm. Interstitial cells located between the tubules produce androgens, primarily testosterone, which stimulates sperm maturation and secondary characteristics. Spermatozoa, characterized by a head (with an acrosomal cap) and a flagellum (tail), travel through the rete testis, efferent ductules, and into the epididymis, where they complete functional maturation, known as capacitation, before moving into the ductus deferens. The accessory glands—the seminal glands, prostate gland, and bulbo-urethral glands—contribute the majority of seminal fluid, which provides nutrients (fructose from seminal glands), activates sperm, and produces buffers (alkaline mucus from bulbo-urethral glands) to counteract acidity. The penis contains erectile tissue, specifically two corpora cavernosa and one corpus spongiosum surrounding the spongy urethra, which become engorged with blood during erection. The female reproductive system includes the ovaries, uterine tubes, uterus, and vagina. The ovaries are stabilized by the broad, ovarian, and suspensory ligaments, and their cortex is where oogenesis occurs monthly as part of the ovarian cycle. Rising FSH levels initiate the development of primordial follicles into primary, secondary, and eventually a single tertiary (Graafian) follicle, which secretes estrogens, especially estradiol. A surge in Luteinizing Hormone (LH) triggers ovulation, releasing a secondary oocyte. The remaining follicular cells form the corpus luteum, which primarily secretes progesterone to prepare the uterus for pregnancy; if fertilization does not occur, it degenerates into a corpus albicans. The oocyte travels through the uterine tube, which is divided into the infundibulum (with fimbriae), the ampulla (the typical site of fertilization), the isthmus, and the uterine part. The uterus is essential for supporting a developing embryo and fetus and is composed of the perimetrium, the myometrium (thick muscle layer), and the inner endometrium, which is subdivided into a constant basilar layer and a functional layer that changes during the uterine cycle (menstrual cycle). The three phases of the uterine cycle are menses (shedding of the functional layer), the proliferative phase (repair under estrogen), and the secretory phase (enhanced secretion under progesterone from the corpus luteum). The vagina, a muscular tube extending to the vestibule, receives the penis and serves as the birth canal, and the surrounding external genitalia (vulva) includes the labia minora and majora and the clitoris. The mammary glands produce milk (lactation) after development stimulated by hormones like prolactin and human placental lactogen (HPL), with ejection stimulated by oxytocin. If pregnancy occurs, the placenta forms, secreting HPL and HCG, the latter maintaining the corpus luteum. Finally, the chapter addresses aging, including menopause in women (cessation of cycles and estrogen decline) and the male climacteric (gradual decline in testosterone).