Reproductive System Help
Gametes, or sex cells, are the functional reproductive cells. They are haploid cells, each containing a half-complement of genetic material (23 single chromosomes). Fertilization of an ovum by a spermatozoon produces a normal diploid cell, the zygote, with 23 paired chromosomes. One out of the 23 pairs of human chromosomes determines sex. Sex chromosomes are of two types, X and Y.
Spermatogenesis is the process by which sperm cells are produced in the testes of a male. Oogenesis is the process by which ova are produced in the ovaries of a female. Both processes involve a special kind of cell division called meiosis. In meiosis (see Figure 3-1), each chromosome duplicates itself as in mitosis. However, in meiosis, the homologous chromosomes are attached to each other and come to lie alongside one another in pairs, producing a tetrad of four chromatids. Two maturation divisions are required to effect the separation of the tetrad into four daughter cells, each with one-half the original number of chromosomes. The nuclear aspects of meiosis are similar in males and females but there are differences in the cytoplasmic aspects such that:
Primary and Secondary Sex Organs
The primary sex organs, or gonads, are the testes in the male and the ovaries in the female. The gonads function as mixed glands, producing both hormones and gametes. The secondary, or accessory, sex organs are those structures that mature at puberty under the influence of sex hormones and that are essential in caring for and transporting gametes. Secondary sex characteristics are features that are considered sexual attractants.
Male Reproductive System
The male sex organs are formed prenatally under the influence of testosterone secreted by the gonads (testes). During puberty, the secondary sex organs mature and become functional. Male reproductive organs (Figure 23-1) and their functions are listed below.
Testes. The testes are located within the scrotum, a pouch of skin that encloses the testes. Each testis is covered by two tissue layers, the outer tunica vaginalis, a thin sac derived from the peritoneum; and the inner tunica albuginea, a tough fibrous membrane that encapsulates the testes. Spermatozoa are produced in the seminiferous tubules of the testes and enter the rete testis for further maturation. They are then transported out of the testes through a series of efferent ductules into the epididymis for the final stages of maturation. Mature spermatozoa are stored in the epididymis and the first portion of the ductus deferens. The testes also produce the andogens, male sex hormones, in the interstitial cells. Sustentacular cells of the testes provide essential molecules to the developing sex cells.
Spermatic Ducts and Accessory Glands. Once spermatozoa are mature, they pass through a series of tubules during ejaculation. From the ductus deferens, spermatozoa enter the ejaculatory duct where secretions from the seminal vesicle are added. The ejaculatory duct empties into the prostatic urethra. The prostate secretions also empty into the urethra. Spermatozoa then pass through the membraneous urethra into the spongy urethra in the penis. The bulbourethral glands are at the base of the penis. The accessory glands contribute alkaline secretions that form semen and function to nourish and enhance motility of spermatozoa and neutralize the acidic environment of the urethra and the vagina.
Penis. The penis consists of an attached root, a free body, and an enlarged tip, the glans penis. The penis is specialized with three columns of erectile tissue to become engorged with blood for insertion into the vagina during coitus. The urethra also passes through the penis as a conduit for urine. Erection of the penis depends on a surplus of blood entering the arteries of the penis as compared to the volume exiting through venous drainage. This is stimulated by parasympathetic innervation. Ejaculation, expulsion of semen through the urethra, is a result of sympathetic innervation.