The defining characteristic of sexual reproduction is that two parents combine their [1] gametes , or sex cells, to form the fertilized egg, or [2] zygote . Thus the offspring are not genetically identical to either parent. The sex cell of the male is the [3] sperm (spermatozoon) and that of the female is the [4] egg (ovum) . The organs that produce the sex cells are called the [5] primary sex organs, or [6] gonads . Other organs that are necessary for reproduction are called [7] secondary sex organs . Features that are not essential for reproduction but attract the sexes to each other are called [8] secondary sex characteristics .

The sex of an individual is fundamentally determined by the two sex [9] chromosomes , denoted X and Y. A person who inherits X from both parents is [10] female , and one who inherits X from the mother and Y from the father is [11] male . However, differentiation of the sexes also depends on exposure to certain prenatal hormones. Embryos of both sexes have a pair of [12] mesonephric (wolffian) ducts with the potential to develop into a male reproductive tract and a pair of [13] paramesonephric (müllerian) ducts with the potential to develop into a female reproductive tract. In pregnancy, the placenta secretes a hormone called [14] testis-determining factor (TDF) . If the embryo is genetically XX, there are no receptors for this hormone on the embryonic gonad, and the 13 ducts develop into the female reproductive tract while the 12 ducts degenerate. If the embryo is XY, the gonad has receptors for this hormone and it begins to secrete another hormone, [15] testosterone . This hormone causes the 12 ducts to develop into a male reproductive tract, while müllerian-inhibiting factor causes the 13 ducts to degenerate.

At 8 weeks, the fetuses of both sexes have identical external structures capable of differentiating into either male or female genitalia. These include the [16] phallus , which becomes a penis in the male and a clitoris in the female; the [17] urogenital folds , which develop to enclose the urethra in a male and become the labia minora in a female; and the [18] labioscrotal folds , which become the scrotum in a male and the labia majora in a female. Male and female organs that develop from the same embryonic structures are said to be [19] homologous to each other. An important event late in male fetal development is the descent of the [20] testes into the [21] scrotum . This is essential because sperm production requires a lower temperature than the core body temperature. If it fails to occur, a boy is said to have [22] cryptorchidism, or undescended testes and will be sterile if the condition is not corrected.

The "crotch," or area bordered by the coccyx, pubic symphysis, and ischial tuberosities, is anatomically named the [23] perineum . The anus opens within its posterior half, the anal triangle, and the genitals occupy the anterior [24] urogenital triangle. The testes occupy a sac, the [25] scrotum , which is divided in two by a median [26] septum . Each testis has a fibrous capsule and is internally divided into compartments containing the [27] seminiferous tubules, where sperm are produced. The walls of these tubules are formed by the [28] sustentacular (Sertoli) cells, which form a protective [29] blood-testis barrier and promote the development of the germ cells. Between the 27 tubules are the [30] interstitial (Leydig) cells, the source of testosterone. Sperm are stored in the [31] epididymis on the posterior side of the testis. During ejaculation, they are transported by a muscular duct called the [32] ductus (vas) deferens , which is one member of the [33] spermatic cord on the posterior side of the scrotum. This cord and the scrotum carefully regulate the temperature of the testes. When it is cold, the [34] dartos muscle of the scrotal wall contracts and tightens up the scrotum while the [35] cremaster muscle of the cord contracts and draws the scrotum closer to the body. The [36] pampiniform plexus of blood vessels in the cord forms a [37] countercurrent heat exchanger, allowing blood to reach the testes but cooling it down about 2ÚC before it gets there.

The accessory glands of the male reproductive system include a pair of [38] seminal vesicles posterior to the urinary bladder; a single median [39] prostate glandr inferior to the bladder, with the urethra passing through it; and a pair of small [40] bulbourethral (Cowper) glands with ducts leading into the urethra in the root of the penis. The root is the internal half of the penis, and the [41] shaft is the external half except for its head, the [42] glans . Most of the penis is occupied by three [43] erectile tissues that engorge with blood, enlarging and stiffening the penis during sexual arousal. One of these, a median ventral body called the [44] corpus spongiosum , encloses the urethra. The other two, the [45] corpora cavernosa , lie side by side on the dorsal side of the penis. All three bodies have blood sinuses called [46] lacunae separated by muscular walls called [47] trabeculae .

Around age 10 to 12, a boy’s hypothalamus begins to secrete [48] gonadotropin-releasing hormone (GnRH) , which triggers the changes of puberty. This hormone stimulates the anterior pituitary to secrete [49] luteinizing hormone (LH) , which stimulates the interstitial cells of the testes to secrete testosterone, and [50] follicle-stimulating hormone (FSH , which makes the testes sensitive to the testosterone. Among other things, this brings on the beginning of sperm production, or [51] spermatogenesis ; growth of the secondary sex organs; development of the secondary sex characteristics; and awakening of the [52] libido , or sex drive. To regulate sperm production without inhibiting the other effects of testosterone, the testes also secrete [53] inhibin , a hormone with a negative feedback effect on pituitary FSH secretion. In middle age, men go through a transition called the male [54] climacteric , thought to result from declining testosterone secretion and rising FSH and LH secretion. For a variety of physiological and sociological reasons, many men in old age experience [55] impotence, or erectile disfunction , the inability to sustain a sufficient erection for intercourse.

Spermatogenesis, Spermatozoa, and Semen

In order to prevent the chromosome number from doubling every generation, sexual reproduction requires a type of cell division called [56] meiosis , which in humans cuts the chromosome number in half during the production of sperm or eggs. Cells with a full set of 46 paired chromosomes are called [57] diploid , while the sex cells, with half as many chromosomes, are called [58] haploid . In prophase I, homologous chromosomes adhere to each other to form [59] tetrads and then exchange portions of their DNA in a process called [60] crossing-over . At the stage called [61] anaphase I , the two members of a pair separate from each other and migrate to separate daughter cells. In meiosis II, each double-stranded chromosome splits apart at its [62] centromere into two strands called [63] chromatids , and one of these strands migrates to each daughter cell. Thus, when a cell with 46 double-stranded chromosomes completes meiosis I, the result is two daughter cells that each have [64; how many?] 23 chromosomes with [65; how many?] 2 strand(s) per chromosome; at the end of meiosis II, there are four daughter cells, each possessing [66] 23 chromosomes with [67] 1 strand(s) per chromosome.

In the male embryo, primordial germ cells form in the yolk sac and colonize the gonad, where they become [68] spermatogonia . These cells divide by mitosis, producing a population of type A and type B 68s. The type A cells maintain a lifetime supply of sex cells, while the type B cells differentiate into larger [69] primary spermatocytes on their way to becoming sperm. These cells pass through the [70] blood-testis barrier between sustentacular cells and then undergo meiosis I, becoming [71] secondary spermatocytes , then meiosis II, becoming [72] spermatids . There are no further cell divisions, but each 72 transforms into a spermatozoon through a process called [73] spermiogenesis , in which it grows a tail and sheds excess cytoplasm. The head of a spermatozoon contains little more than the nucleus and an apical lysosome called the [74] acrosome . The tail consists of a [75] midpiece packed with mitochondria, followed by a long flagellum divided into a principal piece and a short endpiece. The ejaculated fluid, called [76] semen , consists not only of sperm (which are a very small percentage of the total volume), but also of the sugar [77] fructose ; [78] spermine and other bases that neutralize vaginal acidity; [79] prostaglandins , which stimulate peristalsis of the female reproductive tract; and fibrinogen, which causes the 76 to [80] clot and adhere to the female reproductive tract.

When the penis is flaccid, it receives most of its blood from the [81] dorsal arteries. During the excitement phase of sexual response, however, the [82] deep arteries of the erectile tissues dilate, and blood rapidly flows into the lacunae. Swelling of the tissues with blood, called [83] vasocongestion , is accompanied by increasing muscle tension, or [84] myotonia . The [85] bulbourethral glands secrete a clear slippery fluid into the urethra during this stage. The corpora cavernosa become more rigid than the corpus spongiosum partly because they are ensheathed in a fibrous sleeve, the [86] tunica albuginea , which resists expansion, and partly because the corpus spongiosum lacks a/an 82 artery. Orgasm begins with a stage called [87] emission , in which the sympathetic nervous system stimulates peristalsis of the [88] ductus deferens , transporting sperm from the epididymis to the ampulla. Contractions of the ampulla propel the sperm into the urethra, where they mix with secretions of the seminal vesicles and prostate. Semen in the urethra triggers the [89] expulsion stage of orgasm. A spinal reflex causes the [90] bulbospongiosus muscle to give five or six strong contractions and expel semen from the penis. In the [91] resolution phase, blood flow into the penis is reduced and the penis softens; this softening is called [92] detumescence , the opposite of 83. Orgasm in males is followed by a [93] refractory period in which it is usually impossible to attain another erection.