1. Sexual reproduction does not necessarily involve copulation or physical contact between the two parents. What it does involve is that each offspring has two parents and a combination of genes from both.

2. In sexual reproduction, the offspring are not genetically identical to their parents and usually not even to each other.

3. Genetic diversity provides the foundation for the survival and evolution of a species.

1. To reproduce sexually means that the parents must produce gametes that can meet and combine their genes in a zygote. The gametes must provide motility and nutrition to make the combination successful.

2. The smaller, motile gamete is the spermatozoon while that large nutrient-laden one is the egg.

3. By definition, an individual that produces eggs is female and one that produces sperm is male.

4. Genetically, and individual with a Y sex chromosome is classified as male and any individual lacking a Y chromosome is classified as female.

5. In mammals, the female is also the parent that provides a sheltered internal environment for the development and prenatal nutrition of the embryo.

1. The reproductive system consists of primary and secondary sex organs. The primary sex organs (gonads) are those that produce gametes. These are the testes of the male and ovaries of the female.

2. The secondary sex organs are those that are essential to reproduction, such as ducts, glands, and a penis in males, and uterine tubes, uterus, and vagina in females.

3. Secondary sex characteristics are features that are not essential for reproduction but that attract the sexes to each other.

1. Most of our cells have 23 pairs of chromosomes, 23 pairs of which are autosomes, and 1 pair of sex chromosomes (an X or Y).

2. The sex of the child is determined by the male who donates either a X or a Y to the offspring. The female can only donate an X chromosome.

1. Sex determination does not end with fertilization, but requires an interaction between genetics and the hormones produced by mother and fetus.

2. At about 5 to 6 weeks, gonadal ridges appear near the kidneys.

3. The Y chromosome has a gene for testis-determining factor (TDF) which codes for hormone receptors in the embryonic gonad.

4. The placenta secretes human chorionic gonadotropin (HCG). If the fetus is XX, its gonads have no HCG receptors, and it cannot respond to the hormone.

5. If the fetus is XY, however, the gonad has HCG receptors and responds to the hormone. By 8-10 weeks, it begins to secrete mullerian-inhibiting factor (MIF), and androgens. Androgens stimulate the fetus to develop male genitals.

6. The female results from the absence of androgens rather than the presence of estrogens.

1. Most people regard the external genitals as the most definitive characteristics of a male or female.

2. In the fetus, the external genitals begin from identical structures in both sexes (at 8 weeks): a phallus, urogenital folds, and labioscrotal folds.

3. By 9 weeks, the fetus begins to show sexual differentiation, and male or female genitalia are formed by the end of week 12.

4. Male and female organs that develop from the same embryonic structure are said to be homologous. Thus the penis is homologous to the clitoris and the scrotum is homologous to the labias majora.

1. In the embryo, a cord of muscle and connective tissue called the gubernaculum extends from the floor of the scrotum to the gonad. It shortens as the fetus grows and guides the testis through a passageway in the groin called the inguinal canal. This descent of the testes begins in weeks 6 to 10; by 28 weeks, the testes enter the scrotum.

2. In the scrotum, the testes are kept 2^oC cooler than in the pelvic cavity. This is essential for sperm production.

1. The testis itself has a white fibrous capsule called the tunica albuginea. Connective tissue septa divide the organ into 250-300 wedge-shaped lobules.

2. Each lobule contains one to three seminiferous tubules in which sperm are produced.

3. The seminiferous tubules lead into a network called the rete testis, embedded in the capsule on the posterior side. Sperm partially mature in the rete.

4. Each testis is supplied with blood by a testicular artery and drained by a testicular vein.

1. The testes are contained in a pendulous sac, the scrotum. The scrotum is divided into right and left compartments by an internal median septum, which protects each testis from infections of the other one. The location of the septum is externally marked by a seam called the perineal raphe.

2. A structure called the spermatic cord passes up the back of the scrotum, then anterior to the pubis, and through the inguinal ring to the inguinal canal. It contains a sperm duct called the ductus deferens and other structures that descended with the testis as it passed through the inguinal canal.

3. The scrotum has three mechanisms for regulating the temperature of the testes.

1. After leaving the testis, the sperm travel through a series of spermatic ducts to reach the urethra.

1. There are three sets of accessory glands in the male.

1. The penis serves to deposit semen in the vagina. Half of it is an internal root, and half is the externally visible shaft.

2. Distally, the penis ends in a swollen head called the glans. The proximal tip of the glans is the corona.

3. The penis consists mainly of three cylindrical bodies called erectile tissues, which fill with blood during an erection.

4. All three erectile tissues are spongy in appearance and contain numerous tiny blood sinuses called lacunae.

1. As the hypothalamus matures, it begins producing gonadotropin-releasing hormone (GnRH) at puberty. This hormone travels to the anterior pituitary and stimulates the production of follicle-stimulating hormone (FSH) and interstitial cell-stimulating hormone (ICSH), also known as luteinizing hormone (LH). These two gonadotropins, as they are called, stimulate different cells in the testis.

2. The androgens stimulate spermatogenesis (in the presence of ABP), they suppress the secretion of GnRH so FSH and LH are held in check, and they stimulate the development of secondary sex characteristics and other somatic changes of puberty.

1. Testosterone production peaks at about 7 mg/day at age 20 then declines steadily to as little as one-fifth of this level by age 80.

2. As testosterone and inhibin levels decline, so does feedback inhibition of the pituitary. Consequently, FSH and ICSH levels rise significantly in the late 40s to early 50s, producing changes called male climacteric.

3. About 20% of men in their 60s and 50% of men in their 80s experience impotence for a variety of reasons.

1. The most significant event of puberty is the onset of spermatogenesis. This is a process in which germ cells undergo two divisions called meiosis and the four daughter cells differentiate into spermatozoa.

2. In eukaryotes, there are two types of cell division: mitosis and meiosis.

3. The stages of meiosis are fundamentally the same in both sexes. Briefly, it consists of two divisions and the following phases: prophase I, metaphase I, anaphase I, telophase I, interkinesis, prophase II, metaphase II, anaphase II, and telophase II. These events are depicted in Fig. 27.14, p. 977; Transp. 475.

4. At the end of meiosis, there are four haploid cells, each containing 23 single-stranded chromosomes. Fertilization combines 23 chromosomes from the father with 23 chromosomes from the mother, reestablishing the diploid number of 46 in the zygote. The contrasts between meiosis and mitosis are shown in Fig. 27.15, p. 978.

1. The first cells destined to become sperm cells are the primordial germ cells. These differentiate into spermatogonia, which lie along the periphery of the seminiferous tubule, outside the blood-testis barrier (BTB).

2. Spermatogonia multiply by mitosis, producing two types of daughter cells, called type A and type B spermatogonia. Type A cells remain outside the BTB and continue to multiply from puberty until death.

3. Type B spermatogonia migrate closer to the tubule lumen and differentiate into slightly larger cells called primary spermatocytes. Sustentacular cells transfer these cells through the BTB toward the inside of the tubule.

4. After the BTB closes behind it, the primary spermatocyte undergoes meiosis I, giving rise to haploid secondary spermatocytes. Each of these undergoes meiosis II, dividing into two spermatids. Each stage is a little closer to the lumen of the tubule.

5. The rest of spermatogenesis is called spermiogenesis and involves no further cell division but maturation of the sperm cells. Eventually sperm cells are washed out of the seminiferous tubule by fluid from the sustentacular cells.

1. The spermatozoon has a pear-shaped head and a long tail.

1. The fluid expelled during orgasm is called semen or seminal fluid. Its major constituents are as follows.

1. Masters and Johnson divided intercourse into four recognizable phases, which they called excitement, plateau, orgasm, and resolution.

2. The penis is richly supplied with blood from an internal pudendal artery that divides into a dorsal artery and a deep artery within the root of the penis. During arousal, blood enters lacunae from the deep artery. When the penis is flaccid, most of its blood supply comes from the dorsal arteries.

3. The penis is richly innervated by sensory and motor nerve fibers. Impulses from motor fibers can bring about an erection from thoughts and input to special senses.

1. The excitement phase is characterized by vasocongestion, myotonia, and increases in heart rate, blood pressure, and pulmonary ventilation. The bulbourethral glands secrete their fluid during this phase.

2. The most obvious manifestation of male sexual arousal is erection of the penis.

3. In the plateau phase, variables such as respiratory rate, heart rate, and blood pressure are sustained at a high level, or rise slightly, for a few seconds to a few minutes before orgasm.

1. The orgasm, or climax, is a short but intense reaction usually marked by the discharge of semen (ejaculation). This occurs in two stages.

2. Some sperm may seep from the penis prior to ejaculation.

1. Immediately following orgasm comes the resolution phase. Discharge of the sympathetic nervous system reduces blood flow to the penis, and gradually the penis undergoes detumescence.

2. In men, resolution is followed by a refractory period, lasting anywhere from 10 minutes to a few hours, in which it is usually impossible to attain another erection and orgasm.