In addition to the regulation of LH release from the anterior pituitary, testosterone affects sexual development, maturation, and function, and contributes to the maintenance of fertility and secondary sexual characteristics in the adult male (see Figures 8–5 and 8–6). In addition, testosterone exerts overall anabolic effects in muscle and bone.
Sexual Development and Differentiation
Sexual differentiation in humans is genetically and hormonally controlled. Genes on the Y chromosome signal primordial cells in the embryonic gonad ridge to differentiate into Sertoli cells and stimulate newly migrated germ cells to differentiate as spermatogonia, developing into a testis (see Figure 8–5). The cells of the embryonic testis secrete hormones that lead to the development of male secondary sexual characteristics. The Sertoli cells secrete müllerian inhibitory factor or substance (MIF or MIS), causing regression of the müllerian ducts. The Leydig cells secrete testosterone, causing differentiation and growth of the wolffian duct structures. DHT causes growth of the prostate and penis and fusion of the labioscrotal folds (see Figure 8–6).
The processes and factors involved in fetal development and sexual differentiation involve genetic, embryologic, histologic, and anatomic details not covered in this monograph. This section aims to summarize the key events in sexual differentiation as they pertain to endocrine regulation and function and to highlight the role of androgens in the determination of male sexual development.
Mammalian sex determination leading to the development of male or female phenotype involves 3 sequential processes:
- Determination of the genetic sex of the embryo when an X- or a Y-bearing sperm fertilizes the oocyte
- Determination of the fate of the bipotential or nondifferentiated gonad and thus of gonadal sex
- Differentiation of male or female internal and external genitalia, or determination of phenotypic sex.
The determination of genetic sex is mediated through the chromosomal set, which in the normal male is 46,XY. The subsequent sexual differentiation is determined by genetic factors. Several sex-specific genes have been found to regulate gonadal differentiation and subsequent male or female reproductive tract development. One of the first genes involved in sexual differentiation is a gene on the Y chromosome called SRY (sex-regulating region of the Y). The product of the SRY gene is a protein that stimulates the neutral gonadal tissues to differentiate into testes, thereby determining gonadal sex. SRY is necessary and sufficient to initiate the male development cascade. If the SRY gene is mutant or missing on the Y chromosome, the embryo develops into a female.
The process of differentiation of the human male gonads begins in the sixth week of gestation (see Figure 8–5). The first morphologically identifiable event is the development of precursor Sertoli cells, which aggregate to form the seminiferous cords, which are then infiltrated by primordial germ cells. By the end of the ninth week, the mesenchyme that separates the seminiferous cords gives rise to the interstitial cells, which differentiate as steroid-secreting Leydig cells. It is thought that the placenta-derived hormone, human chorionic gonadotropin (hCG), might be responsible for the initial development of Leydig cells because the onset of testosterone production precedes LH secretion. Hence, gonadotropic control of fetal testicular steroidogenesis is mediated initially by placenta-derived hCG and later by LH. The resulting increase in fetal testosterone production stimulates Leydig cell proliferation, increases the expression of steroidogenic enzymes (particularly 3β-hydroxysteroid dehydrogenase and 17α-hydroxylase), and increases the expression of the androgen receptor in the target tissues.
The postgonadal phase of sexual differentiation or external genitalia differentiation is almost exclusively hormone dependent (see Figures 8–5
). Once the gonads have differentiated into testes, the secretion of testicular hormones is sufficient to promote masculinization of the embryo. The production of testosterone and antimüllerian hormone (AMH) during a critical time in early gestation ensures male development. Initially, both the male (mesonephric or wolffian) and female (paramesonephric or müllerian) internal genital ducts are present. In females, the mesonephric ducts regress, and the paramesonephric ducts develop into the uterine tubes, uterus, and upper vagina. In males, starting at the eighth week of gestation, AMH mediates the regression of the paramesonephric or müllerian ducts. AMH is a member of the TGF-β family that is expressed in the Sertoli cell from the beginning of testicular differentiation up to puberty. AMH binds to the type 2 AMH serine/threonine kinase receptors expressed in the surrounding mesenchyme of the müllerian ducts leading to apoptosis and regression of the müllerian ducts. The mesonephric duct system (wolffian ducts) remains and forms the vas deferens, epididymis, and seminal vesicle. This process is dependent primarily on testosterone. In the female, in the absence of androgens, the wolffian ducts regress and the müllerian ducts are spared from apoptosis, developing into the uterus, fallopian tubes, and vagina. Estrogens do not appear to be essential for normal sexual differentiation of either sex, as shown by normal genital development in males with a mutant estrogen receptor gene or with aromatase deficiency. The differentiation of external male genitalia is regulated particularly through the actions of DHT. During the developmental period, as already mentioned, the expression of the type 2 testosterone reductase is higher than that for the type 1 reductase. The capacity for converting testosterone to DHT is greater for the former isoenzyme.
Following müllerian duct regression and androgen-dependent virilization of the urogenital system, the testes migrate from their site of origin next to the kidney into the scrotum. This is the final critical event in male sexual differentiation. The 2-step process of transabdominal migration followed by descent into the extraabdominal scrotal sac finalizes sexual differentiation in the male and is completed during the late gestational period. The descent of the testis results from the regression of the cranial suspensory ligament, which connects them to the abdominal wall through the gubernaculum. Regression of the cranial suspensory ligament, transabdominal migration, and the final descent of the testis into the scrotum are mediated by testosterone and insulin-like growth factor 3. This hormone, also known as Leydig insulin-like hormone or relaxin-like factor, is a member of the insulin and insulin-like growth factor peptide hormone family. In humans, failure of complete testicular descent into the scrotum (cryptorchidism) is one of the most common congenital abnormalities, involving approximately 3% of male births. Cryptorchidism is important because spermatogenesis requires lower temperatures (as in the scrotum) than those found intraabdominally. If untreated, cryptorchidism can lead to infertility, and it has been associated with an increased risk of testicular tumors.
Sexual Maturation and Function
Puberty is the physiologic transition between childhood and adulthood and involves the development of secondary sexual characteristics and the pubertal growth spurt. The process takes place over a period of approximately 4 years. Puberty is triggered by increased pulsatile secretion of GnRH by the hypothalamus, leading to increases in serum gonadotropins and, thus, to increases in gonadal secretion of sex steroids. The hypothalamic-pituitary-gonadal system is active during the neonatal period but enters a dormant state in the juvenile, prepubertal period. During the initial phase of puberty, plasma levels of LH increase primarily during sleep. These sleep-associated surges are later present throughout the day and mediate or result in an increase in circulating testosterone levels.
Puberty is preceded by adrenarche, a period characterized by increased adrenal production of DHEA and androstenedione at around 6–8 years of age that is not associated with increased production of adrenocorticotropic hormone or cortisol. The peak concentrations of DHEA and androstenedione are reached during late puberty and early adulthood. During this stage, there is some conversion of adrenal-derived androgens to testosterone, resulting in a small increase in circulating testosterone levels. The signal that triggers upregulation of the synthesis of DHEA and androstenedione is not known.
The increase in pulsatile release of GnRH is essential for the onset of puberty. However, the mechanism controlling the pubertal increase in GnRH release is still unclear. Leptin, a hormone secreted from adipose tissue (see Chapter 10), has been shown to have a permissive role in the timing of the activation of the GnRH pulse generator. The increase in the amplitude of GnRH pulses triggers a cascade of events including increases in the amplitude of FSH and LH pulses, followed by marked increases in gonadal sex hormone production. Accompanying the increase in gonadal steroid production during puberty is an increase in the amplitude of growth hormone secretory bursts. Together, growth hormone and gonadal steroids are responsible for producing normal pubertal growth. During the adolescent growth spurt, growth velocity increases from the prepubertal rate of 4–6 cm per year to as much as 10–15 cm per year. The physiologic changes associated with puberty are summarized as follows:
- Leydig cell maturation and initiation of spermatogenesis
- Testis enlargement; reddening and wrinkling of scrotal skin
- Pubic hair growth from the base of the penis
- Penis enlargement
- Prostate, seminal vesicle, and epididymis growth
- Facial (mustache and beard) and extremities hair growth, regression of scalp line
- Larynx enlargement, thickening of the vocal cords and deepening of the voice
- Enhanced linear growth
- Increased muscle mass and hematocrit
- Increased libido and sexual potency
Sexual maturity is achieved at approximately age 16–18 years. During this stage, sperm production is optimal, plasma gonadotropins are normal, and most sexual anatomic changes have been completed. Beginning at age 40, there is a gradual decline in circulating testosterone levels, followed at age 50 by a reduction in sperm production. Testosterone levels in healthy, aging men decline on the order of 100 ng/dL per decade, accompanied by increases in SHBG resulting in an overall decrease in free and bioavailable testosterone levels. In addition, aging is associated with decreased testosterone to estradiol ratio, decreased LH pulse frequency, loss of the diurnal rhythm of testosterone secretion, and diminished accumulation of 5α-reduced steroids in reproductive tissues. These hormonal changes are well established by the age of 50 years. This period of androgen deficiency is called andropause and is characterized by diminished sexual desire and erectile capacity; fatigue and depression; decreased intellectual activity, lean body mass, body hair, and bone mineral density; and increased visceral fat and obesity. These age-related physiologic changes are caused by decreased testosterone, DHEA androstenedione, and growth hormone production.
Fertility and Secondary Sexual Characteristics
Spermatogenesis is the process of continuous germ cell differentiation to produce spermatozoa (Figure 8–7). Spermatogenesis is initiated at the time of puberty, is compartmentalized within the blood-testis barrier, and is mainly under FSH regulation. Spermatogenesis involves 4 basic processes:
Schematic representation of key events in spermatogenesis. The process of spermatogenesis involves proliferation (mitosis) of spermatogonia, producing primary spermatocytes (diploid cells; 46 chromosomes). Spermatocytes undergo 2 meiotic divisions to yield spermatids, or haploid cells (23 chromosomes). Spermatids undergo a process of maturation (spermiogenesis) and development into spermatozoa. During this last phase, spermatozoa acquire the key elements for their function (see Table 8–3). This continuous process takes approximately 70 days. At any given time, cells from all steps of spermatogenesis can be identified in the testes.
Proliferation of Spermatogonia (Stem Cells) Giving Rise to Spermatocytes (Diploid Cells)
Spermatogonia line the seminiferous tubule near the basement membrane. They originate at puberty by proliferation of the gonocytes and are derived from primordial germ cells. One or 2 divisions of spermatogonia occur to maintain their population in a stem cell pool (see Figure 8–7). Of the cells resulting from these mitotic divisions, some spermatogonia stay in the “resting” pool, whereas the remaining spermatogonia proliferate several times and undergo 1–5 stages of division and differentiation. After the last division, the resulting cells are termed primary spermatocytes. The “resting” or stem cell spermatogonia remain dormant for a time and then join a new proliferation cycle of spermatogonia. These cycles of spermatogonial divisions occur before the previous generation of cells has completed spermatogenesis, so that multiple stages of the process are occurring simultaneously in the seminiferous tubules. This overlap ensures a residual population of spermatogonia that maintain the ability of the testis to continuously produce sperm.
Meiosis of Spermatocytes to Yield Spermatids (Haploid Cells; 23 Chromosomes)
The primary spermatocytes undergo 2 divisions; the first meiotic division produces 2 secondary spermatocytes. Division of the secondary spermatocytes completes meiosis and produces the spermatids.
Spermiogenesis, or Maturation and Development of Spermatids into Spermatozoa (Sperm)
This phase is characterized by nuclear and cytoplasmic changes that provide spermatozoa with key elements for their function. The main events during this phase involve spermatid condensation of nuclear material, formation of the acrosome, repositioning of the spermatid to allow formation and elongation of tail structures, mitochondrial spiral formation, and removal of extraneous cytoplasm, resulting in spermatozoa. Each of the features acquired during this step plays an important role in sperm function, as summarized in Table 8–3.
Table 8–3. Key Events in Spermiogenesis and Their Functional Importance in Sperm Function ||Download (.pdf)
Table 8–3. Key Events in Spermiogenesis and Their Functional Importance in Sperm Function
|Key event||Functional importance|
|Nuclear chromatin condensation||Haploid chromatin carries either X or Y chromosome|
|Acrosome development||The acrosome is a large secretory vesicle that overlies the nucleus in the apical region of the sperm head and contains enzymes needed for mucus penetration and fertilization|
|Repositioning of spermatids; development and growth of flagellum||Microtubular structure provides motility, allowing sperm movement (3 mm/min) through the genital tract|
|Formation of mitochondrial sheath around flagellum||Provides energy (fructose-derived ATP) for flagellar movement|
This is the final process of release of mature sperm from the Sertoli cells into the tubule lumen.
Throughout spermatogenesis, the germ cells move from the basal to the adluminal region of the seminiferous tubule into the compartment protected by the blood-testis barrier. The mitotic phase occurs in the basal compartment, whereas the meiotic and postmeiotic phases occur in the luminal compartment. The overall results of spermatogenesis are the following: cell proliferation and maintenance of a reserve germ cell population, reduction in chromosome number and genetic variation through meiosis, and production of spermatozoa.
Regulation of Spermatogenesis
Spermatogenesis is dependent on gonadotropin stimulation and testosterone production. FSH stimulates proliferation and secretory activity of the Sertoli cells, whereas LH stimulates the production of testosterone. Testosterone in turn stimulates spermatogenesis through receptor-mediated events in the Sertoli cells. The LH-induced rise in intratesticular testosterone plays an essential role in the initiation and maintenance of spermatogenesis by the Sertoli cells. Testosterone produced by the Leydig cells are transported to the developing germ cells bound to ABP produced by the Sertoli cell in response to FSH stimulation and released into the adluminal compartment. The synthesis of ABP requires that the Sertoli cell be under androgen influence, underscoring the importance of testosterone in Sertoli cell function and the reliance on paracrine mechanisms of hormone action.
Anabolic and Metabolic Effects of Androgens
In bone, the main physiologic effect of testosterone is to reduce bone resorption by increasing osteoblast lifespan and proliferation. Testosterone enhances bone formation, increases periosteal apposition of bone, increases protein synthesis, and decreases protein breakdown, having an overall anabolic effect in bone and skeletal muscle. Much of testosterone’s action on bone results from its aromatization to 17β-estradiol and the estrogen receptor. Testosterone-derived estrogen is a critical sex hormone in the pubertal growth spurt, skeletal maturation, accrual of peak bone mass, and maintenance of bone mass in the adult. It stimulates chondrogenesis in the epiphysial growth plate, increasing pubertal linear growth. At puberty, estrogen promotes skeletal maturation and the gradual, progressive closure of the epiphysial growth plate and the termination of chondrogenesis. In the adult, estrogen is important in maintaining the constancy of bone mass through its effects on remodeling and bone turnover.
Testosterone inhibits lipid uptake and lipoprotein lipase activity in adipocytes, stimulates lipolysis by increasing the number of lipolytic β-adrenergic receptors, and inhibits differentiation of adipocyte precursor cells. Androgens stimulate resting metabolic rate and lipid oxidation and enhances glucose disposal by increasing the expression of glucose transporters on the plasma membrane of adipocytes.