Physiology: Male Reproduction Flashcards
The two major functions of the testis are production of the []; and production of [].
The two major functions of the testis are production of the male germ cell, sperm, or spermatogenesis; and production of male sex steroid hormones, or steroidogenesis.
The seminiferous tubules are [] and are the site of [] and [].
The seminiferous tubules are avascular and are the site of spermatogenesis and sperm transport. They are composed of a tubular wall formed by peritubular cells resting on a basement membrane that encloses both Sertoli cells and spermatogenic cells. It will become apparent that the seminiferous tubule is analogous to the ovarian follicle and Sertoli cells are similar to granulosa cells. The latter are referred to as germinal epithelium because sperm cells are produced there.
The seminiferous tubules empty their contents into the [] through which the contents move to the [] for providing some sperm maturation and sperm transport along the reproductive tract.
The seminiferous tubules empty their contents into the rete testis through which the contents move to the epididymis for providing some sperm maturation and sperm transport along the reproductive tract.
final sperm maturation and the ability to fertilize an egg occurs in the female reproductive tract[], a process known as [].
Final sperm maturation and the ability to fertilize an egg occurs in the female reproductive tract, a process known as sperm capacitation.
•It is interesting that sperm can remain viable in the female reproductive tract for about 48 hours, thus increasing the probability for fertilization
The interstitial compartment contains the cells of [], [], [], and various [] and occupies the space between seminiferous tubules. The interstitium is the site of [] (in [] cells) and is highly [].
The interstitial compartment contains the cells of Leydig, lymphatics, blood vessels, and various connective tissue elements and occupies the space between seminiferous tubules. The interstitium is the site of steroidogenesis (in Leydig cells) and is highly vascularized.
•Therefore, testicular compartmentalization affords the ability of mature sperm to be selectively transported outside the body, and the steroid hormones to have localized effects on spermatogenesis, but also to communicate with distant sites within the body via the circulation. Similarities between testicular Leydig cells and ovarian theca interna cells can be drawn.
- Approximately 80% of testicular mass consists of the seminiferous tubules.
- Examination of these tubules in cross section reveals anatomical progression that parallels the temporal sequence of spermatogenesis. Sertoli cells which line the basement membrane of the tubules provide crucial support for spermatogenesis. Also lining the basement membrane are undifferentiated spermatogonia with each clone of differentiating daughter cells migrating toward the tubule lumen as it moves progressively through the stages of spermatogenesis.
- A major function of Sertoli cells is to maintain tight junctions between themselves and thereby form the blood-testis barrier. This barrier separates the basal and adluminal compartments within the seminiferous tubules, and provides avascular compartmentalization for the tubular lumen. The barrier prevents movement of proteins, charged organic molecules, and ions from interstitial fluid into the seminiferous tubules.
- Consequently, the composition of intratubular fluid differs markedly from interstitial fluid, and this composition may be critical for later stages of spermatogenesis.
- The bloodtestis barrier also provides important immunological protection, preventing the formation of antibodies against the highly differentiated spermatozoa.
Similarities in male and female Meiosis:
•2 meiotic divisions (Meiosis I and Meiosis II) to reduce from diploid to haploid
Differences in male and female Meiosis:
•cytoplasm
- male: Equal division of cytoplasm into 2 secondary spermatocytes and 4 spermatid
- female: Unequal division of cytoplasm into mature oocyte and 2 polar bodies
•spermatogenesis vs. oogenesis
- male: Spermatogenesis occurs throughout a man’s lifetime
- female: Oogenesis is suspended in Prophase I during fetal life and a woman is born with all the oocytes she will have for the rest of her life
•cytoplasmic components
- male: Severe reduction in cytoplasm during spermiogenesis (packaging)
- female: Mature oocyte provides most cytoplasmic components for zygote (e.g., mitochondria, regulatory proteins)
- Testicular cell types can be discussed in two general categories, germ cells (sperm) and somatic cells (Sertoli and Leydig). As with oogenesis, spermatogenesis involves two meiotic divisions to reduce the diploid complement of 46 chromosomes to 23 (haploid). This halving of the diploid chromosome number occurs prior to the complete differentiation of spermatozoa.
- Spermatogenesis (formation of a spermatozoa from a spermatogonia) in humans requires 64 days; however, the time between initiation of two successive spermatogenic generations is approximately 16 days. Thus when one clone of daughter cells is a quarter of the way through gametogenesis the next clone begins to form, and four successive clones, each further along in development and closer to the tubular lumen, are visible in a single histological specimen.
- Spermatogenesis can be divided into three major stages: a) mitotic proliferation, b) meiosis, and c) packaging (also called spermiogenesis). It begins with mitosis of a spermatogonium to form two daughter cells. One of these cells then continues on to form mature sperm, while the other is retained at the basement membrane as a ready source for maintaining the germ cell line. The mitotic capabilities of spermatogonia are obvious since at birth, the testes contain 0.6 x 106 spermatogonia; whereas this number is 12 x 106 at puberty. In fact, a normal adult male produces 100 - 200 x 106 sperm per day. After undergoing a variable number of mitotic divisions, the daughter cell becomes a primary spermatocyte and enters a resting phase during which chromosomes are duplicated in preparation for meiosis. In man, the daughter cell divides once to become a secondary spermatocyte, and again to become a spermatid. These two meiotic divisions are without DNA replication; therefore, four haploid (23 chromosomes) spermatids are produced from one primary spermatocyte. In addition to the reduction in chromosome number, meiosis allows some chromosomes to exchange segments of DNA, thus mixing their genetic information.
•The rate of spermatogenesis is hormone-independent and quite consistent within a species; however, the yield is hormone-dependent and varies from one individual to another.
Production of mature spermatozoa:
•Even after meiosis, spermatids resemble normal epithelial cells. Production of mature spermatozoa requires extensive remodeling of both nucleus and cytoplasm. This packaging (spermiogenesis) involves
1) condensation of chromatin into a tight inert packet
2) formation of the acrosome by aggregation of Golgi-produced enzymatic vesicles
3) growth of the tail out of one centriole
4) extrusion of most of the cytoplasm
[] cells are located in the vascularized interstitial compartment and are the site of synthesis and secretion of the male sex steroid hormones, the []. Similar to ovarian theca interna cells, [] produced by the anterior pituitary, stimulates androgen production in the Leydig cells.
Leydig cells are located in the vascularized interstitial compartment and are the site of synthesis and secretion of the male sex steroid hormones, the androgens (testosterone and androstenedione). Similar to ovarian theca interna cells, luteinizing hormone (LH) produced by the anterior pituitary, stimulates androgen production in the Leydig cells.
- As with TSH and ACTH, LH is required for maintenance of testicular tissue. In males deprived of adequate LH levels (such as those on anabolic steroids which provide negative feedback to the pituitary), reversible testicular atrophy can occur.
- In addition to testosterone, the testes secrete androstenedione, and dehydroepiandrosterone (DHEA); but because of their relatively low secretory rates and potencies, in terms of testicular function, testosterone is the major steroid that needs to be considered. In fact, the adrenal cortex is the major source of DHEA and androstenedione in both men and women.
LH stimulates this pathway via [] and []. As with the adrenal cortex and ovarian theca interna cells, side chain cleavage conversion of [] to [] is the rate limiting step.
LH stimulates this pathway via Gs and cAMP. As with the adrenal cortex and ovarian theca interna cells, side chain cleavage conversion of cholesterol to pregnenolone is the rate limiting step.
- As already discussed, the major male sex steroid hormone produced in the testes is testosterone.
- As with other steroid hormones, testosterone is lipophilic, and; therefore, once released into the blood, circulates largely (98%) bound to serum binding proteins. Most (45%) is bound to sex hormone binding globulin (SHBG), also known as testosterone binding globulin (TeBG, which also binds 17ß-estradiol), which is produced in the liver. Additionally, 53% is bound to albumin (low affinity binding). As a result, only 2% is free, and this is the only active form of the hormone.
- Once secreted, testosterone can arrive at distant sites and be further metabolized.
Testosterone can be converted to two active metabolites, as well as an inactive metabolite, androsterone 17 keto-steroid. In sex accessory tissues, testosterone diffuses into the cell and is converted to dihydrotestosterone (DHT) by the enzyme 5α- reductase. This is a membrane-bound enzyme found primarily in the male reproductive tract and liver. At these sites, DHT is more potent than testosterone in stimulating reproductive tract growth. While this conversion is a critical step in the accessory sex glands, it may not be as important in many of the other actions of androgens. Conversion of testosterone by the enzyme aromatase to another active metabolite, 17ß-estradiol occurs in peripheral tissues. Therefore, testosterone can be viewed in some ways as a pro-hormone because, in addition to its important physiological role it can serve as a precursor for two other active steroid hormones
Our patient’s initial serum testosterone (350 ng/dL) is in the _____ range.
A. low
B. normal
C. high
Our patient’s initial serum testosterone (350 ng/dL) is in the _____ range.
A. low
B. normal low end of normal
C. high
After 1 year of injections, the concentration of testosterone in seminiferous tubule is expected to _____.
A. decrease
B. increase
C. remain the same
After 1 year of injections, the concentration of testosterone in seminiferous tubule is expected to _____.
A. decrease negative feedback –> decreased Lh and FSH –> decreased Sertoli production of ABP
B. increase
C. remain the same
