BIO 104 - Spermatogenesis and Spermiogenesis - Hormonal Control of Testicular Function

Define the process of spermatogenesis and the process of spermiogenesis:

Spermatogenesis is the process by which the seminiferous tubules of the testes produce haploid sperm. In humans, it takes 65-70 days

Spermiogenesis is the final stage of spermatogenesis, and involves the maturation/remodeling of spermatids into sperm

What are the 3 main phases of spermatogenesis?

1) Mitotic Proliferation & Blood-Testis Barrier

2) Meiosis

3) Spermiogenesis

What is Mitotic Proliferation?

Produces large number of cells that provide a continuous supply of germ cells (spermatogonia)

• supply of spermatogonia stays replenished

• primary spermatocytes are produced

• double stranded & at rest

• Testosterone is required

What is the Blood-Testis Barrier?

It is formed by the tight junctions of neighboring Sertoli/Nurse/Sustentacular cells once the primary spermatocyte is “beyond” the BTB

• All the primary spermatocytes are linked together by cytoplasmic bridges making a large syncytium

• This continues until the sperm are released into the lumen of the seminiferous tubules

• This allows for exchange of cytoplasm between the 4 sperm

What is the process of Meiosis?

This process is a type of cell division in which the chromosome number is reduced from 46 (2n) to 23 (n

• One diploid cell gives rise to 4 haploid cells

• In males Testosterone is required

• The cytoplasmic are still present

Starting with a primary spermatocyte , list all the spermatogenic cells (in order), ending with the spermatozoa:

Primary Spermatocyte, Secondary Spermatocyte, Spermatids, Spermatozoa

Are spermatogonia diploid or haploid? Where are these cells located?

Spermatogonia are diploid and are located in the the seminiferous tubules

Which cells arise from the mitotic division of spermatogonia? What is the importance of this mitotic division? Are the primary spermatocytes diploid or haploid?

Primary spermatocytes and the importance of this mitotic division is it occurs during mitotic proliferation which produces large numbers of cells that provide a continuous supply of germ cells (spermatogenesis). They are double strands and diploid (46).

Which cells are formed following the completion of meiosis I by primary spermatocytes? Are these cells diploid or haploid?

Secondary spermatocytes and these cells are double strands and haploid (23).

Which cells are formed following the completion of meiosis II by the secondary spermatocytes? Are these cells diploid or haploid?

Spermatids and these cells are single strands and haploid (23).

What changes occur to spermatids during the process of spermiogenesis? Are the sperm (spermatozoa) capable of traveling through the female reproductive tract and fertilization of an ovum?

As a spermatid differentiates into a sperm cell capable of finding and fertilizing an egg (see Chapter 9), an acrosomal vesicle is formed by the Golgi apparatus. An elongated flagellum, organized by the centriole, grows from the “tail” end of the sperm cell. Mitochondria cluster around the flagellum. The nucleus becomes condensed and elongated, and a droplet of excess cytoplasm is extruded from the cell. During spermiogenesis, the sperm heads become embedded in tiny pockets in the Sertoli cell membrane. Later the heads are released from these cells, an event called spermiation.

Two requirements for the sperm to be motile are an elevated pH and a source of energy (Fructose). Sperm needs to undergo capacitation to be able to fertilize an ovum.

What are the main structural features of a sperm?

Head, Acrosome, Plasma membrane, Nucleus, Mid-piece, Nucleus, Centriole, Mitochondria, Tail, Flagellum, Axial filament, End piece

In summary, 1 primary spermatocyte gives rise to _____ viable gametes

4

Why is the seminiferous tubule considered a “production line”?

If you look at a given region of one seminiferous tubule, you will see that not all areas of the seminiferous epithelium are in the same stage of spermatogenesis (Figure 4.3). Earlier stages of spermatogenesis (spermatogonia) are found at the basal side of the tubule; moving toward the tubular lumen, one can observe germ cells in progressively later stages of differentiation.

List several functions of the Sertoli cells:

• Secretion of Testicular fluid

• Phagocytosis (“pacman/gobbling”)

• Tight junctions (BTB)

• Androgen binding protein (ABP)

  • binds to Testosterone to keep local concentration ∼ 100 x greater than general circulation

• Inhibin

• Produce the enzymes aromatase & 5-alpha reductase

• They have receptors for Testosterone and FSH

What is the importance of gap junctions and tight junctions in Sertoli cells?

These things are important because it is what forms the blood-testis barrier and allows communication between cells.

Describe the significance of the blood-testis barrier:

• Tight junctions between the basal portions of adjacent Sertoli cells provide for a blood–testis barrier.

• This prevents the leakage of certain molecules between Sertoli cells and allows these cells to control the chemical composition of testicular fluid within the seminiferous tubules, resulting in a unique microenvironment in the tubules.

• In addition, the blood–testis barrier protects spermatocytes and spermatids from immune attack, as these haploid cells are recognized as foreign by a man’s immune system.

• Primary spermatocytes are linked together by cytoplasmic bridges making a large syncytium (which continues until the sperm are released into the lumen of the seminiferous tubules) this allows the exchange of the cytoplasm between the 4 sperm.

Name several proteins released by the Sertoli cells:

androgen-binding protein, as well as hormones such as inhibin and anti-Müllerian hormone

Which of the proteins inhibits the release of FSH from the anterior pituitary gland?

Inhibin

What is the role of the androgen binding protein in Sertoli cells?

It binds to testosterone (DHT) to keep local concentration. It’s about 100 times greater than general circulation.

• May be to concentrate androgens near the Sertoli cells so these steroids can influence spermatogenesis more effectively.

• The androgens are kept from diffusing out of the tubules because the blood– testis barrier hinders passage of the ABP–androgen complex out of the tubule.

• Androgens are important in maintaining the blood–testis barrier.

What within the testes are the interstitial (Leydig) cells located? What is the major function of these cells?

The Leydig cells are located in the interstitial spaces between the seminiferous tubules. They produce 95% of Testosterone in Males.

List several examples of androgens:

5-alpha DHT, Testosterone, Androstenedione, DHEA

What are several sources of estradiol in males?

They are present in the testis (spermatocytes, spermatids, Sertoli cells, Leydig cells, rete testis), epididymis, prostate, and seminal vesicles.

Define the process of spermiation:

Is the release of sperm cell from its connection to a Sertoli cell.

• Sperm then enter the lumen of seminiferous tubule and flow towards ducts of the testes

• Requires the breakdown of tight junctions and gap junctions between the sertoli cells and spermatozoa (sperm)

Where are the kisspeptin releasing neurons located?

Arcuate nucleus (ARC)

Which testicular cells are targets for FSH? Which are targets for LH?

Sertoli cells are the targets for FSH and Leydig cells are the targets for LH

Is positive hormonal feedback also involved in male gametogenesis?

No, the primary hormonal feedback loop is a negative feedback cycle. The negative feedback regulation of testosterone and gonadotropins in the male helps to maintain constant production of sperm.

Do sperm have receptors for testosterone? Do Sertoli cells?

Sperm lack Testosterone receptors while Sertoli cells have receptors for Testosterone

What is the effect of LH on Leydig cells?

LH stimulates the Leydig cells to secrete Testosterone

What is the role of FSH in spermatogenesis?

Receptors for FSH are also present on Sertoli cells, and FSH appears to amplify the spermatogenesis-promoting effects of testosterone.

FSH WITH TESTOSTERONE:

• supports survival of spermatogonia and spermatocytes by suppressing apoptosis of the male germ cells

• supports meiotic progression of the germ cells and allows Sertoli cells sustain contact with spermatids as they differentiate into spermatozoa

• helps maintain the function of Testosterone promoting the release of mature sperm during spermiation

Do Sertoli cells have FSH receptors?

Sertoli cells have receptors for FSH

What is the role of inhibin?

They play a role in FSH release by suppressing FSH secretion, possibly by decreasing the sensitivity to GnRH of FSH-secreting cells in the anterior pituitary