Lecture 3: Spermatogenesis
What is spermatogenesis?

Overview:

1. Spermatogenesis is the process of producing sperm with half the number of chromosomes (hapliod) as somatic cells.

a. The germ cells progress first from the diploid to haploid state and then change shape to become spermatozoa.

2. The process of spermatogenesis then allows the recombination of male and female haploid gametes at fertilization.

3. This provides genetic contributions from both parents without increasing the number of chromosomes each generation.

Where does spermatogenesis occur?

1. Spermatogenesis occurs in medullary sex cords known as seminiferous tubules.

2. Seminiferous tubules are part of the male gonad or testes.

Cells involved in spermatogenesis.

1. Sertoli cells—“nurse cells”

a. Nurse cells provide:

i. Support for germ cells

ii. Environment for germ cells to develop and mature

iii. Substances initiating meiosis or the reduction from diploid to haploid cells

iv. Hormonal signals effecting pituitary gland control of spermatogenesis

b. Sertoli cells divide the seminiferous tubules into two environments for the development of spermatozoa

i. Basal compartment is the space in which spermatogonia develop to primary spermatocytes

ii. Adluminal compartment is

c. Sertoli cells produce hormones

i. Estrogen

ii. Inhibin—suppresses pituitary FSH

2. Leydig cells

a. Produce testosterone

b. Located adjacent to seminiferous tubules.

Spermatogenesis in the Sexually Mature Male

1. Function of the testes is to produce the male gametes or spermatozoa

a. This process is termed, spermatogenesis

b. The site of spermatozoa production is the seminiferous tubules

2. The spermatozoa originate from precursor cells that are called spermatogonia

a. These cells line the basement membrane of the seminiferous tubule

3. Spermatogenesis can be divided into three parts:

a. Spermatocytogenesis—proliferative phase

b. Meiosis—production of the haploid gamete

c. Spermiogenesis

i. Spermatids mature into spermatozoa (sperm)

4. The adult male mammal that is a continuous breeder.

a. Males continue to produce spermatozoa throughout life.

Spermatocytogenesis and Meiosis

1. Spermatogonia divide

a. Located near outer surface of seminiferous tubule

b. Originate at puberty

c. One or two divisions of spermatogonia occur to maintain their population in a stem cell pool (type A spermatogonia)

d. These divisions are mitotic

e. Spermatogonia proliferate several times and undergo 1 to 5 stages of division and differentiation

f. After the last division, the resulting cells are termed primary spermatocytes

2. The primary spermatocytes then undergo the first of the two divisions that constitute meiosis
a. The first meiotic division produces two secondary spermatocytes

b. Division of the secondary spermatocytes completes meiosis and produces the spermatids

3. Stem cell spermatogonia remain dormant for a time and then join a new proliferation of spermatogonia

a. This new wave of spermatogonial divisions does not wait for the previous generation of cells to complete spermatogenesis

i. The purpose of this phenomenon is to ensure a residual population of spermatogonia

b. The time required for one spermatogonium to divide and form spermatozoa requires about 4.5 to 5 times that time span between divisions of the stem cell spermatogonia

Spermiogenesis: morphological conversion of round spermatid into spermatozoa without a division

1. This part of spermatogenesis is defined as the nuclear and cytoplasmic changes in the spermatid that results in the spermatozoa

2. Events associated with spermiogenesis:

a. Condensation of nuclear material

b. Removal of extraneous cytoplasm

c. Formation of the acrosome

d. Formation of tail structures

3. Spermiogenesis ends in the testis with release of the spermatozoa from the Sertoli cell

a. Throughout spermiogenesis, spermatozoa are embedded Sertoli cells

b. The process by which spermatozoa are shed into the lumen of the seminiferous tubule for transport out of the testis is spermiation

What is the overall result of spermatogenesis?

1. The overall result of spermatogenesis:

a. Cell proliferation

i. More cells are produced than originally present

ii. Each spermatogonia may produce up to 256 spermatozoa per cycle (2⁵ x 4)

b. Maintenance of a reserve germ cell population

i. Production of new spermatogonia is faster than maturation of spermatozoa

c. Haploid gametes are produced

d. Genetic variability is introduced

i. Independent assortment during meiosis

ii. Crossing-over during Prophase I of meiosis

e. Spermatids mature into spermatozoa

Hormonal Control of Spermatogenesis

1. Hormones that affect spermatogenesis

a. Testosterone

i. Produced by Leydig cells

ii. Androgen

iii. Promotes Sertoli cell function

b. Estradiol

i. Produced by Sertoli cells

c. Inhibin

i. Produced by Sertoli cells

ii. FSH levels in males is decreased by inhibin

d. FSH—follicle stimulating hormone

i. Produced by anterior pituitary

ii. FSH regulates the mitotic divisions and efficiency of type A spermatogonia development

iii. FSH controls entry of stem cell type A spermatogonia into proliferating pool

iv. The yield of spermatozoa is increased by FSH by preventing the degeneration of differentiating A type spermatogonia

v. FSH is necessary during development: required to establish Sertoli cell function

e. LH—lutenizing hormone

i. Produced by anterior pituitary

ii. Increases testesterone production by Leydig cells

f. GnRH—gonadotropic releasing hormone

i. Hypothalamic hormone that controls release of anterior pituitary hormones (LH and FSH)

2. Once the Sertoli function is developed, testosterone alone will maintain spermatogenesis

a. The yield of spermatozoa is increased if FSH is present

Spermatogonia are labile

1. Degeneration of germ cells effected by:

a. Season

i. Reflects hormonal and temperature cycles

b. Disease

c. Trauma or heat

i. Germ cells are temperature sensitive

ii. Testes are external structures to maintain a lower temperature

d. Hormonal

i. Effects of FSF and inhibin

Capacity for Sperm Production

1. Species dependent

2. Size of testis

3. Sperm produced/gm of tissue

a. Involves length and efficiency of spermatogenesis

4. Daily sperm production

a. Involves both size of testis and sperm produced/gm of tissue

b. Boar best followed by ram, human is poor