Human_Sexuality_Lecture_3_Outline

I. Overview of Gametogenesis

A. Definition

1. Process of differentiation that produces cells specialized for reproduction

a. Gametes

i. Oocytes

ii. Spermatocytes

B. Two processes

1. Spermatogenesis

a. Produces sperm

b. Spermatogonium differentiate into spermatazoa

2. Oogenesis

a. Produce ova

b. Oogonium differentiate into ovum

C. Gametogenesis takes place in gonads

1. Male

a. Testes

2. Female

b. Ovary

II. What is spermatogenesis?

A. Overview

1. Spermatogenesis is the process of producing sperm

a. Sperm are haploid

i. Half the number of chromosomes as somatic cells

2. There are multiple steps

a. Germ cells differentiate and are committed to becoming sperm

i. Primary spermatocytes

b. Spermatocytes undergo meiosis

i. Become spermatids

ii. This process is spermatocytogenesis

c. Spermatids change shape to become spermatozoa

i. This process is spermiogenesis

3. Spermatogenesis allows the recombination of male and female haploid gametes at fertilization

4. The process provides genetic contributions from both parents

i. Offspring have the same number of chromosomes as parents

III. Spermatogenesis

A. Where does it occur?

1. Occurs in the male gonad

a. Testes

b. Occurs in medullary sex cords known as seminiferous tubules

B. What types of cells are 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 produce hormones

i. Estrogen

ii. Inhibin-suppresses pituitary FSH

2. Leydig cells

a. Produce testosterone

b. Located adjacent to seminiferous tubule

C. Stages of spermatogenesis

1. Proliferation

a. Cells involved

i. Spermatogonia

ii. Originate at puberty and live until death

b. Process maintains a stem cell population that can become sperm

i. 100’s of millions of sperm are required to fertilize a single egg

c. Cell division is mitotic

2. Differentiation

a. Some spermatogonia will become committed to becoming sperm

i. These cells are referred to as primary spermatocytes

ii. Primary spermatocytes are diploid

iii. Primary spermatocytes will undergo meiosis

3. Spermatocytogenesis

a. Primary spermatocytes undergo the first of the two divisions that constitute meiosis

i. At end of the first meiotic division the cells are known as secondary spermatocytes

ii. Cells are haploid (1N) with 2N DNA

b. Secondary spermatocytes undergo the second meiotic division

i. At end of the second meiotic division the cells are known as spermatids

ii. Cells are haploid (1N) with 1N DNA

c. Spermatocytogenesis is slower than proliferation

i. Cannot exhaust supply of stem cells

4. Spermiogensis

a. Morphological conversion of round spermatid into spermatozoa

i. No cell division involved

b. Nuclear and cytoplasmic changes necessary to become spermatozoa

i. Need acquire new functions to participate in fertilization

ii. Need to be motile

iii. Need to penetrate the outer membranes of the egg

c. Events associated with spermiogenesis

i. Condensation of nuclear material

ii. Removal of extraneous cytoplasm

iii. Formation of the acrosome

iv. Formation of tail structures

5. Spermiation

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

b. Throughout spermiogenesis, spermatozoa are embedded Sertoli cells

c. Spermatozoa are shed into the lumen of the seminiferous tubule for transport out of the testis

IV. 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

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

V. How is Spermatogenesis Controlled?

1. Spermatogenesis is hormonally controlled

a. Testosterone

i. Produced by Leydig cells

ii. Androgen

iii. Promotes Sertoli cell function

iv. Controls the rate at which spermatozoa at made

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 of spermatogonia

iii. FSH controls the number of spermatogonia differentiating into proliferating primary spermatocytes

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

VI. Other Considerations

A. 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 FSH and inhibin

B. 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