Gametogenesis

Question 1

What are the 4 phases of gametogenesis?

Answer 1

1) Extraembryonic origin of germ cells –> migration to gonads
2) Increase in germ cell number by mitosis
3) Reduction in chromosome number by meiosis
4) Structural and functional maturation into eggs and sperm

Question 2

When do primordial germ cells begin to migrate, where from-to?

Answer 2

3rd week of foetal development; from the primary ectoderm (epiblast) to the yolk sac wall (near exit of allantois)

Question 3

What is the allantois?

Answer 3

Small sausage-shaped outpouching from the caudal wall of the yolk sac of the early embryo

Question 4

What happens if primordial germ cells inappropriately migrate?

Answer 4

They may settle at extra-gonadal sites and if they persist at ectopic sites, can give rise to teratomas (tumours).

Question 5

Where do primordial germ cells migrate from-to at week 6 of foetal development?

Answer 5

From the yolk sac wall, along the vitelline and dorsal mesentery to reach the gonadal ridge

Question 6

What happens once the primordial germ cells reach the gonads?

Answer 6

Undergo rapid mitotic division to produce two diploid cells (oogonia in women)

Question 7

When do oogonia undergo intensive mitosis in females during foetal development?

Answer 7

2nd to 5th month of foetal development

Question 8

When do large numbers of oogonia begin to undergo degeneration?

Answer 8

After 5 months of foetal development

Question 9

Provide an overview of meiosis to produce haploid oogonia

Answer 9

Meiosis I (reduction division) as it halves the chromosome number (2n to n).
Meiosis II (equatorial division) where there is independent re-assortment of maternal and paternal chromosomes (leading to variation) and sister chromatids separate

Question 10

Describe the process of meiosis

Answer 10

1) Oogonia are diploid (2n) and 2c (two chromatids per chromosome)
2) Pairing of homologous chromosomes and crossing over in prophase of meiosis I –> tetrad (complex of 4 chromatids) –> crossing over form 4 recombinant chromatids –> cell is 2n, 4c
3) Separation of chromosomes in meiosis I –> 1n, 2c
4) Chromatids are separated as a result of meiosis II –> 4 daughter cells –> haploid with one chromatid; 1n, 1c

Question 11

Describe the time-line of the meiotic stages in a developing female

Answer 11

Foetal period: oogonia enter meiosis I
Birth: oogonia enter prophase I and become primary oocytes
Puberty: Primary oocyte completes meiosis I and begins meiosis II just before ovulation (forms secondary oocyte and 1st polar body)
Conception: Fertilisation triggers completion of meiosis II –> fertilised egg and 2nd polar body

Question 12

When do oogonia enter the first meiotic divison?

Answer 12

Late foetal development, then enter diplotene stage of prophase I in early months after birth

Question 13

When do primary oocytes complete the first meiotic division?

Answer 13

Just before ovulation –> forms secondary oocyte and first polar body

Question 14

When do secondary oocytes enter the second meiotic division?

Answer 14

Just prior to ovulation before arresting at metaphase II

Question 15

When do the secondary oocytes complete the second meiotic division?

Answer 15

Only if there is fertilisation –> fertilised egg and second polar body

Question 16

Describe the structure of oogonia

Answer 16

These cells are ‘naked’; not par of any ovarian substructure

Question 17

Describe the structure of primary oocytes

Answer 17

Surrounded by granulosa cells to form a primary follicle

Question 18

Describe the structure of primary follicles

Answer 18

Have granulosa cells and a primary oocyte which help to form the zona pellucida

Question 19

Describe the structure of secondary and Graafian follicles

Answer 19

A layer of theca cells line the follicle (theca interna and externa), have an antrum which contains serum components, enzymes and proteoglycans (enzymes digest follicular wall in ovulation, proteoglycans draw in water to increase volume of antral fluid to help eject oocyte out into oviduct)

Question 20

When are hormones required for the continuation of oogenesis?

Answer 20

After the primary follicle stage, until then oogenesis is self-regulated

Question 21

What hormones are required to facilitate the change from primary to secondary follicle?

Answer 21

FSH production (anterior pituitary) which causes oestrogen production in granulosa cells surrounding the primary follicle –> causes production of the antrum

Question 22

What hormones are required to facilitate the change from secondary to Graafian follicle?

Answer 22

Production of activin (gonads and pituitary) –> stimulates granulosa cell proliferation. LH production –> testosterone production by theca cells –> testosterone enters granulosa cells –> converted to oestrogen (mainly 17β oestradiol) by aromatase enzyme –> stimulates formation of further LH receptors on granulosa cells –> allows follicle to respond to LH surge –> ovulation

Question 23

What hormone is produced by the Graafian follicle to prepare the reproductive tract for possible fertilisation?

Answer 23

Oestrogen is produced by granulosa cells of the follicle which then prepare the reproductive tract for possible fertilisation

Question 24

What hormones are required to facilitate ovulation from the Graafian follicle?

Answer 24

LH surge (anterior pituitary) caused by high oestrogen levels –> expulsion of secondary oocyte

Question 25

When does the formation of sperm begin?

Answer 25

At puberty, not before

Question 26

Describe spermatogonia

Answer 26

Starting cells of sperm production, kept at the base of the seminiferous tubule by interlocking Sertoli cells, connected by intercellular cytoplasmic bridges

Question 27

What marks the initiation of spermatogenesis?

Answer 27

Spermatogonia (stem cells) forming type A spermatogonia at regular intervals

Question 28

How are type B spermatogonia produced?

Answer 28

Type A spermatogonia undergo a limited number of divisions, and at the last division Type B spermatogonia are formed

Question 29

Describe primary spermatocytes and their migration

Answer 29

Primary spermatoocytes are formed when type B spermatogonia enter meiosis I, and in meiosis I primary spermatocytes become immunologically distinct from the other cells and move through the blood-testis barrier to the interior of the seminiferous tubule

Question 30

How long does it take primary spermatocytes to complete meiosis I?

Answer 30

24 days (making mRNA for future protein production which is stored until required)

Question 31

How are secondary spermatocytes formed?

Answer 31

When meiosis I has completed in primary spermatocytes they become secondary spermatocytes and these immediately enter meiosis II

Question 32

How are spermatids produced?

Answer 32

When secondary spermatocytes finish meiosis II

Question 33

What is spermiogenesis?

Answer 33

Mturation of spermatids into mature, motile spermatozoa

Question 34

Describe spermiogenesis

Answer 34

a. Reduction of nuclear size
b. Golgi apparatus condenses crosome
c. Flagellum grows out of centriole
d. Cytoplasm streams away from nucleus
e. Mitochondria form a spiral arrangement around formed flagellum
f. The head becomes partitioned into domains (acrosomal cap and nucleus)
g. Cytoplasm moves to the residual body which is phagocytosed by Sertoli cells

Question 35

What forms the blood-testis barrier, and what happens if this is compromised?

Answer 35

Sertoli processes in the testis form an immunological barrier and if this is broken then autoimmune infertility can occur

Question 36

What happens if there is early translation of proteins in primary spermatocytes in meiosis I?

Answer 36

Results in sterility

Question 37

Where does the increase in germ cell number occur in spermatogenesis?

Answer 37

Mitosis occurs between type B spermatogonia and the primary spermatocyte

Question 38

Where do the reductions in chromosome number occur in spermatogenesis (meiotic divisions)?

Answer 38

First meiotic division: primary spermatocyte –> forms secondary spermatocyte
Second meiotic division: secondary spermatocyte –> forms spermatid

Question 39

What is capacitation?

Answer 39

Final stage of sperm cell maturation, occurs in female reproductive tract (requires contact with oviduct secretions) –> physiological reaction occurs –> allows sperm to perform acrosome reaction with secondary oocyte (egg)

Question 40

Describe the hormonal control of sperm production

Answer 40

GnRH (hypothalamus) –> FSH and LH (anterior pituitary) –> LH stimulates testosterone production by Leydig cells (surrounding tissue of seminiferous tubule) –> produced testosterone and FSH act on Sertoli cells in seminiferous tubule –> secrete androgen binding protein (ABP) and tubular fluid –> ABP binds to testosterone and carries it to fluid region of seminiferous tubule –> stimulates spermatogenesis

Question 41

What is the role of early theca cells in the primary follicle?

Answer 41

Secrete angiogenic factors to stimulate the growth of blood vessels