Lecture 5: Gametogenesis

Question 1

How are mature gametes made?

Answer 1

• germ cells colonise the gonad in the embryo (male germ cells colonise the medulla of developing gonads, and the female germ cells colonise the cortex)
• proliferate by mitosis
• reshuffle genetically (genetic diversity) ad reduce to haploid by meiosis
• cytodifferentiate into mature gametes

Timing and scale of this varies between sexes

Question 2

Where is the SRY gene found?

Answer 2

On the Y chromosome

-codes for a transcription factor that leads to the development of the testis

Question 3

How does oogenesis differ from spermatogenesis?

Answer 3

Oogenesis: 
-very few gametes (400 in lifetime)
-intermittent production (1/month)
Spermatogenesis:
-huge number made (200 million)
-continuous production
-essentially 'disposable'

Question 4

What is formed during the process of fertilisation?

Answer 4

Zygote (diploid)- from fertilisation of two haploid gametes

Question 5

What determines the sex of the offspring?

Answer 5

The sperm, because it contains XY, whereas female contains only XX

Question 6

What are the 2 main functions of meiosis?

Answer 6

-reduce chromosome number in gamete to 23
-ensures every gamete is genetically unique
Used only in production of sperm and eggs

Question 7

What does meiosis produce?

Answer 7

4 daughter cells

-however in a female only one develops into a mature oocyte, others form polar bodies

Question 8

How does genetic variation occur?

Answer 8

Crossing over: exchanging DNA b/w 2 homologous chromosomes
Independent assortment: random orientationof each bivalent along metaphase plate)
Random segregation: random distribution of alleles amoung the 4 gametes

Question 9

Where does spermatogenesis occur?

Answer 9

Seminiferous tubules, in spaces between adjacent sertoli cells

• spermatogensis occurs in the walls in the basal compartment
• spermatids emerge into lumen from the adluminal compartment

Question 10

Where do the seminiferous tubules coalesce?

Answer 10

Rete testis: concentrates the sperm
-it anastomoses then with the ductili efferentes, where the sperm then enter the head of the epididymis (storage tube for sperm)

Question 11

What is the function of sertoli cells?

Answer 11

Nuture the spermatids

Question 12

What are the compartments in the seminiferous tubules?

Answer 12

Compartments are created due to tight junctions between the sertoli cells which form the blood testes barrier
Basal compartment
Adluminal compartment

Question 13

Why is the blood testes barrier important?

Answer 13

• Different environments are required for the germ cell line (spermatogonium) and the development of the spermatids
• sperm could be recognised as non self generating an immune response

Question 14

What other cells are found in the seminiferous tubule?

Answer 14

Leydig cells are also found in the seminiferous tubules, and produce testosterone to stimulate spermatogenesis

Question 15

What are the male germ cells ‘raw material’ called?

Answer 15

Spermatogonia

-available for up to 70 years

Question 16

What are the 2 different forms of spermatogonia?

Answer 16

• Ad spermatogonium: reserve stock (resting)
• Ap spermatogonium: maintain stock, and from puberty onwards they produce type B spermatogonia which give rise to primary spermatocytes

Question 17

What are the steps in spermatogenesis?

Answer 17

• spermatogonium divide by mitosis giving rise to Ad or Ap spermatogonium
• Ad are reserve stock to regenerate numbers
• Ap go down a meiosis pathway producing type B spermatogonia which give rise to primary spermatocytes
• primary spermatocytes divide by meiosis giving rise to secondary spermatocytes and then to spermatids which are haploid

Question 18

What is the difference between spermatogenesis and spermiogenesis?

Answer 18

Spermatogenesis is describing the entire process of gamete production in males
Spermiogenesis is describing the differentiation of spermatids into spermatozoa

Question 19

How long is the spermatogenic cycle?

Answer 19

16 days

Question 20

How have males evolved to increase chances of fertilisation?

Answer 20

If spermatogenesis was in synchrony, men would only be fertile every 16 days.
We have adapted so that men are fertile all the time

Question 21

How do cells appear in a cross section of the seminiferous tubule?

Answer 21

In groups with the same maturation stage, in a spiral like pattern

Question 22

What is the spermatogenic cycle?

Answer 22

The time taken for reappearance of the same stage within a given segment of the tube

Question 23

What is the spermatogenic wave?

Answer 23

The distance between the same stage of maturation

Question 24

What is spermiation?

Answer 24

Release of spermatids into the lumen of the seminiferous tubule

• spermiogenesis needs to occur (differentiation) to remodel the sperm
• they are non-motile until they reach the epididymis (they are transported via sertoli cell secretions assissted by peristaltic contraction)

Question 25

Where in the testis do the spermatids remodel?

Answer 25

- seminferous tubule - rete testis - ductuli efferentes - into the head of the epididymis where spermatozoa are formed

Question 26

What facilitates movement of the sperm tail?

Answer 26

Dynein

Question 27

How much semen is produced per ejaculate?

Answer 27

2 ml

Question 28

What is the composition of semen?

Answer 28

Seminal vesicle (70%)- amino acids, citrate, fructose- energy source for glycolysis, prostaglandins Prostate (25%)- proteolytic enzymes, zinc Sperm (2-5%)- 200-500 million per ejaculate Bulbourethal gland secretions (Cowper gland)- mucoproteins to help lubricate and neutralise acidic urine in distal urethra

Question 29

What controls the motility of sperm?

Answer 29

Zinc -as the amount of zinc lowers, the motility of the sperm increases, so when they enter a female reproductive tract, conc of zinc falls, stimulating a greater motility

Question 30

What is sperm capacitation?

Answer 30

Final maturation step required before the sperm becomes fertile Conditions in female genital tract stimulate: -removal of glycoproteins and cholesterol from sperm membrane -activation of sperm signalling pathways = allows sperm to bind to zona pellucida of oocyte and initate the acrosome reaction

Question 31

What has to be done to sperm in IVF?

Answer 31

The sperm are infertile as they haven't travelled in the womans reproductive tract -the sperm have to be incubated in capacitation media which mimics the environemnt of female reproductive tract

Question 32

When does maturation of oocytes begin?

Answer 32

Before birth - germ cells (arise from yolk sac), colonise the gonadal cortex and differentiate into oogonia - oogonia then proliferate rapidly by mitosis - at end of 3rd month oogonia arranged in clusters surrounded by flat epithelial cells (which eventually differentiate into follicular cells) - majority continue to divide by mitosis but some enter meiosis which arrest in prophase of meiosis 1: they are now called primary oocytes

Question 33

How many chromatids fo oogonium and primary oocytes have?

Answer 33

Oogonium: 46 chromatids | Primary oocyte: 92 chromatids

Question 34

What is the max number of germ cells reached is females?

Answer 34

7 million, and this is reached by mid gestation - cell death begins and many oogonia and primary oocytes die: atresia (quality control mechanisms as you want the best 400) - at 7th month gestation majority of oogonia have degenerated and 2 million primary oocytes survive - these 2 million have now entered meiosis 1 and are individually surrounded by layer of flat epithelial cells now called follicular cells: now called PRIMORDIAL FOLLICLES

Question 35

How many primordial follicles are left at puberty?

Answer 35

40,000 remain

Question 36

What happens during each mestrual cycle?

Answer 36

``` 15-20 oocytes start to mature each month (but only 1/2 complete maturation each month) 3 stages of maturation -preantral -antral -preovulatory ```

Question 37

What happens in the preantral stage?

Answer 37

As primordial follicles begin to grow, the surrounding follicular cells change from flat to cuboidal and proliferate to produce stratified epithelium of granulosa cells (granulosa cells are follicular cells) -granulosa cells secrete a layer of glycoprotein on oocyte called the ZONA PELLUCIDA

Question 38

What happens in the antral stage?

Answer 38

Fluid filled spaces appear between the granulosa cells and these coaelsce to form the ANTRUM -outer fibrous layer (theca externa) and an inner secretory layer (theca interna) surrounds the outside of the granulosa cells

Question 39

What is the cumulus oophorus?

Answer 39

The granulosa cells around the oocyte

Question 40

What is the function of theca interna?

Answer 40

Interna: receptors to LH, this stimulates production of androgens

Question 41

What happens in the preovulatory stage?

Answer 41

- surge in LH induces this stage - meiosis 1 is now complete resulting in 2 haploid daughter cells of unequal size - one cell receives most of the cytoplasm, the other receives none (first polar body) - the cell then enters meiosis 2 but arrests in metaphase 3 hours before ovulation - meiosis 2 is only completed if the oocyte is fertilised, otherwise cells degenerates around 24 hours after ovulation

Question 42

What happens during ovulation?

Answer 42

- FSH and LH stimulate rapid growth of the follicle several days before ovulation occurs - mature follicle is now 2.5 cm in diameter and is called the graafian follicle - LH surge increases collagenase activity (help rupture of follicle) - prostaglandins increase response to LH and cause local muscular contractions in ovarian wall - oocyte is extruded and breaks free from ovary

Question 43

What is the corpus luteum?

Answer 43

Remaining granulosa and theca interna cells become vascularised and secrete oestrogen and progesterone -this stimulates the uterine mucosa to enter secretory stage in prep for embryo implantation

Question 44

When does the corpus luteum die if not fertilised?

Answer 44

14 days

Question 45

Where does the sperm meet the oocyte?

Answer 45

Ampulla

Question 46

How is the oocyte transported?

Answer 46

- fimbriae sweep over surface of ovary - uterine tube contracts rhythmically - oocyte carried into the tube by sweeping of fimbriae and cilia on epithelial lining - oocyte then propelled by peristaltic muscular contractions of the tube and cilia

Question 47

What happens to the corpus luteum if fertilisation doesn't occur?

Answer 47

- degenerates - forms a mass of fibrotic scar tissue called the corpus albicans - progesterone production decreases, precipitating menstrual bleeding

Question 48

What happens to the corpus luteum if fertilisation does occur?

Answer 48

- degeneration of CL prevented by human chorionic gonadotropin secreted by developing embryo - CL continues to grow and forms the corpus luteum of pregnancy (corpus luteum of pregnancy) - cells continue to secrete progesterone until 4th month, secretion of progesterone by placenta then becomes adequate