Repro: Origin Of The Gametes

Question 1

Where do male and female germ cells enter the gonad?

Answer 1

Female germ cells enter the cortex of the gonad and become oogeonia, whereas male germ cells enter the medulla and become spermatogonia

Question 2

How many gametes are produced from meiosis?

Answer 2

Males: produces 4 daughter cells

Females: only 1 develops into a mature oocyte, the their 3 form disposable polar bodies
1 gets all the cytoplasm

Question 3

What are the ways that genetic variation arises from meiosis?

Answer 3

• crossing over occurs in prophase 1, formation of chiasmata to exchange DNA
• random segregation of chromosomes among 4 gametes
• independent assortment, 2 homologous chromosomes must go int separate gametes

Question 4

Outline the process of spermiogenesis?

Answer 4

Spermatids are released into the seminiferous tubules (spermiation)

As they pass through the rate testis, ductility efferentes and epididymis they remodel (grow a tail)

They are non motile at first so transported via sertoli cell secretions an peristaltic contraction

Question 5

What is the role of the sertoli cell barrier? (Or blood testis barrier)

Answer 5

Allows control of the environment the germ cells develop in - only androgens can penetrate the barrier

Also prevents antigenic products enter the blood stream and causing an autoimmune reaction (have different genes in the sperm)

Question 6

What does the mitotic division of spermatogonia produce?

Answer 6

1. Ad spermatogonia which maintain the stock

2. Ap spermatogonia which give rise to primary spermatocytes

Question 7

What does each primary spermatocyte give rise to?

Answer 7

Each primary spermatocyte divides by meiosis to form 4 haploid spermatids which differentiate into spermatozoa
( NB spermatogenesis = the whole process, spermiogenesis = just the cytodifferentiation into spermatozoa)

Question 8

What’s the difference between the spermatogenic cycle and wave?

Answer 8

Not all stages of spermatogenesis are visible in a cross section of seminiferous tubule at one time

• the spermatogenic cycle is the time take for reappearance of the same stage in a given segment (16 days)
• the spermatogenic wave is the distance between the same stage

Question 9

What is the composition of ejaculate?

Answer 9

~2ml of semen per ejaculate

• 70% is seminal vesicle secretions eg AA, citrate, fructose, prostaglandins
• 25% is prostate secretions eg proteolytic enzymes, zinc
• 5% is sperm, 200-500million
• 1% is bulbourethral gland secretions eg mucoproteins to lubricate and neutralise urine in distal urethra

Question 10

What is sperm capacitation?

Why is this important clinically?

Answer 10

Conditions in the female genital tract stimulate removal of glycoproteins and cholesterol from the sperm membrane and activation of sperm signalling pathways. This allows sperm to bind to the zona pellucida

Therefore in IVF the sperm has to be incubated in capacitation media before fertilisation

Question 11

What is the primordial follicle?

Answer 11

A group of primary oocytes which are arrested in meiosis 1 and surrounded by flat epithelial cells called follicle cells.
The primordial follicle now waits until puberty

Question 12

What are the 3 stages of maturation of oocytes that occurs from puberty onwards?

Answer 12

1. Pre-antral
2. Antral
3. Preovulatory

Question 13

Describe the pre-antral stage of oocyte maturation

Answer 13

The primordial follicles begin to grow and the follicular cells change to produce a stratified epithelium of granulosa cells. The granulosa cells secrete glycoprotein to form the zona pellucida

Question 14

Describe the antral stage of oocyte maturation

Answer 14

A fluid filled space forms called the Antrum

The secondary follicle is now comprised of the oocyte surrounded by the zona pellucida, then the antrum surrounded by granulosa cells then the theca interna (secretory) and then external (fibrous)

Question 15

Describe the preovulatory phase of oocyte maturation

Answer 15

An LH surge causes a growth phase and meiosis 1 is completed resulting in 2 haploid daughter cells (or is a polar body).
The cell then enters meiosis 2 but arrests in metaphase - meiosis 2 only completes if oocyte is fertilised

Question 16

What is the corpus luteum?

WHat does it secrete?

Answer 16

After ovulation when the oocyte is extruded, the remaining follicle is leutanized (by LH) to become the corpus luteum and remains in the ovary.

• the theca cells secrete androgens and progesterone
• the granulosa cells secrete progesterone, lots of oestrogen and inhibin

Question 17

What is the function of the corpus luteum?

Answer 17

Secretes oestrogen and progesterone.

If no fertilisation: the corpus luteum degenerates to form the corpus albicans (scar tissue). The progesterone decreases causing menstrual bleeding.

If fertilisation occurs: HCG maintains the corpus luteum which grows to form the corpus luteum graviditatis. Continues to secrete progesterone for 4 months until placenta becomes adequate

Question 18

How is the oocyte transported to the uterus?

Answer 18

Shortly before ovulation fimbrae sweep over the surface of the ovary to carry the oocyte into the fallopian tubes (which also have cilia)
The oocyte is propelled by peristaltic muscular contractions