Gametogenesis and Assisted Reproductive Technologies

Question 1

What is spermatogenesis?

Answer 1

Spermatogenesis is the production of mature sperm - spermatazoa from spermatogonia (from PGCs)

Question 2

Where does spermatogenesis take place?

Answer 2

In the seminiferous tubules of the mature testes. The differentiation occurs in the walls of the tube and the mature sperm released in to the lumen and stored in epididymis

Question 3

How do PGCs turn into spermatogonia?

Answer 3

During the development of the testes, PGCs become spermatogonia, which act as unipotent cells to form spermatozoa

Question 4

Which cells support the development of mature sperm cells and how do they do this?

Answer 4

In the seminiferous tubules of the testes, there are Sertoli cells which give nutritional support and an appropriate hormonal environment to support the differentiation of spermatazoa by being in close proximity to the spermatogonium

Question 5

Explain the cell types and processes involved in spermatogenesis

Answer 5

PGCs give rise to spermatogonia, which undergo mitosis (maintaining the pool) and differentiate to form a primary spermatocyte, which undergoes meiosis I to form secondary spermatocytes (44XY) which undergo meiosis II to become spermatids (22X/Y) which undergo two stages of differentiation to become spermatozoa

Question 6

Describe the hormonal axis involved in sperm production

Answer 6

GnRH is released by the hypothalamic-pituitary axis which stimulates the release of LH and FSH from anterior pituitary. LH stimulates Leydig cells to release testosterone that synergises with FSH to stimulate the Sertoli cells to optimise spermatogenesis

Question 7

Describe the development of the oogonium

Answer 7

During the devleopment of the ovaries, PGCs tunr into oogonia. The oogonium (ovum stem cell) develops into a primary oocyte (diploid) in pre pubescent years. The primary oocyte is frozen in prophase I of meiosis. By the time the antral follicle ruptures, the primary oocyte completes meiosis (haploid) becoming a secondary oocyte. A polar body is expelled. This becomes frozen in metaphase II and only completes meisosis after fertilisation.

Question 8

Describe the development of the follicle in oogenesis

Answer 8

Oogonium form primordial follicles in pre pubescent years. At puberty, the primordial follicles become primary follicles, which grow, producing a growing follicle, that results in the production of follicular fluid (hyaluronic acid) inside the follicle. The pools join together becoming an antrum (lake of fluid), and the antral follicle bursts, releasing the secondary oocyte. The antral follicle becomes the corpus luteum which stays for about a week before degrading

Question 9

Describe the role of FSH and LH in oogenesis.

Answer 9

Both produced by anterior pituitary in response to GnRH produced by hypothalamus. FSH stimulates the follicle to proliferate, and to produce estrogen, via the production of androgens from cholesterol stimulated by LH, and FSH stimulates conversion of androgens to estrogen. FSH also causes production of follicular fluid

Question 10

What is the role of Estrogen in oogenesis?

Answer 10

Estrogen goes into bloodstream and go to hypothalamus and anterior pituitary, exerting negative feedback, lowering GnRH levels and inhibits pituitary producing FSH and LH. LH starts to rise again, and switches to a positive feedback mechanism, resulting in an LH surge,which brings about the rupturing of the antral follicle

Question 11

Why doesn’t FSH not increase during the LH surge?

Answer 11

Because the Graffian follicle releases Inhibin B which inhibits the release of FSH, so FSH levels stay low during the LH surge

Question 12

How does LH rupture the Graffian follicle?

Answer 12

Increases blood flow and permeability to the follicle, increasing follicular fluid production. Also stimulates proteases to cause rupture of the follicle, releasing the secondary oocyte - ovulation

Question 13

What is the role of LH in regards to the corpus luteum?

Answer 13

LH also stimulates the corpus hemorrhagicum (follicle full of blood) to differentiate and become corpus luteum (accumulation of lipids, cholesterol, fat)

Question 14

What is the role of the corpus luteum?

Answer 14

Corpeus luteum, stimulated by LH to produce progesterone.

Question 15

Describe what occurs after ovulation

Answer 15

The secondary oocyte moves into the fallopian tubes via local currents produced by the fallopian tube clawing, and is moved by the cilia into the correct location where it waits until fertilisation. If not fertilised in about 12-24 hours it begins to break down

Question 16

What is the role of semen?

Answer 16

Semen contains fructose, zinc, and calcium which are neccessary to activate the swimming movements of the sperm and also an alkaline environment optimal for sperm survival

Question 17

Which tissues produce sperm?

Answer 17

Seminal vesicles, prostate, bulboerythral glands

Question 18

What is capacitation?

Answer 18

The second activation needed for the sperm can fertilise the egg. Happens while the sperm are swimming from cervix into fallopian tubes through the uterus

Question 19

Describe capacitation

Answer 19

The head of the sperm is changed so that the acrosome below the cell membrane is activated and the enzymes can digest the outer layers of the oocyte.

Question 20

Describe the process of fertilisaiton

Answer 20

Activated sperm release acrosomal enzymes that digest the corona radiata cells and the zona pellucida, and one sperm fuses with the membrane, entering the egg (fertilisation). As it enters, cortical granules are released which fuse with the membrane, hardening the zona pellucida so other sperm can’t enter

Question 21

Which processes occur after fertilisation in the sperm nucleus?

Answer 21

The tightly packed chromosomes in the sperm nucleus begin to unwind - decondensation. This allows formation of the male pronucleus

Question 22

Which proccesses occur after fertilisation in the oocyte?

Answer 22

The process of meiosis is complete, expelling the second polar body. The female pronucleus is then allowed to form - grouped chromosomes but not tightly packed

Question 23

What happens once both pronuclei are formed in the oocyte?

Answer 23

Haploid chromosome sets are duplicated to form identical chromatids (male and female origin). Pronuclei fuse to form a nucleus that;s breifly tetraploid, and mitosis is quickly initiated and the first cleavage division occurs

Question 24

What is the usual rate of success of human fertilisation?

Answer 24

About 10-15% per menstrual cycle. Decreases after 3 consecutive cycles. After 12 months treatment is considered

Question 25

What are main causes of infertility?

Answer 25

28% unexplained, male problems about 20%, female problems about 40% but more easily identified than male.

Question 26

Describe hormonal treatments for infertility

Answer 26

Aimed at correcting hormonal imbalances preventing normal sperm/egg production. Normalise gonadal function. Use injected LH and FSH (GnRH may work). Steroids may also be needed to normalise gonadal function. More of a fertility treatment than assisted reproduction

Question 27

Describe the process of in vitro fertilisation

Answer 27

It involves the induction of ovulation via hormonal methods, followed by surgical collection of oocytes from mature follicles. They’re then fertilised in vitro by sperm from partner/donor. Multiple follicles used as each cycle is demanding on the woman’s health and expensive.

Question 28

Describe other procedures of assisted reproductive technologies

Answer 28

IVF followed by surrogacy for uterine malfunctions
Intra cytoplasmic (into egg) sperm injection if sperm motility is low or cannot penetrate zona pellucida. Also can inject round spermatids if later stages of spermatogenesis are compromised
Pronuclear transfer with mitochondrial replacement therapy