L5: gametogenesis

Question 1

Explain spermatogenesis

Answer 1

Occurs in the seminiferous tubules of the testes
Spermatogonia, which reside in the basal compartment, divide by mitosis to give two primary spermatocytes
One replaces the spermatogonia and one undergoes meiosis I to produce two secondary spermatocytes
Secondary spermatocytes undergo meiosis II to produce two spermatids each (4 in total)

Question 2

Distinguish between the spermatogenic cycle and spermatogenic wave

Answer 2

Spermatogenic cycle = length of time it takes for spermatids at the same stage in the cycle to ‘show up’ again when looking at a specific point along the seminiferous tubules
Spermatogenic wave = distance between groups of spermatids at the same level of maturation

Question 3

What is spermiogenesis?

Answer 3

Process by which spermatids become spermatozoa
Happens once the spermatids have been released into the lumen of the seminiferous tubules
Allows the non-motile spermatids to become motile – process is complete when they have reached the epididymis

Question 4

Explain oocyte maturation before birth

Answer 4

Oogonia proliferate rapidly by mitosis
Some enter meiosis -> these cells arrest in prophase I = primary oocytes
Cell death begins and many oogonia and primary oocytes degenerate
All surviving primary oocytes have entered meiosis I & each is surrounded by follicular cells = primordial follicle
From puberty onwards, ~15-20 oocytes start to mature each month passing through three stages

Question 5

List the stages of oocyte maturation from puberty

Answer 5

Preantral phase
Antral phase
Pre-ovulatory phase

Question 6

Describe the preantral phase

Answer 6

Primordial follicles grow
Surrounding follicular cells changes from flat to cuboidal = granulosa cells
Granulosa cells secrete layer of glycoprotein called zona pellucida

Question 7

Describe the antral phase

Answer 7

Fluid filled spaces start to appear between the granulosa cells, coalesce to form the antrum
Several follicles begin to develop with each ovarian cycle, usually only one reaches maturity

Question 8

Describe the pre-ovulatory phase

Answer 8

Surge in LH
Meiosis I now complete - 2 haploid cells of unequal size (1 polar body & 1 oocyte)
Cell then enters meiosis II, but arrests in metaphase -> only completes if oocyte is fertilised

Question 9

What happens to the follicle if fertilisation does or doesn’t occur?

Answer 9

Does not: oocyte will degrade approximately 24 hours later
Does: second polar body is produced and discarded because all energy is focused on a single ovum at the end of this process

Question 10

What happens following ovulation?

Answer 10

Remaining granulosa and theca interna cells become vascularised, forming the corpus luteum – secretes oestrogen and progesterone
Will degenerate after 14 days -> forms the corpus albicans, mass of scar tissue

Question 11

Describe transport of the oocyte at ovulation

Answer 11

Oocyte is released when the follicle ruptures & is ‘caught’ by the fimbriae at the distal end of the fallopian tube
If fertilisation occurs, the implanted embryo releases B-hCG, which maintains the corpus luteum until the placenta can take over production of progesterone at ~20 weeks

Question 12

What happens to the oocyte during ovulation?

Answer 12

LH surge increases collegenase activity
Prostaglandins increase response to LH & cause local muscular contractions in ovarian wall
Oocyte extruded & breaks free from ovary