1027 - Ovulation and Spermatogenesis

Question 1

Outline the process of Meiosis as it applies to ova

Answer 1

Aim is Reduce from diploid to haploid. In utero, the oogonium undergoes a single stage of mitosis to produce primary oocytes. This is then frozen until puberty, when several start to mature, and some finish meiosis 1 (46XX) 36-48 hours before ovulation. One of these products becomes a polar body (discarded), whereas the other becomes a secondary oocyte. At ovulation, meiosis II begins, but is frozen at metaphase until fertilisation. At fertilization, the second meiotic division completes, creating a polar body again, as well as a mature oocyte.

Each ovum results in 2 polar bodies. Runs from 7 million in a 20 week foetus, to 20 million at birth, 300,000 at first period, and 0 at menopause.

Question 2

Outline the ovarian cycle

Answer 2

Divided into follicular and luteal phases 
Follicular - Primordial follicles start to grow 8 months before their cycle starts, and number recruited depends on number remaining.  Follicle grows and produces oestrogen, with egg attached to the wall, within antral space and surrounding follicle cells. One follicle becomes dominant, 20mm in size on day 14.
LH surge (pituitary) occurs 36 hours before ovulation. Follicle becomes a corpus luteum and secretes progesterone.  Lasts 14 days.

Question 3

Describe the sperm

Answer 3

3 parts - Head contains nucleus (very densely packed DNA) and acrosome, midpiece contains mitochondria, tail for propulsion.
55 microns long, 5 microns wide at head, move at 2mm per minute (but can be assisted by uterine contraction and fallopian cilia)
Live 3-4 days in female.

Question 4

How is genetic material packed so tightly inside the sperm?

Answer 4

Entirely in nucleus (head). DNA attached to annulus, histones replaced by protamines (condenses further), disulfide bonds form in epididymis.

Question 5

When do testes commence functions? What hormones are key?

Answer 5

Commence testosterone production in-utero - driven by LH and leydig cells.
Mature sperm produced at puberty - driven by FSH and sertoli cells.

Question 6

Outline spermatogenesis

Answer 6

In seminiferous tubules, spermatogonia divide mitotically to form primary spermatocytes (46XY). No further differentiation until puberty.
Primary spermatocytes undergo meiosis 1 to produce 2 secondary spermatocytes, which immediately undergo meiosis 2 to form 2 spermatids (23X or Y).
Spermatids travel through the seminiferous tubules to the epididymis, where they mature, and acrosome and flagella form - takes 10 days.
One primary spermatocyte forms 4 spermatozoa over the course of 48 days.

Question 7

Outline fertilisation

Answer 7

200 sperm reach fallopian tube, with 10 in ampulla at time of fertilisation (must be within 10 hours of ovulation).
Ovulated egg has jelly-like cumulus, and tough zona pellucida for sperm to attach to. Acrosome digests this, and electrical signal causes egg to secrete mucus to smother the egg and prevent further penetration.
Sperm head disintegrates, releasing chromosomes, and egg completes 2nd mitotic division.
2 pro-nuclei fuse to form 46XX or XY.
Fertilised egg divides without increasing in size until 16 cells, which then bind strongly to form morula.
Enters uterine cavity 3 days post-ovulation/fertilisation.
Develops into blastocyst - outer rim becomes placenta, inner ball becomes foetus.