Fertilisation

Question 1

Ejaculated sperm

Answer 1

Ejaculated semen is coagulated
Prostatic and seminal vesicle secretions comprise seminal fluid which coagulates - prevents loss, later liquefies.
Movement through cervical mucus removes seminal fluid, abnormally morphological sperm and cellular debris.

Question 2

Sperm passes into cervix …

Answer 2

Cervical mucus is less viscous in the absence of progesterone allowing sperm to pass.
Sperm can inhabit cervical crypts which may form a reservoir. Some evidence of thermotaxis, but mechanism not yet elucidated.
Fertilisation typically within 24-48 hours but sperm have been recovered alive after 5 days.

Question 3

Journey to the egg

Answer 3

Passage through uterus not well understood, currents set up by uterine or tubal cilia may have a role.
Chemo-attractants released from the oocyte cumulus complex may attract the sperm.
Sperm become hyperactivactivated. Forceful tail beats with increased frequency and amplitude mediated by Ca2+ influx via CatSper channels.

Question 4

Sperm capacitation & acrosome reaction

Answer 4

Capacitation is partly achieved by removing the sperm from the seminal fluid, also uterine or tubal fluid may contain factors which promote capacitation.
Biochemical rearrangement of the surface glycoprotein and changes in membrane composition must occur before the acrosome reaction can occur.
Acrosome reaction occurs in contact with the zona –cumulus complex; the acrosomal membrane on the sperm head fuses releasing enzymes that cut through the complex.
Acrosin bound to the inner acrosomal membrane digests the zona pellucida so the sperm can enter.

Question 5

Ovulation

Answer 5

LH spike causes resumption of meiosis and ovulation. Converts the primary oocyte to secondary oocyte plus 1st polar body.
Basement membrane breaks so blood pours into the middle.
Oocyte cumulus complex extruded out and caught by fimbrae of uterine tube.
Theca and granulosa become mixed and the empty follicle is known as the corpus luteum. It produces progesterone in the luteal phase.

Question 6

Progesterone

Answer 6

• Makes the endometrium secretory and receptive to implantation.
• Supresses cilia in uterine tubes once oocyte has already passed.
• Makes cervical mucus viscous again to prevent further sperm penetration.

Question 7

Estrogen

Answer 7

• Helps to maintain endometrium in luteal phase (causes proliferation in follicular phase).

Question 8

Demise of the corpus lutem

Answer 8

• If fertilisation does not occur, CL has an inbuilt finite lifespan of 14 days.
• Regression of CL essential to initiate new cycle.
• Fall in CL-derived steroids causes inter-cycle rise in FSH.
• Cell death occurs, vasculature breakdown, CL shrinks. Over time it becomes a corpus albicans.
• CL rescued in pregnancy by hCG from embryo binding to its LH receptors. CL continues to produce progesterone and maintain endometrium.

Question 9

Menstrual cycle achieves

Answer 9

• Selection of a single follicle and oocyte.
• Regular spontaneous ovulation.
• Correct haploid number of chromosomes in the oocyte by completing meiosis I and beginning meiosis II.
• Cyclical changes in the cervix and uterine tubes, to enable egg transport and sperm access.
• Preparation of the endometrium of the uterus to receive the fertilised egg.
• Support of the implanting embryo and endometrium by corpus luteum progesterone.
• Initiating a new cycle if fertilisation does not occur.

Question 10

Sperm binding & penetration part 1

Answer 10

After meiosis I the oocyte has 23X chromosomes, but 2 copies of each chromosome arranged as sister chromatids.

Question 11

Sperm binding & penetration part 2

Answer 11

Entry of the sperm causes an increase in Ca2+ via phospholipase Zeta from sperm.
Ca2+ causes the completion of meiosis II expelling the second polar body & cortical

Question 12

Sperm binding & penetration part 3

Answer 12

The sperm nuclear membrane breaks down, the chromatin decondenses and chromosomes separate.

Question 13

Syngamy part 1

Answer 13

4-7 hours after sperm penetration the two sets of haploid chromosomes become surrounded by distinct membranes forming two pronuclei.
These haploid structures synthesise DNA in preparation for the first mitotic division.

Question 14

Syngamy part 2

Answer 14

The pronuclei fuse and the mitotic metaphase spindle forms with the chromosomes assuming their position at its equator.

Question 15

Syngamy part 3

Answer 15

Mitosis is completed and the one cell zygote becomes a two cell embryo.

Question 16

Early embryo development part 1

Answer 16

The fertilized egg has 2 pronuclei.

This is the first sign of fertilization.

Question 17

Early embryo development part 2

Answer 17

The developing embryo contains 6-8 cells 3 days after fertilization

Question 18

The early embryo development part 3

Answer 18

Five days after fertilization it is called a blastocyst and differentiates into inner cell mass, blastocoel and trophoblast.