Gametogenesis, Fertilization And The Blastocyst

Question 1

Gametogenesis in ovary

Answer 1

1 and 2 - primary follicle;
3 - growing follicles, septum pellucidum
4 - corpus atreticum (all but one)
5 - one - fluid filled anthrum; thecal layer - theca interna and theca externa
6 - mature/Graafian follicle; stratum granulosum; cumulus oophorus; secondary oocyte;
7 - release ovum; corona radiata; zona pellucida;
8 - theca interna and stratum granulosum enlarge - corpus luteum;
9. No fert - Corpus albicans
10. Fert - CL sustained by HCG - large

Question 2

Spermatogenesis

Answer 2

Stem cells in testis;
Spermatogonium undergoes mitosis;
Primary spermatocyte undergoes M I;
Secondary spermatocyte (23) undergoes MII;
Spermatid matures;
Spermatozoa

Question 3

Sperm structure

Answer 3

Acrosomal cap - derived largely from Goli apparatus - contains enzymes for penetration of ovum;
Head (4 micrometers) - consists largely of nucleus with a sparse cytoplasmic covering;
Middle piece (7 micrometers) - helically wound mitochondria, axial bundle (9 + 2 arrangement of fibrils) - power plant
Tail (40 micrometers) - axial bundle;

Question 4

Comparison of male and female gametogensis

Answer 4

Males - 
Low cyto:nucleus ratio;
Mitosis, meiosis;
Continuous from puberty - stem cells retained;
Takes 9 weeks;
Motile;
Fluid from other glands added prior to ejaculation;
Haploid spermatozoa;

Female -
V high cyto:nucleus ratio;
Meiosis;
Discontinuous - all primary oocytes present at birth - no stem cells retained - primary oocytes suspended partway through meiosis;
5-12 primary oocytes continue with meiosis with each monthly cycle following puberty - do not complete meiosis until fertilisation;
Non-motile;
Haploid ova;

Question 5

Ovum shedding

Answer 5

Ovum is shed into the abdominal cavity and caught by the fimbria of the Fallopian tube;
Ovum is briefly outside the reproductive system;
In females the abdominal cavity interacts with the exterior;
In males the abdominal cavity is closed;

Question 6

Passage of sperm to fertilisation

Answer 6

Only 1% of sperm deposited penetrate the cervix;
Several hours later the sperm reaches the isthmus and becomes less motile;
Chemoattractants released from cumulus cells on ovulation make sperm motile again and swim to ampulla for fertilisation;
Sperm require capacitation - conditioning in female reproductive tract during which the acrosomal region loses glycoprotein coat;

Question 7

Fertilisation process

Answer 7

Secondary oocyte halted part way through MII;

Penetration by spermatozoan completes MII - acrosomal enzymes disperse corona radiata, PM of gametes fuse and male nucleus injected;

Fertilised ovum (Diploid); Zona reaction - cortical granules release enzymes that digest sperm receptors ZP2 and ZP3 so they can no longer bind to sperm - permeability of zona pellucida altered to prevent further penetration; ovum shrinks - perivitelline space (gemets contribute DNA equally but cyto highly unequal;

Cleavage - 2 blastomeres (30h post fert);

Further cleavage- progressively smaller blastomeres (more DNA synthesis but virtually no protein made);

Morula - solid ball of cells; cyto:nuclear ratio normal; cytoplasm differentially packaged; zona pellucida persists (prevents implantation);

Blastocyst formation - morulla cells undergo impaction (establishment of tight junctions betw surface cells and starts to split);

A cavity develops 5 days post fert;
Zona pellucida ‘hatching; or breaking down;
Beginnings of differentiation of inner cell mass and trophoblast;