Gametogenesis, Fertilization, and the Blastocyst

Question 1

Where is the fundus found in the female reproductive system?

Answer 1

Above the uterine cavity

Question 2

Describe the different parts of the fallopian tubes. (imagine if the ovum shed by an ovary made its journey into and along the fallopian tube)

Answer 2

From opening (fimbrae end) to uterine cavity end: Infundibulum, ampulla, isthmus, interstitial/intramural

Question 3

What are fimbrae?

Answer 3

The finger-like projections of the fallopian tube openings (lead into infundibulum)

Question 4

Where are all female eggs ‘held’ prior to puberty?

Answer 4

All the eggs a female will ever have are present when they are born. They are suspended in Meiosis II, partially divided, and sit there until puberty.

Question 5

What is the primary follicle?

Answer 5

Primary oocyte surrounded by single layer of follicular cells from ovarian epithelium

Question 6

What happens at the beginning of each monthly cycle following puberty?

Answer 6

A few primary follicles becoming growing follicles as they become several-layered and become separated from the oocyte by the zona pellucida (septum pellucida)

Question 7

What happens in the step following the formation of the zona/septum pellucida in the growing follicle?

Answer 7

. All but one of the growing follicles degenerate to form a small corpus atreticum/scar
. The follicular cells of the remaining growing follicle secrete fluid to produce a fluid-filled antrum
. A thecal layer is formed around the follicle (made from ovarian non-gamete cells)
. The thecal layer consists of vascular theca interna (produces oestrogen) and theca externa (avascular and inactive)

Question 8

What happens in the mature/Graafian follicle?

Answer 8

. Majority of follicular cells form the stratum granulosum (produces progesterone)
. The other follicular cells surround the oocyte as the cumulus oophorus
. Now is secondary oocyte

Question 9

When are oestrogen and progesterone produced during the development of the oocyte, and what are they produced by?

Answer 9

. Oestrogen is produced by the theca interna (inner thecal layer of growing oocyte at stage where fluid-filled antrum is formed in growing follicle, forming a matur/Graafian follicle)
. Progesterone is produced by the stratum granulosum in the mature/Graafian follicle (in the stage where most follicular cells from stratum granulosum and the rest form the cumulous oophorus, giving rise to a secondary oocyte)

Question 10

What happens after the secondary oocyte is formed in the mature follicle?

Answer 10

The mature follicle ruptures to release the ovum, which is surrounded by corona radiate (surrounding follicular cells)

Question 11

Describe the structure of the mature/Graafian follicle.

Answer 11

. Secondary oocyte surrounded by cumulous oophorus

. Interior side of thecal layer lined by stratum granulosusm, which secretes oestrogen

Question 12

How is the corpus luteum formed? What is the importance of this?

Answer 12

. Cells of the theca interna and stratum granulosum enlarge and turn yellowish on form the corpus luteum, which combines the effect of both structures and secretes large amounts of progesterone and some oestrogen

Question 13

What happens to the corpus luteum if fertilization doesn’t occur?

Answer 13

. If fertilization doesn’t occur, corpus luteum has 12 days of life
. After these 12 days, it degenerates into the small corpus albicans
. The cessation of the output of progesterone and oestrogen by corpus luteum leads to menstruation

Question 14

What happens to corpus luteum if fertilization occurs?

Answer 14

. Corpus luteum is kept alive by HCG produced by the conceptus (early embryo in uterus)
. Forms large corpus luteum of pregnancy, which will eventually form a large corpus albicans

Question 15

Where is the ovum shed and caught?

Answer 15

Shed by ovaries into abdominal cavity and caught by fimbrae of fallopian tubes

Question 16

Why is pelvic infection more likely to occur in females?

Answer 16

. In females, the abdominal cavity is exposed to the exterior (vagina leads to Fallopian tubes, leads to abdominal and peritoneal cavities)
. Bacteria can travel through fallopian tubes into peritoneal cavity and cause pelvic infection

Question 17

Describe spermatogenesis.

Answer 17

. Stem cells in testes divide into spermatogonium
. Spermatogonium (diploid) divide by mitosis to produce primary spermatocytes (diploid)
. Spermatocytes undergo meiosis I to produce secondary spermatocytes (haploid)
. Secondary spermatocytes undergo meiosis II to produce spermatids (haploid)
. Spermatids mature into spermatocytes/sperm (haploid)

Question 18

Describe the structure of a spermatozoon.

Answer 18

. Acrosomal cap in tip of sperm head contains enzymes that help penetrate ovum
. Middle piece contains helically wound mitochondria, which surround a central bundle of fibrils arranged in a 9+2 arrangement (same as in flagellum or cilium)
. These fibrils continue throughout the motile tail region to give movement

Question 19

What is Kartageners Syndrome?

Answer 19

Where dynein arms don’t work, so sperm can’t swim= infertile male

Question 20

Compare the cytoplasmic:nuclear ratio of spermatozoa and ova.

Answer 20

. Ova have very high cytoplasmic:nuclear ratio

. Sperm have low cytoplasmic:nuclear ratio

Question 21

Describe what happens during fertilization.

Answer 21

. At isthmus of fallopian tubes, sperm become less motile
. Chemoattractants released from cumulus oophorus make sperm more motile and they swim to the ampulla to fertilise the egg
. Shed secondary oocyte halted part way through meiosis II
. Sperm penetrates zona pellucida and prompts completion of meiosis II
. Plasma membranes of gametes fuse and sperm nucleus injected into ovum

Question 22

Describe what happens after the sperm penetrates the egg.

Answer 22

. Plasma membranes of gametes fuse and sperm nucleus injected into ovum
. Diploid zygote formed and zona reaction occurs to prevent further penetration of egg
. Cleavage of zygote to form 2 blastomeres by mitosis
. Cells continue to divide to give progressively smaller blastomeres, become solid to form morula

. In the second week, morula cells undergo compaction to establish tight junctions between surface cells. The zona pellucida starts to split.
. After about 5 days post-fertilisation, a cavity develops and the blastocyst is formed (inner cell mass and trophoblast begin to differentiate)

Question 23

When does the blastocyst form?

Answer 23

About 5 days post-fertilisation

Question 24

Why does the zona pellucida break when the blastocyst develops?

Answer 24

So that the blastocyst can implant

Question 25

The blastocyst consists of an inner cell mass and trophoblasts. What do the inner cell mass and trophoblasts eventually form?

Answer 25

. Inner cell mass eventually develops into embryo

. Trophoblasts eventually develop into placenta and extraembryonic membranes

Question 26

Where does fertilization occur?

Answer 26

In the ampulla of the fallopian tube

Question 27

How does the sperm ‘know’ where the egg is to fertilise it?

Answer 27

The cumulus oophorus releases chemoattractants