Lecture 10: Gametogenesis, Fertilization and the Blastocyst

The female reproductive system

Coronal section of reproductive system

Ovary and the ovarian cycle

Gametogenesis

Gametogenesis in female is called oogensis

Surge in LH leads to completion of meiosis I to convert a 10 oocyte to 20 oocyte

Step 1 of female gametogenesis

The primary oocyte is surrounded by a single layer of follicular cells (from the ovarian epithelium). Together they are known as the primary follicle

Step 2 of female gametogenesis

At the beginning of each cycle, 5-12 primary follicles begin to develop and become growing follicles.

Step 3 of FG

The follicular cells multiply and become several-layered. They also become separated from the ovum by an acellular mucopolysachharode layer, the septum pellucidum

Step 4 of FG

Usually all but one of the growing follicles degenerate to form a small corpus atreticum or scar

Step 5 of FG

The follicular cells of the remaining growing follicle secrete a fluid which produces a fluid-filled antrum within the follicle.

The ovarian non-gamete cells surrounding the follicle also become altered and form a thecal layer around the follicle.

Step 6 of FG

In the mature or Graafian follicle, the majority if the follicular cells form the stratum granulosum, which will produce progesterone.

The remainder surround the oocyte as the cumulus oophorus. It is now a secondary oocyte.

Step 7 of the FG

The mature follicle ruptures to release the ovum. This retains a covering of follicular cells which form the corona radiate. The septum pellucidum has expanded to become the zona pellucida.

Step 8 of the FG

The cells of the theca interna and the stratum granulosum enlarge, turn yellowish and form the corpus luteum. It secrets large amounts of progesterone (and also oestrogen). Prior to ovulation the follicle produced mainly oestrogen.

Step 9 of FG

If fertilisation does not occur the corpus luteum has a life of only 12 days after which it degraded into the corpus albicans. The cessation of its hormonal output leads to menstruation

Step 10 of FG

If pregnancy occurs the corpus luteum is sustained by the HCG produced by the conceptus and forms a large corpus luteum of pregnancy. This will eventually form a large corpus albicans.

Female Gametogenesis overview

1,2,3 : Primary oocyte to primary follicle to growing follicle

4,5 : Growing follicle and corpus atreticum

6 : Graafian follicle and secondary oocyte

7: Release of oocyte

8,9,10 : Corpus luteum and corpus albicans

Female reproductive system overview

Male reproductive system

Seminiferous tubule overview

Gametogenesis in males overview

Spermatogenesis

Spermatogenesis

Rounded spermatic differentiates to become the tadpole-shaped sperm via a process called Spermiogenesis

Comparison of male and female gametogenesis

Female reproductive system

The ovum is shed into the abdominal cavity and is caught by the fimbria of the fallopian tube. So, just for a moment it is ‘outisde’ the reproductive system.

Note that in the female the abdominal cavity communicates with the exterior (vagina, fallopian tube, abdominal cavity)

In the male, the abdominal cavity is closed.

Fertilisation process

Only 1% of the sperm deposited go past the cervix, several hours later sperm reach isthmus - become less motile - chemoattractants released from cumulus cells on ovulation make sperm motile again - swim to ampulla - fertilisation.

Why do sperm require capacitation

Condiitoning in female reproductive tract during which across all region loses glycoprotein coat.

Overview of fertilisation (3 phases)

• Sperm penetration of corona radiata

• Sperm binding and penetration of the zona pellucida

• Fusion of sperm and oocyte cell membranes (syngamy)

Fertilisation & cleavage

Secondary oocyte halted part way through 2nd meiosis division

Fertilisation - What are acrosomal enzymes for?

Acromomal enzymes (acrosin) released via acrosomal reaction help disperse corona radiate and aid penetration of zona pellucida. Plasma membranes of gametes fuse and male nucleus is injected.

What results in completion of the 2nd meiotic division

Mature ovum, penetration by spermatozoon results in completion of 2nd meiotic division

Role of enzymes released by cortical granules

Digest sperm receptor proteins ZP2 and ZP3 so that they can no longer bind sperm.

What does the zona (cortical) reaction

Leads to polyspermy block to prevent more than one sperm penetrating/ fertilising the ovum. The mechanism is not foolproof making diandric triploidy quite a common phenomenon.

Zona reaction overview

2-cell blastula

4-cell blastula

Blastomeres are totipotent up to

The eight-cell stage

A cluster of 16-32 blastomeres forms a

Morula

In the second week, the morula is

Still surrounded by the zona pellucida free within tubal/ uterine cavity. Still a solid ball of cells

Morula cells undergo what in the second week

Compaction - Establishment of tight junctions between surface cells

Zona pellucida starting to split

Morula and compaction images

How does a blastocyst form

5 days post fertilisation, a cavity develops in the morula and blastocyst is formed.

The zona pellucida is beginning to degenerate ‘hatching’

Has probably been responsible for preventing implantation up to now

Photomicrographs of sections of human blastocysts

Summary of female fertilisation

Secondary oocyte halted part way through 2nd meiotic division

Penetration by spermatozoon triggers completion of 2nd meiotic division.

Summary of early cleavage and blastocyst formation