2.4 - Sexual Reproduction Flashcards
What is spermatogenesis?
the production of sperm cells in the seminiferous tubules
What is the process of spermatogenesis?
Sperm production occurs in the testes from puberty onwards.
Testes contain seminiferous tubules where spermatogenesis takes place.
Spaces between tubules contain interstitial (Leydig) cells, which produce testosterone.
Spermatogenesis starts in the germinal epithelium (outer layer of tubules).
Germinal epithelium cells divide by mitosis, forming diploid spermatogonia.
One daughter cell remains in the epithelium.
The other continues developing into a sperm cell.
Spermatogonia migrate toward the lumen of the tubule, moving between Sertoli cells (which line the tubules).
Spermatogonia differentiate into primary spermatocytes.
Primary spermatocytes undergo meiosis:
Meiosis I → secondary spermatocytes
Meiosis II → spermatids
Spermatids remain attached to Sertoli cells as they mature into motile spermatozoa (sperm cells).
Structural changes include:
- formation of acrosome, which contains enzyme for fertilisation
- development of the flagellum for mobility
- condensation of the nucleus to remove any excess cytoplasm
Mature sperm detach from Sertoli cells and enter the seminiferous tubule lumen.
Sperm travels through the epididymis (a coiled tube), continuing to mature.
Eventually, they move toward the sperm duct.
Role of Sertoli cells
Sertoli cells provide structural and nutritional support to developing sperm
They also secrete substances that regulate the process and help transport spermatids to the tubule lumen
What is oogenesis?
the production of oocytes
Explain the process of oogenesis.
Ova production begins in the ovaries of the female foetus before birth.
The germinal epithelium (outer layer of the ovary) divides by mitosis during the first 7 months of foetal development, forming diploid oogonia.
Oogonia migrate throughout ovarian tissue.
Oogonia production stops after 7 months; this is the total number a female will ever produce.
In the months before birth, oogonia:
Grow in size
Enter meiosis I
Get surrounded by follicle cells
These structures (oogonia + follicle cells) form primary follicles.
Once primary follicles form, oogenesis pauses until puberty.
Although millions of primary follicles exist at birth, most degrade over time and never mature.
At puberty, FSH stimulates several primary follicles to continue developing.
Usually, only one reaches maturity per cycle.
Meiosis I resumes, forming:
A secondary oocyte (large)
A polar body (small, little cytoplasm, no further development)
The secondary oocyte begins meiosis II and is released from the ovary with its follicle cells during ovulation.
The remaining follicle in the ovary becomes the corpus luteum.
Meiosis II completes only if fertilisation occurs:
A sperm enters the secondary oocyte
Meiosis II finishes just before the nuclei fuse
A second polar body is formed
The secondary oocyte briefly becomes an ovum before fusion
What is the role of the polar body?
It is to regulate the cytoplasm
Similarities between the two processes?
- They have some processes in common like mitosis, meiosis, cell growth and cell differentiation
- both the production of sex cells
Differences between the two processes
Structure:
ova are large, immobile cells with large cytoplasm and a surrounding layer of follicle cells
sperm are small, mobile cells with little cytoplasm and an acrosome that contains enzymes
Daughter cells per meiotic division:
One ovum is produced at the end of meiosis II, the polar bodies produced in meiosis I and II do not give rise to ova
Four sperm cells are produced at the end of meiosis II.
The frequency at which the process occurs:
Once per menstrual cycle for ova
Continuous from puberty onwards
The events of fertilisation from the first contact between the
gametes to the fusion of nuclei.
Fertilisation is the fusion of one sperm cell and one ovum, combining two haploid nuclei to form a diploid zygote.
During sexual reproduction:
- Many sperm cells are released.
- They are attracted to the secondary oocyte by chemical signals.
At this stage, the female gamete is still a secondary oocyte; meiosis II is not yet complete.
The secondary oocyte is surrounded by:
- A layer of follicle cells
- A zona pellucida (glycoprotein layer)
To reach the oocyte’s surface, sperm must digest the zona pellucida using enzymes from the acrosome – this is the acrosome reaction.
When the first sperm breaks through:
- It binds to complementary receptors on the oocyte membrane.
- The sperm and oocyte membranes fuse, allowing the sperm nucleus to enter.
This triggers completion of meiosis II in the oocyte:
- A second polar body is released.
- The mature ovum is formed.
Fusion of membranes also triggers the cortical reaction:
Cortical granules (vesicles) move to the membrane and release enzymes by exocytosis.
These enzymes:
- Break down binding proteins on the ovum’s surface
- Harden the zona pellucida
The cortical reaction prevents polyspermy (entry of more than one sperm).
Inside the ovum, the male and female nuclei fuse, completing fertilisation.
Formation of embryo to blastocyst stage
After fertilisation, the zygote divides by mitosis to form two diploid nuclei.
Cytoplasm divides equally → two-cell embryo forms.
Mitosis continues:
- Forms a four-cell embryo, then more cells.
- Eventually forms a blastocyst (a hollow ball of cells).
- the inner group of cells = blastomeres, which will become the foetus.
Up to this point, the embryo remains in the oviduct.
Around day 7:
- The blastocyst has about 125 cells.
- It reaches the uterus.
- The zona pellucida (protective layer) breaks down and is lost.
The blastocyst needs nutrients (egg cell supplies are used up).
It implants into the endometrium (uterus lining) to get nutrients.
The outer layer of the blastocyst forms finger-like projections to penetrate the endometrium.
Nutrient and oxygen exchange with the mother’s blood begins.
The embryo continues rapid growth and development after implantation.
