Spermatogenesis & oogenesis, Fertilisation

describe Spermatogenesis

• Spermatogenesis is the process of producing sperm cells in the seminiferous tubules of the testes.

• Spermatogonia (2n) divide by mitosis to replenish themselves and produce primary spermatocytes.

• Primary spermatocytes undergo Meiosis I to form two secondary spermatocytes (n).

• Secondary spermatocytes undergo Meiosis II to form four spermatids (n).

• Spermatids differentiate into spermatozoa (mature sperm), developing a flagellum and many mitochondria.

• Sertoli cells nourish and support developing sperm cells.

• The final result is four genetically unique haploid sperm cells produced by equal cytokinesis.

Describe Oogenesis

• Oogenesis is the process of producing female gametes (eggs) in the ovaries.

• Oogonia (2n) divide by mitosis and develop into primary oocytes.

• Primary oocytes begin Meiosis I before birth and pause in Prophase I.

• Follicle cells surround the primary oocyte to form a primary follicle.

• When Meiosis I is completed, unequal cytokinesis produces a large secondary oocyte and a small polar body.

• The secondary oocyte begins Meiosis II and pauses in Metaphase II.

• The secondary oocyte is released during ovulation.

• If fertilisation occurs, Meiosis II is completed, producing an ovum and another polar body.

• Polar bodies usually degenerate.

• The final result is one large functional egg that retains most of the cytoplasm for embryo development.

What is fertilisation and where does it occur?

Fertilisation is the fusion of a male gamete (sperm) and a female gamete (egg) to form a diploid zygote. It normally occurs in the oviduct (fallopian tube).

Describe the process of fertilisation.

• Sperm reaches the egg.

• Acrosome reaction occurs and enzymes digest the zona pellucida.

• Sperm membrane fuses with the egg membrane.

• Sperm nucleus enters the egg.

• Cortical reaction hardens the zona pellucida and prevents polyspermy.

• Male and female pronuclei fuse.

• A diploid zygote forms and begins mitosis.

What are the acrosome reaction and cortical reaction?

• Acrosome reaction: enzymes from the sperm digest the zona pellucida so the sperm can reach the egg.

• Cortical reaction: cortical granules from the egg harden the zona pellucida and prevent polyspermy.

What is polyspermy and why must it be prevented?

Polyspermy is fertilisation of an egg by more than one sperm. It must be prevented because it would produce an abnormal chromosome number in the zygote.

What happens immediately after fertilisation?

The male and female pronuclei fuse to form a diploid zygote, which begins mitotic divisions and develops into an embryo.

Define: zona pellucida, zygote, polyspermy.

• Zona pellucida: protective glycoprotein layer surrounding the egg.

• Zygote: diploid cell formed after fertilisation.

• Polyspermy: fertilisation of an egg by multiple sperm.