4- Sexual reproduction in mammals

What is gametogenesis?

The process by which diploid germ cells undergo meiosis and differentiation to form haploid gametes (sperm or ova).

Where does spermatogenesis occur?

In the seminiferous tubules of the testes.

What are spermatogonia?

Diploid stem cells that divide by mitosis to maintain the germ cell supply and produce primary spermatocytes.

What are primary spermatocytes?

Diploid cells that undergo meiosis I to produce secondary spermatocytes.

What happens during meiosis II in spermatogenesis?

Secondary spermatocytes divide to form four haploid spermatids.

What is spermatogenesis?

The maturation of spermatids into spermatozoa (sperm cells), involving formation of the acrosome, flagellum, and mitochondrial concentrate.

What are the stages of spermatogenesis?

1. Primordial germ cells (diploid cells which are the precursors to gametes) divide several times by mitosis to form spermatogonia.

2. Spermatogonia grow without further division to form primary spermatocytes.

3. Primary spermatocytes undergo the first meiotic division to form secondary spermatocytes (diploid).

4. Secondary spermatocytes undergo the second meiotic division to form spermatids (haploid, but without a flagellum, acrosome etc.).

5. Spermatids differentiate and grow to form mature spermatozoa.

What is the role of Sertoli cells?

Provide nourishment, regulate development, form the blood-testes barrier, and remove cytoplasm from developing sperm.

What is the role of Leydig cells?

Produce testosterone, which stimulates spermatogenesis.

Where does oogenesis occur?

In the ovaries.

What are the stages of oogenesis?

1. Primordial germ cells divide several times by mitosis to form oogonia.

2. Only one oogonium continues to grow to form a primary oocyte.

3. The first meiotic division forms one secondary oocyte and one polar body (small cells that bud off the oocyte, stick to the oocyte and do not develop into gametes).

4. The second meiotic division of the secondary oocyte forms one haploid ootid and one polar body. The second meiotic division of the polar body forms two more polar bodies. They degenerate and die as the ootid develops.

This meiotic division starts in utero but is halted at prophase and occurs only in response to fertilisation to form the mature ovum.

When are primary oocytes formed?

During fetal development.

At what stage is meiosis halted in primary oocytes?

Prophase I, until puberty.

What triggers completion of meiosis I in oogenesis?

Ovulation, producing a secondary oocyte and a polar body.

At what stage is the secondary oocyte halted?

Metaphase II, until fertilisation.

What completes meiosis II in oogenesis?

Fertilisation by a sperm cell.

Why are polar bodies formed?

To dispose of extra chromosomes while conserving cytoplasm for the ovum.

What is the acrosome?

A specialised vesicle in the sperm head containing enzymes for digesting the zona pellucida of the egg.

What is the zona pellucida?

A glycoprotein layer surrounding the secondary oocyte that prevents polyspermy.

What is capacitation?

Biochemical changes in sperm within the female reproductive tract that increase motility and acrosome responsiveness.

What is the acrosome reaction?

Release of hydrolytic enzymes from the sperm acrosome to digest the zona pellucida.

What is the cortical reaction?

Fusion of cortical granules with the oocyte membrane causing zona pellucida hardening, preventing polyspermy.

What is a zygote?

The diploid cell formed after fusion of sperm and ovum nuclei.

What are the stages of fertilisation?

1) The sperm head contacts the zona pellucida and the acrosome reaction occurs - enzymes digest the zona pellucida as the acrosome fuses with the cell membrane of the sperm and releases the digestive enzymes.

2) The sperm head fuses with the cell membrane of the egg cell, thus allowing the sperm nucleus to enter the egg cell.

3) The cortical reaction occurs which causes the zona pellucida to harden and prevents polyspermy.

4) The nuclei fuse and a full set of chromosomes is restored, thus forming a diploid zygote.

What is double fertilisation?

One sperm fertilises the egg → zygote

What is the function of the endosperm?

Provides nutrients to the developing embryo.

Compare spermatogenesis and oogenesis in terms of cell output.

Spermatogenesis → 4 sperm per primary spermatocyte

Why do sperm contain many mitochondria but ova contain many organelles and nutrients?

Sperm need ATP for motility

Explain how polyspermy is prevented in mammals.

Zona pellucida thickening via cortical granule release (cortical reaction).

What is one key difference between fertilisation in mammals vs flowering plants?

Flowering plants have double fertilisation