11.1 Essential ideas
11.1.4a Sexual reproduction: gametogenesis and fertilisation
- Gametes are haploid cells produced in the gonads – testes and ovaries.
- Spermatogenesis takes place in the seminiferous tubules, starting at puberty and continuing throughout the male life.
- Oogenesis is the production of egg cells (ova) in the ovaries. It starts from primary follicles produced during fetal development, and continues after puberty until menopause.
- Both spermatogenesis and oogenesis involve mitosis, cell growth, two divisions of meiosis, and differentiation.
- Spermatogenesis is stimulated by testosterone secreted by Leydig cells in the interstitial fluid between the seminiferous tubules. (Leydig cells are are not shown in Figure 11.1.4a/a.)
- The outer layer of the seminiferous tubule is called the germinal epithelium. It produces diploid cells that undergo mitosis to become primary spermatocytes.
- As the cells divide through meiosis I and II, they are pushed further down towards the lumen of the tubule.
- Sertoli cells provide support and nourishment for the differentiating secondary spermatocytes. When the spermatozoa are fully mature, they are released from Sertoli cells into the lumen of the tubule and carried away by seminal fluid.
- Each diploid germinal cell produces four haploid gametes with very little cytoplasm.
- The number of primary follicles in each ovary is determined during fetal development. When puberty begins, FSH (follicle-stimulating hormone) stimulates the maturation of the follicle to produce an ovum.
- Meiosis I is an uneven division that results in one secondary oocyte and a polar body, with proportionately less cytoplasm. Ovulation releases an ovum at this stage.
- Meiosis II occurs after fertilisation and produces an even larger ovum and another polar body. Polar bodies normally undergo apoptosis.
- Unlike spermatogenesis, oogenesis results in only one haploid gamete with a lot of cytoplasm.
Fertilisation in animals can be internal or external
- Fertilisation occurs when the egg and sperm nuclei fuse to form a diploid zygote.
- For most marine or freshwater animals, fertilisation tends to take place externally. Gametes are deposited, normally simultaneously, in water, and fertilisation takes place outside of the female’s body.
- The advantage of external fertilisation is a smaller energy investment by the female, but external fertilisation has high risks, including predation and environmental variations.
- Internal fertilisation is an adaptation to terrestrial living. Gametes would dry out if fertilisation took place outside of the body.
- Another advantage of internal fertilisation is greater protection for the embryo.
Fertilisation involves mechanisms to prevent polyspermy
Figure 11.1.4a/c – Overview of fertilisation including the acrosome reaction, fusion of plasma membranes and the cortical reaction
- Sperm swim directionally by sensing the concentration of chemicals released by the cells surrounding the egg. This process is called chemotaxis.
- Thousands of sperm may reach the egg at the same time.
- The egg is surrounded by an extracellular matrix of glycoproteins called the zona pellucida.
- The head of a sperm cell contains hydrolytic enzymes in a membrane-bound sac called the acrosome. The contents of the acrosome are capable of digesting the zona pellucida. This is called the acrosome reaction.
- The acrosome reaction exposes a part of the egg plasma membrane, allowing the plasma membranes of the egg and sperm to fuse. The moment of fertilisation occurs when the nucleus of the sperm cell is released into the egg.
- The cortex of the egg, just below the plasma membrane, contains cortical granules in vesicles that are released by exocytosis after the fusion of the nuclei.
- The cortical reaction results in hardening of the zona pellucida and prevents a second sperm from fusing with the fertilisation membrane.
- Compare spermatogenesis and oogenesis.
- Outline the process of fertilisation.
- Describe mechanisms that prevent polyspermy.
- Polyspermy refers to an egg that has been fertilised by more than one sperm.
- Chemotaxis is a directional movement in response to a chemical stimulus.