Formation of Eggs and Sperm and Fertilisation

Explain the role of meiosis in the production of gametes and genetic variation through recombination of alleles and genes including independent assortment and crossing over (details of the stages of meiosis are not required)   Explain how mammalian gametes are specialised for their functions. Describe the process of fertilisation in mammals and flowering plants (starting with the acrosome reaction in mammals and pollen tube growth in plants and ending with the fusion of nuclei) and explain the importance of fertilisation in sexual reproduction.

Formation of gametes

Spermatozoa structure and Function Sperm cell (= spermatozoon) Specialised lysosome contains proteolytic enzymes Contains paternal set of chromosomes = flagellum to make sperm motile Provides energy from respiration for movement of flagellum

Sperm Production Millions of sperm produced in a lifetime – but very few will fertilise an egg. Sperm are not identical; many sperm do not reach the egg so the production of vast numbers ensures that at least some will get there. Many sperm are defective (e.g. 2 tails) and so only the ‘fittest’ will reach the egg to be in with a chance of fertilising it. 107 sperm per g of testis per day! Sperm are made in the seminiferous tubules of the testes

Spermatids develop into mature spermatozoa and enter the lumen of the tubules for transport to the epididymis where they are stored Spermatozoa in lumen of tubule

Egg production All potential egg cells are present before birth At birth, egg cell development is stopped at prophase of meiosis I Meiosis I completed just prior to ovulation and then stopped at metaphase II It is the 2o oocyte that is ovulated NB: unequal cytokinesis produces oocyte and polar body. Meiosis II is only completed on fertilisation. Only one functional gamete produced.

Egg Cell Human egg. Light micrograph of a human secondary oocyte (mature egg), with its first polar body (oval, centre right). Before ovulation the egg undergoes meiotic division to create two cells: a secondary oocyte and a polar body. The polar body will degenerate, while the larger egg develops in preparation for fertilisation. Around the egg is the zona pellucida layer.

Egg cell, ovum or oocyte = mass of granulosa cells from follicle Rich in protein and lipids for subsequent embryo development post fertilisation = zona pellucida (muco-polysaccharide coating) Contains maternal set of chromosomes 100 mm

Comparison of sperm and ovum Spermatozoa Small so sperm can move easily Sperm does not have a zona pellucida Sperm has a flagellum (tail) so it can move for fertilisation Sperm has an acrosome which contains enzymes to penetrate the ovum No cytoplasm/food reserves Ovum Large, spherical since cytoplasm contains food reserves for embryo development Has a zona pellucida – changes on fertilisation to prevent entry of more sperm No flagellum; non-motile No acrosome Lots of cytoplasm/food reserves

The Acrosome Reaction: Digestion of the Follicle cells Sperm meets the follicle cells (also known as granulosa cells) around the oocyte first Chemicals from these cells causes the acrosome to swell Acrosome bursts and protease enzymes are released. This digests intercellular cement holding the granulosa cells together

The Acrosome Reaction: Digestion of the zona pellucida This creates a pathway for other sperm to get through to the zona pellucida Proteases from their acrosomes digests a way through the zona pellucida to the cell-surface membrane of the oocyte The sperm cell membrane fuses with the oocyte membrane

Fertilization The head containing the nucleus enters oocyte. This triggers an influx of Ca2+ ions which causes the lysosomes of the oocyte to release their contents This causes a rapid change in the composition of the zona pellucida hardening it so that no further sperm can enter. The nuclei do not actually fuse. Both the male and females sets of chromosomes undergo DNA replication to produce double-stranded chromosome and the nuclear membranes break down.

Mammalian fertilization 1. The sperm is attracted to the ovum by hormones released by the follicle cells surrounding the ovum 2. When the sperm reaches the ovum it embeds its head in the zona pellucida, triggering the acrosome reaction 3. The acrosome swells and bursts, releasing proteolytic enzymes 4. The enzymes digest a hole into the ovum 5. Sperm membrane fuses with ovum membrane and the sperm nucleus enters the ovum by endocytosis 6. Lysosomes in the ovum cause the zona pellucida to harden once the sperm’s nucleus has entered the ovum, stopping further sperm from penetrating the ovum.

A spindle forms. The two sets of chromosomes arrange themselves on the equator of the spindle in metaphase of mitosis The first mitotic division is followed by cytokinesis to produce a two-celled embryo about 24 hours after sperm