Biology 4.1 - Fertilisation, Implantation and Pregnancy.

Fertilisation 

Fertilisation - the fusion of a male and female gamete to produce a zygote. 

• Once sperm is deposited in the uterus, it responds to the secondary oocytes chemoattractants and swims into the oviduct.
• Capacitation now occurs, in which the membrane of the sperm head in front of the acrosome layer becomes more permeable to calcium ions and more fluid. This occurs by removing cholesterol and glycoproteins. 
• On contact with the corona radiata the acrosome layer secretes proteases that digest it.
• Acrosin is then released by the acrosome to digest the zona pellucida. 
• The cell membranes of the sperm and oocyte fuse and the sperm head penetrates the secondary oocyte. 
• This triggers the cortical reaction in which the cortical granules fuse with the cell membrane to prevent polyspermy (the entry of more than one sperm).
• The entry of new genetic material stimulates the completion of meiosis II and so the secondary oocyte becomes a fertilised ovum. 
• The nuclei of the sperm and oocyte fuse to form a zygotic nucleus = zygote. 
• The first mitotic division occurs in which two cells are produced. Until organs form, this collection of cells is referred to as an embryo. 

Implantation 

Implantation - the sinking of the blastocyst into the endometrium. 

• As the embryo moves down the oviduct it divides repeatedly by mitosis in a sequence called cleavage. 
• A solid ball of 16 cells is formed = morula.
• After 7 days, the ball of cells becomes hollow. It is now called a blastocyst. 
• The blastocyst is surrounded by a layer of cells called trophoblasts.
• After ovulation, the endometrium thickens and its blood supply increases in preparation for the blastocyst. 
• The trophoblasts of the blastocyst have protrusions (trophoblastic villi) that help to penetrate the endometrium. They also increase the surface area for the exchange of nutrients and respiratory gases from the endometrium. 

Hormones and pregnancy 

• Following implantation, the blastocyst will begin to secrete the hormone Human Chorion Gonadropin (hCG). 
• Following implantation, this is secreted by the chorion of the placenta. 
• Its function is to maintain the corpus luteum for the first 16 weeks of pregnancy. The corpus luteum MUST be maintained as it secretes progesterone that maintains the endometrium. The endometrium provides nutrients to the growing embryo and eventually forms part of the plcaneta. 
• The placenta also secretes progesterone and oestrogen, which inhibit the production of luteinising hormone, LH (so ovulation does not occur), follicle stimulating hormone, FSH (so no more follicles mature) and prolactin (so milk is not synthesised.
• Progesterone also ensures oxytocin is not secreted - the myometrium and walls of the milk ducts cannot contract.
• Oestrogen also stiumulates the growth of the uterus to accomodate the growing foetus, the growth and development of mammary glands and increases their blood supply. 

Summary of the stages of pregnancy 

1ST TRIMESTER (weeks 0 - 12)

• Includes conception, implantation and embryogenesis.
• All major organs are laid down.
• Some electrical activity.
• By day 22 the foetus…