Fertilisation and Implantation

What is fertilisation?

The fusion of haploid gamete nuclei to form a diploid zygote, which then divides by mitosis as it travels to the uterus

What happens during implantation?

• Occurs 7 days after fertilisation

• If the fertilised ovum implants into the uterine wall, pregnancy begins and is maintained by hormonal secretion

How do sperm reach the egg?

• Introduced via semen during copulation

• Vaginal wall contractions assist movement

• Sperm survive 4-5 days in the reproductive tract

• Rheotaxis (upstream movement) helps sperm reach the egg

• Sperm swim via whipping tail motion through cervix → uterus → fallopian tubes

What are the three layers sperm must cross to fertilise an egg?

1. Corona radiata: Sperm push through follicle cells (that produce enzymes)

2. Zona pellucida: Acrosome reaction w/ glycoprotein releases enzymes to break through

3. Plasma membrane: allows only one sperm to penetrate → triggers enzymes that destroy the glycoproteins in zona pellucida & causes electrical changes. Sperm triggers Meiosis II.

What hormone is secreted by the embryo, and what is its function?

Human Chorionic Gonadotropin (HCG): maintains the CL to sustain during pregnancy

What are the functions of progesterone and oestrogen during pregnancy?

• Secreted by the CL until the placenta takes over

• Maintins the endometrium

• Inhibits FSH and LH secretion from the anterior pituitary gland

• High progesterone levels limit uterine activity (e,g. contraction)

What triggers birth (labour)

1. Baby presses on cervix, sending signals to the brain

2. Pituitary gland releases oxytocin

3. Oxytocin stimulates uterine contractions, increasing pressure

4. More oxytocin is released, reinforcing contractions

What hormonal changes occur during birth?

• Oestrogen and progesterone levels drop

• Relaxin soften the cervix and pelvic ligaments

• Prolactin increases to stimulate milk production

• Suckling increases prolactin and oxytocin release

What are the main hormones involved in birth>

Hormone	Secreted from	Role in birth

Oxytocin

Pituitary gland

Increases & stimulates uterine contractions

Progesterone & Oestrogen

Placenta

Decrease in final weeks → stimulates contractions

Relaxin

Ovaries + Placenta

Relaxes ligaments & cervix

Prolactin

Pituitary gland

Stimulates milk production, increases with suckling