What Stops More Than One Sperm From Entering an Egg

A single cell formed at fertilisation grows, over nine months, into a newborn baby. Along the way it must travel to the uterus, embed in its lining, build a placenta, and finally be pushed out and fed.

This part covers fertilisation and sex determination, cleavage and implantation, the placenta and pregnancy, and parturition and lactation.

Fertilisation occurs at the ampullary-isthmic junction of the oviduct when a sperm contacts the zona pellucida of the secondary oocyte; the acrosomal reaction lets it penetrate, a cortical reaction then changes the zona to block other sperm, and the X or Y chromosome carried by the sperm decides whether the zygote is female or male.

Steps:

- The released ovum is swept into the oviduct; fertilisation happens only if sperm reach the ampullary-isthmic junction at the same time
- Acrosomal reaction — enzymes from the acrosome digest the layers around the oocyte
- Cortical reaction — once one sperm enters, changes in the zona pellucida block entry of more sperm, preventing polyspermy
- Sperm entry triggers the secondary oocyte to complete meiosis II, forming the ovum and a second polar body
- The haploid nuclei of sperm and ovum fuse into a diploid zygote

Sex determination:

- Every ovum carries an X chromosome
- Half of the sperm carry X and half carry Y
- An XX zygote develops as a female; an XY zygote as a male

An everyday example. Blaming mothers for the birth of daughters is scientifically wrong — the chromosome from the sperm, not the ovum, decides the baby's sex.

The substance. The ovum completes meiosis II only at fertilisation, which is why the second polar body appears at that moment.

As the zygote moves along the oviduct it divides by mitosis, called cleavage, into 2, 4, 8 and 16 cells; an embryo of 8 to 16 cells is a morula, which becomes a blastocyst with an outer trophoblast and an inner cell mass, and the blastocyst embeds in the endometrium during implantation.

Cleavage:

- Mitotic divisions begin as the zygote travels towards the uterus
- The daughter cells are called blastomeres
- An embryo with 8 to 16 blastomeres is a morula

Blastocyst:

- The morula keeps dividing and becomes a blastocyst as it enters the uterus
- Trophoblast — the outer layer, which attaches to the endometrium and helps form the placenta
- Inner cell mass — attached inside the trophoblast; it becomes the embryo

Implantation:

- The trophoblast attaches to the endometrium
- Uterine cells divide rapidly and cover the blastocyst, embedding it in the lining
- Implantation marks the start of pregnancy

An everyday example. Identical twins arise when a very early embryo splits and each part develops on its own.

The substance. Cleavage increases cell number without increasing size — the cells get smaller with each division until implantation supplies nourishment.

After implantation, finger-like chorionic villi from the trophoblast interlock with uterine tissue to form the placenta, which supplies oxygen and nutrients, removes wastes and secretes hormones such as hCG, hPL, oestrogens and progestogens; the heart forms by the end of the first month and most organ systems by the end of the third.

Placenta:

- Chorionic villi — finger-like projections surrounded by uterine tissue and maternal blood
- The umbilical cord links the placenta to the embryo and carries substances to and from it
- Supplies oxygen and nutrients, removes carbon dioxide and wastes, and acts as an endocrine tissue

Hormones of pregnancy:

- The placenta secretes hCG (human chorionic gonadotropin), hPL (human placental lactogen), oestrogens and progestogens
- Relaxin is secreted by the ovary in the later phase of pregnancy

Landmarks:

- End of month 1 — the heart forms
- End of month 2 — limbs and digits develop
- End of month 3 — most major organ systems and the external genital organs are well developed
- End of month 9 — the foetus is fully developed and ready for delivery

An everyday example. Doctors listen for the foetal heartbeat as one of the first signs of healthy growth.

The substance. The placenta keeps maternal and foetal blood separate — substances pass across a thin barrier instead of the blood mixing.

Parturition is triggered by a neuroendocrine foetal ejection reflex: signals from the fully developed foetus and placenta cause mild uterine contractions, which release oxytocin from the maternal pituitary, which causes stronger contractions and still more oxytocin until the baby is delivered; lactation then supplies milk, beginning with antibody-rich colostrum.

Parturition:

- Signals come from the fully developed foetus and the placenta
- They induce mild uterine contractions — the foetal ejection reflex
- This triggers release of oxytocin from the maternal pituitary
- Oxytocin makes the uterine muscle contract more strongly, which releases more oxytocin
- The cycle continues until the baby is expelled through the birth canal, followed by the placenta

Lactation:

- The mammary glands differentiate during pregnancy and start producing milk towards its end
- Colostrum, the milk of the first few days, contains antibodies essential for building resistance in the newborn
- Breastfeeding during early growth is recommended by doctors for raising a healthy baby

An everyday example. Doctors sometimes give oxytocin to induce or strengthen labour, using the same hormone the body releases naturally.

The substance. Colostrum's antibodies give passive immunity, protecting the newborn before its own immune system matures.