10 min readPathway sheetA 3-mark question on fertilisation or the blastocyst, 1-mark MCQs on the site of fertilisation, the morula and who decides the sex of the baby, and spotting the T.S. of blastula in the practical

Fertilisation, embryo development up to the blastocyst, and implantation

In the ampulla, one sperm fertilises the secondary oocyte to form a zygote. Cleavage gives a morula and then a blastocyst, which implants in the endometrium.

Site of fertilisationThe ampullary region of the fallopian tube

MorulaAn embryo with 8 to 16 blastomeres

BlastocystAn outer trophoblast and an inner cell mass; the trophoblast attaches to the endometrium

Sex of the babyDecided by the sperm: an X-bearing sperm gives XX (girl); a Y-bearing sperm gives XY (boy)

Fertilisation

1
During coitus, semen is released into the vagina (insemination). Motile sperms swim through the cervix and uterus to the ampulla of the fallopian tube.
2
The ovum is also carried to the ampulla. Fertilisation happens only if the ovum and sperms reach it at the same time. That is why not every coitus leads to pregnancy.
3★
A sperm touches the zona pellucida. This causes changes in the membrane that block other sperms, so only one sperm fertilises the ovum.
4
Secretions of the acrosome help the sperm pass through the zona pellucida and plasma membrane into the cytoplasm of the ovum.
5
Sperm entry makes the secondary oocyte complete meiosis II. This division is also unequal: it gives a haploid ovum (ootid) and a second polar body.
6★
The haploid nuclei of the sperm and the ovum fuse to form a diploid zygote.

★ key step

Who decides the sex of the baby?

Mother's gametesA woman is XX, so every ovum carries an X chromosome.

Father's gametesA man is XY, so 50% of sperms carry X and 50% carry Y. X sperm + ovum → XX (girl). Y sperm + ovum → XY (boy).

So, scientifically, the sex of the baby is decided by the father's sperm, not by the mother.

Draw it: from zygote to implantation

After implantation, the inner cell mass differentiates into the embryo, and pregnancy begins.

Activity: spotting the T.S. of a mammalian blastula (permanent slide)

Look for a hollow ball of cells around a fluid-filled cavity.
A single outer layer of flat cells: the trophoblast.
A cluster of cells attached to one side of the inner wall: the inner cell mass, which will form the embryo.
The cavity: the blastocoel.
Spotting comment: identify it as a mammalian blastocyst; at implantation the trophoblast attaches to the endometrium, and the inner cell mass becomes the embryo.

How to draw a blastocyst

Draw a circle with a thin ring of flat cells around it; label it trophoblast.
Inside, at the top, draw a cluster of rounded cells attached to the ring; label it inner cell mass.
Label the empty space blastocoel (cavity).
Title: Mammalian blastocyst.

In real life

In India it is illegal to tell parents the sex of an unborn baby, to stop female foeticide. Biology adds a clear message: the father's sperm, not the mother, decides whether a baby is a boy or a girl.

Exam angle

2 marks: “How is the sex of a human baby decided at fertilisation? Who decides it?” Ova all carry X; half the sperms carry X and half carry Y. X sperm + ovum → XX (girl); Y sperm + ovum → XY (boy). So the father's sperm decides the sex.

3 marks: “Describe the changes in the zygote from fertilisation until implantation.” Cleavage in the isthmus into 2, 4, 8, 16 blastomeres; morula (8 to 16 cells); blastocyst with trophoblast and inner cell mass in the uterus; trophoblast attaches to the endometrium; uterine cells cover it; implantation; pregnancy.

1 marks: “Name the layer of the blastocyst that attaches to the endometrium.” The trophoblast.

Remember it

Zygote → Morula → Blastocyst → Implantation: 'Zebras Make Big Impressions'.