IMAT Biology · Cell Division and Reproduction
Making Sperm and Eggs, Fertilisation and the Early Embryo
Sperm and eggs are made by mitosis then meiosis in the testes and ovaries; fertilisation joins them into a diploid zygote that divides into an embryo.
Why this matters
The papers asked once, in 2017, which cells along the path to an egg are diploid. Spermatogenesis, fertilisation and early development have not been asked yet but are on the syllabus, and they reuse the ploidy rules of meiosis.
Concept 1 of 3: Spermatogenesis: the stages of sperm production in the testis
A stock of stem cells keeps dividing by mitosis so the supply never runs out. Some of them grow, go through meiosis and become four small haploid cells, which then reshape into swimming sperm. The cell only becomes haploid at the end of meiosis I.
Definition
Spermatogenesis takes place in the seminiferous tubules of the testes, continuously from puberty.
- Spermatogonia (2n) divide by mitosis; some grow into primary spermatocytes (2n).
- Meiosis I gives two secondary spermatocytes (n); meiosis II gives four spermatids (n).
- Spermatids mature into spermatozoa without dividing.
- Sertoli cells nourish the developing sperm; Leydig (interstitial) cells between the tubules make testosterone.
| Cell | Ploidy | Made by |
|---|---|---|
| Spermatogonium | Diploid (2n) | Mitosis of germ cells |
| Primary spermatocyte | Diploid (2n), DNA copied | Growth of a spermatogonium |
| Secondary spermatocyte | Haploid (n), two chromatids per chromosome | Meiosis I (2 per primary) |
| Spermatid | Haploid (n) | Meiosis II (4 per primary) |
| Spermatozoon | Haploid (n) | Differentiation of a spermatid, no division |
Practice this conceptself-check · 3 quick reps
Sperm are made by meiosis, not mitosis
Mitosis only builds up the stock of diploid spermatogonia. The step that makes haploid cells is meiosis. Saying sperm are formed by mitosis is wrong in animals (in plants it is different: see life cycles).
Concept 2 of 3: Oogenesis: egg production and its pauses
Egg making starts before birth and then waits, sometimes for decades. Each division of the cytoplasm is unequal: almost all of it goes to one cell, so the future egg keeps the food store, and the small leftover cells (polar bodies) die. One primary oocyte gives only one egg.
Definition
Oogenesis takes place in the ovaries.
- Before birth, oogonia (2n) multiply by mitosis and grow into primary oocytes (2n), which start meiosis I and stop in prophase I.
- From puberty, in each cycle usually one primary oocyte completes meiosis I, giving a large secondary oocyte (n) and a small first polar body.
- The secondary oocyte stops in metaphase II and is released at ovulation.
- Meiosis II is completed only if a sperm enters, giving the ovum (n) and a second polar body.
- The cells of the ovary's germinal epithelium and follicles are ordinary diploid body cells.
| Cell | Ploidy | When |
|---|---|---|
| Oogonium | Diploid (2n) | Multiplies by mitosis before birth |
| Primary oocyte | Diploid (2n), DNA copied | Formed before birth; paused in prophase I until puberty |
| Secondary oocyte | Haploid (n), two chromatids per chromosome | Made by meiosis I; paused in metaphase II; ovulated |
| Polar bodies | Haploid (n) | Small cells from each unequal division; they degenerate |
| Ovum | Haploid (n) | Completed only after a sperm enters |
Practice this conceptself-check · 4 quick reps
Spermatogenesis gives four gametes, oogenesis gives one
One primary spermatocyte gives four sperm. One primary oocyte gives one egg plus polar bodies, because the cytoplasm is divided unequally. Both still go through the same two meiotic divisions.
The secondary oocyte is already haploid
The halving happens in meiosis I, so the secondary oocyte is haploid even though meiosis is not finished. Only the oogonia and primary oocytes (and the body cells of the ovary) are diploid.
Concept 3 of 3: Fertilisation and early development of the human embryo
Fertilisation joins two haploid sets into one diploid cell, and lets in only one sperm. The zygote then divides fast by mitosis without growing, so its cells get smaller. A hollow ball forms, sinks into the uterus wall, and its cells start to sort into three layers that build every organ.
Definition
Fertilisation usually happens in the oviduct (Fallopian tube).
- The acrosome at the sperm's tip releases enzymes that digest a path through the layers round the egg (the zona pellucida).
- After one sperm enters, the cortical reaction hardens the zona and blocks other sperm (polyspermy).
- The egg finishes meiosis II; the two haploid nuclei join to give a diploid zygote. The sperm (X or Y) decides the sex.
- Cleavage: fast mitosis, no growth, giving a solid morula then a hollow blastocyst (inner cell mass plus an outer trophoblast, which helps form the placenta). It implants in the endometrium about a week after fertilisation.
- Gastrulation forms the three germ layers.
- Identical twins come from one zygote that splits; non-identical twins from two eggs fertilised by two sperm.
| Germ layer | Position | Gives rise to |
|---|---|---|
| Ectoderm | Outer | Epidermis of the skin, hair, nervous system (brain, spinal cord, nerves) |
| Mesoderm | Middle | Muscle, bone, heart, blood and vessels, kidneys, gonads |
| Endoderm | Inner | Lining of the gut and airways, liver, pancreas |
Practice this conceptself-check · 4 quick reps
Identical twins share one zygote
Identical twins come from one fertilised egg that splits early, so they have the same genes. Non-identical twins come from two eggs and two sperm, so they are no more alike than other siblings.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (3)
Spermatogenesis: the stages of sperm production in the testis5 rows
| Cell | Ploidy | Made by |
|---|---|---|
| Spermatogonium | Diploid (2n) | Mitosis of germ cells |
| Primary spermatocyte | Diploid (2n), DNA copied | Growth of a spermatogonium |
| Secondary spermatocyte | Haploid (n), two chromatids per chromosome | Meiosis I (2 per primary) |
| Spermatid | Haploid (n) | Meiosis II (4 per primary) |
| Spermatozoon | Haploid (n) | Differentiation of a spermatid, no division |
Oogenesis: egg production and its pauses5 rows
| Cell | Ploidy | When |
|---|---|---|
| Oogonium | Diploid (2n) | Multiplies by mitosis before birth |
| Primary oocyte | Diploid (2n), DNA copied | Formed before birth; paused in prophase I until puberty |
| Secondary oocyte | Haploid (n), two chromatids per chromosome | Made by meiosis I; paused in metaphase II; ovulated |
| Polar bodies | Haploid (n) | Small cells from each unequal division; they degenerate |
| Ovum | Haploid (n) | Completed only after a sperm enters |
Fertilisation and early development of the human embryo3 rows
| Germ layer | Position | Gives rise to |
|---|---|---|
| Ectoderm | Outer | Epidermis of the skin, hair, nervous system (brain, spinal cord, nerves) |
| Mesoderm | Middle | Muscle, bone, heart, blood and vessels, kidneys, gonads |
| Endoderm | Inner | Lining of the gut and airways, liver, pancreas |
Watch out for (4)
- Sperm are made by meiosis, not mitosis→ Spermatogenesis: the stages of sperm production in the testis
- Spermatogenesis gives four gametes, oogenesis gives one→ Oogenesis: egg production and its pauses
- The secondary oocyte is already haploid→ Oogenesis: egg production and its pauses
- Identical twins share one zygote→ Fertilisation and early development of the human embryo
Drill the past-year questions on this topic
Open the bank with this topic pre-filtered.