Oogenesis and spermatogenesis: one ovum or four sperm
Each cell that enters meiosis gives one functional ovum in oogenesis (the making of egg cells) but four sperm in spermatogenesis (the making of sperm).
Remember
In oogenesis the cytoplasm is shared out unevenly, leaving one large ovum and polar bodies (tiny cells with minimal cytoplasm that degrade), whereas in spermatogenesis it is shared equally, so each cell entering meiosis gives four sperm.
Sperm production, spermatogenesis, starts with the spermatogonium, a diploid stem cell (a cell with 46 chromosomes that can keep dividing to renew the supply). One of its daughters becomes a primary spermatocyte (an early stage that goes on to undergo meiosis). Meiosis I gives two secondary spermatocytes (haploid after meiosis I), and meiosis II gives four haploid spermatids (early sperm cells that still look like ordinary round cells). These mature into sperm. Because the cytoplasm is shared equally, all four are functional, and a full cycle takes about 64 days.
Oogenesis is built around unequal division. A primary oocyte (the cell that undergoes meiosis, formed in the foetus from the ovarian stem cells) begins meiosis I before birth and then pauses. The chromosomes are still divided equally, so each cell receives a haploid set (half the normal number, 23 chromosomes), but at both meiotic divisions one cell keeps almost all of the cytoplasm. Meiosis I leaves a large secondary oocyte (the cell that receives nearly all of the cytoplasm) and a tiny first polar body (a small cell that is later discarded). Meiosis II, which is completed only if a sperm enters, leaves one large ovum and a second polar body. A polar body is a small cell with almost no cytoplasm that cannot develop and soon degrades.
The first polar body may or may not divide again, so up to three polar bodies can be formed, and the textbook outcome is one ovum and three polar bodies. The single large ovum keeps the nutrients and other material that the early embryo will need. Spermatogenesis, with its equal sharing, gives four sperm.
Key facts
- Oogenesis output
- one large ovum and small polar bodies from each primary oocyte, rather than four equal gametes.
- Polar body count
- three in the textbook outcome, although the first polar body may or may not divide again, so up to three polar bodies can be formed.
- Spermatogenesis output
- four haploid sperm, each with 23 chromosomes, from each primary spermatocyte.
- Cytoplasm
- shared very unevenly in oogenesis but equally in spermatogenesis.
- Meiosis II in the oocyte
- completed only if a sperm enters.
- Sperm timing
- about 64 days from spermatogonium to formed sperm, roughly two months.
Exam traps
- Four equal functional gametes are the outcome of spermatogenesis, not oogenesis, because in oogenesis the cytoplasm is shared unevenly and one cell becomes a functional ovum.
- Three ova with one polar body reverses the textbook ratio of one ovum to three polar bodies, because the cells that receive almost no cytoplasm cannot become ova.
- An ovum does not form on its own: the other products of meiosis are real cells, the polar bodies, which form and then degrade.
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