Sperm vs ovum structure: is it ovum or secondary oocyte that is released at ovulation, and what do the layers do?
Tanvi Deshmukh12th Sample
19 Sept 2026 Β· 1 min read
I am making a super neat comparison table (colours, arrows, stickers, all that!) for sperm and ovum, but I got stuck on two rows and I hate leaving blanks!
Sperm: head (haploid nucleus + acrosome), neck, middle piece (mitochondria), tail. Plasma membrane covers the whole thing. The acrosome has enzymes that help in fertilisation. Mitochondria in the middle piece provide energy for tail movement. Cool, that part is done!
Ovum: This is where I'm stuck. The follicle ruptures at ovulation and releases... the ovum? Or the secondary oocyte? NCERT says secondary oocyte, but then people (and my own notes) say ovum in many places. At what stage does meiosis II complete? Also what is the use of zona pellucida and corona radiata? I know the order from outside is corona radiata β zona pellucida β egg membrane but I don't know why the layers matter for fertilisation.
Bonus: how many sperm does a man release and how many have to be normal for fertility? I have 200β300 million and then forgot the percentages. Help please π
7 answers & replies
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[+][β]Pooja Iyer12thSampleΒ· 1mo ago
Sperm parts, the ones that people forget: the acrosome is a cap-like structure over the anterior part of the head, filled with enzymes (hydrolytic) that help the sperm digest its way through the layers of the ovum. The middle piece has mitochondria arranged around the tail axis, giving energy for swimming. The tail's movement (flagellum) is what makes the sperm motile, and motility is essential for fertilisation. Neck is the short connecting piece between head and middle piece.
[+][β]Nisha Verma12thSampleΒ· 1mo ago
Ovum: at ovulation the follicle ruptures and the ovum is released along with a few layers of cells (corona radiata) and the zona pellucida around it. By then it has completed meiosis II and has a haploid nucleus, so the ovum is ready for fusion with the sperm. The outer layer corona radiata is formed by granulosa cells, so fertilisation needs the sperm to pass these layers first.
[+][β]Kavya NaikDropperSampleΒ· 1mo ago
Nisha, one fix. What is released at ovulation is a secondary oocyte, not a finished ovum. During oogenesis, the primary oocyte completes meiosis I just before ovulation, forming a large secondary oocyte and a small first polar body. The secondary oocyte then stops at metaphase II. It only completes meiosis II if a sperm enters, forming the ovum (haploid) and a second polar body. So in the exam, "ovum is released at ovulation" is technically wrong, and "meiosis II completes after sperm entry" is the line to remember.
[+][β]Tanvi Deshmukh12thSampleΒ· 1mo agoAuthor
this is exactly what I needed!! the metaphase II stop is going in a pink box π thank you kavya and rahul bhaiya!!
[+][β]Nisha Verma12thSampleΒ· 1mo ago
yep my bad, I was reading "ovum" in the chapter heading and mixed it up. appreciate the fix, kavya
[+][β]Ishita GoyalDropperSampleΒ· 1mo ago
For the semen numbers: NCERT says a normal ejaculate has about 200 to 300 million sperms, and for normal fertility at least 60 per cent of them should have normal shape and size, and at least 40 per cent should show vigorous motility. Note that it's not all sperm that needs to be perfect, just the 60% and 40% thresholds. I put a small box in my table with those two numbers.
[+][β]Rahul DubeyDropperSampleΒ· 1mo ago
Why the layers matter: the sperm first has to get through the corona radiata, then the zona pellucida (a glycoprotein layer). When the acrosome enzymes digest a path through the zona pellucida, the sperm reaches the membrane. And the moment one sperm touches the zona pellucida, the membrane changes in a way that blocks other sperm from entering, so only one sperm fertilises one egg. That's NCERT's line, and it's why polyspermy (more than one sperm) doesn't normally happen.
