Question

The covering of ovum at ovulation is

• A. chorion
• B. endometrium
• C. zona radiata
• D. zona pellucida

Hint:

• Acellular vs. Cellular: The inner zona pellucida is non-cellular (glycoprotein-based), whereas the outer corona radiata is cellular (composed of granulosa cells).

• Function of Zona Pellucida: It contains species-specific sperm receptors (primarily ZP3) and undergoes the "cortical reaction" after sperm entry to prevent polyspermy.

• It also prevents premature implantation (ectopic pregnancy) of the blastocyst while it travels through the fallopian tube.

Answer 1

The Correct Option is D

Step 1: Picture the ovum at ovulation.
At ovulation, the Graafian follicle ruptures and releases the secondary oocyte, which is wrapped in protective layers.
Step 2: Recall the layers around the oocyte.
The innermost acellular glycoprotein coat is the zona pellucida, and outside it lie the follicular cells called the corona radiata.
Step 3: Rule out chorion.
The chorion is an extra-embryonic membrane that forms much later during embryonic development and helps make the placenta, so it does not cover the ovum at ovulation.
Step 4: Rule out endometrium.
The endometrium is the inner lining of the uterus, not a covering of the egg, so it is wrong.
Step 5: Rule out zona radiata.
Zona radiata refers to striated egg membranes in some non-mammalian vertebrates, not the standard mammalian covering.
Step 6: Choose zona pellucida.
The acellular glycoprotein membrane directly covering the oocyte at ovulation is the zona pellucida, which the sperm must penetrate during fertilisation.
\[ \boxed{\text{zona pellucida}} \]