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

Concept:

• Oocyte Membranes: During follicular development, the secondary oocyte is surrounded by distinct protective layers that play crucial roles in fertilization and early development.

• Primary Membrane (Zona Pellucida): An acellular, gel-like layer composed of glycoproteins (ZP1, ZP2, ZP3, and ZP4) secreted by the secondary oocyte itself.

• Secondary Membrane (Corona Radiata): An outer layer of radially arranged follicular (granulosa) cells that remains adhered to the zona pellucida upon ovulation.

Step 1: Analyze the state of the ovum at the time of ovulation
During ovulation, the Graafian follicle ruptures to release the secondary oocyte (often referred to as the ovum at this stage) into the peritoneal cavity, where it is swept into the fallopian tube. At this moment, the innermost specialized protective covering of the oocyte is the acellular glycoprotein envelope.

Step 2: Evaluate the options systematically

• (A) Chorion: This is the outermost extra-embryonic membrane formed later during embryonic development. It participates in the formation of the placenta and is not present on the unfertilized ovum at ovulation.

• (B) Endometrium: This refers to the glandular inner mucosal lining of the uterine wall, which prepares for implantation, rather than a membrane covering the ovum.

• (C) Zona radiata: While sometimes confused with the cellular *corona radiata*, "zona radiata" historically refers to striated membranes in the eggs of certain non-mammalian vertebrates (such as teleost fish) and is not the standard primary mammalian cover.

• (D) Zona pellucida: This is the non-cellular glycoprotein membrane that directly covers the plasma membrane of the mammalian secondary oocyte at ovulation.

Step 3: Confirm the correct choice
The primary acellular coat that encapsulates the oocyte at ovulation and must be penetrated by the sperm during fertilization is the zona pellucida. Thus, option (D) is the correct answer.