Correct option is D
Explanation:
Statement A: Incorrect
- Xenopus eggs have yolk, but they undergo holoblastic (not meroblastic) cleavage.
- Holoblastic cleavage occurs when the entire egg divides, though unevenly due to yolk presence.
- Meroblastic cleavage occurs when only part of the egg divides (e.g., in birds and fish).
Statement B: Incorrect
Embryonically transcribed β-catenin in the blastomeres of sea urchin embryos regulates the autonomous specification of micromeres.
· β-catenin itself does not directly regulate autonomous specification in sea urchin micromeres. Instead, it is a key player in conditional specification and later mesodermal and endodermal lineage specification.
Statement C: Correct
The sodium pump activity in the trophoblast helps in the formation of the blastocoel of a mammalian blastocyst.
· the formation of the blastocoel in a mammalian blastocyst is driven by sodium (Na⁺) and water transport through the trophoblast (specifically, the trophectoderm layer).
Statement D: Correct
- The zona pellucida (ZP) prevents polyspermy and ectopic (tubal) pregnancy.
- The ZP hardens after fertilization to prevent further sperm entry (cortical reaction).
- It keeps the fertilized egg from implanting too early, ensuring it reaches the uterus before attachment.
Information Booster
· Cleavage in Xenopus → Holoblastic cleavage, not meroblastic, as cleavage occurs throughout the egg despite yolk presence.
· β-Catenin in Sea Urchins → Regulates mesoderm and endoderm fate, but micromere specification is controlled by maternal factors (Disheveled, Pmar1), not embryonically transcribed β-catenin.
· Sodium Pump in Trophoblast → Na⁺/K⁺ ATPase in the trophectoderm creates an osmotic gradient, drawing water into the blastocoel, essential for blastocyst formation.
· Zona Pellucida in Humans → Prevents polyspermy, regulates sperm binding, and prevents premature implantation (reducing risk of tubal pregnancy).
