Question

Assertion in a morula stage, cells divide without increase in size.
Reason: Zona pellucida remains undivided till cleavage is complete.
A. Both Assertion and reason are correct and reason is the correct explanation for reception
B. Both assertion and reason are correct. But reason is not the correct explanation for assertion.
C. Assertion is a correct but reason is incorrect
D. Both the assertion and reason are incorrect. Early stage embryo consisting of 16 cells (called blastomeric) in a solid ball contained within the zona pellucida.

Answer 1

Hint: An early stage in post-fertilization development when cells have rapidly mitotically divided to produce a solid mass of cells (16 or more) with a “mulberry” appearance is called the morula stage.

Complete Answer:
- During compaction, cells on the outer part of the morula become bound tightly together with the formation of desmosomes and gap junctions, becoming nearly indistinguishable.
- These changes in cell morphology and cell-cell adhesion initiates the formation of the solid ball of cells.
- With time, the cells on the outside and inside become differentially fated into trophoblast (outside) and inner cell mass (inside) progenitors.
- A key event prior to morula formation is "compaction", where the 8 cell embryo undergoes changes in cell morphology and cell-cell adhesion that initiates the formation of this solid ball of cells.
- The cleavage divisions of the individual cells do not have to happen synchronously and thus phase shiftings can occur. The result is that the cell divisions do not happen according to the sequence 4, 8, 16, 32, 64, 128..
- But rather at any given moment any number of cells can be present in the embryo.After fertilization, the zona pellucida undergoes a striking change. From the moment of fusion of the gametes onwards, the zona pellucida changes to become an impermeable membrane, separating the zygote physically and physiologically from the environment.

Therefore, from the above explanation the correct option is (B) Both assertion and reason are correct. But reason is not the correct explanation for assertion.

Note: The name of morula stage comes from its resemblance to a mulberry, since it really looks like a collection of spherical cells.