Correct option is B
Explanation-
Statement A: Zygote is a totipotent stem cell
The zygote is the single-cell embryo formed immediately after fertilization. It is called totipotent because it has the ability to give rise to all embryonic tissues (ectoderm, mesoderm, endoderm) and all extraembryonic tissues (like placenta and chorion)
Totipotency is limited to the zygote and the first few divisions (2-cell to 8-cell stage).
While biologically correct, some multiple-choice questions exclude this if they're focused on stem cells in culture or specific stages beyond zygote.
Statement B: The cells of inner cell mass are said to be pluripotent - CORRECT
The inner cell mass (ICM) is a group of cells inside the blastocyst (around day 5 in human development). ICM cells are pluripotent, meaning that they can give rise to all three germ layers (ectoderm, mesoderm, endoderm)
but they cannot give rise to extraembryonic tissues (like placenta). These are the cells from which embryonic stem cells (ESCs) are derived. In vitro, pluripotent stem cells can differentiate into any body cell type, but not into an entire organism.
Statement C: The three regulatory transcription factors, Oct4, Nanog and Sox2 help maintain pluripotency of the inner cell mass - CORRECT
These three transcription factors work together to:
Maintain the stemness/pluripotency of ICM cells.
Prevent premature differentiation.
Their roles:
Oct4: Maintains undifferentiated state; downregulation causes differentiation.
Nanog: Reinforces pluripotency and self-renewal.
Sox2: Partners with Oct4 to activate other pluripotency genes.
These three are often called the core pluripotency triad.
Statement D: Cdx2 upregulates Oct4 and Nanog - INCORRECT
Cdx2 is a transcription factor expressed in the trophectoderm (outer layer of the blastocyst). It promotes the differentiation of cells into trophoblast lineage (for placenta). Cdx2 actually represses Oct4 and Nanog in those outer cells. This ensures those cells don’t remain pluripotent and instead differentiate into extraembryonic tissues.
So, Cdx2 downregulates Oct4 and Nanog, not upregulates.
So, the correct answer is option b - B and C
