In mammalian development, the zygote represents the earliest developmental stage, possessing totipotency—the unique ability to give rise to an entire organism including both embryonic and extraembryonic tissues. This totipotency is endowed through epigenetic reprogramming immediately post-fertilization, involving processes like DNA demethylation to reset the parental genomes.

The cells of the inner cell mass (ICM), which develop later within the blastocyst, exhibit pluripotency. They retain the capacity to differentiate into nearly all cell types that form the embryo proper but cannot generate extraembryonic structures like the placenta.

Maintaining pluripotency in the ICM depends on a core network of transcription factors—Oct4, Nanog, and Sox2—that regulate gene expression patterns essential for preserving the undifferentiated state and self-renewal capabilities.

Conversely, Cdx2 is a transcription factor involved in trophoblast specification and does not upregulate Oct4 or Nanog; in fact, it antagonizes these factors to promote lineage segregation.

Thus, the correct statements combining these facts are:

(1) (A) Zygote is a totipotent stem cell.

(B) The cells of inner cell mass are said to be pluripotent.

This combination accurately reflects current developmental biology understanding, emphasizing distinct but complementary roles of totipotency and pluripotency in embryogenesis.

Answer: (1) A and B