In the early stages of mammalian embryogenesis, the inner cell mass (ICM) of the blastocyst holds the key to forming all the tissues of the developing organism. This ability, termed pluripotency, is tightly regulated by a network of transcription factors that maintain stem cell self-renewal and suppress differentiation into specialized cell types too early.

The three transcription factors at the center of this network are Oct4, Sox2, and Nanog. These factors cooperate to regulate gene expression programs that ensure the ICM cells remain pluripotent. Oct4 (also known as Pou5f1) functions as a master regulator, directly activating pluripotency-associated genes and maintaining the undifferentiated state. Sox2 partners with Oct4 on regulatory DNA elements, reinforcing the pluripotency network and driving embryonic stem cell identity. Nanog further stabilizes pluripotency by inhibiting genes that promote differentiation and enabling the cells to proliferate but remain undifferentiated.

This core set establishes an autoregulatory loop, wherein the three transcription factors bind to their own genes as well as to each other’s promoters, maintaining consistent expression levels conducive to the pluripotent state. Disrupting any one of these transcription factors typically causes loss of pluripotency and initiation of differentiation pathways.

Other factors such as Cdx2 are critical for specifying alternative lineages like the trophoblast but do not maintain pluripotency. The unique interplay among Oct4, Sox2, and Nanog ensures the inner cell mass cells retain the potential to become any cell type in the body, a fundamental prerequisite for normal development and regenerative capabilities.

In conclusion, the pluripotency of the inner cell mass in mammals is maintained primarily by:

(1) Oct4, Sox2, and Nanog.

This foundational knowledge underpins much of stem cell biology, developmental research, and advances in regenerative medicine.

Answer: (1) Oct 4, Sox 2 and Nanog