#Question id: 7185

Following statements are regarding to establishing the three separate cell lineages in the mouse blastocyst.

A. The first two cell types to be specified are the inner cell mass and the trophectoderm. This involves progressive restriction of expression of the genes that code for transcription factors Oct4 and Cdx2.

B. At the eight-cell stage, all the cells express both genes at low levels, but by the late morula/early blastocyst stage, Cdx2 expression has increased in, and eventually becomes restricted to, the cells on the outside of the embryo, and these will form trophectoderm. Oct4

expression becomes restricted to the inner cell mass cells at early blastula stage.

C. Within the inner cell mass, two separate cell types arise: the epiblast cells, from which the embryo will develop; and the primitive endoderm, which will form extra-embryonic structures. This involves a progressive restriction of expression of the genes for the transcription factors Nanog and Gata6.

D. At the early blastula stage, some of the inner cell mass cells apparently at random start to express higher levels of Nanog, whereas the other cells express higher levels of Gata6.

E. By the late blastula, the cells expressing the two different transcription factors have sorted out within the inner cell mass so that cells expressing high levels of Nanog form an outer epithelial layer of primitive endoderm enclosing the epiblast cells expressing high levels of Gata6.

Which of the following statements are correct?