Maternal X chromosome expression in mouse chorionic ectoderm

An electrophoretic variant of the X‐linked enzyme phosphoglycerate kinase (PGK‐1) has been used to study regulation of X chromosome expression in the diploid derivatives of the trophectoderm at 8–8.5 days post coitum in the mouse. These derivatives included the chorionic ectoderm and the polar trophoblast. The biochemical analysis suggests that only the maternally derived X chromosome (X m ) is expressed in the diploid trophectoderm derivatives. Cell selection and maternal tissue contamination were ruled out as possible causes of the observed X m expression. From these and other results, we conclude that all derivatives of the trophectoderm, along with the primitive endoderm, express only X m , whereas derivatives of the primitive ectoderm show random X chromosome expression.