Positioning of inner cell mass determines the development of mouse blastocysts in vitro.

We have explanted mouse blastocysts into culture dishes and studies the positioning of the inner cell mass (ICM) at the time of attachment in order to establish what effect the location of the inner cell mass has on the subsequent development of embryos in vitro. We show that blastocysts may attach to the substrate with any portion of the trophectoderm. The location of ICM within the attaching blastocysts is unpredictable. The directions of egg-cylinder development may be either upward or downward. The axis assumed by a developing egg cylinder in vitro depends to a large extent on the initial positioning of the ICM at the time of attachment. Egg cylinders grown from blastocysts that have the ICM in the lower position (close to the plastic dish) reach advanced stages of development in greater numbers than egg cylinders grown from blastocyst with their ICM's in the upper lateral or upper position. Only upward-growing egg cylinders can develop further into somitic stage (which is equivalent to 8 1/2- or 9-day in vivo embryos). In contrast, downward growing egg cylinders become thwarted in their further development by the surrounding trophectoderm. If the downward growing egg cylinders are freed of trophectoderm and repositioned to face upwards they will develop in a manner similar to the originally upward growing egg cylinder. These findings indicate that the initial positioning of the ICM at the time of attachment is an important factor in governing the development of blastocysts to somite stage in vitro.