In vitro development of inner cell masses isolated immunosurgically from mouse blastocysts: I. Inner cell masses from 3·5-day p.c. blastocysts incubated for 24 h before immunosurgery

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Abstract
This paper describes the in vitro development of inner cell masses isolated immunosurgically from mouse blastocysts which had been collected on 3 ·5 days p.c. and then incubated for 24 h. The inner cell masses continue to grow in culture and develop through a series of stages with increasing complexity of internal organization. By day 1 all of the cultured ICMs have an outer layer of endoderm, and by day 3 some of them have two distinct kinds of inside cells; a columnar epithelial layer and a thin hemisphere of elongated cells. Later, mesodermal cells appear to delaminate from a limited region of the columnar layer, close to where it forms a junction with the thinner cells. By day 5, about 25 % of the cultured ICMs have a striking resemblance to normal 7 ·5-day p.c. C3H embryos, with embryonic ectoderm, extra-embryonic ectoderm and chorion, embryonic and extra-embryonic mesoderm, and visceral endoderm. When mechanically disrupted and grown as attached clumps of cells in a tissue dish, these embryo-like structures give rise to trophoblast-like giant cells. These results suggest that the inner cell mass of 4 ·5-day p.c. blastocysts contains cells which can give rise to trophoblast dérivâtes in culture.