The distribution of ingested horseradish peroxidase in the 16-cell mouse embryo

Cells of the 16-cell mouse embryo endocytose horseradish peroxidase (HRP) which becomes localized in most cases to a juxtanuclear position. Cells that have ingested HRP in intact embryos, and cells dissociated from embryos prior to culture in HRP, showed similar patterns of cytoplasmic distribution of the ingested enzyme. Cells in the embryo in situ were incubated in HRP, and then labelled with fluorescent antibody either before (to label the outside surface of the embryo) or after (to reveal populations of outer polar and inner apolar cells) their disaggregation into single cells. The population of polar outside cells from the morula includes more cells with a highly restricted localization of HRPcontaining vesicles than does the population of inside cells, and this restricted localization underlies the exposed surface or pole of the cell. A 2/16 couplet formed by division in vitro of a 1/8 cell is comparable to the pairs of cells dissociated from 16-cell embryos; most couplets from either source consisted of a larger cell that showed polarized surface binding of fluorescent ligand (fluorescent pole) and a smaller cell with a uniform distribution of bound ligand. The incidence of restricted patterns of HRP staining was highest among populations of both larger and polar cells. When 1/8 cells labelled with HRP are observed during division to 2/16, the previously clustered vesicles of ingested HRP become more dispersed throughout the cytoplasm and, although the two cells of some couplets can stain differently very soon after their formation, the patterns of distribution of HRP take about 1 h after division to stabilize. These observations are consistent with cells of the 16-cell embryo inheriting different features of cytoplasmic organization.