Micromere cell membranes in Lychechinus pictus embryos have distinct properties

Developing embryos undergo numerous cell divisions following distinct differentiation pathways before maturing into complex organisms.  In sea urchins, beginning as early as the fourth division, specialization occurs in the vegetal cells as an unequal division gives birth to macromeres and micromeres.  These micromeres lose adhesion, migrate inside of the blastula, and give rise to the larval skeleton and germ cells.  They have been found to vary from other cells of the embryo in more than just size and transcriptome.  Others have noted that these micromeres generally lack microvilli.  Since microvillar membranes found on the apical surfaces of epithelial cells contain lipid rafts rich in gangliosides and cholesterol, we questioned whether membranes of micromeres lack such lipids.  Using spinning disc confocal microscopy we observed the presence of gangliosides through use of fluorescent cholera toxin B in fixed and in live dissociated blastomeres giving birth to micromeres. In fixed blastomeres, the presence of an even actin rich cortex was observed on all cells by the use of fluorescent phalloidin. In contrast to the even distribution of actin, the cell membranes in micromeres were found to lack cholesterol rich lipid rafts observed on all other cells of the embryo likely due to the loss of apical membrane components caused by the unique division that leads to these cells.  The process of cell division, where new membrane is delivered via exocytosis, may result in micromeres lacking “apical” cell surfaces found in the cells of the blastula.