4-DIMENSIONAL MICROSCOPIC ANALYSIS OF THE FILOPODIAL BEHAVIOR OF PRIMARY MESENCHYME CELLS DURING GASTRULATION IN THE SEA-URCHIN EMBRYO

During gastrulation of the sea urchin embryo, primary mesenchyme cells (PMCs) migrate from the vegetal pole to a site below the equator of t he embryo where they form a ring-like structure and begin producing th e larval skeleton. As these cells migrate, they extend and retract fil opodia which appear to interact with the basal lamina and underlying e ctoderm. To better characterize this behavior in vivo, we studied PMC migration using differential interference contrast (DIG) microscopy in combination with four-dimensional imaging (x,y,z space and time). We were able to determine the persistence and direction of extension of e ach filopodium and were also able to observe the dynamic behavior of e ach using colorized movie loops. This analysis showed that: (1) Most f ilopodia are quite transient, usually persisting for less than 0.5-6.0 min, during which time they continuously survey their surroundings; ( 2) PMCs extend an average of 121 filopodia/ hr during migration; (3) t he initial direction of extension of filopodia from the cell body is r andom, with just as many filopodia projecting toward as away from the direction of migration; (4) as a consequence of (2) and (3) above, eac h PMC explores the area surrounding its cell body approximately once e very 5 min; (5) PMCs nearer to the target site migrate faster than tho se located farther away. To further investigate filopodial distributio n, confocal microscopy was used to collect z series of PMCs transplant ed to different locations in the embryo and fixed during migration. We found that more filopodia tended to be distributed toward the target site as cells approached the ring, suggesting that filopodial distribu tion may reflect regional differences in directional cues. (C) 1995 Ac ademic Press, Inc.