Worm Breeder's Gazette 13(4): 48 (October 1, 1994)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
MRC, Cambridge, England
Cell division axes are often controlled precisely in embryos, by intracellular cues (for example, Hyman and White, 1987; Dan, 1979), or by extracellular cues. Extracellular cues have been found to work either by mechanical deformation of cell shape (Symes and Weisblat, 1992) or by induction, as changes in cell fate are often followed by changes in cell division patterns (Sternberg and Horvitz, 1986). I have found evidence that the division axes of some early cells, EMS and E, are controlled by specific cell-cell contacts with their posterior neighbors (EMSP2 or E-P contact). Altering the orientation of contact between these cells(1) alters the orientation in which the EMS or E cell divides. I have been following this up with timelapse videomicroscopy of centrosome movements and anti-tubulin immunofluorescence to visualize asters and centrosomes These have been done primarily in the EMS cell, in both intact embryos and isolated cell pairs. Contact-dependent mitotic spindle orientation appears to work by establishing a site of the type described by Hyman and White (1987) and Hyman (1989) in the cortex of the responding cell: one centrosome moves toward the site of cell-cell contact during rotation, both in intact embryos and re-oriented cell pairs(2). The effect is especially apparent when two donor cells are placed on one side of the responding cell. Both centrosomes are "captured", pulling the nucleus to one side of the cell. No centrosome rotation occurs in the absence of cell-cell contact, nor in nocodazole-treated cell pairs. The relationship between gut induction and spindle orientation in EMS is being examined, as both require contact between P and EMS. When P and EMS are isolated in the first five minutes of the EMS cell cycle, neither effect occurs. Placing P onto EMS soon after this time still rescues gut induction, but can no longer rescue the spindle orientation effect. In these cell pairs EMS divides in various orientations, yet gut differentiation generally occurs, suggesting that proper spindle orientation is not necessary for gut induction. There is however one spindle orientation which appears to be incompatible with gut induction: when the EMS cleavage furrow forms directly through the site of cell-cell contact, gut differentiation does not occur (0/10 cases, compared to 14/14 of other orientations). The results suggest that some of the cortical sites described by Hyman are established cell-autonomously (in P[l], P, and P), and some are established by cell-cell contact (in EMS and E). Contact-dependent mitotic spindle orientation appears to play a role in ensuring that developmental information received via induction is partitioned between daughter cells. It might also play a role later in morphogenesis, generating lines of cells in the embryo. Notes: (1) That orientation of cell pairs was altered has been confirmed by seeing that no whole cell rotation occurs in high magnification timelapse recordings of cell pairs, and by noting in live and fixed cells the random position of the centrosomes after cells are apposed. (2) The direction of rotation in EMS is at odds with Waddle's finding that actin-capping protein localizes to the anterior side of EMS during rotation (Waddle et al., 1994). Schierenberg previously noted that the nucleus in these cells moves posteriorly to the site of cell-cell contact, and is dependent on contact with P2 in EMS (Schierenberg, 1987). References: Dan (1979) Dev Growth Diff 21(6):527-535. Hyman and White (1987) J Cell Biol 105:2123-2135. Hyman (1989) J Cell Biol 109:1185-1193. Schierenberg (1987) Dev Biol 122:452-463. Sternberg and Horvitz (1986) Cell 44:761-772. Symes and Weisblat (1992) Dev Biol 150:203-218. Waddle et al. (1994) Development 120:2317-2328.