E.E. Just's “independent irritability” revisited: The activated egg as excitable soft matter

Abstract

Ernest Everett Just's experimental work on post-fertilization events in invertebrate eggs led him to posit a dynamic and directive role for the zygotic "ectoplasm" (cortical cytoplasm), in subsequent development. His perspective was neglected during the years that followed his early death not only because of his well-documented marginalization as an African-American in U.S. science, but because his ideas were at odds with the growing gene-centrism of developmental biology in the latter half of the 20th century. This essay reviews experimental work that shows that the egg cortex in many animal groups is a chemically and mechanically active medium that sustains both spatiotemporal calcium ion transients and periodic deformations in the time leading up to cleavage. These wave phenomena are seen to play regulatory roles in germ plasma localization and gene expression, and influence the reliability and success of developmental outcomes. Just resisted vitalistic explanations for the active processes he observed and inferred regarding the egg cortical cytoplasm, but recognized that the physics and chemistry of his time were inadequate to account for these phenomena and anticipated that expansions of these fields would be necessary to explain them. Here again he proved prescient. Late 20th century developments in the physics of "excitable media" and "soft matter" have provided the bases for models, some of which are described here, of chemical and mechanochemical wave propagation in the activated egg cortex. Lastly, the implications of these post-fertilization phenomena for animal evolution, a problem also addressed by Just, are discussed.

Development and Evolution: The Physics Connection

“…DPMs that refer to individual cell functions such as the POLs and OSC, are to be understood as designating the multicellular consequences of those functions (Based on Newman and Bhat 2008. See Newman and Bhat 2008, 2009 connection between evolution and development has led us to conclusions at odds with the Modern Synthesis, though our framework finds support from findings by investigators working within Evo-devo employing different paradigms from ours: (i) Morphological evolution does not necessarily track genetic evolution; largescale morphological change can occur with a minimum of genetic change, while morphology can be static despite extensive genetic change (e.g., Kuraku and Meyer 2008;Cardoso et al 2009) (ii) Phenotypic change can precede associated genotypic change (e.g., WestEberhard 2003;Palmer 2004). (iii) Macroevolutionary change can be very rapid (e.g., Rokas et al 2005).…”

“…D'Arcy Wentworth Thompson (1860Thompson ( -1948 suggested that viscous flow and environmentally induced mechanical deformation, among other physical factors, could explain the shapes of organisms and morphological transformations between different species (Thompson 1942). The embryologist E. E. Just described the animal egg as a purely physical system that was nonetheless "self-acting, self-regulating and self-realizing" (Just 1939, 237;Newman 2009). One implication of these views-that much biological form was nonadaptive-had no place in the emerging standard model, however, and these figures were relegated to the scientific margins during their lifetimes.…”

E. E. Just's broad, yet hidden, influence on modern cell and developmental biology

“…All that was needed to activate the egg and initiate the developmental process, then, was a stimulus, either a spermatozoon or a nonspecific trigger (hypertonic or hypotonic seawater, UV light, temperature change), which would set in motion a whole series of events that was latent in the system. (See Newman () for a description of some of the ways in which changes at the egg's surface during fertilization influence or determine later events taking place in the embryo.) The cell was poised and ready to be triggered by these nonspecific stimuli.…”

Conceptual Change in Biology

“…William Bateson (1861Bateson ( -1926 proposed that certain tissues exhibited oscillatory excitations that could cause them to organize into segmental and other repeating patterns (Bateson and Bateson 1928;Newman 2007). The embryologist E. E. Just described the animal egg as a purely physical system that was nonetheless "self-acting, self-regulating and self-realizing" (Just 1939, 237;Newman 2009). The embryologist E. E. Just described the animal egg as a purely physical system that was nonetheless "self-acting, self-regulating and self-realizing" (Just 1939, 237;Newman 2009).…”