215Measurements of relative cortical displacement using superficially or deeply
placed fluorescent dyes found that cortical rotation begins about halfway though the
first cell cycle, eventually covering an average 30° of arc over the dense yolky cyto-
plasm. Also, cortical movement progressed along an animal–vegetal meridian, and
toward the future dorsal side of the egg (generally away from the sperm entry point),
with the region of greatest movement correlating with the position of the axial midline
(Vincent et al. 1986 ; Gerhart et al. 1989 ). It is thought that changes in fluid dynamics
in the fertilized egg result in a low viscosity/high elasticity shear zone in the subcorti-
cal region as well as an increase in firmness in the deep cytoplasm, allowing this dif-
ferential movement between two parts of the egg (Elinson 1983 ; Gerhart et al. 1989 ).
In an extensive comparison of axis forming mechanisms, Clavert ( 1962 ) indicates
that, in addition to amphibians, primitive fish including lampreys, lungfish, and
chondrostean fish (sturgeons and paddlefish) likely form gray crescents, suggesting
that these organisms also likely undergo cortical rotation. With these older compara-
tive studies and more recent molecular data taken together, it is apparent that the
basics of the amphibian cortical rotation model are conserved throughout the anam-
niotes (icthyopsids). And vestiges may exist even in some amniotes. It is also now
generally appreciated that cortical rotation establishes a self-organizing, transient
microtubule polarity in the zygote that is critical for the transport of cortical cyto-
plasmic dorsal determinants and activation of Wnt/beta-catenin signaling (reviewed
in Gerhart 2004 ; Weaver and Kimelman 2004 ; Houston 2012 ). Wnt/beta- catenin
signaling (see Sect. 6.3.5) also plays a key role in bird and mammal axis formation,
but this is likely controlled by mechanisms other than cytoplasmic localization.
6.2.1 Mechanisms of Amphibian Cortical Rotation
6.2.1.1 Cortical Rotation in Anurans
A number of studies have shown that cortical rotation is primarily controlled by the
assembly of parallel microtubule arrays in the vegetal cortex (Fig. 6.3). Treatment
of fertilized eggs during the middle of the first cell cycle with microtubule-
depolymerizing agents such as ultraviolet irradiation (UV), exposure to anti-
microtubule drugs, cold and high pressure, can inhibit gray crescent formation and/
or block axis formation in both frog and salamander eggs (Malacinski et al. 1975 ;
Manes et al. 1978 ; Manes and Elinson 1980 ; Scharf and Gerhart 1983 ; Vincent and
Gerhart 1987 ). Correspondingly, impressive arrays of parallel microtubules are
assembled in the vegetal cortical region during the period of cortical rotation in
Xenopus and Rana (Lithobates) (Elinson and Rowning 1988 ).
Microtubules are completely disassembled in the egg at fertilization, but pro-
gressively repolymerize over the first 35 min, approximately when relative cortical
movement begins. Microtubules in the cortical region initially form a disorganized
network that gradually becomes organized as cortical rotation progresses. By mid-
cortical rotation, microtubules are predominantly oriented with the plus ends
6 Vertebrate Axial Patterning: From Egg to Asymmetry