|Publication Type:||Journal Article|
|Year of Publication:||2001|
|Authors:||S. Shigeno, Kidokoro, H., Tsuchiya, K., Segawa, S., Yamamoto, M.|
An atlas of the developing brain up to hatching stage is established using conventional histological methods in the oegopsid squid, Todarodes pacificus. The brain originates from placodal thickenings in the ectoderm at the end of epiboly. The neuroblasts composing the placodes ingress in a group and accumulate into ganglia under the proximal surface of the surface epithelium. Four pairs of the ganglia, pedal, palliovisceral, cerebral, and optic, form the brain primordium. These ganglia come into contact with one another, and eventually accumulate into a ring-like cluster (circumesophageal cluster) encircling the oral ingrowth and the inner yolk around the surface of the head. The circumesophageal cluster regionally differentiates into brain lobe anlagen through formation of neuropiles and nerve tracts. The neuropiles form a ladder-like structure with two longitudinal columns situated in the ventrolateral parts of the circumesophageal mass and some axonal tracts bridging the left and right columns (ladder-like framework). The brain is quite premature at the time of hatching, especially in the supraesophageal part. Though most brain nerves are already present, many brain lobes and commissures found in the adult brain are not yet differentiated. The present results show that the morphological processes of the brain formation are essentially common among the coleoid cephalopods. The similarity of the embryonic brain in Todarodes to the adult brain in Nautilus suggests that the coleoid brain has evolved on the basic plan as seen in the nautiloid brain.