The chemoarchitecture of the pretectal complex of the rabbit was examined in sections stained by acetylcholinesterase (AChE) and reduced nicotinamide adenine dinucleotide (NADH) diaphorase in the coronal, horizontal and sagittal plane. Twelve different subdivisions can be identified in the rabbit prectectum on the basis of the distribution of both histochemical markers. According to the standard terminology, the pretectal complex of the rabbit consists of: the nucleus of the optic tract; the anterior, posterior, olivary and medial pretectal nuclei; the nucleus of the posterior commissure; the periventricular subcommissural gray; the suprageniculate and internal suprageniculate nuclei, and the dorsal, lateral and medial terminal nuclei of the accessory optic system. The combined use of several sectioning planes and the histochemical mapping of AChE and NADH diaphorase have been of value in resolving the structural limits within transitional regions of the pretectum.

In the developing brain, the phenomenon of neurogenesis is manifested heterotopically, that is, much the same neurogenetic steps occur at different places with a different timetable. This is due apparently to early molecular regionalization of the neural tube wall in the anteroposterior and dorsoventral dimensions, in a checkerboard pattern of more or less deformed quadrangular histogenetic areas. Their respective fate is apparently specified by a locally specific combination of active/repressed genes known as ‘molecular profile’. This leads to position-dependent differential control of proliferation, neurogenesis, differentiation, and other aspects, eventually in a heterochronic manner across adjacent areal units with sufficiently different molecular profiles. It is not known how fixed these heterochronic patterns are. We reexamined here comparatively early patterns of forebrain and hindbrain neurogenesis in a lizard (Lacerta gallotia galloti), a bird (the chick), and a mammal (the rat), as demonstrated by activation of acetylcholinesterase (AChE). This is an early marker of postmitotic neurons, which leaves unlabeled the neuroepithelial ventricular cells, so that we can examine cleared wholemounts of the reacted brains to have a birds-eye view of the emergent neuronal pattern at each stage. There is overall heterochrony between the basal and alar plates of the brain, a known fact, but, remarkably, heterochrony occurs even within the precocious basal plate among its final anteroposterior neuromeric subdivisions and their internal microzonal subdivisions. Some neuromeric units or microzones are precocious, while others follow suit without any specific spatial order or gradient; other similar neuromeric units remain retarded in the midst of quite advanced neighbors, though they do produce similar neurogenetic patterns at later stages. It was found that some details of such neuromeric heterochrony are species-specific, possibly related to differential morphogenetic properties. Given the molecular causal underpinning of the updated prosomeric model used here for interpretation, we comment on the close correlation between some genetic patterns and the observed AChE differentiation patterns.

Connections of the rabbit suprageniculate pretectal nucleus (SP) with the superior colliculus were explored by means of retrograde transport of horseradish peroxidase or Fluorogold. Large injections centered in the superficial and intermediate tectal layers resulted in bilateral retrograde transport to the medium-size multipolar neurons of the suprageniculate pretectal nucleus. Horseradish peroxidase was also transported anterogradely into the ipsilateral and contralateral neuropiles of the suprageniculate pretectal nucleus. The labeled cells in SP were dispersed throughout the nucleus, including its dorsal, wedge-shaped, internal portion. Labeling was mainly ipsilateral, and less abundant on the contralateral side.

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