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Methylazoxymethanol acetate (MAM) is a substance that inhibits migration of neurons in the embryonic brain. After intraperitoneal injection of two different doses of MAM to pregnant rats, microcephaly with or without complete development of the cerebral cortex was observed in every litter. High MAM doses (30 mg/kg) resulted in the lack of superficial layers (II-IV) of the cerebral cortex when the deep layers (V, VI) were seen. The claustrum was present but composed of loosely packed, medium-size, triangular or fusiform neurons with anarchic oriented long axes. After administration of low MAM doses (14 mg/kg) two different parts (medial and lateral) of the insular claustrum were observed. Our results suggest that neurons of the insular claustrum create two different subpopulations of cells, which were similar to that observed in primitive insectivore (e.g., hedgehog), but fuse in development.

Cyto- and myeloarchitectonics as well as acetylcholinesterase activity of the nucleus limitans thalami--an important part of the nociceptive system--were studied in insectivora (Sorex araneus, Erinaceus europaeus, Talpa europaeus), rat, rabbit, cat, monkey and man. Our results suggest that nucleus limitans is phylogenetically a rather new structure. In insectivora and in the rat we could not find any evident group of neurons resembling nucleus limitans. In the rabbit and cat nucleus limitans is composed of few layers of elongated cells. In macaca and man it is a larger, more complicated structure possessing various types of neurons, and arranged into many layers.