MORPHOLOGICAL-CHARACTERISTICS OF NEOCORTICAL LAMINAE WHEN STUDIED IN TANGENTIAL SEMITHIN SECTIONS THROUGH THE VISUAL-CORTEX OF THE RABBIT

In semithin sections cut tangentially with respect to the surface of the cerebral cortex and with a side length of at most a few mm, it is extremely difficult to be certain which lamina has been sectioned, for instance lamina III or upper lamina IV. In order to provide criteria to overcome this difficulty, a systematic study was carried out in which 1 .mu.m semithin sections through the visual cortex of the rabbit were cut in both the frontal and tangenital planes and compared with 8-12 .mu.m paraffin sections stained for cells and/or myelin. It was found that the arrangement of thick dendrites, myelinated fibers and unmyelinated profiles which may be either dendrites or neurites, permits subdivision of the visual cortex into 3 characteristic horizontal zones which are termed zones A, B and C. The relation between these 3 zones and the cytoarchitectonic pattern is as follows: Zone A corresponds to the cytoarchitectonic lamina I; zone B comprises lamina II/III, lamina IV and the upper half of lamina V; zone C corresponds to the lower half of lamina V and all of lamina VI. Zones A and B can further be subdivided: Zone A consists of 2 layers, whereas zone B can be divided into 3 tiers, each of which is characterized by a particular arrangement of the myelinated and unmyelinated profiles. It is shown that the morphological criteria thus obtained facilitate orientation in tangenitally cut semithin sections.