Neostriatal projections from cytoarchitectonically defined gyri in the prefrontal cortex of the dog

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Abstract
The cytoarchitecture and neostriatal projections of the canine prefrontal cortex (PFC) were examined by using Nissl, silver degeneration, autoradiographic, and horseradish peroxidase techniques.Cytoarchitectonically, the PFC can be divided into six major gyral areas, with each area corresponding to one of the major gyri–proreal, polar, pregenual, subproreal, paraorbital, and orbital–defined myeloarchitectonically by Kreiner (J. Comp. Neurol. 116:117–133, ′61). Of the six major areas, only the proreal gyrus displays a distinctly granular layer IV. In all other gyri, layer IV is more difficult to distinguish. In both the orbital and polar gyri, layer IV is poorly defined and consists of scattered clusters of small cells between layers III and V. In the pregenual, subproreal, and paraorbital gyri, layer IV is not detectable. In all gyri, layer V consists of a thin lamina of small and medium‐size pyramidal cells. The transition from layer HI to layer V is marked by changes in cell density and size as well as staining intensity. Borders between layers V and VI are generally less distinct than those between layers III and V, with the exception of the pregenual gyrus, in which layer V is separated from layer VI by a distinct cell‐poor lamina. In the subproreal and paraorbital gyri, the border between the underlying white matter and layer VI is particularly distinct, with large numbers of neurons oriented tangentially to the white matter. In contrast, the border between layer VI of the orbital gyrus and the white matter is less distinct and consists of palisades of neurons extending well into the white matter.The silver degeneration and autoradiographic methods revealed that prefrontostriatal projections terminate as dorsoventrally oriented longitudinal bands within the medial half of the head and body of the caudate nucleus. Projections from the polar and dorsal proreal gyri terminate most medially in the nucleus while projections from the lateral part of the proreal, subproreal, orbital, and paraorbital gyri terminate progressively more laterally within the medial half of the nucleus. There is apparent overlap between adjacent projection fields. A sparse projection was also noted to a small part of the putamen adjacent to the internal capsule.Injections of horseradish peroxidase into the medial and ventromedial parts of the head of the caudate nucleus resulted in widespread retrograde labeling in parts of the proreal, pregenual, paraorbital, subproreal, and central precruciate gyri. Labeled neurons were located primarily in the superficial part of layer V with smaller numbers of labeled cells in layers III, IV, and VI.These results indicate that the PFC in the dog can be divided into several Cytoarchitectonically distinct gyri that project topographically and with some degree of overlap to the medial part of the head and body of the caudate nucleus. Data from other studios in the cat show that the medial part of the caudate nucleus receives preferential projections from prelimbic and infralimbic cortical regions as well as from the basolateral amygdaloid nucleus. These findings, combined with those reported in the present study, indicate that the medial part of the carnivore caudate nucleus may be closely associated with limbic function.