The hypothalamic arcuate nucleus of rat—A quantitative golgi analysis

The hypothalamic arcuate nuclei of several hundred rats were studied with several types of Golgi impregnations in young and adult animals. In addition to qualitative observations, a computer-assisted quantitative analysis of dendritic arbors and axonal trajectories in coronal and horizontal sections was performed on 1,702 neurons in young rats. With data on large numbers of neurons, and using a product-moment analysis of dendritic trees, we found statistically reliable differences in the mean size, shape, center of mass, skewness, and orientation of dendritic trees of neurons in different parts of the arcuate nucleus. Despite differences within the nucleus, neurons of the arcuate nucleus characterized as a group were different from neurons of the hypothalamic ventromedial nucleus. Generally two or three, but sometimes one or four, primary dendrites arise from somata in the arcuate nucleus. The total dendritic tree is usually fairly small. Some dendrites are covered with spines, while others appear to have smooth surfaces. Medially located neurons often have dendritic trees oriented in a dorsoventral direction, while more lateral cells, especially at the ventral surface of the hypothalamus, have mediolaterally oriented dendritic arbors; a graded transition of orientation occurs between the two extremes. Dendrites originating in the arcuate nucleus may enter the cellpoor zone between the ventromedial and the arcuate nucleus, and therefore may share afferents with dendrites of the ventromedial nucleus projecting into the cell-poor zone. The mean center of mass for dendritic trees in all parts of the arcuate nucleus in coronal and horizontal sections was situated lateral, and, in most areas, ventral to the cell body. Dendritic fields tend to be elongated; if an elliptical shape is fitted to the dendritic arbors with the product-moment analysis, the major axis of the ellipse is four to five times greater than the minor axis. The orientation of dendritic trees in horizontal sections is generally in a rostrocaudal direction; this is less pronounced laterally than medially. Near the third ventricle in a horizontal plane, dendrites tend to run orthogonal to the tanycytes; tanycytes, as studied with Golgi impregnations and scanning electron microscopy, constitute a large percentage of total tissue volume medially. Tanycyte distal processes run in a very orderly progression through the arcuate nucleus. Axons originate from either the perikaryon or a proximal dendrite. Some axons arising in the arcuate nucleus also terminate within the nucleus. Axons of arcuate nucleus origin may bifurcate, with one branch leaving the nucleus and the other terminating within it. Axons entering the median eminence are more likely to divide into collaterals within the median eminence rather than before entering it. The trajectory of axons in single thick coronal sections originating from arcuate neurons is more often ventral than dorsal. In the medial arcuate nucleus in horizontal sections, axons of intrinsic origin generally run in a rostrocaudal direction. Taken together, the data in the present study indicate intrinsic regional variation of dendritic structure and a complex level of internal organization of the arcuate nucleus.