The hypothalamic suprachiasmatic nucleus of rat: Intrinsic anatomy

The internal structure of the suprachiasmatic nucleus (SCN) was studied qualitatively and quantitatively in several hundred rat brains with a variety of methods including several types of Golgi impregnations, Nissl stains, horseradish peroxidase application, and electron microscopy. One suprachiasmatic nucleus has a volume of about 0.068 mm3 and contains close to 8,000 neurons. Dorsomedially, cells tend to be smaller and more tightly packed than ventrolaterally; significantly more somatic appositions occur in the dorsomedial SCN than in other parts of the nucleus. Two neurons with an extended region of somatic apposition may have no intercellular specializations, or they may be held together with various attachment plaques. Chains of neurons with long regions of somatosomatic apposition are found in the dorsomedial SCN with Nissl and silver stains, and with EM. The length of these chains is generally oriented in an antero-posterior direction. Interspersed with the neurons are astroglia. Astroglia studied with Golgi impregnations, Cajal's gold sublimate stain, and EM in SCN may have a rich cytoplasm falling in between the organelle-rich cytoplasm of some large neurons and the organelle-poor cytoplasm of some of the smaller SCN neurons. Nuclei of SCN glia and neurons are often invaginated and multiple nucleoli are a prominent feature of a large number of SCN neurons. With Golgi impregnations a number of relatively simple dendritic arbors exist. These include the simple bipolar cell, curly bipolar cell, radial neuron, monopolar neuron, and spinous cell. At the borders of the nucleus some dendrites may travel into the adjacent anterior hypothalamus; similarly, dendrites from cells outside SCN may enter into the nucleus boundaries. Compared with the rest of the hypothalamus, axons in SCN stain very poorly with conventional histological methods including Luxol blue, Bodian, Sevier-Munger, Loyez, and Golgi, in part because of the fine caliber of the fibers. Golgi impregnated and silver-stained axon fascicles composed primarily of unmyelinated axons may divide up within the nucleus or may pass through without maintaining local collaterals. Several different types of Golgi-impregnated axonal arborizations terminate within SCN. Some have an extensive number of boutons ending on SCN somata, dendrites, and dendritic appendages. Other single axons pass through SCN without leaving any collaterals, or terminals in the nucleus. The majority of Golgi-impregnated axons arising from SCN neurons maintain locally terminating collaterals, with boutons en passant and termineaux; these axons originate with equal frequency from the perikaryon or from a proximal dendrite. Golgi, horseradish peroxidase, and silver staining methods reveal axons connecting left and right SCN. The SCN is a complex nucleus with recognizable subdivisions that contain neurons with several types of relatively simple dendritic arbors. Within any area of SCN, ultrastructural differences can be found between neighboring cells, suggesting a heterogenous population of neurons. Combining the results of the present study with previously reported data, neurons of the SCN have a large number of possibilities for intercellular communication between cells within the nucleus. These include a few presynaptic dendrites and somata, frequent local circuit axons, and possibly ephaptic interaction between closely apposed cells.