Ultrastructural characterization of gonadotropin‐releasing hormone (GnRH)‐producing neurons

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Abstract
By means of preembedding immunohistochemistry, 2 types of gonadotropin-releasing hormone (GnRH) positive neurons in the rat were identified and characterized in the preoptic region and in the diagonal band: a smooth GnRN neuron with relatively even cytoplasmic contours and a spiny GnRH neuron with thorn-like protrusions of the perikaryon and cell processes. Both cell types contain the same organelles in similar number and distribution, but they differ in the number of synaptic contacts. GnRH cell bodies have a large round or ovoid nucleus, well-developed rough endoplasmic reticulum arranged in multilayered stacks or as individual cisternae and several Golgi complexes. Lysosomes are not numerous under the conditions studied. Specializations include kinocilia, nematosomes and lamellar whorls. Throughout the cytoplasm, scattered dense core vesicles with a diameter of 100 nm and clear vesicles with a diameter of 30-40 nm can be seen with a preferential localization close to the cell membrane. The cell processes of smooth GnRH cells close to the perikaryal appear as extensions of the perikaryal cytoplasm with all organelles except the nucleus. The 2 neurites originate from the perikaryon as tapering cones over a distance of 200-300 .mu.m, until they reach a diameter of 0.5-3 .mu.m. Cell processes of spiny GnRH cells show bifurcations, protrusions or invaginations, and contain clear and dense core vesicles in their spines. In areas distant from the perikaryon, immunoreactive fibers with a large number of dense core and clear vesicles can occasionally be seen to terminate synaptically or asynaptically on other neurons. The GnRH neurons show postsynaptic specializations at the level of the perikaryon and at cell processes, when apposed by a presynaptic terminal. Such synaptic contacts are seen less frequently on smooth cells than on spiny cells. Large areas of the GnRH cell may be covered by a thin glial lamella, which separates the cell body from the surrounding neuropil. The existence of 2 populations of GnRH cell bodies with different patterns of innvervation suggests different integrative capacities.