The terminal nerve (TN) of several dolphins was examined using gross dissection aided by osmium staining, routine light and electron microscopy, and immunocytochemistry with antibodies to mammalian luteinizing-hormone-releasing hormone (LHRH). The TN consists of numerous pial strands which emanate from large paired ganglia located in the dura near the frontal lobe of the hemisphere. The strands are largely composed of myelinated axons which extend to basal forebrain areas including the anterior perforated substance. Peripheral branches of the ganglia run through foramina in the ethmoid bone into the region of the nasal sacs and blowhole. Round to oval ganglion cells are scattered along the nerve and thousands of similar cells are found in the dural ganglia where they are encapsulated by satellite cells. A second, less prevalent cell type is also found in the ganglia. These neurons are fusiform, lack a well-defined capsule and are LHRH-immunoreactive. The results are compared to observations of the anatomy and functions of the TN in other mammals, which unlike toothed whales have retained an olfactory system. Involvement in reproduction and control of secretions and/or circulation of the nasal sac vocalization system are suggested functions of the TN in dolphins.
SINCE the initial report that LHRH-expressing neurons migrate during development from their birth place on the medial side of the olfactory placode into the brain (1), two sorts of questions have received additional experimentation and discussion. First, what light does this unusual migration into the developing brain shed on the nature of LHRH-expressing cells, and what are the implications for their normal roles in the adult brain? Second, what would be the implications of migration disturbances for human reproductive physiology, including Kallmann's and other syndromes (see companion paper by Dr. Crowley)? Accordingly, the purposes of this review are to reexamine some of the evidence concerning normal controls over LHRH neurons in light of what these cells are supposed to do in the adult brain; to review the burgeoning evidence that they actually originate in the olfactory pit and migrate into the developing forebrain; and to introduce the subject of Kallmann's syndrome (see companion paper by Dr. Crowley).
Abstract Light and electron microscopic immunocytochemistry were used to examine the structure of LHRH neurons and fibers in the nervus terminalis of the gray short‐tailed opossum ( Monodelphis domestica ). LHRH‐immunoreactive neurons and fibers form a loose plexus within the fascicular network of the ganglion terminale on the median surface of the olfactory bulb. There are at least two populations of LHRH‐immunoreactive neurons within the network of the ganglion terminale: fusiform and round neurons similar to those described in the forebrain. At the ultrastructural level, axosomatic and axodendritic contacts were seen between LHRH‐immunoreactive and nonimmunoreactive elements in the ganglion terminale. These contacts were classified as (1) synaptic input, with asymmetric synapses seen between a nonimmunoreactive axon terminal and a LHRH‐immunoreactive cell body or a nonimmunoreactive axon terminal and a LHRH‐immunoreactive dendritic process. (2) synaptic output, with symmetric synapses seen between LHRH‐immunoreactive and nonimmunoreactive processes. This study is the first systematic examination of the ultrastructure of the LHRH‐immunoreactive neurons and their synaptic contacts in the nervus terminalis. The possible integrative roles for this LHRH‐immunoreactive system are discussed.
