Experimental evidence for an early commitment of gonadotropin-releasing hormone neurons, with special regard to their origin from the ectoderm of nasal cavity presumptive territory.

The origin and the migration of gonadotropin-releasing hormone (GnRH)-producing neurons were studied using the indirect immunoperoxidase method in normal and surgically operated chick embryos. In normal embryos, during early embryonic development, GnRH neurons were located only in the respiratory and the olfactory epithelia. Then, these neurons followed the nearest nerve bundle and occupied, thereafter, the dorsal, medial or ventral part of the olfactory nerve according to the time and area of the olfactory epithelium they emerged from. At the junction with the forebrain, the majority of GnRH neurons passed ventromedially round the olfactory bulb. Therefore, they penetrated through the interhemispheric space and coursed obliquely toward caudal and dorsal telencephalon from where they will be later distributed to reach their adult-like position. In view of the large distribution of these neurons in the nasal region, unilateral surgical ablation either of the whole or of each presumptive territory of nasal structures was performed from 2 to 4 somite stages. As expected, when both olfactory placode and ectoderm of nasal cavity presumptive territories were unilaterally removed, olfactory nerve, nasal structures and GnRH neurons failed to develop in the operated side. After the unilateral removal of the olfactory placode anlage, the distribution pattern of GnRH neurons was not disturbed in the operated as well as in the control side although ipsilateral olfactory structures were greatly reduced. In contrast, when the presumptive ectoderm of nasal cavity was unilaterally removed, GnRH neurons were detected only in the control side where this territory was left intact. Therefore, from early neurogenesis, GnRH neurons seem to be already committed, and they originate from the ectoderm of nasal cavity presumptive territory.