Olfactory placode formation

Abstract: Gonadotropin-releasing hormone (GnRH), which is essential for reproductive function, is made by neurones that migrate from the nasal region into the brain during early embryonic development. This migration begins in chick when the olfactory pit is formed. This is approximately the time that GnRH neurones can be detected immunocytochemically. The present study investigated (i). how early in development the GnRH gene is expressed and (ii). the sites of its expression. Accordingly, reverse transcriptase-polymerase chain reaction (PCR) and in situ hybridization were performed on chick embryos before gastrulation up until the stage by which GnRH neurones have begun to migrate into the central nervous system. Primers were made to the 5'- and 3'-UTR region of the message for cGnRH-I, the form of the peptide that is essential for reproductive function in the chicken. PCR product was found in all stages and the sequences of products from all stages were identical. Thus, the GnRH gene is expressed continuously throughout embryonic development. In situ hybridization with a digoxygenin labelled riboprobe revealed staining along the primitive streak immediately before gastrulation. In later stages, cGnRH-I gene expression was seen in association with the anterior neural ridge. The expression was subsequently restricted to a narrow, clearly defined region, which is associated with the presumptive nasal cavity and olfactory placode. Later, GnRH neurones could be seen in their migratory routes by both in situ hybridization and immunocytochemistry. Expression of the GnRH gene has been described in preimplantation stages in mammals and there is evidence that the neuropeptide plays a role in formation and maintenance of the placenta. What role (if any) it may play in early avian development remains unknown. The demonstration of sites of GnRH expression during the early period of neurulation suggests that GnRH neurones arise before olfactory placode formation.

Abstract: Bell,1 using embryos of Rana esculenta, attempted to show the relation of the brain to the development of the olfactory placode. He transplanted “that part of the brain which forms the olfactory lobe” to other regions of the body without obtaining any induction of an olfactory placode. His conclusion was that the olfactory placode developed independently of the brain. He did not, however, preclude the possibility of an early stimulus for olfactory placode formation being imparted by the brain to the ectoderm above it.Recently we have been able to show that the part of the fore-brain median and slightly anterior to the optic vesicle of the intermediate neurula stage of Rana pipiens has the power to induce an olfactory placode in the ectoderm of the flank (Fig. 1).After one accidental induction (from a transplant of an optic vesicle which evidently included some of the rest of the fore-brain) 3 series of operations were performed in an attempt to localize the olfactory placode inducing part of the brain. Th...