Anatomy and development of the terminal nerve and Jacobson's organ.
A comparative study on bats and other mammals.

Holger Jastrow and Helmut H. A. Oelschläger
Departments of Anatomy, J. Gutenberg-University, D-55099 Mainz and J.W. Goethe-University, D-60590 Frankfurt am Main, FRG,

The vomeronasal or Jacobson's organ (Jo) is a small sac-like recess in the nose of vertebrates with sensory epithelium specialised for perception of pheromones. It is the origin of fibres of the vomeronasal- as well as the terminal nerve (tn), both of which are derivatives of the olfactory placode and play an important role in the establishment of the hypothalamo-hypophysial-gonadal axis. This study focuses on the development of Jo and the tn including counts of tn perikarya. Conventionally stained microslide series of a human foetus (116 mm crown-rump length, CRL) and different bat genera (n = 29), mainly Myotis myotis, from 6.5 mm (CRL) up to adulthood were investigated. The comparison of the results obtained with literature data of other mammals revealed major deviations from the general developmental scheme of the derivatives of the olfactory placode. Usually the placode appears in young embryos at about the same time as the somites. Within a few days it forms the olfactory pit, from which tn neuroblasts and then olfactory fibres emerge to sprout in the direction of the rostrobasal forebrain. Then a small invagination in the medial part of the placode indicates the formation of Jacobson's organ close to the newly formed main nasal ganglion of the tn (mng). In all the bat genera under scrutinity Jo and the mng were formed in the nasal roof and not, as in other mammals, in the septum. Further, the investigated bats developed neither a vomeronasal nerve nor an accessory olfactory bulb, thus the tn is the only connection of Jo to the brain. The innervation of Jo by the tn, the close vicinity of the mng to Jo and the fact that both structures are in an unusual location for mammals allow the hypothesis that in bats perception of pheromones is different. It is discussed which functional conclusions may be drawn from this deviation from the general developmental scheme. The quantification of tn perikarya revealed a reduction beginning in the late fetal period that can be explained by immigration of cells into the brain and apoptosis.

This oral presentation was at the 94th Meeting of the Anatomical Society at Hamburg, Germany, 29.3.1998, published in Ann Anat 181 (Suppl), p. 75. 

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