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,
E-mail: jastrow@mail.zdv.uni-mainz.de,
http://www.uni-mainz.de/~jastrow/
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.