Holger Jastrow1 and Helmut A. Oelschläger2
1) Department of Anatomy, Johann Wolfgang Goethe-University,
Theodor Stern-Kai 7, D-60590 Frankfurt a.M.;
2) Department of Anatomy, J. Gutenberg-University, Becherweg
13, D-55128 Mainz.
The terminal nerve (tn; nervus terminalis) plays an important
role in the establishment of the hypothalamo-hypophyseal-gonadal axis as
it is the source of Luteinizing Hormone-Releasing Hormone (LHRH)- immunoreactive
(-ir) neurons that immigrate into the CNS. The brains and meningeal portions
of the tn were analysed in 3 early postnatal (2 m,
1f) and 3 non-reproductive adult female specimens
of the African mole-rat (Cryptomys spec.), a subterranean rodent
with only one reproductive female in a family, looking for differences
in LHRH-ir cell distribution. LHRH-ir neuron counts in the CNS of 5 animals
with a bilateral tn revealed that the spreading of ir neurons was more
or less equal on both sides. In one of the adult females, however, the
right tn was lacking, and within the CNS only very few ir neurons were
found on the right side, whereas an exceptionally large number was present
on the left side. Thus the total number of LHRH-ir neurons in this female
did not differ significantly from that in the other adult animals. In the
3 adult non-reproductive females (including the animal with the unilateral
tn) an additional population of LHRH-ir cells was detected in the area
of the parafascicular and dorsomedial thalamic nuclei. These cells were
present on both sides of the brain in about the same quantity. Their perikarya
had large or small vacuoles heavily labelled for LHRH, thus they were named
"dark-spot cells" (DSCs). This investigation shows that unilateral absence
of the tn is correlated with an almost total lack of ipsilateral LHRH-ir
neurons but does not affect the DSC population. The results obtained provide
strong evidence for ipsilateral migration of LHRH neurons. The larger number
of LHRH neurons in the specimen with the unilateral tn raises the question
as to the regulation of LHRH cell numbers (decreased apoptosis?). The distribution
of the newly described DSCs suggests that they are independent of the tn.