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Neurotrophin-3 mRNA:
As shown in Figure 2, NT3 mRNA was expressed
at much higher levels than BDNF mRNA in the developing olfactory
system. At embryonic day 14, dense
hybridization of NT3 cRNA occurred in the olfactory epithelium, in the respiratory
epithelium, and in the epidermis of the snout and jaw. There was also diffuse
labeling in the superficial neocortex at this age (Figure 2D). By embryonic day 16,
the olfactory bulbs were clearly visible and exhibited distinct labeling in the outer
layers (Figure 2E). However, only sparse labeling was seen where the olfactory nerve
fibers contacted the ventral bulb; very little hybridization was seen in the accessory
olfactory bulb. At embryonic day 16, expression of NT3 mRNA was still very high in
the olfactory epithelium, with slightly less labeling seen in the underlying mesoderm
(Figure 2E). The dense distribution of silver grains throughout the olfactory
epithelium made individually labeled cells unidentifiable. By embryonic day 20, the
density of NT3 cRNA hybridization had declined in the olfactory epithelium, although the
surrounding mesoderm was still moderately labeled. At this
age, dense labeling was seen in the developing cingulate cortex (Figure
2F).
Figure 3
Sections hybridized with cRNA for trkB (A-B) and trkC (C-D). High levels of trkB
mRNA are seen in the developing olfactory bulb (dOB) by E14 (A), and by E16 (B) expression
is higher in the outer layer (B). TrkC cRNA labels most of the brain at E14 (C) and E16
(D), including the bulbs. At E16 (D), the mesoderm (m) under
the olfactory epithelium is densely labeled. LV, lateral ventricle;
BTel, basal telencephalon; Tg,
tegmentum.
Tyrosine receptor kinase B mRNA: Hybridization
of trkB cRNA occurred throughout most of the neuraxis at embryonic
day 14 to 16 (Figure 3A). At embryonic
day 14, labeling was denser in the superficial region of the developing
bulb than in deeper lay
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ers. Hybridization density was also higher in the superficial
portions of the neocortex compared to deeper portions. By embryonic day 16, many regions
of the brain and spinal cord were heavily labeled (Figure 3B). At
this age, a distinct laminar pattern of labeling was seen in the
olfactory bulb, with the outer region
more densely labeled than the inner region; by embryonic day 18,
this labeling appeared denser. Hybridization of trkB cRNA in the
olfactory
epithelium was not detected at any of
the time points examined.
Figure 4
High magnification, light field photomicrographs of tissue
sections through the olfactory epithelium (oe in A) and olfactory
bulb (B) of a postnatal day 4 rat. (A) NT3
immunoreactivity is detected in olfactory nerve axons (ax) located below and within the
lamina propria (lp). (B) TrkB antibody stains the mitral cells
(m) of the olfactory bulb, with cell bodies more densely labeled
than their dendrites which are distributed in
the external plexiform layer (epl) and within individual glomeruli
(g). No trkB immunostaining is detected in the olfactory nerve layer
(onl). [gcl: granule cell layer]
Tyrosine receptor kinase C mRNA:
At embryonic day 14, trkC cRNA hybridized throughout
most of the brain and spinal cord (Figure 3C). Dense labeling was also seen in areas of
mesoderm. At embryonic day 16, high levels of trkC cRNA hybridization
occurred in the mesoderm underlying the olfactory epithelium and developing
neocortex, while moderate
levels of hybridization were seen in the olfactory bulb
 
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