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).

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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


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.

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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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Kavita Singh - TNeurotrophin Expression in the Developing Olfactory... [1] [2] [3] [4] [5] [6]