Ultrastructural and cytochemical identification of apoptotic cell death accompanying development of the fetal rat olfactory nerve layer

From the periphery to the brain: Wiring the olfactory system

The olfactory system represents a perfect model to study the interactions between the central and peripheral nervous systems in order to establish a neural circuit during early embryonic development. In addition, another important feature of this system is the capability to integrate new cells generated in two neurogenic zones: the olfactory epithelium in the periphery and the wall of the lateral ventricles in the CNS, both during development and adulthood. In all these processes the combination and sequence of specific molecular signals plays a critical role in the wiring of the olfactory axons, as well as the precise location of the incoming cell populations to the olfactory bulb. The purpose of this review is to summarize recent insights into the cellular and molecular events that dictate cell settling position and axonal trajectories from their origin in the olfactory placode to the formation of synapses in the olfactory bulb to ensure rapid and reliable transmission of olfactory information from the nose to the brain.

Caspase-3 and caspase-9 mediate developmental apoptosis in the mouse olfactory system

Neuronal apoptosis is a key component in the sculpting of tissues during embryonic and postnatal development and is driven largely by the action of caspases. In the mouse olfactory system, caspase-3 and -9 are expressed in olfactory receptor neurons (ORNs) of adult mice, and their selective retrograde activation drives ORN apoptosis following ablation of their target, the olfactory bulb (OB). Here, we show that both of these caspases are expressed at the earliest stages of ORN embryonic development, and their expression is concentrated in outgrowing ORN axons. The retention, in null mice for both caspases, of a population of ORNs that would normally undergo developmental apoptosis beginning at E13 of development, results in a permanently expanded population of ORNs. In turn, in some caspase-3 null mice, the ORN target organ, the OB, also develops abnormally, resulting in the formation of secondary, apparently functional, extracranial ectopic OBs.

Cell death in the nasal septum of normal and naris-occluded rats

Blocking airflow through an external naris of a newborn rat results in a thinning of the olfactory mucosa by postnatal day (P) 20, at least partially due to a reduction in mitotic rates. We employed the TUNEL method to examine patterns of cell death. By P20-30, significant differences in the numbers of labeled profiles were found in the middle 50% of the tissue, a region primarily containing olfactory sensory cells.

Apoptosis in the development of the mouse olfactory epithelium

Apoptotic cells were detected in the mouse olfactory epithelium (OE) at different embryonic and postnatal stages by in situ nick translation (ISNT) and Tdt-mediated dUTP nick end-labeling (TUNEL) techniques. During development, the apoptotic process presented two peaks. One at E12 during the invagination of the olfactory placode and the second at E16 corresponding to olfactory axon synaptogenesis. Then, from E18, a sharp decrease in the number of apoptotic cells was observed and at E19 the apoptotic index reached low values that were maintained in postnatal stages, P1 and P8, and in the adult. Apoptotic nuclei belonged to mature as well as immature olfactory receptor neurons (ORNs). Indeed, double-labeling experiments evidenced apoptotic neurons immunopositive for olfactory marker protein (OMP), carnosine and GAP-43. According to our data, two apoptotic phases occur during early development. One is involved in the morphogenesis of the OE when this last is not yet, or poorly, connected to its target, the olfactory bulb (OB). The second peak of apoptosis is more closely dependent on the interplay between OE and OB.

Basic fibroblast growth factor in the primary olfactory pathway: mitogenic effect on ensheathing cells

The mitogenic effect of basic fibroblast growth factor and nerve growth factor (2.5S) on olfactory ensheathing cell culture was examined by bromodeoxyuridine uptake. It was found that, at 10 ng/ml, basic fibroblast growth factor elicited about a three-fold increase in proliferation, while the stimulatory effect of nerve growth factor was considerably less. The increased proliferation resulting from basic fibroblast growth factor could be attributed to perlecan, which was shown to be expressed by ensheathing cell in culture. Perlecan is known to induce high-affinity binding of basic fibroblast growth factor to receptors on cell membranes. Immunohistochemical staining demonstrated that basic fibroblast growth factor was abundantly expressed in select regions of the lamina propria underlying the olfactory epithelium. In these regions, contiguous patches of olfactory epithelium also showed the presence of basic fibroblast growth factor. Although basic fibroblast growth factor was present on the periphery of nerve bundles in the olfactory nerve layer of the bulb, all other laminae did not demonstrate the presence of this factor. The immunohistochemistry and cell culture results show that regions of the lamina propria and small patches of the olfactory epithelium, by their presence of basic fibroblast growth factor, are potential sites of ensheathing cell proliferation in vivo.

B-50, the growth associated protein-43: modulation of cell morphology and communication in the nervous system

The growth-associated protein B-50 (GAP-43) is a presynaptic protein. Its expression is largely restricted to the nervous system. B-50 is frequently used as a marker for sprouting, because it is located in growth cones, maximally expressed during nervous system development and re-induced in injured and regenerating neural tissues. The B-50 gene is highly conserved during evolution. The B-50 gene contains two promoters and three exons which specify functional domains of the protein. The first exon encoding the 1-10 sequence, harbors the palmitoylation site for attachment to the axolemma and the minimal domain for interaction with G0 protein. The second exon contains the "GAP module", including the calmodulin binding and the protein kinase C phosphorylation domain which is shared by the family of IQ proteins. Downstream sequences of the second and non-coding sequences in the third exon encode species variability. The third exon also contains a conserved domain for phosphorylation by casein kinase II. Functional interference experiments using antisense oligonucleotides or antibodies, have shown inhibition of neurite outgrowth and neurotransmitter release. Overexpression of B-50 in cells or transgenic mice results in excessive sprouting. The various interactions, specified by the structural domains, are thought to underlie the role of B-50 in synaptic plasticity, participating in membrane extension during neuritogenesis, in neurotransmitter release and long-term potentiation. Apparently, B-50 null-mutant mice do not display gross phenotypic changes of the nervous system, although the B-50 deletion affects neuronal pathfinding and reduces postnatal survival. The experimental evidence suggests that neuronal morphology and communication are critically modulated by, but not absolutely dependent on, (enhanced) B-50 presence.