P.P.C. Graziadei

Neurogenesis and neuron regeneration in the olfactory system of mammals. I. Morphological aspects of differentiation and structural organization of the olfactory sensory neurons

SummaryThe neurogenetic process leading to the formation of primary sensory neurons persists into adult life in the olfactory epithelium of mammals. The morphological stages of maturation and ageing of this exceptional neuron have been described both at light and electron microscopical levels. For descriptive purposes the neural elements have been classified as: (1) basal cells proper, (2) globose basal cells, and (3) neurons. Intermediate stages, however, have been identified. Autoradiographic observations complement the morphological studies and provide a time sequence of the morphological stages leading to the mature neurons.A typical columnar arrangement of the sensory neurons has been described. Furthermore, active and quiescent zones have been recognized in the neuroepithelium. In the active zones the neurogenetic process is vigorous, and the zones are characterized by the presence of immature elements. However, in the quiescent zones there exists a population of mature elements while immature neurons are sparse.

Neurogenesis and neuron regeneration in the olfactory system of mammals. II. Degeneration and reconstitution of the olfactory sensory neurons after axotomy.

SummaryThis report describes the retrograde degeneration affecting olfactory sensory neurons of rats after severance of their axons and illustrates the reconstitution of new neurons originating from stem cells located at the base of the olfactory neuroepithelium.Degeneration of the mature, axotomized neurons, signalled by an increased electron density of their cytoplasmic matrix and by the appearance of lipofuscin-like granules, can be detected in the neuroepithelium as early as 24 h after surgery and becomes conspicuous between the second and the third day. Degenerating neurons can be observed in decreasing number up to the tenth post-operative day. They are removed by macrophages which invade the epithelium. The reconstitution of new neurons begins to occur after eight days, when the stem cells undergo vigorous mitotic activity and differentiate into neurons. The morphology of the reconstituted neurons has been described in detail at different stages of their maturation. After 30 days, the olfactory epithelium appears similar to controls. On the basis of both morphological (in rats) and autoradiographic (in mice) observations, the basal cells have been recognized as stem cells of the olfactory neurons.

Continuous Nerve Cell Renewal in the Olfactory System

The olfactory system has been long reported as one of the most phylogenetically primitive (Ariens Kappers et al., 1967), because of the peripheral location of its neurons and the relatively conspicuous development in lower vertebrates

Neurogenesis and neuron regeneration in the olfactory system of mammals. III. Deafferentation and reinnervation of the olfactory bulb following section of thefila olfactoria in rat

SummaryAxotomy at the level of thelamina cribrosa in rat induces rapid degeneration of the olfactory sensory axons in the bulb. The phenomenon, which is limited to the layer of olfactory fibres and to the glomeruli of the bulb, can be observed as early as 15–24 h after surgery, and peaks at 3–4 days. The glomeruli located in the rostro-ventral portion of the bulb are affected first, and the process extends to the dorso-caudal portion with a delay of 12–24 h. Reactive hypertrophy of the glia coincides with removal of the degenerating terminals, and is maximal 48 h after axotomy.Axotomy does not preclude reinnervation of the bulb by axons originating from new, reconstituted neurons in the olfactory neuroepithelium. These new axons begin to reach the periphery of the bulb approximately at the 20th day post-operative and then reinnervate the glomeruli. The rostro-ventral portion of the bulb is the first to be reinvaded by the new axons. The glomeruli reacquire a morphological pattern similar to controls between 20 and 30 days.

Denervation in the primary olfactory pathway of mice. IV. Biochemical and morphological evidence for neuronal replacement following nerve section

Unilateral olfactory nerve section was performed in the mouse. Three biochemical markers of the olfactory chemoreceptor neurons: carnosine, carnosine synthetase activity and the olfactory marker protein, were measured in the olfactory bulb and epithelium. Parallel observations were made by light microscopy as well as at the ultrastructural level. The specific biochemical markers decrease rapidly in both bulb and epithelium and reach a minimum by the end of the first week after surgery. They then slowly return to 80% of control values by one month. Carnosinase activity in epithelium was essentially unaffected. These biochemical observations coincide temporally with the onset of degenerative changes seen morphologically, in both the bulb and epithelium. The degenerative changes persist for up to two weeks in the bulb and for about one week in the epithelium. At this time basal cell division and differentiation begins in the epithelium with subsequent regrowth of olfactory axons into the glomerular layer of the olfactory bulb with ther reappearance of olfactory axon terminals. The temporal coincidence of these biochemical and morphological observations suggests they are manifestations of the same process, and is consistent with the idea that the olfactory chemoreceptor neurons are perhaps unique in being able to be replaced from undifferentiated stem cells.

Plasticity of connections of the olfactory sensory neuron: regeneration into the forebrain following bulbectomy in the neonatal mouse.

Abstract Neonatal mice underwent unilateral bulbectomy, which included the main and accessory olfactory bulbs. From 5 days of survival onward, there was a marked anterior displacement of the frontal cortex into the cavity previously occupied by the bulb. As a result of the bulbectomy and consequent damage to olfactory axons, the olfactory perikarya underwent retrograde degeneration. New neurons were then reconstituted from stem cells within the olfactory neuroepithelium. By 20 postoperative days the new olfactory axons had reached the level of the lamina cribrosa and by 30 days the fibers had penetrated into the telencephalon and had formed typical glomerular structures within the host tissue. Fibers were directed to either the paleo- or neocortex where they were observed in close proximity to large cortical neurons. The formation of glomeruli persisted over the course of the study (180 days) and showed an expansion within the cortical tissue up to 60 days of survival. The identification of these fibers and glomerular structures as olfactory was confirmed by immunohistochemical techniques using antisera to the specific olfactory protein. Ultrastructural observations clearly indicated the typical glomerular pattern of the structures and demonstrated synaptic contacts between the sensory terminals and dendritic processes, as yet unidentified, originating from the surrounding cerebral matrix. Our observations thus demonstrate that following bulbectomy and retrograde degeneration of olfactory neurons, the cells can regenerate in the absence of their normal target. Furthermore, the newly formed axons can penetrate a ‘foreign’ environment, the cerebral cortex, and form typical glomerular structures and corresponding sensory synapses. The findings suggest a heretofore unsuspected degree of plasticity in the olfactory system as well as in the cerebral cortex.

Influence of the olfactory placode on the development of the brain in Xenopus laevis (Daudin): I. Axonal growth and connections of the transplanted olfactory placode

Abstract The olfactory placode of Xenopus laevis larvae at different stages of development has been removed and transplanted onto the head of host larvae to determine the effect of the donor/host age on the viability of the transplant. Viability of the transplant (defined as penetration of the nerve of the transplanted placode in the hosts CNS) has been found to be maximal when both donor and host were at about stage29/30. Using stage 29/30 larvae, three distinct zones have been mapped on the hosts head from which the transplanted olfactory placode consistently sent its nerve to three different regions of the brain, namely to the telencephalon, the diencephalon and to the myelencephalon, respectively. The additional olfactory input induced a hypertrophy where it penetrated the telencephalon. When the olfactory input penetrated the diencephalon a marked hyperplasia of the neuronal elements and formation of glomeruli was observed in the dorsal thalamus. There was no overall change in the organization of the myelencephalon when the input reached this region. Donors up to and including stage 41 remade the placode. Removal of the placode after this stage resulted in permanent loss of the organ and hypoplasia of the olfactory bulb and telencephalon. The results illustrate the unique property of the olfactory sensory neurons to induce hyperplasia in anomalous regions of the hosts CNS and to determine, in these regions, the characteristic terminal structures which are commonly described as glomeruli.

The influence of the olfactory placode on the development of the telencephalon inXenopus laevis

Removal of the sensory plate in Xenopus laevis embryos was performed to study the influence of the olfactory anlage on the development of the forebrain. Embryos, which at stage 22-23 underwent removal of the olfactory anlagen, were killed from stage 47 to 60. In 79% of the animals, two olfactory organs reformed and gave origin to two olfactory nerves which contacted the forebrain. In this instance, the telencephalic hemispheres developed normally. In 14% of the animals, one olfactory organ reformed which contacted the brain by means of one olfactory nerve. This resulted in the development of a unique, reduced in size, cone-shaped telencephalic lobe. In the remaining animals, only a rudiment of the olfactory organ, unconnected with the brain, was present; in these cases, the telencephalon did not develop. Similar results were observed in embryos where olfactory anlagen removal was coupled with damage to, or partial removal of, the prosencephalic vesicle. In animals where lesion of the forebrain was performed without placodal removal, normal development of the forebrain was observed. The developmental relationship observed between the olfactory organ and the forebrain suggests an active role of the nose on the development of the brain.

Neurogenesis of sensory neurons in the primate olfactory system after section of the fila olfactoria

Axotomy of the olfactory sensory neurons in the adult primate squirrel monkey induces retrograde degeneration of the perikarya in the nasal neuroepithelium. The process of neuronal degeneration is rapid and by the 10th day the olfactory neuroepithelium is deprived of all mature neurons. Basal cells, supporting cells and the Bowmans glands are unaffected by the surgical procedure. The degeneration of the neurons is followed by intense mitotic activity of the basal cells of the neuroepithelium. At 30 days survival several young, mature neurons are present again in the neuroepithelium. At 60--90 survival days the neuroepithelium reacquires a population of neurons similar to controls. The persistence of neurogenesis and the replacement of experimentally degenerated neurons in an adult, non-human primate is briefly discussed.

Reinnervation of the olfactory bulb after section of the olfactory nerve in monkey (Saimiri sciureus)

Section of the fila olfactoria in squirrel monkey, a non-human primate, induces rapid degeneration of the sensory axon terminals in the olfactory bulb glomeruli. A population of axons, from newly formed sensory neurons in the olfactory neuroepithelium, regrow, passes the lamina cribrosa and, upon reaching the olfactory bulb, reinnervates the glomeruli. A new set of synaptic contacts is reformed between the sensory terminals and the post-synaptic dendritic processes of the glomeruli. Our observations indicate that this portion of the CNS of a non-human primate can be reinnervated after deafferentiation, and that active synaptogenesis occurs.