Naturally occurring neuronal death during the development of the inferior olive in the chick

Development of the human accessory olivary nuclei: A morphometric and computerized 3D-reconstruction study

INTRODUCTION

This study described the prenatal development of the accessory olivary nuclei (AO) in humans.

MATERIALS/METHODS

Serial brain sections from ten pre- and full term infants aged 21-43 postmenstrual weeks (PW) were stained using the Klüver-Barrera method. A computerized 3D-reconstruction technique and morphometry were adopted for the study.

RESULTS

The medial AO (MAO) and dorsal AO (DAO) were identified at 21 PW. The dorsal cap was clearly differentiated from the main body (MB) of the MAO in neuronal cytoarchitecture. Pyknotic neurons were diffusely observed in the AO at 21 PW and were most concentrated in the MB. These neurons became infrequent from 28 PW onward. Neuronal nests existed in clusters between the AO and the medial lemniscus at 21 PW, which reduced progressively in size and number with age. The 3D-reconstructions showed that the AO are separated into caudal and rostral parts, and that this separation is achieved by mid-gestation in the DAO. Nuclear volume increased exponentially with age in the AO, although the rate of increase was half that of the principal nucleus (PO). Neuronal numerical density decreased rapidly 21-28 PW. The total neuronal number showed a weak correlation with age. The mean neuronal profile area increased linearly with age.

CONCLUSION

The human AO are separated into caudal and rostral parts in the fetal period. The nuclear volume and neuronal profile areas increase with age, although the rate of this increase is lower than in the PO. Natural neuronal death may occur at mid-gestation in the AO.

Ontogeny of subunits 2 and 3 of the AMPA-type glutamate receptors in Purkinje cells of the developing chick cerebellum

Several molecules, involved in cellular communication in the mature nervous system, appear to play important roles during neural development. These roles include neuronal growth, morphological changes of neurites, and neuronal survival. Such plasticity processes seem to be in part the result of activation of different receptor subtypes, which could cause Ca(2+) influx, a major candidate to be an outgrowth promoter. In this context, we performed immunohistochemical and in situ hybridization experiments to examine the following aspects of the development of chick cerebellum Purkinje cells: (i) expression of AMPA-type glutamate receptor GluR2/3 proteins; (ii) the levels of mRNAs coding for the GluR2 and GluR3 flip/flop isoforms; and (iii) expression of calbindin (CB) and parvalbumin (PV). Expression of GluR2/3 proteins, CB, PV, and the mRNAs coding for GluR2 and GluR3 splice variants all revealed a differential expression during development in chick Purkinje cells. GluR2/3 proteins and the GluR3 flop variant start to be expressed at E10, while the expression of CB, PV, the GluR3 flip isoform and the splice variants of GluR2 all started around E12-E14. All proteins showed an increasing expression from embryonic stages into the posthatching period. These results reveal a developmentally regulated expression of GluR2/3 proteins, including their splice variants, and of CB and PV in Purkinje cells. These findings may suggest a relationship between these proteins and specific cerebellar developmental processes.

A quantitative study of the purkinje cells in the cerebellum and the inferior olivary neurons in the chicken

A single olivocerebellar fiber branches off several climbing fibers. One Purkinje cell receives input from only one climbing fiber. A single inferior olivary neuron, therefore, synapses with several Purkinje cells, so that there are more Purkinje cells than the inferior olivary neurons. We aimed to elucidate the numerical ratio of the inferior olivary neurons to Purkinje cells in the chicken. The total numbers were 353,834 +/- 5,274 in the Purkinje cells per the cerebellum and 21,553 +/- 904 in the inferior olivary neurons of both sides. The numerical ratio of inferior olivary neurons to Purkinje cells was 1:16. The ratio of those neurons in mammals is about 1:4-17, so that the ratio in the chicken is within the range of mammals.

Naturally occurring neuronal death during the postnatal development of Purkinje cells and their precerebellar afferent projections

Naturally occurring neuronal death plays a substantial developmental role in the building of the neural circuitries. The neuronal death caused by different cerebellar mutations is mostly of an apoptotic nature. Apart from the identity of the intrinsic mechanisms of the mutations, adult cerebellar mutants are a powerful tool to causally study the development of the cerebellar connectivity. Thus, studies on adult cerebellar neuronal cell death occurring in mouse mutants elucidate: (i) the dependence of the postsynaptic neurons on their partners, (ii) the 'en cascade' postsynaptic transneuronal degeneration after target-deprivation, and (iii) the close relationship between the molecular modular organization of the cerebellar cortex and dying Purkinje cells. Neuronal cell death has been extensively studied in developing olivocerebellar system. However, less data are available on the occurrence of naturally occurring neuronal death during the in vivo normal development of the Purkinje cells and the mossy fiber system neurons. The developmental role of neuronal death during the establishment and refinement of the olivocerebellar projection is currently discussed. Moreover, the occurrence of neuronal death during the development of the basilar pontine nuclei and its role in the acquisition of the adult pontocerebellar projection is still poorly understood. In the present review, we correlate the dates of Purkinje cells death with the inferior olivary and basilar pontine neuronal apoptosis, discussing their developmental relationships during the elaboration of the fine-grained maps of the cerebellar afferent connections.

Ontogeny of two calcium-binding proteins (calbindin D-28K and parvalbumin) in the human inferior olivary complex and their distribution in the adults

The inferior olivary complex (IOC) is a prominent nuclear relay system of the medulla oblongata. Anatomically, it is connected to the cerebellum for coordination of motor activities. Calbindin D-28K (CALB) and parvalbumin (PV) are cytosolic calcium-binding proteins (CBP) that play a role in Ca2+ homeostasis. We examined their ontogeny and distribution in the fetal, postnatal and adult human IOC by immunohistochemistry. At 11-12 weeks of gestation (wg), calbindin immunoreactivity was present in the principal olive and the medial accessory olive, it was absent in the dorsal olive. Parvalbumin immunoreactivity developed at 16-17 wg in the ventral lamella and the lateral bulge of the principal olive only. Calbindin expression gradually increased from 20 to 37 wg, whilst by contrast, parvalbumin expression was moderate. By 37 wg, all three IOC subnuclei were immunopositive for both proteins. In a 3-month-old infant, parvalbumin was intensely developed in the olivary axons. In the adults (40- to 59-year-old), calbindin was distributed in most neurons, and olivocerebellar fibres, whereas parvalbumin was present in some neurons and few fibres. Parvalbumin expressed till 51 years, and disappeared by 59 years of age. Calbindin immunoreactivity in the olivary axons was declined at 70 years of age. The data suggest a differential distribution and requirement of these proteins in the human IOC maturation. It may be that the IOC utilizes mainly calbindin for Ca2+ buffering. The loss of parvalbumin with ageing might influence the excitability of the spared IOC neurons.

Subtle developmental abnormalities in the inferior olive: an indicator of prenatal brainstem injury in the sudden infant death syndrome

Subtle quantitative abnormalities in neuronal populations derived from the rhombic lip (i.e. arcuate nucleus at the ventral medullary surface, external granular layer of the cerebellum) have been reported in victims of the sudden infant death syndrome (SIDS). In this study, we examined the inferior olive, a major rhombic lip derivative, to determine if subtle rhombic lip abnormalities also involve this nucleus in SIDS. We analyzed the number and density of neurons and reactive astrocytes in the inferior olive in 29 SIDS cases and 29 controls. Computer-assisted cell counting procedures were used in sections stained with hematoxylin and eosin/Luxol fast blue. There was a significant difference in the postconceptionally age-adjusted mean for neuronal density between SIDS cases (7,687 +/- 255 neurons/mm(3)) and controls (8,889 +/- 255 neurons/mm(3)) (p = 0.002). The difference in age-adjusted mean neuronal number between SIDS cases (1,932 +/- 89 neurons/2 sections) and controls (2,172 +/- 89 neurons/2 sections) was marginally significant (p = 0.063). Reactive astrocytes were present in the inferior olive in SIDS cases, but their number, density, and developmental profile were not significantly different from that of control infants dying of diverse known causes. SIDS victims found dead in cribs, beds, and sofas, prone or supine had subtle olivary abnormalities, suggesting that affected infants are at risk in various sleeping situations. We propose that at least some SIDS victims experience intrauterine brainstem injury including the olivo-arcuato-cerebellar circuitry derived from the rhombic lip. These observations provide future directions for SIDS research concerning the role of early insults in pregnancy, the rhombic lip, and the interactions of the ventral medulla and cerebellum in cardioventilatory control.

Increased inferior olivary neuron and cerebellar granule cell numbers in transgenic mice overexpressing the human Bcl-2 gene

Neuron-target interactions during development are critical for determining the final numbers of neurons in the nervous system. To investigate the role of Purkinje cells and programmed cell death in the regulation of afferent neuron numbers, we have counted olivary neurons and granule cells in two lines of transgenic mice (NSE73a and NSE71) that overexpress a human gene for bcl-2 (Hu-bcl-2) in Purkinje cells and olivary neurons, but not in granule cells. Bcl-2 overexpression in vivo reduces naturally occurring neuronal cell death and cell death following axotomy, target removal, or ischemia. Olivary neuron numbers in NSE73a and NSE71 transgenic mice are significantly increased compared to controls by 28% and 27%, respectively, while granule cell numbers are only increased in NSE73a mice (29% above controls). We have previously shown that Purkinje cell number is increased by 43% in NSE73a transgenics and by 23% in NSE71 transgenics. The ratio of Purkinje cells to olivary neurons is not significantly different between the control and transgenic mice, while the ratio of granule cells to Purkinje cells is significantly decreased in the NSE71 transgenic mice compared to controls and NSE73a transgenics. The increased numbers of olivary neurons suggest that bcl-2 overexpression rescues these neurons from programmed cell death. The increase in granule cell number in only one transgenic line is discussed with respect to hypotheses that Purkinje cells regulate both granule cell progenitor proliferation and the survival of differentiated granule cells.