Changes in nucleolar morphology and volume of the supraoptic nucleus neurons during postnatal development of the rat

Regulation of ribosomal RNA gene copy number, transcription and nucleolus organization in eukaryotes

One of the first biological machineries to be created seems to have been the ribosome. Since then, organisms have dedicated great efforts to optimize this apparatus. The ribosomal RNA (rRNA) contained within ribosomes is crucial for protein synthesis and maintenance of cellular function in all known organisms. In eukaryotic cells, rRNA is produced from ribosomal DNA clusters of tandem rRNA genes, whose organization in the nucleolus, maintenance and transcription are strictly regulated to satisfy the substantial demand for rRNA required for ribosome biogenesis. Recent studies have elucidated mechanisms underlying the integrity of ribosomal DNA and regulation of its transcription, including epigenetic mechanisms and a unique recombination and copy-number control system to stably maintain high rRNA gene copy number. In this Review, we disucss how the crucial maintenance of rRNA gene copy number through control of gene amplification and of rRNA production by RNA polymerase I are orchestrated. We also discuss how liquid-liquid phase separation controls the architecture and function of the nucleolus and the relationship between rRNA production, cell senescence and disease.

Nucleolar alterations are reliable parameters to determine the cytogenotoxicity of environmental pollutants

Pollution generated by deposition of industrial activity waste in the environment without due care can lead to serious environmental consequences. Bioassays in higher plants are means of understanding the cytogenotoxic effects of these substances. In the present work, Allium cepa L. was used as a model species to assess nucleolar changes induced by environmental pollutants. The substances used were Methyl Methane Sulfonate (MMS), cadmium (Cd), Spent Potliner (SPL) and the herbicide Atrazine. Water was used as a negative control. The silver-stained nucleolar organizer region (AgNOR) assay was used making it possible to evaluate how nucleolar parameters (number of nucleoli per nucleus and nucleoli area) behave when facing stress caused by such pollutants. The results obtained showed a variation in the observed parameters: an increase in the number of nucleoli in the treated cells and tendency to a reduction in nucleolar area, indicating that the tested pollutants may have impaired nucleolar activity. In addition, it was possible to establish a relationship between the behavior of the nucleolus with other changes as plantlet growth, cell proliferation, and DNA damage.

Expression and localization of dystrophins and β-dystroglycan in the hypothalamic supraoptic nuclei of rat from birth to adulthood

Dystrophins (Dps) are the sub-membranous proteins that work via the dystrophin-associated proteins complex, which comprises β-dystroglycan (β-DG), a cell surface receptor for extracellular matrix. Recently, we have revealed β-DG decrease and central function impairment of supraoptic nucleus (SON) in Dp71 deficient adult mice, opening the question on the profiles of Dps and β-DG during SON development. At birth and the age of 10, 20 and 60 days, we examined the expression by RT-PCR and Western-blotting, and the distribution by immunohistochemistry of Dps and β-DG. Also, we analyzed, by immunohistochemistry and Western-blotting, the neuropeptide, arginine vasopressin (AVP), in the SON at the different ages. At birth, Dp71 and to a lesser extends, Dp140 and Dp427, and also β-DG are revealed in the SON. They are localized in the magnocellular neurons (MCNs), astrocytes and vessels. From birth to adulthood, the AVP raise in the SON coincides with the progressive increase of Dp71 level while the level of Dp140 and Dp427 increased only at D20, D10 post-natal development, respectively, and β-DG expression did not change. Moreover, the location of Dps or/and β-DG in the cell compartments was modified during development: at D10, Dps appeared in the astrocytes end-feet surrounding MCNs, and at D20, Dps and β-DG codistributed in the astrocytes end-feet, surrounding MCNs and vessels. Such a distribution marks the first steps of post-natal SON development and may be considered essential in the establishment of structural plasticity mechanisms in SON, where astrocyte end-feet, vessels, magnocellular neurons, are physiologically associated. The disappearance of β-DG in the MCNs nucleus marks the adulthood SON and suggests that the complex of Dps associating β-DG is required for the nucleoskeleton function in the post-natal development.

Suckling induced activation pattern in the brain of rat pups

OBJECTIVES

The aim of the study was to understand the effects of suckling on the brain of the pups by mapping their brain activation pattern in response to suckling.

METHODS

The c-fos method was applied to identify activated neurons. Fasted rat pups were returned to their mothers for suckling and sacrificed 2 hours later for Fos immunohistochemistry. Double labeling was also performed to characterize some of the activated neurons. For comparison, another group of fasted pups were given dry food before Fos mapping.

RESULTS

After suckling, we found an increase in the number of Fos-immunoreactive neurons in the insular and somatosensory cortices, central amygdaloid nucleus (CAm), paraventricular (PVN) and supraoptic hypothalamic nuclei, lateral parabrachial nucleus (LPB), nucleus of the solitary tract (NTS), and the area postrema. Double labeling experiments demonstrated the activation of calcitonin gene-related peptide-ir (CGRP-ir) neurons in the LPB, corticotropin-releasing hormone-ir (CRH-ir) but not oxytocin-ir neurons in the PVN, and noradrenergic neurons in the NTS. In the CAm, Fos-ir neurons did not contain CRH but were apposed to CGRP-ir fiber terminals. Refeeding with dry food-induced Fos activation in all brain areas activated by suckling. The degree of activation was higher following dry food consumption than suckling in the insular cortex, and lower in the supraoptic nucleus and the NTS. Furthermore, the accumbens, arcuate, and dorsomedial hypothalamic nuclei, and the lateral hypothalamic area, which were not activated by suckling, showed activation by dry food.

DISCUSSION

Neurons in a number of brain areas are activated during suckling, and may participate in the signaling of satiety, taste perception, reward, food, and salt balance regulation.

Nucleolar Enrichment of Brain Proteins with Critical Roles in Human Neurodevelopment

To study nucleolar involvement in brain development, the nuclear and nucleolar proteomes from the rat cerebral cortex at postnatal day 7 were analyzed using LC-MS/iTRAQ methodology. Data of the analysis are available via ProteomeXchange with identifier PXD002188. Among 504 candidate nucleolar proteins, the overrepresented gene ontology terms included such cellular compartmentcategories as "nucleolus", "ribosome" and "chromatin". Consistent with such classification, the most overrepresented functional gene ontology terms were related to RNA metabolism/ribosomal biogenesis, translation, and chromatin organization. Sixteen putative nucleolar proteins were associated with neurodevelopmental phenotypes in humans. Microcephaly and/or cognitive impairment were the most common phenotypic manifestations. Although several such proteins have links to ribosomal biogenesis and/or genomic stability/chromatin structure (e.g. EMG1, RPL10, DKC1, EIF4A3, FLNA, SMC1, ATRX, MCM4, NSD1, LMNA, or CUL4B), others including ADAR, LARP7, GTF2I, or TCF4 have no such connections known. Although neither the Alazami syndrome-associated LARP7nor the Pitt-Hopkins syndrome-associated TCF4 were reported in nucleoli of non-neural cells, in neurons, their nucleolar localization was confirmed by immunostaining. In cultured rat hippocampal neurons, knockdown of LARP7 reduced both perikaryal ribosome content and general protein synthesis. Similar anti-ribosomal/anti-translation effects were observed after knockdown of the ribosomal biogenesis factor EMG1 whose deficiency underlies Bowen-Conradi syndrome. Finally, moderate reduction of ribosome content and general protein synthesis followed overexpression of two Pitt-Hopkins syndrome mutant variants of TCF4. Therefore, dysregulation of ribosomal biogenesis and/or other functions of the nucleolus may disrupt neurodevelopment resulting in such phenotypes as microcephaly and/or cognitive impairment.

Emerging roles of the neuronal nucleolus

Although, the nucleolus has been observed for almost 200 years in neurons, studies that directly address the neuronal roles of this subnuclear structure have appeared only recently. The aim of this review is to discuss recent progress and identify some critical questions that remain to be answered. As expected for the cellular center of ribosome biogenesis, the nucleolus is essential for the growth of developing neurons, including neurite morphogenesis and long-term maintenance of mature neurons. In addition, the nucleolus contributes to neuronal stress responses, including the regulation of apoptosis. Hence, disrupted neurodevelopment or neurodegeneration are among the likely consequences of nucleolar dysfunction. Conversely, the presence of active nucleoli may determine the potential for neurorepair.

RNA polymerase 1-driven transcription as a mediator of BDNF-induced neurite outgrowth

Neurite outgrowth is essential for development of the nervous system. Neurotrophins including BDNF are among extracellular signals that regulate neurite outgrowth. The ERK1/2 pathway contributes to intracellular signaling networks transducing the pro-neuritic effects of BDNF. In the nucleolus, RNA polymerase-1 (Pol1)-mediated transcription regulates ribosomal biogenesis, enabling cellular protein synthesis and growth. Hence, we tested the possibility that Pol1 is an effector for pro-neuritic signals such as BDNF. We report that Pol1-mediated nucleolar transcription was increased by BDNF in an ERK1/2-dependent manner in rat forebrain neurons. Conversely, in cultured hippocampal neurons, knockdown of a Pol1 coactivator, transcription initiation factor 1A (TIF1A), attenuated BDNF- or ERK1/2-induced neurite outgrowth. Also, upon overexpression, a constitutively active mutant of TIF1A strongly promoted neurite outgrowth, including increases in total neurite length and branching. Finally, overexpression of wild-type TIF1A enhanced the pro-neuritic effects of ERK1/2 activation. These observations indicate that the Pol1-mediated nucleolar transcription regulates neurite outgrowth and serves as a major pro-neuritic effector of the BDNF-activated ERK1/2 pathway. Thus, development of the nervous system appears critically dependent on the nucleolus.

Using the nucleolar biomarker and the micronucleus test on in vivo fish fin cells

This study was aimed at developing the nucleolar biomarker and the micronucleus test on in vivo fish fin cells for assessing water cytotoxicity and genotoxicity. Both biomarkers can be used either jointly or separately on fins of the same fish during the experiment. For studying the nucleolar characteristics, small pieces of the fin edge were cut several times during 30-180 min of fish exposure. For micronucleus testing, the fin tissue regenerating after its cutting was investigated after 2-5 days of fish incubation. Effects of copper (0.1 and 2.5 mg/L), cadmium (0.005 and 1.0 mg/L) ions and chloral hydrate (400 and 800 mg/L) solutions were studied on cells of common carp (Cyprinus carpio L.), crucian carp (Carassius auratus gibelio Bloch.), and Mozambique tilapia (Tilapia (Sautherodon) mossambica) using a set of nucleolar characteristics (the number of nucleoli per cell, the size of a single nucleolus, and the percentage of cells with heteromorphic paired nucleoli) and the frequencies of cells with micronuclei and double nuclei. Substantial changes in parameters of nucleolar activity of fin cells were found to be caused by cadmium and copper impact. In comparison to blood cells, gill and fin cells were more sensitive as demonstrated by their nuclear damages after the chloral hydrate influence. Fin cells were useful to determine periodically cytotoxic and genotoxic effects of organic and inorganic substances in the same individual fish without any disruption of its physiological functions.

A novel nucleolar biomarker in plant and animal cells for assessment of substance cytotoxicity

The cytotoxicity of three substances (mercury(II), metolachlor, and 4-nitroquinoline-N-oxide) was assessed with a set of nucleolar parameters: the average number of nucleoli, the average volume of a single nucleolus, and the proportion of cells with heteromorphic-paired nucleoli (PNhet). Their toxic impact was studied on cells of animal and plant test organisms: onion (Allium cepa), lettuce (Lactuca sativa), and hydra (Hydra attenuata). In general, at concentrations near IC/LC(50) the three chemicals produced similar cytogenetic effects after 30-360 min of contact. For instance, in plant cells (Allium cepa and Lactuca sativa) the toxicants increased the percentage of cells with PNhet, decreased the volume of single nucleoli, and exerted no significant impact on the nucleolar number. In animal cells (Hydra attenuata), they reduced the size of nucleoli, produced no effect on the number of nucleoli, but decreased the share of cells with PNhet. Also, the chemicals affected the cells of the three test organisms to different degrees. Thus, the effectiveness of our approach of using nucleolar biomarker (use of the proposed set of parameters and time schedule of several determinations in the first hours of toxicant contact, etc.) as a means of assessing cytotoxicity was confirmed.

The magnocellular neurosecretory system of the hamster hypothalamus: an ultrastructural and morphometric study during lifetime

A quantitative study regarding the age-related changes occurring in the nucleus and the somatic organelles of neurosecretory magnocellular neurons of the hypothalamo neurohypophyseal system (HNS) was carried out in the hamster at six age-points during animal life. The magnocellular cells of both parts of the supraoptic nucleus (SON) and the paraventricular nucleus (PVN) of male Syrian hamsters between 3 and 30 months of age were examined ultrastructurally. Cells of all age groups present the same morphological ultrastructure. Standard manual morphometric techniques are used to calculate the following parameters related directly or indirectly with cellular activity: nuclear area, nucleolar area, nuclear invagination index and volumetric fractions of some intracellular structures (Golgi apparatus, mitochondria, rough endoplasmic reticulum and lipofuscin). With respect to the cell nucleus, the parameters are not modified during aging. No significant differences in the volume density of subcellular components, except lipofuscin, were detected at the age groups studies. However, there is a positive linear trend among all parameters and age except for the rough endoplasmic reticulum. Our results suggest maintenance of the synthetic activity of the magnocellular neurons in the hamster during aging but in no case an increase in their metabolic activity.

Dendritic regression dissociated from neuronal death but associated with partial deafferentation in aging rat supraoptic nucleus

As neurons are lost in normal aging, the dendrites of surviving neighbor neurons may proliferate, regress, or remain unchanged. In the case of age-related dendritic regression, it has been difficult to distinguish whether the regression precedes neuronal death or whether it is a consequence of loss of afferent supply. The rat supraoptic nucleus (SON) represents a model system in which there is no age-related loss of neurons, but in which there is an age-related loss of afferents. The magnocellular neurosecretory neurons of the SON, that produce vasopressin and oxytocin for release in the posterior pituitary, were studied in male Fischer 344 rats at 3, 12, 20, 27, 30, and 32 months of age. Counts in Nissl-stained sections showed no neuronal loss with age, and confirmed similar findings in other strains of rat and in mouse and human. Nucleolar size increased between 3 and 12 months of age, due, in part, to nucleolar fusion, and was unchanged between 12 and 32 months of age, indicating maintenance of general cellular function in old age. Dendritic extent quantified in Golgi-stained tissue increased between 3 and 12 months of age, was stable between 12 and 20 months, and decreased between 20 and 27 months. We interpret the increase between 3 and 12 months as a late maturational change. Dendritic regression between 20 and 27 months was probably the result of deafferentation due to the preceding age-related loss of the noradrenergic input to the SON from the ventral medulla.

Ultrastructural evidence of sexual dimorphism in supraoptic neurons: a morphometric study

We have recently shown that in spite of the absence of receptors for gonadal steroids in the supraoptic nucleus (SON) of the rat hypothalamus, the volume of the nucleus and the size of its neurons are larger in males than in females, and that these differences between male and female rats are correlated with body weight and dependent on the vasopressinergic neurons. As supraoptic neurons and their organelles enlarge when they are engaged in active peptidergic secretion we have carried out a morphometric ultrastructural analysis to determine if cell structures involved in the synthesis and storage of neurosecretory material also display weight-dependent sex dimorphism. Groups of six male and six female rats aged 30, 60 and 180 days were used. Nucleoli, rough endoplasmic reticulum and neurosecretory granules were analysed and we estimated their volume or surface densities and the total volume of nucleoli and rough endoplasmic reticulum, and total surface area of rough endoplasmic reticulum. We found that, with the exception of neurosecretory granules, the densities of the organelles did not differ among the groups studied, but total values were higher in males. These differences were found to be weight-dependent. Since the organelles studied are regarded as reliable indicators of the neurosecretory activity of supraoptic neurons, our data fully support the view that the weight-dependent sexual dimorphism observed in this nucleus reflects greater synthetic activity of its vasopressinergic neurons associated with the need to maintain water balance in larger bodies.

Intranuclear inclusions in paramyxovirus-induced encephalitis: evidence for altered nuclear body differentiation

Intranuclear inclusion bodies (INB) are frequently encountered in viral infections, where they are thought to be accumulations of viral particles. However, for RNA viruses replicating in the cytoplasm, this compartmentalization represents a paradox not consistent with the viral replication cycle. To define the basis for intranuclear paramyxoviral inclusion bodies in astrocytes, natural cases of canine distemper virus subacute encephalitis were examined by light and transmission electron microscopy, and by quantitative confocal immunofluorescence microscopy. Although INB were viral antigen positive, they were not composed of structurally recognizable paramyxoviral nucleocapsids. The structural basis for the INB was instead viral antigen-associated forms of nucleolar development known as nuclear bodies. Three variants of the light microscopic Cowdry type A INB were complex nuclear bodies, giant beaded nuclear bodies (sphaeridia), and nuclear body-associated granulofilamentous matrices. In the latter, the granulofilamentous matrix frequently filled the nucleus, resulting in a fourth morphological INB variant, and was associated with morphological evidence of nuclear degeneration. These findings suggest a novel mechanism of virus-induced cytopathology whereby intranuclear viral protein exerts deleterious effects upon nucleolar differentiation in infected cells and hence altered host cell RNA metabolism.

Organization of nucleoli and nuclear bodies in osmotically stimulated supraoptic neurons of the rat

This study has analyzed variations in the number of nucleoli and nuclear bodies, as well as in their ultrastructural and cytochemical organization, after the osmotically induced activation of supraoptic nucleus (SON) neurons of the rat. The number of nucleoli and nuclear bodies and also the nucleolar size were determined on smear preparations of previously block-impregnated SON. The mean number of nucleoli per cell was 1.35 +/- 0.6 (mean +/- SDM) in control rats. No significant variations in this value were registered either in dehydrated or rehydrated rats. The mean nucleolar volume and the total nucleolar volume per cell showed a significant increase in dehydrated rats with respect to the controls, whereas these two parameters tended to return to control values in rats rehydrated after dehydration. The mean number of nuclear bodies per cell increased significantly from 0.56 +/- 0.50 (mean +/- SDM) in control rats to 1.54 +/- 1.1 after 6 days of dehydration. By electron microscopy, SON neurons displayed a reticulated nucleolar configuration. After the osmotically induced neuronal activation, there was an increase in the proportion of the total nucleolar area occupied by the granular component, and also a reduction in the mean fibrillar-center area. The most characteristic nucleolar features in rehydrated rats were the tendency for the granular component to be segregated and the occurrence of intranucleolar vacuoles. Ultrastructural cytochemistry with a specific silver method revealed a selective silver reaction on the coiled threads of the nuclear bodies--identified as "coiled bodies"--and on the nucleolar fibrillar components in all animal groups studied. Since nucleoli play a major role in ribosome biogenesis, a relationship between these nucleolar changes and the level of cellular activity of SON neurons is proposed. Furthermore, the response of nuclear "coiled bodies" to neuronal activation suggests their participation in the processing and transport of rRNA precursors.

Cytology and organization of reactive astroglia in human cerebellar cortex with severe loss of granule cells: a study on the ataxic form of Creutzfeldt-Jakob disease

In order to investigate the cellular basis of human astrogliosis, we have selected the cerebellar cortex because it provides a relatively simple and geometrical organization of both neuronal and glial populations. A pathological system with severe and progressive loss of granule cells was studied: the ataxic form of Creutzfeldt-Jakob disease, where the tissue geometry is minimally disturbed. The quantitative study revealed a drastic reduction in the numerical density of granule cells in the Creutzfeldt-Jakob disease cerebellum, and a significant increase in the numerical density of astrocytes. Karyometric analysis showed that the nuclear area was significantly greater in reactive astroglial cells than in normal astroglia. Glial fibrillary acidic protein immunocytochemistry revealed astroglial hypertrophy, but the geometry and spatial domains of astroglial subtypes were strictly preserved. Vimentin expression was detected in Bergmann glia and in certain astrocytes of the granular layer. Ultrastructural analysis showed that reactive astroglia had large nuclei, with expanded interchromatinic regions which contained clusters of interchromatin granules and nuclear bodies, and prominent reticulate nucleoli. In the cytoplasm, hypertrophied bundles of intermediate filaments were observed, some of them associated with the nuclear envelope. Numerous adhering and gap junctions were also found among reactive astroglial cells. Perivascular glial processes showed a terminal web of intermediate filaments and a conspicuous plasmalemmal undercoat. Interendothelial tight junctions were preserved. Our results suggest that the severe loss of granule cells induces a highly ordered astroglial response which tends to preserve the geometry of the astroglial scaffold, the domains of each astroglial subtype, the neuronal microenvironmental conditions and the efficiency of the blood brain barrier, in order to promote neuron survival.

Ultrastructural and morphometric analysis of nucleolar and nuclear changes during the early growth period in hamster facial neurons

In this study, progressive developmental changes in the nucleus and associated organelles, including the nucleolus, coiled bodies, nuclear envelope, and nucleoplasm, of hamster facial motor neurons were characterized by two parallel analyses: ultrastructural and morphometric. Golden hamsters (Mesocricetus auratus) used for this series were the 14-day fetus, newborn (less than 6 hr), and 1, 2, 3, 4, 5, 7, 9, 11 and 13 days postnatal ages, with 3 animals per group. Following anesthesia and perfusion fixation, facial nuclear groups were dissected and processed for electron microscopy. Electron micrographs and camera lucida tracings of nuclear profiles were collected and analyzed. The ultrastructural analysis revealed progressive changes in the nucleolus from a compact, segregated type to a reticulated form characteristic of actively protein-secreting cells. Nucleolar microbodies and fibrillar centers were seen at all ages; the latter structures appeared to decrease in size and increase with age in the series. The nucleolus-associated chromatin became less condensed, suggesting an increase in the incorporation of rDNA into the nucleolus proper. Coiled bodies, both free and attached to nucleoli, were found in varying frequencies. The nucleoplasm of neurons at the earliest stages contained large numbers of heterochromatin clumps, which decreased concomitantly with an increase in interchromatin granules and fibrils during the later stages. Nuclear envelope invaginations, polarized along one side of the nucleus, increased throughout the developmental period examined. These changes occurred in concert with a 61% increase in nuclear size and a 47% increase in the length of nuclear envelope. The sequence of nuclear changes observed during this early period of normal facial neuronal growth completes the study of a series of distinctly defined cytomorphic events in this cell type, the lability of which can be experimentally tested for their functional roles in neuronal development.

Association between the nucleolus and the coiled body

By means of light and electron microscopic immunocytochemistry, we have localized p80-coilin, a specific protein marker for coiled bodies, in mammalian cell lines as well as in primary rat neuron cultures. p80-coilin-stained nuclear bodies, which also contained fibrillarin, could be subsequently silver stained by a method specific for the visualization of nucleolar organizer regions. In cycling cells, most coiled bodies were not associated with nucleoli, whereas in rat neurons such as association was frequent. The treatment of cycling cells with actinomycin D or 5,6-dichloro-1-beta-D-ribo furanosyl-benzimidazole led to nucleolar segregation and/or disintegration, and to an association of p80-coilin staining structures with nucleoli. p80-coilin-positive structures contained fibrillarin in both untreated and treated cells. These results support the opinion that there might be a special association between coiled bodies and nucleoli, particularly in neuronal cells.

Nucleolar organization in granule cell neurons of the rat cerebellum

The present light and electron microscopic study deals with the morphology and cytochemical properties of the nucleolus in mature granule cells of the rat cerebellum. Nucleolar organization has been examined by several different methodological approaches: (a) determination of the number of nucleoli per cell from smear preparations, (b) morphometric estimation of nucleolar size, and (c) ultrastructural and cytochemical characterization of nucleolar components. The mean number of nucleoli per cell was found to be 1.46 +/- 0.02 (mean +/- SEM). The morphometric measurement of the nucleolar area showed an average value of 0.359 +/- 0.07 microns 2 (mean +/- SDM). At the electron microscope level, most granule cell nucleoli displayed a fine texture which does not differ substantially from the 'ring-shaped' configuration usually found in cells with low levels of protein synthesis activity; i.e., a large round fibrillar centre surrounded by the dense fibrillar component, and small masses of granular component segregated from the preceding constituents. The meaning of this nucleolar configuration is discussed in the light of recent ultrastructural and biochemical data on nucleolar function. A relationship between this pattern of nucleolar organization and metabolic characteristics of the granule cells is suggested.

Nucleoli numbers and neuronal growth in supraoptic nucleus neurons during postnatal development in the rat

We present a quantitative study of the variations in the number of nucleoli in supraoptic nucleus neurons during the postnatal period, as well as a morphometric and stereological analysis of the nuclear and cytoplasmic volume changes of these maturing neurons. The mean number of nucleoli per cell was 1.59 +/- 0.28 (mean +/- S.D.) at P1; it then began to decrease until P14 (1.32 +/- 0.67) at which age the adult pattern in the number of nucleoli was attained. The mean nuclear volume increased steadily from 214.56 +/- 6.48 microns 3 (mean +/- S.E.) at P1 to 326.1 +/- 10.93 microns 3 at P14 where it remained constant. The average cytoplasmic volume underwent a remarkable increase during postnatal period from 256.38 +/- 12.66 microns 3 at P1 to 3791.18 +/- 204.88 microns 3 at P90. It is noteworthy that the stabilization of the number of nucleoli coincides with the termination of the nuclear growth phase of supraoptic neurons. We suggest that these nuclear and nucleolar changes reflect the attainment of the fully-differentiated state of the protein synthesis machinery in these neurosecretory neurons.