Connections of the representational area of the distal section of the forelimb in the rat sensorimotor cortex

Using horseradish peroxidase, studies have been made on the distribution of retrogradely labeled nervous cells in the sensorimotor cortex of rats. The enzyme was injected into electrophysiologically identified zone of representation of the distal part of the forelimb in areas S2 and S1. It was found that this zone in S2 contains afferent connections mainly from representation of the same extremity in S1 and only a few afferents from other areas of S1, S2 and M1 of the same hemisphere. Single labeled neurones were found in areas S2, S1 and M1 of the contralateral hemisphere. Representation of the forelimb in S1 receives mainly cortical afferents from the same region of S1 and from single cells of homologous zones S2 of the same and S1 of the contralateral hemisphere. Connections from S1 to S2 are more numerous than the opposite ones. In contrast to cats and monkeys, in rats afferent cortical fibers to zone S2 pass not only from the third layer, but also from the fifth and sixth layers of the cortex. It is suggested that during progressive development of the neocortex in mammals, the increase in the degree of separation of neurones (which give origin to corticofugal and cortical connections) among different layers of the cortex takes place.     

The projection of spinocerebellar neurons from the sacrococcygeal region of the spinal cord in the cat. An experimental study using anterograde transport of WGA-HRP and degeneration.

The projection from the sacro-coccygeal region of the spinal cord to the cerebellum was studied by two different techniques in the cat. In five cats wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) was injected caudal to a preceding unilateral cordotomy at the sacral level, aimed at interrupting the spinocerebellar tracts on one side completely, and the distribution of WGA-HRP labeled mossy fibers and mossy fiber terminals was studied in the cerebellum. In three additional cats, degenerating fibers were examined in Fink-Heimer stained sections following unilateral transection of the lateral and ventral funiculi at L7 or S3 level. In the WGA-HRP experiments the labeled mossy fiber terminals were located bilaterally in lobules I-V. Most of them were found in the anterior part of lobule II. In addition, labeled terminals were observed in sublobule VIIIB and in pars copularis of the paramedian lobule, contralateral to the cordotomy. The terminals in the anterior lobe were concentrated in longitudinal zones parallel to the mid sagittal plane. In lobule II, the terminals were most abundant in the superficial, apical parts of the folia. Some presumed terminals were also seen in the cerebellar nuclei. Labeled fibers were found contralateral, but not ipsilateral to the cordotomy in the superior and inferior cerebellar peduncles, as well as in the spinal cord rostral to the cordotomy. The results of the degeneration experiments were the same as those of the WGA-HRP experiments with regard to the detailed projections in the cerebellar cortex. This is strong support against the possibility that WGA-HRP labeled cerebellar mossy fiber terminals, following WGA-HRP injections in the spinal cord, would represent terminals of collaterals of retrogradely labeled neurons. It also lends strong support in favour of WGA-HRP as a reliable anterograde tracer for studying cerebellar cortical projections of spinocerebellar neurons in the cat.     

Organization of the sensory and motor nuclei of the glossopharyngeal and vagal nerves in lampreys

Anterograde and retrograde transport of horseradish peroxidase was used to examine the afferent and efferent projections of the glossopharyngeal and vagal nerves in the lamprey, Lampetra japonica. Except for the ganglion cells and motoneurons, the distribution patterns of HRP-positive elements differed little between the two nerves. Afferent fibers mainly terminated in the ipsilateral cerebellar area, medial octavolateralis nucleus, and between the ventral octavolateralis nucleus and descending tract and nucleus of the trigeminal nerve (dV). In the cerebellar area, most of the labeled fibers were located in the molecular zone, but some penetrated into the granular zone. In the rostral part of the medial octavolateralis nucleus, labeled fibers coursed from the middle to the lateral area, and in the caudal part, they were localized in the dorsal area of the nucleus. In the area between the dV and ventral octavolateralis nucleus, labeled fibers coursed near the dorsal margin of the rostral part of the dV, and in the caudal part, they shifted dorsally. Ganglion cells and motoneurons of each nerve were also labeled.     

Afferent connections of the optic tectum in channel catfish Ictalurus punctatus

Summary Horseradish peroxidase was injected unilaterally into the optic tectum of the channel catfish, Ictalurus punctatus. The sources of tectal afferents were thereby revealed by retrogradely labeled neurons in various brain centers. Retrogradely labeled cells were seen in both the ipsilateral and contralateral telencephalon. The superficial pretectal area was labeled on both sides of the brain. Ipsilateral projections were also observed coming from the entopeduncular nucleus. Both the anterior thalamic nucleus and the ventro-medial thalamic nucleus projected to the ipsilateral optic tectum. Cells in the ipsilateral nucleus of the posterior commissure were seen to project to the tectum. Labeled fibers were visualized in the lateral geniculate nucleus ipsilateral to the injected tectum, however, no labeled cell bodies were observed. Therefore, tectal cells project to the lateral geniculate nucleus, but this projection is not reciprocal. No labeled cells were found in the cerebellum. Labeled cells occurred in both the ipsilateral and contralateral medial reticular formation; they were also observed in the ipsilateral nucleus isthmi. A projection was seen coming from the dorsal funicular nucleus. Furthermore, labeled cells were shown in the inferior raphe nucleus.Abbreviations AP Area pretectalis - C Cerebellum - DPTN Dorsal posterior tegmental nucleus - H Habenula - IRF Inferior reticular formation - LI Inferior lobe - LGN Lateral geniculate nucleus - LR Lateral recess - MB Mammillary body - MRF Medial reticular formation - MZ Medial zone of the telencephalon - NC Nucleus corticalis - NDL-M Nucleus opticus dorsolateralis/pars medialis - NI Nucleus isthmi - NPC Nucleus of the posterior commissure - OPT Optic tectum - OT Optic tract - PC Posterior commissure - PN Pineal organ - PrOP Preoptic nucleus - PT Pretectum - TBt Tectobulbar tract - TEL Telencephalon - TL Torus longitudinalis - TS Torus semicircularis - VC Valvula cerebelli - VLTN Ventrolateral thalamic nucleus - VMTN Ventromedial thalamic nucleus     

Growth,mitosis and morphogenesis of the simple liver acinus in neonatal rats

Radioautography after ³H-thymidine injection, blockage of mitosis due to colchicine and camera lucida drawings were used to study growth, mitosis and morphogenesis in the simple liver acinus of Rappaport in neonatal rats. Hepatic parenchymal cell plates are irregularly arranged and thick from birth to 4 days postpartum. By 10 days the plates begin to assume the adult configuration, irregular and thick in acinar zone 1 (periportal) but straight and thin in acinar zone 3 (pericentral). After a single injection of ³H-thymidine at birth the distribution of labeled nuclei among the hepatic acinar zones was such that zone 1 contained most, zone 3 least, and zone 2 an intermediate amount. This relationship remained constant out to 10 days postpartum. The amount of labeled cells within each zone varied, reaching its highest values at 2 days in zones 1 and 2 but not until 4 days in zone 3. Similar to the distribution of labeled nuclei, frequency of mitosis also exhibited a constant relationship of zone 1 > zone 2 > zone 3, but peaks of cell division within each zone were not always present. All three zones displayed a peak of mitosis at 4 days, whereas a second mitotic peak at 10 days was attained only by cells in zones 1 and 2.Conclusions are: (1) The neonatal liver is an expanding cell population with most of the expansion confined to acinar zones 1 and 2, (2) the period of 4–6 days postpartum is critical and could be the time when acinar metabolic zones are forming, (3) the irregular arrangement of cell plates in the center of the acinus (zone 1) may act to dampen arterial pulsations and allow adequate mixing of arterial and venous blood, (4) immediate postnatal growth of the liver acinus, as shown by an increase in its width, is due primarily to an increase in cell number since individual cell size does not increase.     

Appearance of retrogradely labeled neurons in the rat superior cervical ganglion after injection of wheat-germ agglutinin-horseradish peroxidase conjugate into the contralateral ganglion

Summary Injection of wheat-germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) into the superior cervical ganglion (SCG) of the rat results in accumulation of WGA-HRP in sympathetic postganglionic neurons in the contralateral SCG. The sympathetic pathways involved and the mechanism underlying the labeling were investigated. The labeling in neurons in the contralateral SCG was apparent 6 h after injection and increased in intensity with longer survival times. The number of labeled neurons reached 1300 at 72 h after the injection. Transection of the external (ECN) or internal carotid nerves (ICN) resulted in considerable reduction in the number of labeled neurons. Combined transection of both ECN and ICN virtually eliminated labeling in the contralateral SCG. This provides strong evidence that these two nerves are the major pathways for WGA-HRP transport out of the SCG. No labeling was observed in the contralateral SCG following injection of horseradish peroxidase (HRP). Therefore, it seems unlikely that a direct nerve connection exists between the bilateral ganglia. Instead, the labeling of contralateral SCG neurons appears to depend on the transneuronal transport capacity of WGA-HRP, which conveys the marker in an anterograde direction along the postganglionic fibers to terminals in sympathetic target organs, and then delivers it transneuronally to contralateral SCG neurons. We suggest that the sympathetic nerve fibers originating in the bilateral SCGs run intermingled and are in close contact in their peripheral target organs.     

Anatomical Studies of the Spinocervical Tract of the Rat

The retrograde transport of horseradish peroxidase (HRP) was used to identify and examine the cells of origin of the spinocervical tract (SCt) in the rat. Initially, precise data on the boundaries of the rat lateral cervical nucleus (LCn) were gathered after injecting HRP into the ventrobasal thalamus. These data indicated that the LCn of the rat is restricted to a region on the extreme lateral edge of the dorsalmost portion of the lateral funiculus (DLf) within spinal segment C₂ Following small iontophoretic injections of HRP that were restricted to this area, labeled SCt neurons were found in the ipsilateral nucleus proprius at all levels of the spinal cord but were most numerous in the cervical enlargement. Lesion studies indicated that the overwhelming majority of SCt axons ascend to the LCn within the DLf. In an attempt to determine whether our injection techniques labeled a significant number of cells through axons of passage, HRP injections were made in the DLf ventral to the LCn. Such injections labeled, presumably through axons of passage, cells in several areas of the spinal cord gray matter, including a large number in the contralateral marginal zone Injections in areas immediately rostral to the LCn labeled 20% or less of the total number of cells within the enlargements that were labeled by injections into the LCn. Thus, the majority of cells labeled by injections of HRP into the LCn were labeled through preterminal fibers or terminals themselves. The cells of origin of the SCt in the rat are similar in location to those in the cat but far fewer in number.     

Localization and Central Projections of Primary Afferent Neurons That Innervate the Temporomandibular Joint in Cats

Primary afferent neurons that innervate the temporomandibular joint (TMJ) in cats were labeled by injecting a 2-5% solution of wheatgerm agglutinin bound to horseradish peroxidase into the joint capsule and capsular tissues in 14 cats and processing the brain stem and trigeminal ganglia using the tetramethylbenzidine method described by Mesulam (1978). The perikarya of ganglion cells that innervate the TMJ ranged in diameter from 15 to 109 μm and were primarily located in the posterolateral portion of the trigeminal ganglion. The central processes of these neurons entered the brain stem in middle pons and were distributed to all portions of the sensory trigeminal nuclei. However, the majority of labeled fibers and greatest density of terminal labeling were observed in the dorsal part of the main sensory nucleus and the subnucleus oralis of the spinal trigeminal nucleus. Very few labeled fibers were observed in the spinal tract of the trigeminal nerve below the obex. However, evidence for axon terminals was consistently observed in laminae I, II, and III of the medullary dorsal horn. These findings concur with physiological evidence showing that information from the TMJ influences neurons in rostral (Kawamura et al, 1967) and in caudal (Broton et al, 1985) portions of the trigeminal sensory nuclei.     

Horseradish peroxidase-labeled cells as sources of the spinoreticular and spinothalamic systems of the brain]

The cells of spinoreticular and spinothalamic fibrous systems of the cat brain were studied by the method of axone transmission of horse-radish peroxidase (HP). A dense accumulation of HP-labeled neurons establishing direct relations with the reticular formation and thalamus was seen in the upper segments of the spinal cord. In the lower segments these zones were confined to the medial part of the ventral horn and the intermediate zone of the gray matter. The neurons established direct connections with contralateral nuclei of the reticular formation as well as with the thalamus ipsi- and contralateral nuclei. Possible pathways of transmitting somatic and pain sensitivity are discussed.     

黄土高原不同植被带土壤团聚体有机碳和酶活性的粒径分布特征

为探究土壤中各粒级团聚体不同形态有机碳和酶活性的分布特征,以黄土高原延河流域森林带、森林草原带、草原带土壤为对象,研究了不同粒级团聚体总有机碳、易氧化碳和腐殖质碳含量,以及纤维素酶、β-D葡糖苷酶、过氧化物酶、蔗糖酶和脲酶活性,分析了土壤团聚体有机碳及其组分与酶活性之间的相关关系.结果表明： 3种植被带土壤团聚体有机碳及其组分含量表现为森林带＞草原带＞森林草原带,3种形态有机碳含量在0.25～2 mm粒径均最高;不同植被带土壤团聚体有机碳及其组分含量和酶活性在0～10 cm土层大于10～20 cm土层;3种植被带纤维素酶、β-D葡糖苷酶、蔗糖酶和脲酶活性表现为森林带＞草原带＞森林草原带,过氧化物酶活性表现为森林带＞森林草原带＞草原带;3种植被带土壤中各种酶活性随着粒径的减小呈递增趋势.土壤纤维素酶、过氧化物酶、蔗糖酶和脲酶活性与团聚体各种形态碳含量均呈显著正相关.
     

Experience with the use of horseradish peroxidase for light and electron microscope studies of interneuronal connections]

By the method based on a retrograde axonal transport of exogenous horseradish peroxidase (HRP), the origins of afferentation of the motor cortex of adult cats, kittens and albino rats were studied. HRP-positive neurons were found by light and electron microscopy in the somatosensory cortex (C1) of the ipsilateral hemisphere and in the portions of the cortex of the contralateral hemisphere which were symmetrical to the site of injection of HRP. The disposition of neurons, marked by HRP, in the Vth layer of the motor cortex suggest that these neurons may send their axons into the bundles of comissural fibres going to the motor cortex of the opposite hemisphere. This method considerably expands possibilities of revealing the origins of afferentation of the investigated portion of the nervous system and allows more complete and reliable investigation of interneuronal connections.     

A method of quantitating Paneth cell metaplasia of the stomach by image analysis

Paneth cells are one of the histologic components of intestinal metaplasia of the stomach, as are mucin-producing goblet cells. With the aid of an image quantifier, the distribution of Paneth cells histochemically labeled with acid fuchsin was analyzed for a gastrectomy specimen containing an adenocarcinoma of the intestinal type; the topographic distribution of goblet cells histochemically labeled with Alcian blue (pH 2.5) was also analyzed. The specimen was cut into 63 blocks (0.5 X 4.0 cm) in four zones; antrum (zone I), intermediate region (zone II) and fundus (zones III and IV). Paneth cells were found only in sections containing mucin-producing goblet cells. Paneth cells were found in 12.5% of the 16 sections from the antral zone I containing Alcian blue-positive goblet cells. The rates were 44.4% for the intermediate zone II and 55.5% for the distal fundic zone III. The total area occupied by Paneth cells was significantly lower in the gastric mucosa as compared to the duodenal mucosa. The "Paneth cell index" (total Paneth cell area/total goblet cell area) was highest in the duodenum, followed by the distal fundic zone III. This method of quantitating Paneth cell metaplasia of the stomach will be used to investigate the topographic distribution of those cells in populations with low and high incidences of intestinal metaplasia.     

Morphology of calretinin-immunoreactive neurons in the superficial layers of hamster superior colliculus after enucleation: lack of co-localization with GABA

We recently reported on the distribution and effects of eye enucleation on the immunoreactivity of calretinin in the superficial layers of the hamster superior colliculus (SC). In the present study, we describe the types of labeled cells and compare this labeling to that of GABA, the major inhibitory neurotransmitter in the central nervous system. An almost complete depletion of calretinin-immunoreactive (IR) fibers in the superficial layers of the contralateral SC was found following unilateral enucleation. On the contralateral SC, many calretinin-IR cells were newly appeared. The majority of the newly-appeared cells had small- to medium-sized round, oval, or vertical fusiform cell bodies. Two-color immunofluorescence revealed that none of these newly-appeared cells were labeled with an antibody to GABA. The present results show that the calretinin-IR cells are unique in the superficial hamster SC when compared to most of the other brain areas, where many calretinin-IR cells are GABAergic interneurons.     

Distribution of growth associated protein (GAP-43) immunoreactivity in nerve fibers supplying the pig pineal gland

An immunohistochemical study of the pig pineal gland was carried out using monoclonal mouse antiserum against growth-associated protein GAP-43. The pineal glands were obtained from the 3, 5, 8 weeks old piglets. The immunopositive nerve fibers were observed in the pineal gland as well as in the habenular and the posterior comissural areas. They formed a dense network in the habenular area and the proximal part of the pineal gland. In the comissural area and in the apical part of the gland. single positive fibers were observed. The obtained results may suggest a difference in the plasticity of innervation between the particular regions of the pineal gland.     

Detection of immature dendritic cells in the enamel organ of rat incisors by using anti-cystatin C and anti-MHC class II immunocytochemistry.

Dendritic cells in the enamel organ of rat incisors were examined with immunocytochemistry using an anti-cystatin C antibody for immature dendritic cells and macrophages, OX6 for MHC Class II, ED1 for macrophages and dendritic cells, and ED2 for macrophages. Single cells positive for anti-cystatin C appeared in the enamel organ in zones at which ameloblasts secrete enamel matrix proteins. They were also present in transition and enamel maturation zones. In addition, ameloblasts, osteocytes, and osteoclasts were labeled by anti-cystatin C. ED1 and ED2 immunocytochemistry revealed that there was no macrophage population in the enamel organ of secretion, transition, or enamel maturation zone. A double labeling study showed that most anti-cystatin C-positive cells in the enamel maturation zone were also positive for OX6, whereas anti-cystatin C-positive and OX6-negative cells were prevalent in the secretion zone. The results suggest that immature dendritic cells penetrate the enamel organ of the secretion zone and begin to mature in the zones of transition and enamel maturation. (J Histochem Cytochem 48:1243-1255, 2000)     

Effects of unilateral lesion of the inferior olive on L-[3H] aspartate receptors binding in synaptic membranes of cat cerebellar cortex

In adult cats, local injection of kainic acid (KA) in the inferior olive (IO) of one side, from which the crossed olivocerebellar projection originates, produced asymmetric postural and motor deficits, attributed to selective damage of the olivary neurons. Since aspartate is one of the putative transmitters of the olivocerebellar fibers, experiments were performed to find out whether 6-8 days after injection of KA within the IO of one side produced changes in aspartate receptors binding in different zones of the cerebellar cortex. In particular, binding in the contralateral zones of the cerebellar cortex was referred to proteins contained in membrane suspensions and compared with the control values obtained in the same experiments from the ipsilateral zones. Binding of L-[3H] aspartate decreased on the average to 53.4% of the control value in the medial zone and to 86.1% of the control value in the intermediate and lateral zones of the cerebellar cortex. This reduction varied in different experiments according to the side of the injection, in agreement with the well known pattern of regional distribution of the olivocerebellar projection within the cerebellar cortex. These findings favour aspartate as the putative neurotransmitter of the climbing fibers. The demonstration that binding of aspartate decreased in the cerebellar cortex of one side, 6-8 days after injection of KA in the corresponding IO, indicates that plastic events occur at this level following destruction of the olivocerebellar pathway. In particular, the reduced binding can be attributed either to a decrease in number of the postsynaptic receptor sites for aspartate or to a decreased affinity of this amino acid for the corresponding receptors. These findings, however, do not exclude that an hypersensitivity by denervation may occur at the level of individual Purkinje cells when they are deprived of the climbing fibers input. In order to answer this question further experiments are required to find out how the binding for aspartate is modified at increasing time intervals after the olivary lesion.     

Muscle tissue regeneration of the lymph hearts in adult Rana temporaria frogs. A study by light and electron microscopy autoradiography methods

The regeneration response of adult frog lymph heart muscle tissue was studied from 2 to 3 weeks after mechanical injury. High resolution autoradiographic studies showed that regenerative necrotic zones have many actively proliferating mononuclear cells deprived of cytoplasmic myofilaments. Some of them have numerous free ribosomes, so they might be identified as myoblasts. On the 13th day after injury newly-formed myotubes with chains of myonuclei and pictures of active sarcomerogenesis were observed. On the other hand, the surviving muscle fibers of the perinecrotic zone were rich in myonuclei at their growing ends. In the vicinity of nuclei, accumulation of a mass of non-differentiated cytoplasm rich in free ribosomes and polysomes, rough endoplasmic reticulum, Golgi apparatus, and centrioles are seen. Tritiated thymidine pulse-labeling showed that only rare myonuclei of the perinecrotic zone muscle fibers were labeled, whereas numerous non-differentiated cells of granulation tissue and myosatellites incorporated thymidine. The number of labeled myonuclei markedly increased 96 hours after 3HTdr administration. These data evidence that the myoblastic mechanism is predominant in the regeneration of adult frog lymph heart muscle tissue. It is necessary to emphasize that during the lymph heart muscle tissue reparative myogenesis some of the perinecrotic myonuclei are able to synthesize DNA and to divide mitotically, which distinguishes this type of muscle from skeletal muscle tissue of vertebrates.     

Mesenchymal stem cells transplantation influences upon dynamics of morphological changes in rat brain after stroke

Study of dynamic morphological changes if the brain after ischemic stroke is an important phase of pre-clinical trial of mesenchymal stem cell (MSC) therapy for this widespread disease. Experoments were carried out in inbred Wistar-Kyoto rats. MSCs were isolated, expanded in culture and labeled with vital fluorescent dye PKH26. Animals were subjected to middle cerebral artery occlusion (MCAO) followed by injection of 5 x 10(6) rat MSCs into the tail vein on the day of MCAO. Control group of animals received PBS injection (negative control). Animals were sacrificed in 1, 2, 3 and 5 days and in 1, 2, 4 and 6 weeks after operation. MSCs were revealed in the brain on the third day transplantation. They distributed around brain vessels in both the ipsilateral and contralateral hemispheres. This pattern of distribution remained unchanged during 6 weeks of observation. It was demonstrated that inflammation process and scar formation in the experimental group progressed 25-30 % faster than in the control group. MSC transplantation stimulated endogenous stem cell proliferation on the subependimal zone of lateral ventricles (subventrecular zone). What is more, MSC injection showed neuroprotective effect: almost all penumbra neurons in animals treated with cell therapy retained their normal structure, whereas in animals of control group penumbra neurons died or had signs of serious damage.     

Anterograde tracing of retinal axons in the avian embryo with low molecular weight derivatives of biotin

Several reactive biotin esters were injected into the eyes of chick and quail embryos at various stages of development. Four of the biotin esters reacted with molecules of the eye tissue and were detected with light and electron microscopy in fluorescein isothiocyanate and peroxidase-avidin incubated sections and whole mounts. Intra and extracellular components of the lens, the vitreous body, and the retina were labeled to different degrees. Three of the biotin esters (biotin-N-hydroxysuccinimidester, biotin-epsilon-aminocaproic acid-N-hydroxysuccinimidester, and desthiobiotin-N-hydroxysuccinimidester) prominently marked the optic fiber layer in the retina and the biotin labels were transported along the optic pathway. The tracers were detected up to the growth cone of axons 24 to 36 hr after injection. Explants from biotin marked retinas were cultured on collagen or basal laminae. During culturing axons grew out from these explants into the substratum showing that labeled tissue and nerve fibers were viable. The development of the optic pathway at the chiasma of quail embryos was studied using the biotin/avidin tracing. The bulk of fibers emerging from the retina crossed as shown by double labeling of both optic nerves in a complex pattern of segregated and interdigitizing axon bundles at the chiasma toward the contralateral side of the brain. From stage 25 onward a minor ipsilateral projection was found. At the same developmental stage a few fibers traveled into the contralateral optic nerve and grew retrogradely toward the contralateral eye. The percentage of specimens having this retino-retinal projection increased during development from 53% (stage 24 to 27; E3.5-E5.5) to 89% (stage 29 to 35; E6-E8) and declined to 40% at late embryogenesis (stage 37 to 41; E9-E12). The fact that all retinal axons were found within predictable pathways with some of them running in the wrong direction suggests that nerve fiber pathways provide accurate positional information, but at best weak directional information for growing nerve fibers.     

The activity of deoxyribonucleic acid polymerase and deoxyribonucleic acid synthesis in nuclei from brain fractionated by zonal centrifugation

1. The DNA polymerase (EC 2.7.7.7) activity in purified intact brain nuclei from infant rats was investigated. The effects of pH, Mg(2+), glycerol, sonication and storage of the nuclei under different conditions were examined and a suitable assay system was established. 2. The nuclei from infant brain cells were fractionated by zonal centrifugation in a discontinuous sucrose gradient into five zones: zone (I) contained neuronal nuclei (59%) and astrocytic nuclei (41%); zone (II) contained astrocytic nuclei (81%) and neuronal nuclei (19%); zone (III) contained astrocytic nuclei (82%) and oligodendrocytic nuclei (18%); zone (IV) contained oligodendrocytic nuclei (92%) and zone (V) contained oligodendrocytic nuclei (100%). 3. The content of DNA, RNA and protein for each fraction was measured. 4. The distribution of DNA polymerase activity in the fractionated infant and adult rat brain nuclei was determined. The highest activity was found in the neuronal nuclei from zone (I) and the following zones exhibited a progressive decline. In contrast with the nuclei from infant rats those from adults had a much higher activity and expressed a preference for native DNA as template. 5. The deoxyribonuclease activity in all classes of nuclei was measured with [(3)H]DNA as substrate. A general correspondence in the pattern of the relative activities in the nuclear fractions with the distribution of DNA polymerase was found. 6. The incorporation of [(3)H]thymidine into nuclear DNA in infant and adult rat brain was investigated. The specific radioactivity of the DNA in the 10-day-old rats was highest in zone (V) whereas in the nuclei of adult rats, which exhibited a comparatively low incorporation, the highest specific radioactivity was associated with zones (I) and (V).