SPECIALIZED MEMBRANE JUNCTIONS BETWEEN NEURONS IN THE VERTEBRATE CEREBELLAR CORTEX

"Gap" junctions, the morphological correlate for low-resistance junctions, are demonstrated between some mossy fiber terminals and granule cell dendrites in some lower vertebrate cerebella (gymnotid and frog). Most of the gap junctions (GJs) seen in the gymnotid-fish cerebellum exhibit an asymmetrical configuration, the electron-opaque cytoplasmic material underlying the junction being more extensive in the dendritic than in the axonal side. In the frog cerebellum, the GJs have a symmetrical distribution of such electron-opaque material. In both species the GJs are encountered at the same synaptic interface as the conventional synaptic zone (CSZ), constituting "mixed synapses" in a morphological sense. The axonal surface covered by CSZs is larger than that covered by GJs. In mammalian cerebellum, GJs are observed only in the molecular layer, between perikarya, dendrites, or perikarya and dendrites of the inhibitory interneurons. These GJs are intermixed with attachment plates and intermediary junctions interpreted as simply adhesive. In the mammalian cerebellum, a new type of junction which resembles the septate junctions (SJs) of invertebrate epithelia is observed between axonal branches forming the tip of the brush of basket fibers around the initial segment of the Purkinje cell axon. It is suggested that such junctions may be modified forms of septate junctions. The physiological implications of the possible existence of high-resistance cross-bridges between basket cell terminals, which may compartmentalize the extracellular space and thus regulate extracellular current flow, must be considered.     

金鱼精巢支持细胞间连接和血睾屏障

本文主要采用冷冻断裂-蚀刻方法,同时结合超薄切片和镧标记实验,研究金鱼精巢内的支持细胞间连接特点及血-睾屏障的形成。结果发现:1)金鱼精巢内支持细胞间连接呈紧密连接、桥粒和间隙连接同时存在的形式。2)各种连接的数量、面积、分布密度会随着小囊内生精细胞的发育而改变。3)紧密连接在精子发生过程的各个阶段都存在,但在形态上表现为Ⅰ型和Ⅱ型两种。4)血-睾屏障是在粗线期精母细胞期后形成,它是Ⅰ型紧密连接发育为Ⅱ型紧密连接的结果。     

ON THE MECHANISM OF TAURINE TRANSPORT AT BRAIN CELL MEMBRANES

One non-saturable and two saturable transport systems were demonstrated for taurine in rat brain slices. One of the saturable systems, designated β, is characterized by a high asnity for taurine and a low transport capacity, while the other, designated ω, by a low affinity and a high transport capacity. 2,4-Dinitrophenol inhibited the saturable transport of taurine non-competitively, while hypotaurine. β-alanine, γ-aminobutyric acid, N-methyl-taurine and L-cysteic acid inhibited transport competitively. It is thus inferred that the hypothetical carrier sites of taurine at cell membrane recognize to an equal degree strongly ionized electropositive and electronegative ends of an acceptable molecule separated by two or three carbon atoms. Two is the minimal and also the optimal carbon chain length in an acceptable molecule.     

CELL JUNCTIONS IN AMPHIBIAN SKIN

Cell junctions have been investigated in the amphibian epidermis, a stratified squamous epithelium, and compared to those described previously in simple columnar epithelia of mammalian cavitary organs. In adult frogs and toads, and in larvae approaching metamorphosis, belts of membrane fusion or zonulae occludentes of considerable depth are regularly found between adjoining cells of the outermost layer of the stratum corneum, binding the cells together into a continuous, uninterrupted sheet. Another set of occluding zonules appears in the second cornified layer (when such a layer is present), and a third set usually occurs in the outermost layer of the stratum granulosum. Specialized elements described as "modified" and "composite" desmosomes are encountered along the lateral and basal aspects, respectively, of the cornified cells; ordinary desmosomes and maculae occludentes (i.e., spots of membrane fusion) are found in all other strata. The usual 200 A intercellular gap is generally maintained between the cells of the stratum germinativum at the basal ends of the intercellular spaces. Hence, the intercellular spaces of the epidermis form a largely continuous network, closed to the external medium and open to the dermal interstitia. The situation is comparable to that found in columnar epithelia, except that the intercellular spaces are much more extensive, and an extracellular subcompartment (or two) apparently exists in the stratum corneum and between the latter and the stratum granulosum. The last subcompartment is usually filled with a dense substance, probably derived from discharged secretory granules. The tripartite junctional complex characteristic of lumen-lining epithelia (i.e., a zonula occludens followed by a zonula adhaerens, and desmosome) is seen only in early larvae; in adults and in larvae approaching metamorphosis, the occluding zonule is followed directly by a series of modified desmosomes. Interpreted in the light of current physiological data, these findings suggest that the diffusion of water, ions, and small, water-soluble molecules is impeded along the intercellular spaces of the epidermis by zonulae occludentes while it is facilitated from cell to cell within the epidermis by zonulae and maculae occludentes.     

CELL JUNCTIONS IN OMMATIDIA OF LIMULUS

The intercellular relationships in the ommatidia of the lateral eye of Limulus have been investigated. The distal process of the eccentric cell gives origin to microvilli which interdigitate with the microvilli of the retinular cells. Therefore, both types of visual cells contribute to form the rhabdom and may have an analogous photoreceptor function. Quintuple-layered junctions are found within the rhabdom at the lines of demarcation between adjoining microvilli, whether the microvilli originate from a single retinular cell, from two adjacent retinular cells, or from a retinular cell and the eccentric cell. Furthermore, quintuple-layered junctions between the eccentric cell and the tips of the microvilli of the retinular cells occur at the boundary between the distal process and the rhabdom. These findings are interpreted to indicate that the rhabdom provides an extensive electrotonic junction relating retinular cells to one another and to the eccentric cell. Quintuple-layered junctions between glial and visual cells, as well as other structural features of the ommatidial cells, are also described.     

RELATIONSHIPS BETWEEN CELL CYCLE DURATION, S-PERIOD AND NUCLEAR DNA CONTENT IN ERYTHROBLASTS OF FOUR VERTEBRATE SPECIES

Erythroblasts of four vertebrate species (Triturus cristatus, Rana esculenta, Lacerta viridis and Gallus domesticus) differing markedly in their nuclear diploid DNA content, are used to study a possible relationship between cell cycle duration and DNA content. DNA is determined cytophotometrically and fluorometrically. The cell cycle is analysed by evaluating labelled mitoses after an injection of tritiated thymidine and also by double labelling with ¹⁴C- and ³H-thymidine. A direct but non-linear relationship is demonstrated between DNA content of erythroblast nuclei and the duration of DNA-synthesis.     

EFFECT OF UNDERNUTRITION ON CELL FORMATION IN THE RAT BRAIN

Abstract— Rats were undernourished by approximately halving the normal food given from the 6th day of gestation throughout lactation. Growth of the foetuses was nearly normal, in marked contrast to the severe retardation caused by undernutrition during the suckling period. In comparison with controls the size and the DNA content of the brain were permanently reduced by undernutrition during the suckling period: this effect was relatively small, approx. 15 per cent decrease at 21 and 35 days. The rate of ¹⁴C incorporation into brain DNA at 30 min after administration of [2-¹⁴C] thymidine was taken as an index of mitotic activity; compared with controls there was severe reduction in mitotic activity (maximal decrease by about 80 per cent at 6 days in the cerebrum and by 70 per cent at 10 days in the cerebellum). The rate of acquisition of cells was calculated from the slopes of the logistic curves fitted to the estimated DNA contents. In normal animals the maximal slope was attained at 2·7 days and at 12·8 days after birth in cerebrum and cerebellum respectively; the daily acquisition of cells at these times was 4·8 × 10⁶ and 18 × 10⁶ cells respectively. The fractional increase in cell number at the maximum was 5·4 percent per day in the cerebrum and 15·2 per cent per day in the cerebellum. The rate of acquisition of cells relative to the rate of mitotic activity was higher in the brains of undernourished animals than in controls. One of the compensatory mechanisms for the severe depression of mitotic activity in the brain of undernourished animals Seems to involve a reduction in the normal rate of cell loss.     

A FINE STRUCTURAL STUDY OF ADHESIVE CELL JUNCTIONS IN HETEROTYPIC CELL AGGREGATES

Mixed suspensions of cells obtained by dissociation of 7 day chicken embryo heart and pigmented retina were allowed to reaggregate in tissue culture. The reaggregates which resulted contained both kinds of cells. Establishment of homogeneous tissues by cell sorting out in these reaggregates was advanced by 20 hr in culture and was complete within 2 days. When sorting out was advanced, heterotypic aggregates were fixed, sectioned, and examined in the electron microscope. Particular attention was paid to the morphology of regions of contact between cells. No qualitative differences were observed in the contact junctions between like cells (heart-heart or pigmented retina-pigment retina junctions) and unlike cells (heart-pigmented retina junctions). Broad areas of undifferentiated cell contact with cell membranes separated by a 100–200 A gap were formed regardless of cell type. Specialized junctions of the fascia and macula adherens type were also present, not only between like cells but also between unlike cells.     

THE STRUCTURE OF BRAIN DIHEXOSYLCERAMIDE IN GLOBOID CELL LEUKODYSTROPHY1

Abstract— Dihexosylceramide from the brain of a case of globoid cell leukodystrophy was isolated and characterized. Studies utilizing partial acid hydrolysis and methylation indicated the carbohydrate sequence to be galactosylglucosylceramide. The carbohydrate moiety was liberated by ozonolysis and identified to be lactose by GLC. 4-Sphingenine and sphinganine were the only long chain bases detected. The chain length of the fatty acids present ranged from 14 to 25 carbons; C₁₈, and C₂₁ fatty acids composed one-half of the mixture. All analyses were performed by GLC and TLC utilizing micro-techniques.     

有尾两栖类细胞连接和神经管形成

在神经管形成的过程中,从原肠晚期到神经管即将关闭,我们观察了预定神经上皮和神经上皮细胞的细胞连接。我们的结果和过去的报道不同,我们看到紧密连接和间隙连接在所观察的各个时期一直存在,而且它们是处于持续的形态变化之中,就此论证了紧密连接在神经管形态建成中的作用,并讨论了紧密连接和间隙连接的相互关系。     

RENEWAL OF FATTY ACIDS IN THE MEMBRANES OF VISUAL CELL OUTER SEGMENTS

The renewal of fatty acids in the visual cells and pigment epithelium of the frog retina was studied by autoradiographic analysis of animals injected with tritiated palmitic, stearic, or arachidonic acids. Most of the radioactive material could be extracted from the retina with chloroform-methanol, indicating that the fatty acids had been esterified in lipids. Analysis of the extracts, after injection of [³H]palmitic acid, revealed that the radioactivity was predominantly in phospholipid. Palmitic acid was initially concentrated in the pigment epithelium, particularly in oil droplets which are storage sites for vitamin A esterified with fatty acid. The cytoplasm, but not the nucleus of these cells, was also heavily labeled. Radioactive fatty acid was bound immediately to the visual cell outer segment membranes, including detached rod membranes which had been phagocytized by the pigment epithelium. This is believed to be due to fatty acid exchange in phospholipid molecules already situated in the membranes. Gradually, the concentration of radioactive material in the visual cell outer segment membranes increased, apparently as a result of the addition of new phospholipid molecules, possibly augmented by the transfer from the pigment epithelium of esterified vitamin A. Injected fatty acid became particularly concentrated in new membranes which are continually assembled at the base of rod outer segments. This localized concentration was short-lived, apparently due to the rapid renewal of fatty acid. The results support the conclusion that rods renew the lipids of their outer segments by membrane replacement, whereas both rods and cones renew the membrane lipids by molecular replacement, including fatty acid exchange and replacement of phospholipid molecules in existing membranes.     

LACK OF PHOSPHOLIPID TRANSPORT MECHANISMS IN CELL MEMBRANES OF THE CNS1

The possible transport role of phospholipid-protein complexes, present in the cell supernatant of rat brain was investigated using labelled choline as precursor of phosphatidyl choline. Results obtained after the intracranial injection of choline gave no indication of a sequence of events compatible with a transport of phospholipid molecules from the possible site of synthesis (microsomes) to the supernatant and subsequently to myelin. Chase experiments using rat brain slices incubated in vitro with radioactive choline agreed well with the above mentioned results. Contrariwise, when Na³⁵₂SO₄ was used as precursor, the results clearly indicated that synthesis of sulphatides takes place in microsomes, followed by transfer of the radioactive lipid to sulphatide-containing lipoproteins in the supernatant and finally to myelin. Results presented in this paper seem to give further support to the idea that other subcellular fractions, besides microsomes, can autonomously synthesize part of their own provision of phospholipids. Possible reasons which might explain the marked differences between the mechanisms of addition of phospholipids and sulphatides to myelin are discussed in relation to results obtained by other investigators.     

层粘连蛋白与膜受体结合下膜受体构象与膜脂有序性的研究

配体蛋白与细胞膜受体蛋白结合后,可引起膜受体的构象与膜脂的有序性变化.本文研究外源性层粘连蛋白与腹水肝癌细胞膜受体结合后膜热量变化,膜序参数改变和膜电荷及细胞迁移率的变更.就膜蛋白构象与膜脂有序性以及膜电荷等方面改变的生理意义与层粘连蛋白抗癌细胞脱落转移寻找理论关系.本文应用微量量热法、顺磁共振和细胞电泳等技术,得知层粘连蛋白与癌细胞膜作用后细胞膜有放热效应,膜流动性增大,细胞电泳动变慢.癌细胞膜的这些变化对于限制癌的恶性生长与脱落均起重要作用.     

THE RELATIONSHIP BETWEEN STEM CELL SEEDING EFFICIENCY AND POSITION IN CELL CYCLE

The seeding efficiency of colony-forming cells from normal, regenerating and velocity-sedimented cycling and non-cycling narrow preparations was compared. Colony-forming cells in cycle were found to exhibit a 50% reduction in splenic seeding when compared to normal marrow or sedimented non-cycling cells. The results of this study indicate that the spleen colony assay underestimates the total number of colony-forming cells by a fraction which is directly related to the number of cells in cycle.