A critical role for the EphA3 receptor tyrosine kinase in heart development

Eph proteins are receptor tyrosine kinases that control changes in cell shape and migration during development. We now describe a critical role for EphA3 receptor signaling in heart development as revealed by the phenotype of EphA3 null mice. During heart development mesenchymal outgrowths, the atrioventricular endocardial cushions, form in the atrioventricular canal. This morphogenetic event requires endocardial cushion cells to undergo an epithelial to mesenchymal transformation (EMT), and results in the formation of the atrioventricular valves and membranous portions of the atrial and ventricular septa. We show that EphA3 knockouts have significant defects in the development of their atrial septa and atrioventricular endocardial cushions, and that these cardiac abnormalities lead to the death of approximately 75% of homozygous EphA3(-/-) mutants. We demonstrate that EphA3 and its ligand, ephrin-A1, are expressed in adjacent cells in the developing endocardial cushions. We further demonstrate that EphA3(-/-) atrioventricular endocardial cushions are hypoplastic compared to wildtype and that EphA3(-/-) endocardial cushion explants give rise to fewer migrating mesenchymal cells than wildtype explants. Thus our results indicate that EphA3 plays a crucial role in the development and morphogenesis of the cells that give rise to the atrioventricular valves and septa.     

SEPTUM FORMATION IN THE DESMID XANTHIDIUM (CHLOROPHYTA): EFFECTS OF CYTOCHALASIN D AND LATRUNCULIN B SUGGEST THE INVOLVEMENT OF ACTIN MICROFILAMENTS

Septum formation and mitosis were investigated by light and electron microscopical techniques as well as through the completion of inhibitor experiments in the unicellular desmid Xanthidium armatum (Bréb.) Rabenh. In untreated cells, numerous endoplasmic reticulum (ER) cisternae permeated the mitotic apparatus and secretory vesicles, and ER formed a band in front of the linearly growing septum, indicating the predetermined direction of septum growth. Under the influence of cytochalasin D (CD), the vesicle/ER band lost its proper orientation, which led to a malformed septum wall; moreover, abnormal septum branches could potentially have developed. Whereas the septum of an untreated cell only grew at its edge, the CD-induced branches (also with a vesicle/ER band in front) represented additional growing zones. These observations indicated that actin filaments were involved in establishing, maintaining, and orienting the "preforming" vesicle/ER band and, thus, the later septum. Latrunculin B (LB) had more severe effects on septum formation than did CD. Only small accumulations of septum material were found at the septum edge, and no aberrant growth of the septum occurred in LB-treated as in CD-treated cells. This could be explained by the more rapid disturbance of all actin-driven processes after LB treatment, which was assumed, because even low concentrations of the drug rapidly inhibited cytoplasmic streaming.     

Subcellular Localization and Characterization of Vasopressin Binding Sites in the Ventral Septal Area, Lateral Septum, and Hippocampus of the Rat Brain

[Arg8]-Vasopressin (AVP) has been shown to exert characteristic central physiological actions in the ventral septal area of the rat brain. This study reports the characterization of receptors for AVP in synaptic plasma membranes prepared from the ventral septal area, the lateral septum, and the hippocampus. Binding of [3H]AVP was temperature and time dependent, linearly related to protein concentration, saturable, and specific. Scatchard plot analysis suggested the presence of a population of binding sites in the three brain areas with dissociation constants and maximal binding capacities, respectively, of 1.06 +/- 0.39 nM and 24.0 +/- 7.01 fmol/mg of protein (mean +/- SEM; n = 3 for the ventral septal area, 0.92 +/- 0.13 nM and 47.0 +/- 4.96 fmol/mg of protein (n = 3) for the lateral septum, and 0.91 +/- 0.14 nM and 25 +/- 5.02 fmol/mg of protein (n = 3) for the hippocampus. In all three brain regions, the rank order of potencies of several vasopressin analogs, unrelated peptides, and other compounds for competitive displacement of ligand indicated a receptor with properties resembling those of the V1-like receptor for AVP. These data document the presence of a high-affinity, V1-like vasopressin receptor in the rat ventral septal area for which the pharmacological properties are similar to those previously reported in physiological studies.     

[3H]Ketanserin (Serotonin Type 2) Binding Increases in Rat Cortex Following Basal Forebrain Lesions with Ibotenic Acid

The response of the serotonergic system following injury to the basal forebrain cholinergic system was investigated in rats. The density of 5-hydroxytryptamine (serotonin) type 2 (S2) receptor sites in the frontal cortex and hippocampus was determined 1 week and 4 months after production of lesions by injections of ibotenic acid into the medial septum and nucleus basalis magnocellularis. One week later, the number of S2 receptor sites in the frontal neocortex, as defined by [3H]ketanserin binding, was unchanged. Four months later, the number of [3H]ketanserin binding sites (and Bmax) was increased and high-affinity [3H]serotonin uptake was decreased in the frontal neocortex, but not in the hippocampus, relative to unlesioned controls. Choline acetyltransferase (acetyl-CoA:choline O-acetyltransferase; EC 2.3.1.6) activity was decreased significantly in the frontal neocortex and hippocampus 1 week and 4 months after surgery. The change in frontal neocortical S2 receptor site density was inversely related to the level of choline acetyltransferase activity, was specific for cholinergic denervation associated with the cortex but not the hippocampus, and may represent a localized denervation supersensitivity due to degeneration of median raphe cortical afferents.     

Sporogenesis in Streptomyces melanochromogenes

The mode of spore differentiation in a strain of Streptomyces melanochromogenes was followed by analysis of ultrathin sections of sporulating aerial hyphae at various stages of sporogenesis. A special accent was laid on the formation of the sporulation septum and its alterations in the course of spore delimitation and separation. Distinct differences in formation and substructure have been observed between the cross walls of vegetative hyphae and the sporulation septa.Cross walls of vegetative hyphae are formed in a way typical for Gram-positive bacteria by a centripetal annular ingrowth of cytoplasmic membrane, on which wall material immediately is deposited. The development of the sporulation septa is characterized by the accumulation of amorphous material in addition to the newly synthesized wall layer inside the invaginating cytoplasmic membrane. This amorphous septal material will later be decomposed presumably by two lytic systems which cause the separation of the spores. The central region of the finished sporulation septum is perforated by microplasmodesmata. Spores are released by a break down of the surface sheath. The complete spores are enveloped by a twolayered cell wall and the spiny surface sheath.     

Experimentelle und biometrische Untersuchungen über die systematische Relevanz von Samen- und Fruchtmerkmalen anArabis glabra var.glabra und var.pseudoturritis (Brassicaceae)

Arabis glabra (L.)Bernh. var.pseudoturritis (Boiss. & Heldr.)Fiori (till now treated as a separate Mediterranean species:Arabis pseudoturritis Boiss. & Heldr.) differs fromA. glabra var.glabra only in the larger ± pleurorhizal seeds with a ± conspicuous wing that are arranged in one row in each loculus of the (sometimes longer) pods, the pedicels of the siliquae being shorter than in var.glabra. These closely correlated characters are of little systematic relevance according to biometrical and statistical analyses of both taxa and their experimental hybrids. Furthermore, there is correspondence in pod septum anatomy and chromosome number (2n = 12). This, the lack of intrinsic crossing barriers and sympatric occurence (outside the Mediterranean area!) in North America confirm the classification of these taxa as varieties withinA. glabra. Obviously, the seed and fruit characters discussed, are of even less value as generic characters for a separate genus Turritis (comprising Turritisglabra L.).

Herrn Univ.-Prof. Dr.Hermann Merxmüller anläßlich der Vollendung seines 60. Lebensjahres gewidmet.     

Light- and electron-microscopic study of electrophysiologically characterized neurons in the mediolateral part of the lateral septum of the guinea-pig

In slices of guinea-pig brains, 36 neurons located in the mediolateral part of the lateral septum were stained intracellularly with horseradish peroxidase (n=28) or biocytin (n=8) after electrophysiological characterization. These neurons belonged to class A neurons (n=23), which generated pronounced Ca⁺⁺-dependent high-threshold spikes in control medium, or to class C neurons (n=9), which were recognized by the occurrence of small-amplitude sodic spikes followed by slower larger calcic spikes. The present results demonstrate that, despite the variety of individual cell types, the major morphological population (30/36 cells) was composed of a homogeneous class of large-sized neurons that displayed thick primary dendrites and abundant dendritic appendages. The remaining 6 cells were small-sized, poorly-spiny neurons. Somatic spines were observed on 5 out of the 30 large cells and on one out of the six smaller cells. Labeled axons were mainly oriented to the anterior commissure. The axons of nine cells richly collateralized near the perikaryon. Ultrastructural examination of 3 horseradish peroxidase-injected cells showed indented nuclei, classic organelles and somatic spines. Terminal boutons established symmetric synapses with the injected cells. These results describe the morphological features of electrophysiologically identified neurons and indicate that class A and class C neurons are distributed among morphological populations differing in perikaryal size. This suggests that the different electrical properties of class A and class C neurons reflect recordings from different parts of the neuron rather than from neurons of different types. Furthermore, the present findings demonstrate that, in the guinea-pig, electrical and morphological characteristics of somatospiny neurons are comparable with those of non-somatospiny neurons. Somatospiny neurons have a recognized integrative role in the hippocampo-septo-hypothalamic complex.     

Vasoactive intestinal peptide-immunoreactive cerebrospinal fluid-contacting neurons in the reptilian lateral septum nucleus accumbens

By means of immunocytochemical demonstration of vasoactive intestinal peptide (VIP) an accumulation of cerebrospinal fluid (CSF)-contacting neurons was found in a circumscribed region of the nucleus accumbens/lateral septum of eleven reptilian (chelonian, lacertilian, ophidian, crocodilian) species. Basal processes of these cells contribute to a subependymal plexus whose density displays considerable interspecific variation. VIP-immunoreactive nerve fibers occur also in the lateral septum and the nucleus accumbens where they encompass immunonegative cells in a basket-like pattern. The CSF-contacting neurons are surrounded by columnar ependymocytes frequently arranged in a pseudostratified manner. These specialized arrays of ependymal cells, however, occupy a more extended area than the VIP-immunoreactive CSF-contacting neurons and can be traced from the rostro-ventral pole of the lateral ventricle to the interventricular foramen. These observations suggest the existence of a telencephalic site of CSF-contacting neurons which may be more widespread than hitherto thought and which may participate in a circumventricular system of the lateral ventricle. Previous studies mainly performed with birds indicate that the VIP-immunoreactive CSF-contacting neurons of the nucleus accumbens might form a part of the encephalic (extraretinal and extrapineal) photoreceptor. However, further experiments are required to test this supposition since the VIP-immunoreactive neurons of the nucleus accumbens remained unlabeled by antibodies against bovine rodopsin and chicken cone-opsin in all eleven species analysed in this investigation.     

Fine filaments observed within the cytoplasm surrounding the leading edge of the septum in telophase cells of Spirogyra (Zygnematales, Chlorophyta)

Electron microscope observation revealed the presence of many fine filaments within the cytoplasm surrounding the leading edge of the septum in telophase cells of Spirogyra verruculosa Jao. These filaments, about 7 nm each in diameter, ran parallel to one another along the leading edge of the septum and, sometimes, they appeared to be gathered into at least two bundles. These filament distribution patterns coincided well with those of the fluorescence of rhodamine-labeled phalloidin in the vicinity of the septum in telophase cells. The present results suggest that the fine filaments observed within the cytoplasm surrounding the leading edge of the septum may be actin filaments.     

外侧隔—海马CA1通路电生理分析

电刺激外侧隔区可在海马 CA1区锥体细胞层记录到群锋电位,在 CA1辐射层顶树突记录到兴奋性突触后电位(EPSP)。侧脑室注射微量海人酸损毁海马 CA3-CA4区锥体细胞后,电刺激外侧隔区在 CA1顶树突不再诱发 EPSP,由此认为外侧隔-CA1顶树突的神经联系是通过同侧海马 CA3锥体细胞侧支实现的。但 CA3-CA4损毁后,电刺激外侧隔区在海马 CA1起层锥体细胞底树突仍可记录到 EPSP。这一在正常情况下被掩盖的外侧隔-CA1底树突神经联系及其来源尚有待探讨。