Obermiiller-Wildn, H . 1979. A neurosecretory system in the brain of the lancelet, Branchiostoma lanceolatum. (Department of Zoology, University of Stockholm, Sweden.) -Acta zool. (Stockh.) 60( 3) : 187-196.Ultrastructural and histochemical studies indicate a neurosecretory system exists in the lancelet brain with basal properties resembling a primitive hypothalamic system. A nucleus of secreting neurons, containing peptide granules (115 nm), is prominent in the dorsal walls of the brain. The axons establish contacts with the ventral brain surface,, probably releasing their secretory product out of the brain. The neurons are innervated by dopaminergic "boutons en passant" often very active with a high number of electron translucent vesicles as well as dense-core vesicles (90 nm) . Ventrally located cellbodies containing what are probably secretory peptide granules (110 nm) establish contacts with their basal processes on the ventral brain surface.
The caudal end of the neural tube of the tunicate Oikopleura, the cephalochordate Branchiostoma and newly hatched fry of the clupeiform teleosts Clupea, Engraulis and Sardinops was studied by means of the electron microscope. In Oikopleura and the teleost larvae either Reissner's fiber or an amorphous mass of fiber substance leaks out of the neural tube into the surrounding tissue spaces. In Branchiostoma the disintegrated fiber material is apparently engulfed by the caudal ependymal cells. A relationship seems to exist between the degree of fiber disintegration within the neural tube and the degree of specialization of the caudal neural tube ependymal cells, the two extremes being represented by Branchiostoma with a “closed” Reissner's fiber system with highly specialized caudal ependymal cells and a teleost fry with the intact fiber leaving the neural tube between almost undifferentiated ependymal cells. These observations on lower chordates are in accordance with the hypothesis that Reissner's fiber acts as a detoxicator for the neural tube fluid.
Analyses of the fine structure of the posterior choroid plexus in the teleost Leuciscus rutilus and the determination of the presence and function of the enzymes acid and alkaline phosphatases, ATPase and glucose‐6‐phosphatase confirm similarities between these epithelial cells and the saccus dorsalis and also with the epithelial cells of the choroid plexus found in mammals. The teleost plexus cells contain coated vesicles which are derived from the plasmalemma as well as from the Golgi complex. Moreover, they contain multivesicular bodies and Iysosomes. These organelles function in the absorption of substances from the cerebrospinal fluid and in the breakdown of these substances within the cells. The investigated enzymes play an important role in the secretion of electrolytes into the cerebrospinal fluid by active membrane transport.
The infundibular organ of adult and larval Branchiostoma was studied by means of the electron microscope. The secretion produced by the infundibular cells is released into the ventricular fluid from secretory vacuoles, fusing with the apical plasmalemma and forming a Reissner's fiber in the ventricle. The basal cell processes reach the external limiting membrane but no basal release of secretory material has been observed. No synapses are in contact with the infundibular cells. The organ seems to function autonomously with neither nervous control from the brain tissue nor chemical regulation from the ventricular fluid.
Saccus dorsalis in Leuciscus rutilus is a very folded area enclosing the epiphysis within its foldings. The dorsal sac consists of a single layer of cuboidal ependymal cells which have cilia and very irregularly shaped microvilli. The presence of vesicles in longitudinal rows indicate that these microvilli are very active. Bulbous outpocketenings from the apical surface contain glycogen. These bulgings may be pinched off into the lumen of the third ventricle forming rounded drops which contain richly glycogen. To increase the surface there are protrusions covered with microvilli. In the cytoplasm between the basis of those microvilli there are several coated vesicles which possibly sign micropinocytotic activity. Coated vesicles are also to be found in the lateral and basal parts of the cell, where they often have taken contact with the cell membrane. The ependymal cells rest upon a broad glycocalyx. In the subependymal connective tissue are wide capillaries with thinwalled fenestrated endothelial cells. The functional significance of these structures is discussed. A PAS‐reaction has been done. The ependymal cells and rounded drops in the ventricular lumen showed a positive reaction. Zusammenfassung Der Saccus dorsalis ist bei den Leuciscus rutilus ein stark gefaltetes Gebiet. Er umschliesst mit seinen Falten die Epiphyse, so dass diese im Dorsalsack versenkt liegt. Die Zellen bestehen aus einem einschichtigen kubischen Ependym. Diese sind mit Zilien und unregelmässigen Mikrovilli versehen, welche dadurch, dass sie Bläschen in langen Reihen enthalten, grosse Aktivität zeigen. Ausbuchtungen der apikalen Zellenoberfläche sind mit Glykogenkörnchen gefüllt. Diese Ausbuchtungen werden wahrscheinlich abgeschnürt und bilden dann runde, mit Glykogen gefüllte Tropfen im Ventrikellumen. Zellauswüchse, mit Mikrovilli bekleidet, vergrössern die Oberfläche. Akanthosome (coated vesicles), welche möglicherweise mikropinocytotisch aktiv sind, kommen reichlich zwischen den Basen der Mikrovillis vor, aber auch in den lateralen und basalen Teilen der Zelle, wo sie sich in Kontakt mit der Zellwand befinden. Die Zellen stehen auf einem breiten Glykokalyx. Im subependymalen Bindegewebe laufen Kapillare, deren Endothelzellen gewöhnlich mit Poren versehen sind. Die Funktionelle Bedeutung wird diskutiert. Eine PJS‐Reaktion wurde ausgeführt, bei welcher die Ependymzellen sowie runde Tropfen im Ventrikellumen positiv reagierten.
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