The spermatozoon of Branchiostoma lanceolatum L

Under the electron microscope, the spermatozoon of Branchiostoma lanceolatum shows a spherical nucleus deeply grooved along its caudal third, a bistratified acrosome enriched by plentiful subacrosomal material, two centrioles, mitochondria fused into a single mass surrounding the centriolar region which is highly asymmetrical, a 9 + 2 flagellum tilted with respect to the longitudinal symmetry axis of the nucleus. The sperm of Branchiostoma shares the overall features of that of the Tunicata and fits in perfectly with the phylogenetic position of the Leptocardia.     

The Wheel Organ and Hatschek's Groove in the Lancelet,Branchiostoma lanceolatum (Cephalochordata)

The wheel organ is a specialized epithelium in the roof and sides of the adult lancelet oral cavity. It borders the oral epithelium proper, separated by a thin strip of margin cells which are not ciliated but contain a few large dense-cored vesicles apically. The wheel organ cells are tall and strongly ciliated and have dark, heterochromatin-rich nuclei. Dorsally, and slightly paramedially, the organ is further specialized, forming the so-called Hatschek's groove (pit), which consists of two ciliated cell types. The first type synthesizes a dense granular material, the granules being approximately 95 nm in diameter. This is stored basally and apparently it is also released through the basal cell membrane into the blood cavities. The cells at the bottom of Hatschek's groove have peculiar rod-shaped apical cellular regions. Each cell bears one tall cilium surrounded by microvilli and it is apparently involved in the production of secretory material into the groove. It is evident that the histology, and probably also the function, of the wheel organ and its groove is much more complex than hitherto believed.     

The Circulatory System of Amphioxus (Branchiostoma lanceolatum (Pallas))

The morphology of the circulatory system of Amphioxus ( Branchiostoma lanceolatum (Pallas)) has been investigated using a new intravascular injection technique. A survey of the vessels of Amphioxus using this technique is given. The dorsal arteries and their ramifications are described in detail. The new injection technique brought to light myoseptal plexi, supplied from the dorsal arteries, between every two myomeres. Also the ventral parietal arteries have a much more complicated course than hitherto accepted. They are connected with an atrial plexus which is a continuous net of small vessels in the whole length of the dorso-lateral wall of the atrial cavity. It is postulated that this plexus has a supplementary function in respiration. Plexi of minute vessels in the gonads and a real blood circulation with afferent and efferent gonadal vessels have been demonstrated. Two vessels connecting the liver plexus with the cardinal vein (or the atrial plexi) have been noticed, the v. communicans accessoria anterior mentioned in 1900 by Burchardt, and a so-called oblique vessel never described before. The vessels of the caudal region are analyzed completely and also here a real blood circulation appears possible.     

Excitation-contraction coupling in a pre-vertebrate twitch muscle: The myotomes of Branchiostoma lanceolatum

Summary The segmented trunk muscle (myotome muscle) of the lancelet (Branchiostoma lanceolatum), a pre-vertebrate chordate, was studied in order to gain information regarding the evolution of excitation-contraction (EC) coupling.Myotome membrane vesicles could be separated on isopycnic sucrose gradients into two main fractions, probably comprising solitary microsomes and diads of plasma membrane and sarcoplasmic reticulum, respectively. Both fractions bound the dihydropyridine PN 200/110 and the phenylalkylamine (–)D888 (devapamil) while specific ryanodine binding was observed in the diad preparation only. Pharmacological effects on Ca²⁺ currents measured under voltage-clamp conditions in single myotome fibers included a weak block by the dihydropyridine nifedipine and a shift of the voltage dependences of inactivation and restoration to more negative potentials by (–)D888. After blocking the Ca²⁺ current by cadmium in voltage-clamped single fibers, the contractile response persisted and a rapid intramembrane charge movement could be demonstrated. Both responses exhibited a voltage sensitivity very similar to the one of the voltage-activated Ca²⁺ channels.Our biochemical and electrophysiological results indicate that the EC coupling mechanism of the protochordate myotome cell is similar to that of the vertebrate skeletal muscle fiber: Intracellular Ca²⁺ release, presumably taking place via the ryanodine receptor complex, is under control of the cell membrane potential. The sarcolemmal Ca²⁺ channels might serve as voltage sensors for this process.We thank Drs. H.Ch. Lüttgau and L.M.G. Heilmeyer, Jr. for stimulating discussions during the work, Dr. N.R. Brandt for helpful suggestions, and Drs. A.H. Caswell and M. Michalak for their generous gifts of antibodies. We also thank Ms. P. Goldmann, Mr. R. Schwalm, and Mr. U. Siemen for technical support and Ms. E. Linnepe for editorial help. This work was supported by grant G1 72/1-5 of the Deutsche Forschungsgemeinschaft. R. Benterbusch was recipient of a scholarship by the Studienstiftung des Deutschen Volkes.     

The Oral Papilla of the Lancelet Larva (Branchiostoma lanceolatum) (Cephalochordata)

The oral papilla is a prominent larval organ in the lancelet, which develops early during ontogenesis and disappears completely at metamorphosis. Electron microscopic studies indicated that the prevailing theories of its function are unlikely. Its cells lack the cytological characteristics of sensory or mucus producing cells, and the organ is not equipped with a tuft of flagella. It was not possible to reveal the function of the papilla, but its cells seem to be involved in a pronounced substance transport through the basal cell membrane as well as in the cytoplasm. Apical specializations indicate exo- or endocytotic activity and vesicles imply some exchange also between papilla cells and the sea water.     

The connection between spinal cord and notochord in Amphioxus (Branchiostoma lanceolatum)

Summary Silver impregnation of nerves, histochemical reactions for acetylcholinesterase, and electron microscopy reveal an efferent innervation of the notochord in amphioxus. Extensions of the notochordal lamellae end in groups (the notochordal horns) just below the ventro-lateral surface of the spinal cord where they are opposed to large nerve terminals originating as short collaterals of axons running longitudinally in the nerve cord. This neurochordal junction resembles an ordinary neuro-muscular junction in several respects and is interpreted as a part of the muscular system found in the notochord itself. Acknowledgement. The author is indebted to the staffs at the Marine laboratory in Plymouth and the Biological station at Helgoland for supply of material. The expert advices and criticism of Q. Bone, Ph. D., Plymouth, and Dr. U. Welsch, Kiel, are also greatly appreciated.     

A Neurosecretory System in the Brain of the Lancelet, Branchiostoma lanceolatum

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 ultrastructure of the blood vessels of Branchiostoma lanceolatum (Pallas) (Cephalochordata)

Summary The ultrastructure of the blood vessels in the caudal region of Branchiostoma is described in specimens injected with indian ink. None of the vessels have endothelial cells delimiting the luminal surface. The vessels are delimited either by dense connective tissue or by the characteristic basement lamella underneath the basal lamina of the myocoelic epithelium. It is proposed that the main blood flow in the caudal region follows different pathways depending on the activity of the animal. During swimming the muscle activity of the caudal muscles may have the effect that more blood flows from the aorta to the myoseptal plexi and is drained to the caudal vessel. In the resting animal it is possible that the blood flow through the myosepta is insignificant, and that the caudal blood flow is more or less restricted to the direct connections between the aorta and the caudal vessel: the dorsoventral anastomosis and the segmental connecting vessels.Supported by a grant from the Danish Natural Science Research Council     

The myosepta in Branchiostoma lanceolatum (Cephalochordata): 3D reconstruction and microanatomy

Zoomorphology - Myosepta have been subject to comparative and evolutionary studies in aquatic groups of the Craniata, because they are likely to play a role in transmission of muscular forces to...     

The fine structure of lamellate cells in the brain of amphioxus (Branchiostoma lanceolatum,Cephalochordata)

Summary The lamellate cells of amphioxus have round nuclei, and cytoplasm with many mitochondria and a large amount of glycogen. Each of these cells projects a highly modified, branched cilium into the central canal, where it characteristically forms lamellar structures. Primary branches and secondary lamellae often contain accessory microtubules that are not derived from the axonema. The functional and evolutionary significance of this cell type is discussed in relation to the ciliary photoreceptors found in other chordates.This work is dedicated to Professor A. Carrato, Universidad Complutense, on the occasion of his 80th birthday     

The infundibular organ of the lancelet (Branchiostoma lanceolatum,Acrania): an immunocytochemical study

Reissner's fibers are secretions produced by different ependymal areas of the chordate brain, viz., in adult vertebrates, by the dorsal subcommissural organ, and in all stages of cephalochordates (Branchiostoma lancelets), by the ventral infundibular organ. Fibers produced by these different organs are seemingly identical and the two fiber sources also share some immunocytochemical and lectin-binding properties. The secretions in these two glands are, however, not identical; the infundibular organ cells are strongly reactive with antibodies against vertebrate Reissner's fibers, but they do not react with antibodies raised against the source of the vertebrate fibers, viz., the subcommissural organ. The results support the possibility that, in adult vertebrates, the Reissner's fibers are composed of material not only from the subcommissural organ, but also from another, not yet identified, source that is identical or equivalent to the infundibular organ of the lancelet. There are indications that the infundibular organ is immunocytochemically closely akin to some secretory cells in the vertebrate embryonic brain and also to those that produce the juvenile vertebrate Reissner's fibers, viz., secretory cells in the flexural organ.     

Nocturnal behavior and rhythmic period gene expression in a lancelet, Branchiostoma lanceolatum

The authors here present the first anatomical, molecular biological, and ethological data on the organization of the circadian system of a lancelet, Branchiostoma lanceolatum, a close invertebrate relative of vertebrates. B. lanceolatum was found to be a nocturnal animal and, since its rhythmic activity persisted under constant darkness, it also appears to possess an endogenous, circadian oscillator. The authors cloned a homolog of the clock gene Period (Per), which plays a central (inhibitory) role in the biochemical machinery of the circadian oscillators of both vertebrates and protostomians. This gene from B. lanceolatum was designated as amphiPer. Both the sequence of its cDNA and that of the predicted protein are more similar to those of the Per paralogs of vertebrates than to those of the single protostomian Per gene. A strong expression of amphiPer was found in a small cell group in the anterior neural tube. The amphiPer mRNA levels fluctuated in a rhythmic manner, being high early in the day and low late at night. The authors' data suggest a homology of the amphiPer expessing cells to the suprachiasmatic nucleus of vertebrates.     

The role of the notochard in forward and reverse swimming and burrowing in the amphioxus Branchiostoma lanceolatum

Analyses of high speed cinefilm have shown that amphioxus swims either forward or backward with undulatory movement generated at the leading end, the wave of displacement passing along the body with increasing amplitude. The leading end, whether this is "head" or tail, is evidently more rigid than the trailing end, flexibility at each end changing with reversal in direction of swimming. It is suggested that control of the amplitude of the waves of displacement in different regions of the body in swimming is a function of the notochord, contraction of the muscular notochordal plates increasing its stiffness. Connections between the central nervous system and the notochordal plates via the notochordal pits are already known to exist.
As exposure to light invariably induces swimming in dark–adapted animals, it seems probable that the eyes function in initiating movement. The rate of increase in number and size of the eye cups during larval and adult growth and their pattern of distribution in the nerve cord are given. In the adult the eye cups occur predominantly in the anterior and posterior regions of the body. This may be of significance in providing the stimulus for changes in flexibility of these regions in swimming.
High speed cinefilm has also shown that amphioxus can burrow "head" or tail-first and move through sand in a forward or a reverse direction. It is suggested that rapid reversal of direction is of greater importance in movement through sand than in swimming.     

Primary Sensory Cells in the Skin of Amphioxus (Branchiostoma lanceolatum (P))

Ciliated cells in the rostral epidermis of amphioxus have been serial sectioned and examined in the electron microscope. The cells have a basal axonic process, which can be traced to the subcutaneous nepve-bundles, and hence these cells are primary sensory cells. Apically only the cilium, which takes its origin from an invagination, and a surrounding corolla of microvilli are exposed to the surface. Cross-striated filament bundles closely associate with a basal body and accessory centriole. One such bundle continues into the central part of the cell, while another bundle is attached at the lateral membrane. Between adjoining cells there are apical zonulae adhaerentes, and also poorly developed septate junctions. In addition, a third cell junction is described. Rod-like structures in the surface interdigitations are shown to be continuous with a peripheral layer of microfilaments. The choanocyte-like appearance of the ciliated cells and their resemblance to similar cells in various invertebrates are discussed.