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PubMed1999

Evidence for conservation of the vasopressin/oxytocin superfamily in Annelida.

Annetocin is a structurally and functionally oxytocin-related peptide isolated from the earthworm Eisenia foetida. We present the characterization of the annetocin cDNA. Sequence analyses of the deduced precursor polypeptide revealed that the annetocin precursor is composed of three segments: a signal peptide, an annetocin sequence flanked by a Gly C-terminal amidation signal and a Lys-Arg dibasic processing site, and a neurophysin domain, similar to other oxytocin family precursors. The proannetocin showed 37.4-45.8% amino acid homology to other prohormones. In the neurophysin domain, 14 cysteines and amino acid residues essential for association of a neurophysin with a vasopressin/oxytocin superfamily peptide were conserved, suggesting that the Eisenia neurophysin can bind to annetocin. Furthermore, in situ hybridization experiments demonstrated that the annetocin gene is expressed exclusively in neurons of the central nervous system predicted to be involved in regulation of reproductive behavior. These findings confirm that annetocin is a member of the vasopressin/oxytocin superfamily. This is the first identification of the cDNA encoding the precursor of an invertebrate oxytocin-related peptide and also the first report of the identification of an annelid vasopressin/oxytocin-related precursor.

Amino Acid Sequence↗
PubMed1961

Fine structure of the spermatozoon of Hydroides hexagonus (Annelida), with special reference to the acrosomal region.

This paper describes in some detail the structure of the acrosomal region of the spermatozoon of Hydroides as a basis for subsequent papers which will deal with the structural changes which this region undergoes during fertilization. The material was osmium-fixed and mild centrifugation was used to aggregate the spermatozoa from collection to final embedding. The studies concern also the acrosomal regions of frozen-thawed sperm prepared by a method which previously had yielded extracts with egg membrane lytic activity. The plasma membrane closely envelops four readily recognizable regions of the spermatozoon: acrosomal, nuclear, mitochondrial, and flagellar. The acrosome consists of an acrosomal vesicle which is bounded by a single continuous membrane, and its periphery is distinguishable into inner, intermediate, and outer zones. The inner and intermediate zones form a pocket into which the narrowed apex of the nucleus intrudes. Granular material adjoins the inner surface of the acrosomal membrane, and this material is characteristically different for each zone. Centrally, the acrosomal vesicle is spanned by an acrosomal granule: its base is at the inner zone and its apex at the outer zone. The apex of the acrosomal granule flares out and touches the acrosomal membrane over a limited area. In this limited area the adjoining granular material of the outer zone is lacking. The acrosomal membrane of the inner zone is invaginated into about fifteen short tubules. The acrosomal membrane of the outer zone is closely surrounded by the plasma membrane. At the apex of the acrosomal region a small apical vesicle is sandwiched between the plasma membrane and the acrosomal membrane. Numerous frozen-thawed specimens and occasional specimens not so treated show acrosomal regions at the apex of which there is a well defined opening or orifice. Around the rim or lip of this orifice plasma and acrosomal membranes may even be fused into a continuum. The evidence indicates that the apical vesicle and the parts of the plasma and acrosomal membranes which surround it constitute a lid, and the rim of this lid constitutes a natural "fracture line" or rim of dehiscence. Should fracture occur, the lid would be removed and the acrosomal vesicle would be open to the exterior.

Acrosome↗
PubMed1961

Changes in the spermatozoon during fertilization in Hydroides hexagonus (Annelida). II. Incorporation with the egg.

This, the last of a series of three papers, deals with the final events which lead to the incorporation of the spermatozoon with the egg. The material used consisted of moderately polyspermic eggs of Hydroides hexagonus, osmium-fixed at various times up to five minutes after insemination. The first direct contact of sperm head with egg proper is by means of the acrosomal tubules. These deeply indent the egg plasma membrane, and consequently at the apex of the sperm head the surfaces of the two gametes become interdigitated. But at first the sperm and egg plasma membranes maintain their identity and a cross-section through the region of interdigitation shows these two membranes as a number of sets of two closely concentric rings. The egg plasma membrane rises to form a cone which starts to project into the hole which the spermatozoon earlier had produced in the vitelline membrane by means of lysis. But the cone does not literally engulf the sperm head. Instead, where they come into contact, sperm plasma membrane and egg plasma membrane fuse to form one continuous membranous sheet. At this juncture the two gametes have in effect become mutually incorporated and have formed a single fertilized cell with one continuous bounding membrane. At this time, at least, the membrane is a mosaic of mostly egg plasma membrane and a patch of sperm plasma membrane. The evidence indicates that the fusion of the two membranes results from vesiculation of the sperm and egg plasma membranes in the region at which they come to adjoin. Once this fusion of membranes is accomplished, the egg cytoplasm intrudes between the now common membrane and the internal sperm structures, such as the nucleus, and even extends into the flagellum; finally these sperm structures come to lie in the main body of the egg. The vesiculation suggested above appears possibly to resemble pinocytosis, with the difference that the vesicles are formed from the plasma membranes of two cells. At no time, however, is the sperm as a whole engulfed and brought to the interior of the egg within a large vesicle.

Animals↗
PubMed1957

Electron microscope studies of early stages of sperm penetration in Hydroides hexagonus (annelida) and Saccoglossus kowalevskii (enteropneusta).

1. The early events of sperm entry in Saccoglossus and Hydroides are described and examined in relation to present knowledge of the acrosome reaction and of egg membrane lysins. In Saccoglossus and several other species these events occur in two phases. First. The acrosome filament of the spermatozoön spans the egg membrane barriers, reaches the reactive egg protoplasm, and causes the egg to begin its fertilization reaction. Second. The filament and its connected sperm head move through the egg membrane barriers and enter the egg proper. The first phase is completed in a matter of seconds but the second phase usually requires several minutes. 2. The peripheral areas of the eggs of the two species differ as seen in sections. In Hydroides, but not in Saccoglossus, the vitelline membrane is bounded by a distinct outer border layer of small concentrically differentiated bodies and penetrated by microvilli from the egg. 3. The acrosome filament, seen in the living condition as a delicate thread in Hydroides and as an exceedingly tenuous thread in Saccoglossus, appears to be tubular in both species when seen in electron micrographs of thin sections. 4. The acrosomal region of Hydroides appears to consist of two components-a peripheral one, which may collapse during the acrosome reaction, and a central one related to the acrosome filament. 5. Deliberately induced polyspermic material was used to increase the probability of finding examples of sperm penetration in thin sections. 6. As seen in sections, areas of low electron density, interpreted as spaces or pits from which the material of the membrane is absent, surround the attached or penetrating spermatozoa. (a) In Hydroides the spaces vary greatly in many characteristics including shape, position in the membrane, and size with relation to the enclosed sperm head. In one specimen a portion of the membrane is missing from border to border; no spermatozoön is seen but immediately beneath the space is the apex of a fertilization cone. (b) In every case in which a determination could be made, the spermatozoön in the membrane has undergone its acrosome reaction. (c) In Saccoglossus some pits are found with which several spermatozoa are associated. Generally, where the spermatozoa are more numerous the pit is larger. (d) Pits similar to those seen in Saccoglossus sections are observed in living eggs. They remain in Membrane I after sperm entry. (e) From the above and other considerations it is suggested that the pits and spaces are formed by local action of a lysin or lysins emanating from the individual spermatozoön at the site of sperm entry. 7. It is considered that the suggested lysin would participate in sperm entry by eroding the membrane barrier in the vicinity of the sperm head, thus permitting the sperm head to pass through the membrane. Since the acrosome filament much earlier stimulates the egg's initial fertilization response, this lysin would facilitate the second phase of the early events of sperm entry.

Acrosome↗
PubMed2004

Embryogenesis of GABAergic elements in the nervous system of Eisenia fetida (Annelida, Oligochaeta).

The appearance and development of the GABA-immunoreactive nervous elements in the central nervous system of Eisenia fetida were studied by immunocytochemistry. The nervous system originates from the neuroectoderm situated on the ventral side of the embryo. The organization of the circumpharyngeal ring starts earlier than that of the ventral cord. In the elementary ring the first GABA-immunopositive neurons can be observed (E1 stage) around the mouth. Later the cell number gradually increases and parallel to this process the elementary ring is separeted into a superficial and a deeper portion. The brain and the subesophageal ganglion will be organized from the superficial ring, while the nervous elements of the deeper ring will give rise for the first GABA-immunoreactive elements of the stomatogastric nervous system. In the early stages of the embryogenesis the immunoreactive cells of the developing brain appear solitary, while from the stage E4 they gradually are observed in groups. According to their position, these cell groups are similar to those observed in the brain of the adult earthworms. During embryogenesis the level of the ventral cord ganglia depends on their position in the ectodermal germ bands. It means, that the more organized ganglia are near the circumpharyngeal ring, mean while less developed ganglia are situated caudally from them. By the end of the embryogenesis all ganglia of the ventral cord will be equally well organized. The nerve tracts of the ganglia are built up from contra- and ipsilateral by projected fibres. From E3 stage the medial tracts, mean while from the E4 stage the lateral tracts begin to be formed. During the next stages, more and more fibres connect to the both tracts. At hatching, the development of the central nervous system of Eisenia fetida is not completed, the process is continued during the postembryonic development.

Animals↗
PubMed2001

Molecular evidence that Sclerolinum brattstromi is closely related to vestimentiferans, not to frenulate pogonophorans (Siboglinidae, Annelida).

Siboglinids, previously referred to as pogonophorans, have typically been divided into two groups, frenulates and vestimentiferans. Adults of these marine protostome worms lack a functional gut and harbor endosymbiotic bacteria. Frenulates usually live in deep, sedimented reducing environments, and vestimentiferans inhabit hydrothermal vents and sulfide-rich hydrocarbon seeps. Taxonomic literature has often treated frenulates and vestimentiferans as sister taxa. Sclerolinum has traditionally been thought to be a basal siboglinid that was originally regarded as a frenulate and later as a third lineage of siboglinids, Monilifera. Evidence from the 18S nuclear rDNA gene and the 16S mitochondrial rDNA gene presented here shows that Sclerolinum is the sister clade to vestimentiferans although it lacks the characteristic morphology (i.e., a vestimentum). The rDNA data confirm the contention that Sclerolinum is different from frenulates, and further supports the idea that siboglinid evolution has been driven by a trend toward increased habitat specialization. The evidence now available indicates that vestimentiferans lack the molecular diversity expected of a group that has been argued to have Silurian or possibly Cambrian origins.

Animals↗