Shigetoyo Amano

Metamorphosis of Coeloblastula Performed by Multipotential Larval Flagellated Cells in the Calcareous Sponge Leucosolenia laxa

The calcareous sponge Leucosolenia laxa releases free-swimming hollow larvae called coeloblastulae that are the characteristic larvae of the subclass Calcinea. Although the coeloblastula is a major type of sponge larva, our knowledge about its development is scanty. Detailed electron microscopic studies on the metamorphosis of the coeloblastula revealed that the larva consists of four types of cells: flagellated cells, bottle cells, vesicular cells, and free cells in a central cavity. The flagellated cells, the principal cell type of the larva, are arranged in a pseudostratified layer around a large central cavity. The larval flagellated cells characteristically have glutinous granules that are used as internal markers during metamorphosis. After a free-swimming period the larva settles on the substratum, and settlement apparently triggers the initiation of metamorphosis. The larval flagellated cells soon lose their flagellum and begin the process of dedifferentiation. Then the larva becomes a mass of dedifferentiated cells in which many autophagosomes are found. Within 18 h after settlement, the cells at the surface of the cell mass differentiate to pinacocytes. The cells beneath the pinacoderm differentiate to scleroblasts that form triradiate spicules. Finally, the cells of the inner cell mass differentiate to choanocytes and are arranged in a choanoderm that surrounds a newly formed large gastral cavity. We found glutinous granules in these three principal cell types of juvenile sponges, thus indicating the multipotency of the flagellated cells of the coeloblastula.

Metamorphosis of calcareous sponges I. Ultrastructure of free-swimming larvae

Summary Free-swimming larvae of two calcareous sponges were studied by electron microscopy. The larvae are composed of four kinds of cells, namely flagellated cells, granular cells, four cruciform cells, and several yolk-containing cells. In the anterior hemisphere of the larvae, the columnar flagellated cells are arranged in a single layer. Their nucleus and Golgi apparatus are located in close proximity to the flagellar rootlets. There are granular cells in the posterior hemisphere of the larvae. They have a nucleus with a nucleolus, large phagosomes, well-developed Golgi apparatus, and numerous RER cisternae. There is a cruciform cell in each quadrant of the larvae. From its characteristic arrangement of organelles, it is suggested but not concluded that the cruciform cells participate in photoreception. The yolk-containing cells are, most probably, nutritive cells derived from the mother sponge. The roles of these four kinds of cells in habitat selection and metamorphosis are discussed.

Metamorphosis of calcareous sponges II. Cell rearrangement and differentiation in metamorphosis

Summary The free-swimming larva of the calcareous sponge turns into a sessile juvenile during metamorphosis. Electron microscopic observations of metamorphosing larvae reveal the rearrangement and differentiation of larval cells. About 12 h after the larvae were released from a mother sponge, the settled larvae without flagella consist of an inner cell mass and an enveloping layer of pinacocytes. The inner mass cells have residual flagellar rootlets which clearly show the origin of the cells. On the other hand, the pinacocytes still show the intracellular profile characteristic of the granular cells of the swimming larva. One day after release, scleroblasts and other mesohyl cells differentiate in the peripheral region of the inner cell mass. Two days after release, the central cells of the inner cell mass differentiate into choanocytes. Three days after release, a large gastral cavity is formed and lined by a layer of choanocytes. These results demonstrate the cell lineage in the metamorphosis of the cal...

Morning Release of Larvae Controlled by the Light in an Intertidal Sponge, Callyspongia ramosa

The intertidal sponge Callyspongia ramosa releases larvae in the morning under natural light. The photic control ofthis morning release was studied under experimental light-dark (LD) cycles. Under LD 12: 12h cycles (light period, from 6:00 to 18:00), release peaked about 6:00. But the release was not stimulated by the illu mination in the morning because the sponge colonies re leased larvae even in the darkness. The experiments un der various light regimes showed that the photic stimulus is not the onset of darkness but, unexpectedly, the onset oflight the day before. In fact, C. ramosa colonies invari ably released larvae about 24 hours after the onset of light under all illumination regimes tested. The tidal cycle and the daily cycle of the seawater temperature did not in fluence the time of release. Therefore, in nature, the dawning light most likely stimulates larval release on the next day. This photoadaptation in the larval release of C. ramosa suggests that the morning release is advanta geous for their free-swimming larvae to seek out and set tle on the suitable substratum in the intertidal region within their short dispersive period.

LARVAL RELEASE IN RESPONSE TO A LIGHT SIGNAL BY THE INTERTIDAL SPONGE HALICHONDRIA PANICEA

The intertidal sponge, Halichondria panicea, regularly begins releasing larvae shortly after dawn, and ejects most of them during morning hours under a natural light-dark (LD) cycle. Its diurnal periodicity was confirmed under an artificial LD 12:12h cycle. In search of a trigger that stimulates the sponge colonies to release larvae, the colonies were subjected to experimentally modified LD cycles. Under continuous darkness, only a single release peak was observed about fifteen hours after the beginning of darkness. Further, the colonies invariably released larvae about fifteen hours after the change from light to dark on the preceding day under all illumination regimes examined. The timing of their larval release was independent of both the tidal cycle and the daily cycle of seawater temperature. These results indicate that the trigger is a light signal: the onset of darkness (not onset of light) of the preceding day under natural illumination. Subsequent to this stimulus, H. panicea needs a period of fi...

Self and Non-Self Recognition in a Calcareous Sponge, Leucandra abratsbo

Discrimination between self and non-self has been shown in many demosponges, but calcareous sponges have not been studied. Allorecognition in a calcareous sponge, Leucandra abratsbo, was analyzed in allogeneic combination assays. Most allogeneic combinations were incompatible, and the low rate (4.8%) of allogeneic acceptances suggests an extensive polymorphism in those genes that may control allorecognition. However, histological studies of the rejection process revealed that the first reaction consisted of strong adhesion of allogeneic pieces. Thereafter, the rejection reaction that followed was accompanied by the accumulation of archeocytes in the contact region. Vigorous cytotoxic reactions occurred within this region, and the degenerated cells were probably phagocytosed by archeocytes, which suggests that they are the primary effector cells for cytotoxicity and phagocytosis. Because L. abratsbo is a solitary sponge, armed with protruding spicules that prevent contact of the pinacoderm with that of conspecific individuals, allorecognition may not prevent the formation of allogeneic chimeras in the natural habitat.

Contribution of neurons to habituation to mechanical stimulation in Caenorhabditis elegans

In Caenorhabditis elegans, a light touch induces a locomotor response. Repeated touches, however, result in an attenuation of response, that is, habituation. Withdrawal responses elicited by anterior touch are controlled by anterior mechanosensory neurons (AVM and ALMs), and by four pairs of interneurons (AVA, AVB, AVD, and PVC) (Chalfie et al., 1985; White et al., 1986). To identify the neurons that participate in habituation, we ablated these neurons with a laser microbeam and investigated the resulting habituation of the operated animals. The animals lacking both left and right homologues AVDLR were habituated more rapidly than intact animals. We propose that chemical synapses at AVD play a critical role in the habituation of intact animals.

Hemocyte differentiation in the juveniles of the ascidian Halocynthia roretzi

Summary We studied hemocyte differentiation in the juveniles of the ascidian Halocynthia roretzi by light and electron microscopy. In the 2d juveniles post-metamorphosis, the hemocoel opens and hemocytes firstly differentiate in it. Those hemocytes were classified into vacuolated cells, granular cells, giant cells, and undifferentiated mesodermal cells. Around day 5 after metamorphosis the mesodermal cells gradually transform to hemoblasts. Therefore the juveniles have only three categories of differentiated hemocytes and the mesodermal cells or the hemoblasts. The scarcity of the hemocyte types and an abundance of immature hemocytes in the juveniles allowed us to follow their differentiation pathways distinctly. We revealed the process of cell differentiation characteristic to each hemocyte category. The circulating hemoblasts most likely function as hematopoietic stem cells in the early stages of ascidian hematopoiesis.

Hierarchy of Habituation Induced by Mechanical Stimuli in Caenorhabditis elegans

Abstract C. elegans becomes habituated to repetitive mechanical stimuli. We compared the habituated states induced by three types of mechanical stimuli: touch on the head (head-touch), touch on the anterior body (body-touch), and mechanical tapping of the Petri dish, all of which evoke backward movement. The habituation patterns were similar, but differed in retention period and/or the rate of recovery. We found a hierarchy between the habituated states induced by the three types of mechanical stimuli in the decreasing order of head-touch, body-touch, and tap stimulus. Evidence is presented that the hierarchy is brought out by the magnitude of stimuli rather than by independent neural pathways.