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Ultrastructure and Presumed Origin of the Phagocytic Cells Involved in the Regression of Headless Stalks of Globiferous Pedicell

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Ultrastructure and Presumed Origin of the Phagocytic Cells Involved in the Regression of Headless Stalks of Globiferous Pedicell Echinoderm Reseanh,De Ridder, Dubois, Lahaye & Jangoux (edsJ O 1990 Balkema, Rotterdam. ISBN 90 6191 14 1 9 Ultrastructure and presumed origin of the phagocytic cells involved in the regression of headless stalks of globiferous pedicellariae in the echinoid Sphaerechinus granularis (Echinodermata) M.Ghyoot & Ph. Dubois Laboratoire de Biologie marine (CP 160), UniversitkLibre & BruxeUes, BruxeUes, Belgique M. Jangoux Laboratoire & Bwlogii marine, Universite'& Mons-Hainwt, Mons, Belgique & Laboratoire & Biologie marine (CP 160), Universite' Libre & Bmlles, Bmlles, Belgique ABSTRACT: When globiferous pedicellariae of Sphaerechinus granularis inject their venom, their head autotomizes whereas their stalk remains on the test. Following autotomy, the headless stalk shows repair of the apical wound. resorption of the stalk tissues, and production of a collagen-rich fibrous tissue. Three cell types are involved in these processes. The phagocytic cells perform the resorption of both the joint tissues and the intradermal nerve tracts; the skeletoclastic cells are responsible for the resorption of the calcareous rod; and the collagen-associated cells are involved in both the production and the resorption of collagen fibrils. The three cell types share morphological characteristics between each other and with phagocytic coelomocytes. It is suggested that they could either correspond to migrating coelomocytes or share a common origin with them. 1 INTRODUCTION 2 MATERIALS AND METHODS Globiferous pedicellariae are defensive Individuals of Sphaerechinus granularis appendages located on the outer body were collected intertidally on a rocky surface of most echinoids. They consist shore at La Pointe de lqArmorique of a rigid stalk that supports a (Brittany, France). They were maintained three-jawed head. each jaw being in an open circuit marine aquarium at provided with a well-developed the laboratory of Roscoff until venom-apparatus (Campbell 1983). When required. Stalk regression was initiated globiferws pedicellariae discharge by inducing head autotomy of their venom, they often autotomize at pedicellariae using tube feet from the the base of the stalk (Chia 1970. starfish Marthasterias glacialis as Hilgers & Splechtna 1982). However, in stimuli (Jensen 1966). Headless stalks some species, such as Lytechinus pictus were removed from the test 3 and 15 days and Sphaerechinus granularis. after autotomy. For microscopical globiferous pedicellariae are known to investigations, headless stalks were autotomize only the head after venom fixed for 3h at O0C in 3% glutaraldehyde injection (Holland & Holland 1975, in cacodylate buffer (0.1 M, pH 8.0. Hilgers & Splechtna 1982). Preliminary 1030 m0sm). Fixed stalks were washed in evidence indicates that, after head buffer, postfixed for 1 h in 1% Os04 in autotomy. the pedicellarial stalk cacodylate buffer, and dehydrated in remains on the test and progressively graded ethanol. Stalks were embedded in regresses (Holland & Holland 1975). Spurr's resin and decalcified according The present paper deals with the stalk to the method of Holland and Grimmer regression in headless globiferous (1981) (double embedding method). pedicellariae of Sphaerechinus Sectioning was done with a LKB V granularis. Its goal is to describe the Ultratome using glass or diamond knives fine structure and consider the origin for semi-thin or ultrathin sections. of the cells which are involved in the respectively. Semi-thin sections were regression of the stalk. stained in a 1:l methylene-blue - azur I1 mixture. Ultrathin sections were . Senior Research Assistant NFSR contrasted with uranyl acetate and lead (Belgium) citrate, and observed in a. Philips EM Fig.1: Longitudinal semi-thin section in the distal part of the headless pedicellarial stalk three days after autotomy. Fig.2: Longitudinal semi-thin section in the distal part of the headless pedicellarial stalk fifteen days after autotomy. Figs 3.4: Phagocytic cells (TEM views); Fig.3: Cell process of a phagocytic cell resorbing the head flexor muscles; Fig.4: Phagocytic cell resorbing an intradermal nerve tract. c: intranuclear crystal; cp: cell process; d: dermis; e: epidermis; f: fibrous tissue; fl: head flexor muscles; h: healing epidermis; 1: lysosome-like granule; n: nerve; p: phagosome; pm: phagosome enclosing a muscular fibre; r: calcareous rod; t: tendinous connective tissue. 300 transmission electron microscope. nerve tracts. They are large and elongated cells that develop processes invading tissues in the process of 3 RESULTS AND DISCUSSION resorption (Figs 3, 4). Their cytoplasm contains abundant RER cisternae and a 3.1 General aspects juxtanuclear Golgi apparatus, as well as lysosome-like granules and numerous The stalk of the globiferous phagosomes (Figs 3. 4). The nucleus of pedicellariae of Sphaerechinus most phagocytic cells encloses a crystal granularis consists of an inner (Fig.4). After resorption of both the calcareous rod embedded in a thick layer joint tissues and the intradermal nerve of connective tissue (i.e. the dermis) tracts, clots of phagocytic cells occur that is covered by the epidermis. The in the apical dermis as well as in the dermis contains three epidermal glands, epidermis. These cells are provided with namely the stalk glands (Per& 1950). an intranuclear crystal and their and several longitudinal nerve tracts cytoplasm is filled with numerous (these are parts of the epineural residual bodies. plexus) that extend from the jaw nerves The phagocytic cells resemble the down to the basal part of the stalk phagocytes already described in the (Ghyoot & Jangoux 1988). The integumentary lesions of Psammechinus pedicellarial head is attached to the miliaris (Maes et al. 1986). in the stalk by joint tissues formed by both ovary of Arbacia punctulata (Karasaki the head flexor muscles and tendinous 1965). and in the axial organ of collagen fibres. When the head Sphaerechinus granularis (Bachmann et autotomizes, the joint tissues al. 1980). Like the phagocytic cells, disconnect from the base of the head and they contain phagosomes and residual remain anchored to the distal end of the bodies, and are generally provided with calcareous rod (Hilgers & Splechtna an intranuclear crystal. Similar cells, 1982). that also contain an intranuclear Three days after autotomy, a new crystal, were reported in intact stalks epidermis spreads over the apical wound of globiferous pedicellariae of and covers the underlying dermal tissues Sphaerechinus granularis (Ghyoot & (Fig.1). At the same time, the joint Jangoux 1988) tissues and the intradermal nerve tracts are submitted to intense phagocytosis, Skeletoclastic cells and resorption starts in the distal end of the calcareous rod. Skeletoclastic cells occur in the stroma Fifteen days after autotomy, the joint of the distal part of the calcareous tissues, most of the intradermal nerve rod. They form large syncytia which tracts, and the distal end of the develop conspicuous cell processes that calcareous rod are resorbed. The distal closely surround trabeculae in the part of the stalk is invaded by a process of resorption (Figs 5-71. The collagen-rich fibrous tissue (Fig.2). cytoplasm of the syncytia typically Regression of headless stalks thus contains a large number of mitochondria consists in three processes. i.e., the and several lysosome-like granules repair of the apical wound, . the (Fig.6). It also encloses a few RER resorption of the stalk tissues, and the cisternae, juxtanuclear Golgi production of a collagen-rich fibrous apparatuses and numerous clear vesicles. tissue. A few phagosomes are observed in some syncytia (Fig.5). Some nuclei of the skeletoclastic syncytia are provided 3.2 Cell types involved in the with a crystal (Fig.6). At the regression of headless stalks resorption site, the plasma membrane adjacent to the resorbing trabecula Three cell types are involved in the shows numerous invaginations (Figs 5-7). regression of headless stalks, namely, These generally contain cytoplasmic phagocytic cells, skeletoclastic cells. profiles Fig7 some of them and collagen-associated cells. connecting the invaginated membrane, suggesting a ramified organization of Phagocytic cells these invaginations. The cytoplasm surrounding the invaginations shows Phagocytic cells are observed in both numerous clear vesicles (Fig.7) the joint tissues and the intradermal Skeletoclastic cells have been reported by Mike1 & R6ser (1983b) in membrane-bound profiles are observed in the damaged spine of the echinoid the cell processes (Figs 9, 10). Eucidaris tribuloides. These cells are Occasionally, they also occur in the also organized in syncytia. They are cell bodies. (These profiles never occur provided with numerous mitochondria. in the same cell process.) several membrane-bound homogeneous The first type of membrane-bound bodies (reminiscent of the lysosome-like profile (FTMP) (Fig.9) occurs in cell granules observed in this study) and processes which are generally devoid of clear vesicles occurring near the phagosomes and lysosome-like granules. resorption site. However, contrary to These profiles consist of what happens in the resorbing membrane-limited spaces enclosing pedicellarial rods of Sphaerechinus collagen fibrils (Fig.9). They are found granularis, they do not surround the both in the deep cytoplasm of the cell trabeculae but send thin cytoplasmic process and close to its plasma processes that etch a thin cleft through membrane. They contain collagen fibrils the trabecula, without massive 25 to 80nm in diameter
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