Michael J. Cavey

Organization of a phyllobranchiate gill from the green shore crab Carcinus maenas (Crustacea, Decapoda)

SummaryThe phyllobranchiate gills of the green shore crab Carcinus maenas have been examined histologically and ultrastructurally. Each gill lamella is bounded by a chitinous cuticle. The apical surface of the branchial epithelium contacts this cuticle, and a basal lamina segregates the epithelium from an intralamellar hemocoel. In animals acclimated to normal sea water, five epithelial cell types can be identified in the lamellae of the posterior gills: chief cells, striated cells, pillar cells, nephrocytes, and glycocytes. Chief cells are the predominant cells in the branchial epithelium. They are squamous or low cuboidal and likely play a role in respiration. Striated cells, which are probably involved in ionoregulation, are also squamous or low cuboidal. Basal folds of the striated cells contain mitochondria and interdigitate with the bodies and processes of adjacent cells. Pillar cells span the hemocoel to link the proximal and distal sides of a lamella. Nephrocytes are large, spherical cells with voluminous vacuoles. They are rimmed by foot processes or pedicels and frequently associate with the pillar cells. Glycocytes are pleomorphic cells packed with glycogen granules and multigranular rosettes. The glycocytes often mingle with the nephrocytes. Inclusion of the nephrocytes and glycocytes as members of the branchial epithelium is justified by their participation in intercellular junctions and their position internal to the epithelial basal lamina.

Ascidian larval tunic: Extraembryonic structures influence morphogenesis

SummaryThe larval tunic of Corella inflata is composed of two cuticular layers, extracellular filaments and ground substance. It lies outside the epidermis and most of it is known to be produced by the epidermis. The dorsal, ventral and caudal fins are specialized parts of the tunic that are essential for larval locomotion. The following hypothesis was tested: Morphogenesis of the larval fins is dependent upon the presence of extraembryonic structures (test cells, chorion or follicle cells) before completion of the late tail bud stage of development. We tested this by dechorionating embryos of Corella inflata and Ascidia paratropa. The operation removes all extraembryonic structures. It was performed mainly on neurula, early tail-bud and late tail-bud stages.Fin formation is inhibited when neurulae are dechorionated but not when late tail-bud or older embryonic stages are dechorionated. Dechorionated neurulae produce all of the major components of the tunic (cuticular layers, filaments and ground substance) but they are unable to form functional fins. At the time of dechorionation, in all experiments, the embryos had no fins.Removal of the follicle cells does not inhibit fin formation. The test cells are known to secrete granular “ornaments” that attach to the surface of the tunic. The fibrous, acellular chorion may serve to contain the test cells and their products or products of the embryo that are not firmly attached. The test cells may induce or control the morphogenesis of the larval fins in ascidians before the late tail-bud stage of development. We suggest ways of testing this hypothesis and an alternative hypothesis.

Evolutionary derivation of the American lobster cardiovascular system: an hypothesis based on morphological and physiological evidence

The cardiovascular system of the American lobster includes a large muscular heart that pumps blood into seven arteries, each of which ramifies extensively. Portions of the system may be viewed as relatively primitive, while others are highly derived. We have confirmed earlier findings that the sternal artery is not a single vessel, but a paired structure. The sternal artery and its partner closely resemble the medial branches of the segmental lateral vessels from the dorsal abdominal artery in anterior segments of the abdomen, and they may be homologous. We report that the walls of the dorsal abdominal artery contain blocks of striated muscle cells and that the artery can be induced to contract in response to electrical stimulation or perfusion with proctolin. These observations provide the basis for an attempt to trace the evolution of the heart and arteries from that of primitive malacostracans to its state of development in lobsters. Additional key words: Crustacea, Homarus americanus, evolution, heart, dorsal abdominal artery, sternal artery We present here anatomical and physiological evidence supporting the hypothesis that the cardiovascular system in adult decapod crustaceans, as illustrated for Homarus americanus (e.g., McLaughlin 1983), is derived, by migration and regional specialization, from the evolutionarily primitive plan of a dorsal, longitudinal, tubular, muscular heart with a pair of ostia in each segment running from the head to the telson (Siewing 1963; Hessler et al. 1982). It has been postulated, based on the Cephalocarida, that crustacean ancestors displayed strong serial homology in body plan (Sanders 1955), possibly arising from annelid ancestral antecedants such as an extended nauplius-like larva (Hessler et al. 1982). The Cephalocarida and Branchiopoda (Anostraca, fairy shrimps) are closest to this prototype. This primitive plan is still the general plan in stomatopods (Siewing 1963; McLaughlin 1980). In the ancestral plan, the heart tube lies immediately dorsal to the gut and extends the entire length of the body. The heart tube supplies hemolymph to a short non-muscular anterior median artery to the brain, to some form of serially homologous segmental lateral arterial supply in each body segment, and to an unpaired posterior artery to the telson. The segmental a Author with whom to correspond. lateral arteries are short and do not branch extensively. Paired ostia admit returning hemolymph to the heart tube in each segment. In adults of H. americanus, the single-chambered heart is suspended in the pericardial sinus in the dorsal thorax by several pairs of alary ligaments (Lochhead 1950, pp. 428-431; McLaughlin 1980, pp. 138-141). These elastic ligaments are stretched during systole, and they expand the heart during diastole. During diastole, the heart passively fills with hemolymph from the pericardial sinus through three pairs of muscular,

Ultrastructure and differentiation of ascidian muscle

SummaryThe larval caudal musculature of the compound ascidian Diplosoma macdonaldi consists of two longitudinal bands of somatic striated muscle. Approximately 800 mononucleate cells, lying in rows between the epidermis and the notochord, constitute each muscle band. Unlike the caudal muscle cells of most other ascidian larvae, the myofibrils and apposed sarcoplasmic reticulum occupy both the cortical and the medullary sarcoplasm.The cross-striated myofibrils converge near the tapered ends of the caudal muscle cell and integrate into a field of myofilaments. The field originates and terminates at intermediate junctions at the transverse cellular boundaries. Close junctions and longitudinal and transverse segments of nonjunctional sarcolemmata flank the intermediate junctions, creating a transverse myomuscular (TMM) complex which superficially resembles the intercalated disk of the vertebrate heart.A perforated sheet of sarcoplasmic reticulum (SR) invests each myofibril. The sheet of SR spans between sarcomeres and is locally undifferentiated in relation to the cross-striations. Two to four saccular cisternae of SR near each sarcomeric Z-line establish interior (dyadic) couplings with an axial analogue of the vertebrate transverse tubular system. The axial tubules are invaginations of the sarcolemma within and adjacent to the intermediate junctions of the TMM complex.The caudal muscle cells of larval ascidians and the somatic striated muscle fibers of lower vertebrates bear similar relationships to the skeletal organs and share similar locomotor functions. At the cellular level, however, the larval ascidian caudal musculature more closely resembles the vertebrate myocardium.

Development of the subdigital adhesive pads of Ptyodactylus guttatus (Reptilia: Gekkonidae)

Subdigital adhesive pads play an important role in the locomotion of many species of gekkonid lizards. These pads consist of integrated components derived from the epidermis, dermis, vascular system, subcuticular tendons, and phalanges. These components become intimately associated with each other during the developmental differentiation of the digits and the sequence of this integration is outlined herein in Ptyodactylus guttatus. The pads initially appear as paired swellings at the distal tips of the digits. Subsequently, a fan‐like array of naked scansors develops on the ventral surface of each digit, at about the same time that scales differentiate over the surface of the foot as a whole. At the time of appearance of the naked scansors, the vascular sinus system of the pad also differentiates, along with subcuticular connective tissue specializations. At this stage the digits, along with the rest of the body, are clad in an embryonic periderm. Only after hatching and as the periderm is shed, do the epidermal setae and spines appear. The developmental sequence described here is consistent with predictions previously advanced about the evolutionary origin and elaboration of subdigital pads in gekkonid lizards. The paucity of available staged embryonic material leaves many questions unresolved.

Elasticity, unexpected contractility and the identification of actin and myosin in lobster arteries.

SUMMARY Lobster arteries, which exhibit non-uniform elasticity when stretched, have a trilaminar organization. The inner layer is an elastic connective tissue and the outer layer is a collagenous connective tissue; the middle layer of an artery is an aggregation of cells containing microfilaments. Arterial cells possess actin, myosin and tropomyosin. Except for the dorsal abdominal artery, striated muscle cells are not evident in the walls of any of the vessels. The neurotransmitter glutamic acid and the neurohormone proctolin elicit slow circumferential contractions in all of the arteries leaving the lobster heart. Only the dorsal abdominal artery contracts when stimulated electrically. Longitudinal strips of the arteries do not respond to either drugs or electrical stimulation. Arterial contraction will have profound effects on resistance to blood flow and may be an important component of the control mechanisms regulating blood distribution.

Structure and contractile properties of the ostial muscle (musculus orbicularis ostii) in the heart of the American lobster

Abstract“Venous” blood enters the crustacean heart through bivalved ostia. Each ostium is a discrete anatomical unit that remains functional even when isolated from the heart. Muscle fibers produce overshooting action potentials that have a plateau of variable duration in response to nervous drive from the cardiac ganglion or during trains of electrical stimuli. Contractions show summation and facilitation when stimulated by trains of stimuli delivered at rates greater than 0.5 s−1 and 0.2 s−1, respectively. Contraction amplitude increases with stimulating impulse frequency and train duration. Maximum force occurs at 1.2 times the slack length. The morphology of ostial fibers resembles that of myocardial fibers. Interconnected bundles of myofilaments occur in both the ostial fibers and the myocardial fibers. In ostial and myocardial fibers, the myofilament bundles are invested by perforated sheets of sarcoplasmic reticulum, and these sheets interface with a network of sarcolemmal tubules to form dyadic interior couplings at the level of the sarcomeric H-bands. The contractile apparatus originates and terminates at intermediate junctions on the transverse cellular boundaries, and the lateral surfaces of the muscle fibers are linked by a modest number of communicating (gap) junctions.

Specializations for Excitation-Contraction Coupling in the Podial Retractor Cells of the Starfish Stylasterias forreri

SummaryUltrastructural examination of the podium of the asteroid echinoderm Stylasterias forreri has revealed that cells of the coelomic epithelium and cells of the retractor muscle should be considered as components of a single epithelium. The podial retractor cells are, therefore, myoepithelial in nature. This report concentrates on those ultrastructural features of the retractor cells that are most likely involved with excitation-contraction coupling. The spatial arrangement of the sarcoplasmic reticulum, the couplings between the sarcoplasmic reticulum and sarcolemma, and an intramembranous specialization of the sarcolemma are documented and discussed. Current concepts regarding the innervation of the retractor cells of the podium and the protractor cells of the ampulla are reviewed, and specific proposals for further investigation of podial innervation are outlined.

Ultrastructure of transrectal coelomoducts in the sea cucumber Parastichopus californicus (Echinodermata, Holothuroida)

SummaryThe perivisceral coelom of the sea cucumber Parastichopus californicus is connected to the lumen of the hindgut by as many as 200 short transrectal ducts. Each duct is lined by a pseudostratified epithelium composed of: (i) monociliated, tonofilament-containing cells, (ii) myoepithelial cells, (iii) bundles of neurites, and (iv) granule-containing cells. In most places the lumen of each duct is lined by the monociliated, tonofilament-containing cells. The myoepithelial cells are predominantly basal in position and circular in orientation, but some border the lumen and parallel the long axis of the duct. The epithelium of a duct consists of the same types of cells as occur in the peritoneum covering the rectum and differs markedly from the nonciliated, cuticularized epithelium that lines the lumen of the rectum. Based on ultrastructural characteristics, the transrectal ducts represent evaginations of the peritoneum overlying the rectum and are thus “coelomoducts” sensu Goodrich. The possibility is discussed that perivisceral coelomoducts of holothuroids function in regulating coelomic volumes.

Custom silicone rubber molds for epoxy resin embedding

A two-component silicone rubber kit makes possible the fabrication of custom embedding molds for use with epoxy resins. The kit contains a base material and a curing agent, which are combined by weight, degassed under vacuum (if necessary), and poured into the reservoir of a cast. The silicone rubber mixture cures within 24 hours at room temperature, producing a strong mold that is both flexible and tear-resistant. A three-piece aluminum cast, especially designed for mold-making, consists of a frame surrounding the template that rests on the wings of a platform and forms a reservoir to hold the silicone