Allison L. Burnett

The role of mesoglea in mass cell movement in Hydra.

Abstract Grafting experiments on H. viridis were performed to determine whether the epitheliomuscular cells of the epidermis and the digestive cells of the gastrodermis normally moved independently. In one set of experiments testes were employed as an epidermal marker and the border between algae-laden and algae-depleted digestive cells as a gastrodermal marker. Testes were often observed to move proximally at a faster rate than the border. In another set of experiments composite animals were made from normal and C 14 -labeled animals. Autoradiograms often showed that the epitheliomuscular cells moved proximally faster than the digestive cells. Moreover, in H. pseudoligactis , the epitheliomuscular cells had more mitotic figures per unit area than the digestive cells. This independent movement and growth makes it impossible to imagine the mesoglea as a “cement” holding the two cell layers together. Mesogleas were isolated in order to see if they had properties consistent with their being a substratum for cell movement. They were found to be elastic and tough and to retain the cylindrical shape of the organism even in the absence of cells. A collagen-like protein is present in the mesoglea as shown by the conditions for its extraction, by the behavior of its heat-denatured products on disc electrophoresis, and by the presence of hydroxylysine and hydroxyproline. The role of mesoglea as an endoskeleton in Hydra is discussed.

Dedifferentiation and Redifferentiation of Cells in Hydra viridis

The interstitial cell of the green hydra is formed by dedifferentiation of specialized gastrodermal cells. Similarly, the epidermal epitheliomuscular cells are probably formed by direct difjerentiation of algae-laden digestive cells that lose their algae and enclosed food droplets, migrate to the periphery of the animal, and begin the mucous secretion characteristic of epidermal cells.

A study of growth and cell differentiation in the hepatopancreas of the crayfish

Abstract The histology of the crayfish hepatopancreas has been presented. Also, cell types within the tubule have been distinguished on a histochemical basis. Labeling studies employing tritiated thymidine and uridine have been carried out. There is strong evidence that all the cells in the tubules of the digestive gland arise from embryonic cells at the apex. During their lifetime the embryonic cells are first absorptive, then secretory, then fibrillar, and finally they atrophy and die at the base of the tubule. Thus, the tubule grows continually but is still able to maintain a constant form. These results are discussed in terms of our traditional concepts of cell differentiation. We suggest that the term cellular ontogeny may be more appropriate than cellular differentiation in reference to the present study.

A histological and ultrastructural study of dedifferentiation and redifferentiation of digestive and gland cells in Hydra viridis

Abstract The isolated gastrodermis of the green Hydra is capable of regenerating into a normal animal containing epidermis, mesoglea, and gastrodermis. Since the gastric region of the isolated gastrodermis contains only two cell types, namely, digestive cells and gland cells, our primary interest was to observe the cellular changes during dedifferentiation and redifferentiation into other cell types. Digestive cells at the periphery of the explant differentiate directly into epitheliomuscular cells, while some of the digestive cells in the interior of the mass are destroyed during the formation of the enteron. The peripheral gland cells undergo a complex cellular transformation. The granular endoplasmic reticulum is broken down; in some areas fragments of the endoplasmic reticulum are isolated in vacuoles associated with the Golgi apparatus and all secretory droplets are voided. The gland cell dedifferentiates into an interstitial cell, the cytoplasm of which is characteristically packed with free ribosomes and without endoplasmic reticulum. The interstitial cells subsequently divide and redifferentiate into cnidoblasts. The mechanism by which the endoplasmic reticulum of the gland cells is broken down is not known. Also, it is not known whether new ribosomes are synthesized during the redifferentiation of interstitial cells into cnidoblasts or that the ribosomes from the original gland cells are capable of the protein synthesis involved in nematocyst formation. The digestive cells and gland cells of the explant remain discretely located as they are in the normal animal. For this reason only peripheral gland and digestive cells undergo cellular transformation; while, except for the formation of the enteron, the cells in the interior of the mass become the future gastrodermis of the regenerate.

Behavior in Hydra: Contraction Responses of Hydra pirardi to Mechanical and Light Stimuli

Hydra pirardi contracts in response to light and mechanical agitation. The animals show a reduction in the number of contractions in response to mechanical agitation on repeated testing but continue to contract in response to a light stimulus. Excision of all the tentacles of the animal completely inhibits contraction in response to mechanical agitation but does not affect contraction in response to light. The results of these experiments suggest that H. pirardi has different receptors for light and for mechanical agitation and that the control mechanisms for the contraction responses to these two stimuli are independent.

The Acquisition, Maintenance, and Lability of the Differentiated State in Hydra

The title of this collection of papers, “The Stability of the Differentiated State”, implies that the organisms discussed herein contain differentiated cells. However, one of the central questions we shall be discussing throughout the text is: does differentiation signify a fixed, irreversible state of a cell? For me, using Hydra as an experimental model, to accept the definition that a differentiated cell cannot dedifferentiate would be tantamount to saying that the animal probably contains no differentiated cells. If this is true then none of the material presented here will be relevant to a discussion of differentiation. There is one thing to be said, however, for the foregoing definition — it is a clear formulation and not subject to semantic bickering. All other definitions of differentiation are fraught with exceptions and lead invariably to long, often tedious hours of endless debate that leave symposium participants with ragged nerves, bad digestion and an urge to get back into the laboratory and get to work because the discussion accomplished nothing.

THE ORIGIN OF THE BLASTEMAL CELLS IN DUGESIA TIGRINA.

Abstract Neoblasts in planaria are not necessarily persistent embryonic stock, since they can be formed by dedifferentiation of specialized intestinal gland cells as well as by division of pre-existing neoblasts. This dedifferentiation is a continuous process in the normal, uninjured animal and in regenerating worms. In regenerating worms the relationship between the number of specialized intestinal gland cells and the number of embryonic cells at the peak of neoblast production is one of inverse proportion, i.e., as gland cells decrease in number, neoblasts increase. Mitosis in neoblasts cannot account for the large numbers of these cells in regenerating worms.

An electron microscopic and histochemical study of the secretory cells inHydra viridis

SummaryThe fresh water coelenterateHydra viridis possesses a unique distribution of mucous and serous secretory cells in the gastrodermis. The mucous cells are found only in the hypostome, a region devoid of the serous zymogen cells. On the other hand, the zymogen cells are found extending from the tentacles to the peduncle. Histochemical stains indicated that the two hypostomal mucous cells, spumous and granular, secreted an acid mucopolysaccharide, and incorporated radiosulfate. The radiosulfate label was not sensitive to hyaluronidase digestion, but was removed by acid methanolysis. In contrast, the secretory product of the zymogen cell was rich in proteins and a PAS-positive moiety (unsulfated).The ultrastructure of these cells was correlated with their histochemical staining properties. It was demonstrated that glutaraldehyde preserved the ultrastructure of the secretory granules better than osmium, and also preserved more components within the granules. The mucous cell granules contained an electrolucent and an electron dense component. The cells were both PAS-positive and alcianophilic. After osmium fixation the dense component was lost and the cells were primarily alcianophilic. Osmium also failed to preserve the electron dense component in the zymogen cells.Observations of corresponding thick and thin sections showed a cell containing granules similar to the granules seen in mouse Paneth cells. The dense core was osmiophilic and the lighter halo was alcianophilic.These results lead us to conclude that the electrolucent filamentous component is an alcianophilic acid mucopolysaccharide and the dense granular component is probably a PAS-positive material.ZusammenfassungDer FrischwassercölenteratHydra viridis weist eine einzigartige Verteilung von mukösen und serösen sekretorischen Zellen in der Gasterodermis auf. Die mukösen Zellen finden sich nur im Hypostom, in welchem seröse Zymogenzellen fehlen. Die Zymogenzellen andereseits finden sich von den Tentaklen bis zum Pedunkulus. Histochemische Methoden zeigten, daß die zwei Typen hypostomaler muköser Zellen, d.h. spumöse und granuläre, ein saures Mukopolysaccharid ausscheiden und radioaktives Sulfat einbauen. Der Radiosulfat-Markierer war nicht sensitiv gegenüber Hyaluronidase, konnte aber entfernt werden mit saurem Methanol. Im Gegensatz dazu war das Produkt der Zymogenzellen reich an Proteinen und enthielt PAS-positives Material.Die Feinstruktur dieser Zellen war korreliert mit diesen histochemischen Befunden. Glutaraldehyd erhielt die Feinstruktur der Sekretgranula besser und fixierte mehr Komponenten als Osmium. Die Granula der mukösen Zellen enthielten elektronendichte und -durchsichtige Komponenten; diese Zellen färbten sich mit PAS und Alcyan. Nach Osmium-Fixierung war die elektronendichte Komponente abwesend und die Zellen waren hauptsächlich alcyanophil. Auch in den Zymogenzellen vermochte Osmium nicht, die elektronendichte Komponente zu erhalten. Beobachtungen an alternierenden dicken und dünnen Schnitten zeigten eine Zelle mit Körnern ähnlich den Granulen von Maus Paneth-Zellen. Das dichte Zentrum dieser Granula war osmiophil, der hellere Halo alcyanophil.Diese Resultate lassen uns schließen, daß die elektronen-durchsichtige filamentöse Komponente ein alcyanophiles Mukopolysaccharid ist; das dichte, zentrale Material ist wahrscheinlich PAS-positiv.

An Examination of Polymorphism in the Hydroid Hydractinia Echinata

Several aspects of polymorphism in Hydractinia echinata (Fleming) have been examined. Polymorphism in Hydractinia appears to be the result of inhibition of the normal developmental sequence and not the result of direct genetic control. The inhibition is mediated by both intrinsic and extrinsic factors. The former are responsible for the formation of reproductive zooids in the colony, but this process may be accelerated by extrinsic factors (crowding, increased pCO 2 ). It is suggested that intrinsic factors consist of an inducer or growth stimulator produced in the hypostomal region of the polyps and an inhibitor of growth and differentiation produced by dividing cells beneath the hypostome. The ratio of these two factors throughout the colony ultimately determines the polyp type which will be formed.

An electron microscopic and radioautographic study of hypostomal regeneration inHydra viridis

SummaryThe gastrodermal secretory cells inHydra viridis are limited to specific regions in the body column. There are two types of mucous cells present, and they are limited to the hypostome. The zymogen cells are absent from the hypostome, but they extend along the body column from the tentacles to the peduncle. Transection beneath the tentacles produces a proximal portion of the hydra devoid of mucous cells. This piece regenerates new tentacles and a normal hypostome, filled with mucous cells, within four days.The following events were observed during regeneration. The zymogen cells formed an aggregate within twenty-four hours in the region of the presumptive hypostome. These cells organized and formed lobes of zymogen cells that were positioned similarly to the arrangement of mucous cells in the normal animal. Sparsely distributed small basophilic cells were also present in the reforming hypostome. Using corresponding thick and thin sections we identified the cells incorporating radiosulfate: 1) The zymogen cells in the distal aggregate. 2) Small basophilic cells, some filled with free ribosomes, and others with a well-developed E. R. 3) Secretory cells containing both mucous and serous granules. 4) Secretory cells with granules similar to the granules in mouse Paneth cells.The fate of the secretory granules in the zymogen cells in the distal aggregate is unknown. Some are autolysed within the cell, and others are extruded. However, some observations suggest that there may be a direct transformation of some of the serous granules to mucous granules. The E. M. observations, the radiosulfate incorporation data, and the migrations of cells to the wound site, suggest that both the zymogen cells and basophilic cells transform to mucous cells. Identification of the early stages of mucous synthesis in these basophilic cells enabled us to study the sequence of mucous granule maturation of both the hypostomal mucous cells.The two most significant questions which we feel remain unaswered are: 1) What are the ultrastructural events during the zymogen cell transformation to a mucous cell ? 2) What is the origin of the small gastrodermal basophilic cells ?ZusammenfassungDie gastrodermalen sekretorischen Zellen von Hydra viridis kommen nur in spezifischen Regionen der Körpersäule vor. Es gibt zwei Typen muköser Zellen, und diese findet man ausschließlich im Hypostom. Zymogene Zellen gibt es nicht im Hypostom, aber erstrecken sich längs der Körperachse von den Tentakeln zum Pedunkulus. Sektion unterhalb der Tentakel produziert eine proximale Region von Hydra ohne muköse Zellen. Dieses Stück regeneriert neue Tentakel und ein normales Hypostom mit mukösen Zellen innerhalb vier Tagen.Die folgenden Vorgänge wurden beobachtet während der Regeneration. Die Zymogenzellen bildeten ein Aggregat in der Gegend des präsumptiven Hypostoms innerhalb 24 Std. Diese Zellen bildeten Lappen von Zymogenzellen in ähnlicher Anordnung wie die mukösen Zellen im Normaltier. Ebenfalls vorhanden im neu sich bildenden Hypostom waren locker verteilte, kleine basophile Zellen. Durch Verwendung alternierender dicker und dünner Schnitte identifizierten wir die Zellen, die radioaktives Sulfat einbauten: 1. Zymogenzellen im distalen Aggregat. 2. Kleine basophile Zellen, einige mit freien Ribosomen angefüllt, andere mit gut entwickelten endoplasmatischen Retikulum. 3. Sekretorische Zellen mit mukösen und serösen Granula. 4. Sekretorische Zellen mit Granula, die ähnlich aussehen wie die Granula von Maus Paneth-Zellen. Das Schicksal der sekretorischen Granula in den Zymogenzellen des distalen Aggregates ist unbekannt. Einige werden innerhalb der Zellen autolysiert, andere ausgestoßen. Es scheint aber, daß einige der seriösen Körner sich direkt in muköse umwandeln. Die elektronenoptischen Bilder, die Ergebnisse des Sulfat-Einbaus, und die Wanderung von Zellen zur Wunde weisen darauf hin, daß sowohl Zymogenzellen, als auch basophile Zellen sich in muköse Zellen verwandeln. Identifikationen der Frühstadien von Mukus-Synthese in diesen basophilen Zellen erlaubte uns, die Sequenz der Reifung der Mukus-Granula beider hypostomalen Mukuszellen zu studieren.Die zwei wichtigsten, noch unbeantworteten Fragen verbleiben: 1. Was für feinstrukturelle Veränderungen finden statt während der Transformation von Zymogenzellen in muköse Zellen ? 2. Woher stammen die kleinen gastrodermalen, basophilen Zellen ?