Coelomic Lining Research Articles

The origin of the endothelial cells: an evo‐devo approach for the invertebrate/vertebrate transition of the circulatory system

Circulatory systems of vertebrate and invertebrate metazoans are very different. Large vessels of invertebrates are constituted of spaces and lacunae located between the basement membranes of endodermal and mesodermal epithelia, and they lack an endothelial lining. Myoepithelial differentation of the coelomic cells covering hemal spaces is a frequent event, and myoepithelial cells often form microvessels in some large invertebrates. There is no phylogenetic theory about the origin of the endothelial cells in vertebrates. We herein propose that endothelial cells originated from a type of specialized blood cells, called amoebocytes, that adhere to the vascular basement membrane. The transition between amoebocytes and endothelium involved the acquisition of an epithelial phenotype. We suggest that immunological cooperation was the earliest function of these protoendothelial cells. Furthermore, their ability to transiently recover the migratory, invasive phenotype of amoebocytes (i.e., the angiogenic phenotype) allowed for vascular growth from the original visceral areas to the well-developed somatic areas of vertebrates (especially the tail, head, and neural tube). We also hypothesize that pericytes and smooth muscle cells derived from myoepithelial cells detached from the coelomic lining. As the origin of blood cells in invertebrates is probably coelomic, our hypothesis relates the origin of all the elements of the circulatory system with the coelomic wall. We have collected from the literature a number of comparative and developmental data supporting our hypothesis, for example the localization of the vascular endothelial growth factor receptor-2 ortholog in hemocytes of Drosophila or the fact that circulating progenitors can differentiate into endothelial cells even in adult vertebrates.

Developmental relation between palatine tonsils and kidney

The evolutionary changes of the immune process in the whole line of representatives of the organisms were followed. The immune organs responsible for processing the antigens evolved from the derivatives of coelomic lining over the gut folds to the immune organs developed in the gill region of the digestive tract. Among them the tonsils have to be considered as a very modern evolutionary immune organ. They develop also as the latest in ontogeny even though their primordium contributes very early to the IgA production. The results of the host-antigen reactions have to be eliminated by the excretory organs, which are in invertebrates represented by body poruses and various types of simple segmentally arranged nephridia which are in contact with the gills. These nephridia have evolved in lower vertebrates in pronephros, further in mesonephros and finally in kidneys with filtrating nephrons. In case of defective elimination, the more voluminous immunocomplexes can form the subendothelial or mesangial deposits.

Ultrastructure of the Circulatory System of the Phoronid Phoronopsis harmeri Pixell, 1912. 2. Main vessels

The ultrastructure of the wall of the main blood vessels of the phoronid Phoronopsis harmeri is described. The walls of the lophophoral and left lateral vessels consist of myoepithelial cells of the coelomic lining (peritoneal cells), a thin basal lamina, and an incomplete endothelial lining. In the head region of the body, the wall of the medial vessel consists of myoepithelial cells of the coelomic lining (peritoneal cells), a basal lamina, and true muscular endothelial cells. The anterior part of the medial vessel functions as the heart. In the anterior part of the body, the medial vessel wall consists of five layers: the external nonmuscular coelothelium, a layer of the extracellular matrix, the internal muscular coelothelium, an internal layer of the extracellular matrix, and an incomplete endothelial lining. The complicated structure of the medial vessel wall may be explained by the superimposition of the lateral mesentery on the ordinary vessel wall.

Ultrastructure of the Blood System in the Phoronid Phoronopsis harmeri Pixell, 1912: 1. Capillaries

Four types of blood capillaries of the phoronid Phoronopsis harmeri are described. These are capillaries of the tentacles, of the body, of the stomach plexus, and of the vasoperitoneal tissue. The wall of capillary consists of cells of the coelomic lining, a layer of extracellular matrix, and separate endothelial cells. Myoepithelial coelomic cells of tentacle capillaries contain cross-striated fibers. In capillaries of the body and the stomach plexus, the myofilaments are smooth. In the cells of the wall of vasoperitoneal tissue capillaries, myofilaments are lacking. The cells of the vessel wall of the tentacles, the body, and the vasoperitoneal tissue bear a single cilium. The cells of capillaries of the stomach plexus lack a cilium. The ultrastructure of erythrocytes and amebocytes is described. In the cytoplasm of erythrocytes, there is a basal body. It is assumed that erythrocytes originated from the ciliary cells of the wall of the blood vessels.

The peristomial membrane of regular sea‐urchins: Functional morphology of the epidermis and coelomic lining inParacentrotus lividus(Lamarck)

Abstract The functional organization of both the epidermis and the coelomic lining of the peristomial membrane in the regular sea‐urchin Paracentrotus lividus is described and correlated with the extreme deformability of the peristomial membrane and with the ≪mutability≫ of its related dermis. The epidermis consists of a monolayer of columnar supporting cells mixed with ciliated sensory cells, the wide interspaces between these epithelial cells being occupied apically by a variety of cell types and basally by a conspicuous basiepithelial nerve plexus. The coelomic lining is a peripharyngeal coelomic epithelium consisting of apical monociliated peritoneocytes and subapical myocytes, and has an associated basiepithelial nerve plexus. Although the basic morphology of these epithelia is similar to that of corresponding tissues in other echinoderms, they both have significant structural adaptations and display the histological pattern of typical transitional epithelia, which enables them to match the wide rang...

Epithelial Origin of Mesodermal Structures in Arrowworms (Phylum Chaetognatha)

SYNOPSIS. The classical theory that coelomates evolved from pseudocoelomate and acoelomate ancestors implies that mesodermally derived tissues such as body wall muscles, somatic reproductive tissues, and connective tissues are homologous throughout the Triploblastica and distinct from the epithelial coelomic lining. Ultrastructural studies reveal the mesoderm of hatchling chaetognaths to consist entirely of an epithelial coelomic lining that includes the longitudinal body wall muscles. Comparisons of hatchling and adult mesodermal organization are used to infer that all adult mesodermally derived tissues and organs form by localized elaboration of the hatchling coelomic lining. These findings provide no basis for inferring that any adult mesodermally derived structures evolved independently of or prior to the coelomic epithelium. The presence in hatchlings of myoepithelial body wall muscles supports the theory that coelomates evolved by elaboration of an epithelial coelomic lining from the myoepithelial gastrodermis of diploblastic ancestors.

Ultrastructure of hatchling chaetognaths (Ferosagitta hispida): Epithelial arrangement of the mesoderm and its phylogenetic implications.

The trunk and tail mesoderm of hatchling chaetognaths consists of a simple myoepithelium containing four stereotypically arranged cell types, each matching in position a specific adult tissue. The trunk mesoderm includes lateral cells, longitudinal muscle cells, dorsal and ventral medial cells, and peri-intestinal cells. These correspond, respectively, to the lateral fields, longitudinal body wall muscles, dorsal and ventral perimysial cells, and periintestinal muscles of adults. Because the developing intestine does not extend into the tail, tail cells equivalent in position to peri-intestinal cells in the trunk are designated mesenterial cells. Numerous small spaces situated among the apices of hatchling mesodermal cells have the same position relative to surrounding cells as both the coelomic cavities of early embryos and the adult body cavities. We infer that these spaces in hatchlings expand and coalesce to form the definitive adult body cavities, and that these spaces and the adult body cavities derive from the embryonic coeloms. Because hatchlings lack mesenchymal mesoderm, we infer that all adult mesodermal tissues develop by elaboration of the coelomic lining of hatchlings. Because hatchlings lack cells corresponding to the squamous peritoneocytes overlying the body wall muscles of adults, we conclude that peritoneocytes are specialized adult cells that are not equivalent to cells of the embryonic coelomic lining. Finally, hatchlings contain a complete trunk/tail septum. This observation contradicts reports that this septum forms several days after hatching. It also weakens arguments that chaetognaths are bimeric rather than trimeric. © 1994 Wiley-Liss, Inc.

Erratum: Organization of the adluminal and retractor cells in the coelomic lining from the tube foot of a phanerozonian starfish, Luidia foliolata

Erratum: Organization of the adluminal and retractor cells in the coelomic lining from the tube foot of a phanerozonian starfish, <i>Luidia foliolata</i>

Organization of the adluminal and retractor cells in the coelomic lining from the tube foot of a phanerozonian starfish, Luidia foliolata

The coelomic lining in the tube feet of the phanerozonian starfish Luidia foliolata is a complex pseudostratified myoepithelium consisting largely of flagellated adluminal cells and myofilament-bearing retractor cells. The adluminal cells, joined by zonular intermediate and septate junctions, line the water–vascular canal. Basal processes of the adluminal cells penetrate the underlying layers of retractor cells to terminate as bulbous pedicels at the myoepithelial basal lamina. The longitudinally oriented retractor cells, linked by macular and fascial intermediate junctions, also direct processes obliquely to the basal lamina for anchorage. In keeping with its myoepithelial classification, the coelomic lining of the tube foot is devoid of connective tissue. The organization of the coelomic lining in this primitive species has not substantively improved our understanding of the events of excitation–contraction coupling. Both the coelomic lining and the podial connective tissue lack nerves, and there is no ultrastructural evidence of communicating (gap) junctions between the retractor cells. The sarcoplasmic reticulum of a retractor cell is a plexiform network of agranular cisternae lodged between the contractile apparatus and the sarcolemma. Peripheral couplings between the sarcoplasmic reticulum and the sarcolemma can be identified in ultrathin sections by the presence of electron-dense plaques in the sarcoplasmic gaps between the apposed membranes. In freeze-fracture replicas, the placement of these structural couplings is correlated with aggregations of large intramembranous particles over regions of cisternal confluence. Conceptual problems in understanding excitation–contraction coupling in a modern starfish are compounded in this primitive representative by the presence of many more layers of retractor cells and by a burrowing life-style which requires the tube feet to respond in a rapid, highly coordinated manner.

Ultrastructural Observations on the Dwarf Male of Bonellia viridis (Echiura)

AbstractThe organization of the dwarf male of Bonellia viridis was studied by electron microscopy. The epidermis is formed by two types of epithelial cells: the majority are multiciliated cells; highly vacuolated, non‐ciliated cells are less abundant. The body wall musculature consists of an outer circular, a diagonal, and a longitudinal layer. As a unique feature in coelomate spiralians it was found that the perikarya of all muscle cells are located internal to the entire contractile muscular layer. The muscles are solitary myocytes embedded in extracellular matrix. Masses of secretory and indifferent cells occur inside the muscles. Two types of secretory cells were distinguished. Both of them apparently undergo holocrine secretion. A complete lining of thin peritoneal cells delimits the body cavity. Also, the gut and sperm sac have a complete peritoneal lining. The coelomic lining of the gut is a single‐layered myoepithelium, that of the sperm sac a pseudo‐stratified myoepithelium. The vas deferens was seen to be ciliated. The entrance of the sperm sac is formed by a ciliated funnel that leads into the reservoir by means of a thin, ciliated canal. The existence of repeated transverse nerves and of four longitudinal nerve cords is described for the first time.

Ultrastructure of coelomic lining in echinoderm podia: significance for concepts in the evolution of muscle and peritoneal cells

Ultrastructural data are presented on the histological organization of coelomic lining in the podia of ten species of the five major groups of extant echinoderms. Further evidence of the incorporation of podial retractor muscle cells (myocytes) into a monociliated myoepithelial coelomic lining is provided. In the podia of the crinoid Nemaster rubinginosa and the ophiuroid Ophiophragmus wurdemani as well as in the feeding tentacles of the holothurian Leptosynapta tenuis, coelomic linings are organized as simple myoepithelia consisting of non-contractile peritoneal cells (peritoneocytes) and myocytes. Coelomic linings in the holothurian Thyonella gemmata, the echinoids Eucidaris cf. tribuloides and Lytechinus variegatus, and the asteroids Asterias forbesi and Astropecten sp. are pseudostratified or bipartite pseudostratified myoepithelia consisting of subapical myocytes and apically situated peritoneocytes. The ophiuroid podia of Ophioderma brevispinum and Ophiothrix angulata exhibit transitions from simple myoepithelia to partially pseudostratified epithelia. Intermediate forms between the extremes in myoepithelial organization also occur in the podial lining of single species (e.g. Eucidaris cf. tribuloides). These data supplement recent ultrastructural studies on the podial lining of echinoderms and, in conjunction with published ultrastructural data on the myoepithelial organization of other coelomic linings in echinoderms and in other coelomates, suggest myoepithelial organization of the coelomic lining is a plesiomorph feature in Bilateria.

An autoradiographic investigation of the proliferative activity of pyloric caeca and gondas of Asterias rubens.

The proliferative activity of the pyloric caeca of Asterias rubens was investigated. Autoradiographic experiments using intracoelomically injected (methyl-3 H)-thymidine were performed throughout the year and incorporation into pyloric caeca and into gonads was studied. Tritiated thymidine was found to be incorporated mainly in the coelomic lining of both organs. Cell divisions in the coelomic lining may be necessary for the growth of these organs, for the production of coelomocytes or, in the case of the pyloric caeca, for growth of the digestive epithelium. Proliferative activity of the digestive epithelium of the pyloric caeca was only observed in the median duct. It is hypothesized that new cells, arising from mitosis, grow from the median duct to the side lobes and differentiate into storage cells, for example. The existence of a mitosis-inducing or mitosis-stimulating substance is discussed. In the ovaries follicle cells were found to incorporate (methyl-3 H)-thymidine; in the testis, proliferation of the germinal epithelium occurred simultaneously in all spermatogenic columns. First, the spermatogonia and then later the spermatocytes became labeled. Absorption of substances from the coelomic fluid is discussed.

Primordial germ cells and early stages of oogenesis in Ophryotrocha puerilis (Polychaeta, Dorvillediae)

Each setigerous segment of the protandric polychaete Ophryotrocha puerilis contains two primordial germ cells. A ventral furrow in the gut wall together with the peritoneal lining of the gut forms a genital blood vessel. The gonocytes are located within the peritoneum of this genital blood vessel. At sexual maturity the gonocytes undergo a proliferation cycle, the first division of which gives rise to a cell which is extruded into a forming outpocketing of the coelomic lining. The stem cell remains within the peritoneum. Inside the forming gonad the detached cell goes through a series of four mitotic divisions. The resulting 16 cells are interconnected by cytoplasmic bridges. These bridges are arranged in a very regular pattern which allows the mitotic cycles to be followed. While remaining still within the gonad the 16 cells begin to synthesize yolk and to take up exogenous yolk precursors. At this stage a differentiation into oocytes and nurse cells becomes visible. The oocytes deposit yolk platelets of the definitive size whereas the polyploid nurse cells produce only small yolk bodies that are passed to the adjacent oocytes. In a later stage the cell bridges between adjacent nurse cells are cut and pairs of one oocyte and one nurse cell are released to the coelomic cavity during breakdown of the gonadal sac. Oocyte-nurse cell-complexes then freely float in the coelomic fluid. The proliferation of gonadal cells is well synchronized within one segment. In anterior segments, however, gonadal proliferation usually begins earlier than in posterior segments but smaller oocytes in posterior segments catch up within a few days. Finally a batch of oocytes is produced in which all the oocytes are of the same size (120 μm). The origin of the primordial germ cells remains unknown.

Ultrastructure of the coelomic lining in the podium of the starfish Stylasterias forreri.

Ultrastructural examination of the podium of the asteroid echinoderm Stylasterias forreri reveals that cells of the coelomic epithelium and cells of the retractor muscle are, in fact, components of a single epithelium. The basal lamina of this unified epithelium adjoins the connective tissue layer of the podium. The principal epithelial cells in the coelomic lining are the flagellated adluminal cells and the myofilament-bearing retractor cells. Adluminal cells interdigitate extensively with each other and form zonular intermediate and septate junctions at their apicolateral surfaces. The adluminal cells emit processes which extend between the underlying retractor cells and terminate on the basal lamina of the epithelium. Retractor cells exhibit unregistered arrays of thick and thin myofilaments. The periphery of the retractor cell is characteristically thrown into keel-like folds which interdigitate with the processes of neighboring cells. Specialized intermediate junctions bind the retractor cells to each other and anchor the retractor cells to the basal lamina of the epithelium. The retractor cells are not surrounded by external laminae or connective tissue envelopes. It is concluded that the coelomic lining in the podium of S. forreri is a bipartite epithelium and that the retractor cells of the podium are myoepithelial in nature. There are no detectable communicating (gap) junctions between the epithelial cells of the coelomic lining.

A light- and electron-microscopic study of primordial germ cells in the early mouse embryo

ABSTRACT The primordial germ cells (PGC’s) of the early mouse embryo have been identified in semi-thin and ultra-thin plastic sections on the combined bases of location and a distinctive set of morphological features. These cells originate extra-gonadally and their path of migration during the stages investigated agrees with results of previous histological and experimental studies. At 8–9 days of development, PGC’s are observed among the endoderm cells of the mid- and hindgut and the yolk stalk; at 10–11 days, PGC’s are seen in the dorsal mesentery, the dorsal coelomic lining, and in the rudimentary genital ridge; by 13 days, the gonad is abundantly populated with germ cells. During the migratory period the PGC’s appear as small (10–12 pm diameter), oval, basophilic cells which have a large nucleo-cytoplasmic ratio. These cells possess numerous free ribosomes and polysomes, a filamentous ground cytoplasm, and few profiles of endoplasmic reticulum. Mitochondria display small oval and thread-like profiles. A Golgi complex is not prominent in the PGC’s. The densely granular and fibrillar nucleus is often oval in outline but sometimes shows irregular contours. A large nucleolus is present. Although located among groups of cells that exhibit extensive cell junctions, the PGC’s have not been found to share such junctions with neighboring cells. Furthermore, the PGC’s possess small cytoplasmic processes that contain numerous microfilaments. These observations are interpreted as morphological correlates to the migratory activity of these cells. In the early PGC’s located in the gut, regions of the endoplasmic reticulum where membranes are closely apposed, perhaps fused, often present the appearance of annulate cisternae. In addition, compact aggregates of granulofibrillar material are found in the cytoplasm of these same cells. Neither of these structures is detected in the mesenteric PGC’s, the gonadal germ cells, or any other cell of the embryo at the stages studied. It is suggested that these two cytoplasmic elements may be related to further differentiation of the germ cells. The germ cells of the developing gonad are large, oval cells which possess a nucleus of very round contour. Both the nucleus and the cytoplasm retain a densely granular appearance. The expanded cytoplasm contains a large Golgi complex but still few profiles of endoplasmic reticulum. Free ribosomes and polysomes are abundant. The germ cells share extensive gap junctions with one another and with adjacent stromal cells.

Attachment and metamorphosis of the cheilo-ctenostome bryozoan bugula neritina (linné).

The structure, attachment and subsequent metamorphosis of larvae of the marine bryozoan Bugula neritina were studied by light and electron microscopy. Two points of larval anatomy are of special significance to proper interpretation of the metamorphosis: 1 Two cytologically similar blastemal tissues, each laden with free ribosomes, occur as parts of the apical organ complex. The upper blastema directly contacts the larval surface, forming the non-ciliated rows of the apical organ. The lower blastema is internal and is oral to and contiguous with the upper blastema. 2 The epidermal tissues of the larva are joined in the following sequence, beginning at the aboral pole: a. apical organ complex; b. apical-connecting cell; c. infolded pallial sinus epithelium; d. vesicular-connecting cell; e. aboral vesicular epithelium; f. corona; g. oral vesicular epithelium; and i., j., and k. internal sac neck, wall and roof regions. The initial stages of metamorphosis involve a complex sequence of morphogenetic movements, including: 1 eversion of the internal sac, permanently attaching the larva to the substrate; 2 inrolling of the aboral vesicular epithelium, corona, oral vesicular and ciliated epithelia, and neck region of the internal sac into the larval interior; concomitantly the pallial sinus epithelium evaginates; 3 loss of connection between the invaginated tissues and the surface; 4 fusion of the pallial sinus epithelium with the wall region of the internal sac, maintaining the integrity of the body surface; 5 retraction of the apical organ complex and invagination of the pallial sinus epithelium with the simultaneous elevation of the internal sac wall region to the aboral pole. At the conclusion of these events the preancestrular surface is covered by the wall and roof regions of the internal sac. Cells of the wall region form the epidermis of the body wall except for the attachment disc and secrete a cuticular exoskeleton that is secondarily calcified; the attachment disc is formed by the roof region of the internal sac. Internally, the ectodermal upper blastema differentiates into the lophophore and digestive tract of the ancestrular polypide, while the lower blastema forms the lining of the lophophoral coelom and the splanchnic (but not the somatic) lining of the visceral coelom. The visceral somatic peritoneum is formed from cells that may originate from the mesodermally derived pigmented cells of the larva to which they are similar in pigmentation and cytology. Such a composite derivation of a coelomic lining has not been described previously.

Formula omitted] studies on [formula omitted] (Brandt) coelomocytes, with remarks on revolving cysts

Coelomic fluid from several Patiria miniata , pooled in test tubes and maintained at 4°C, will form sheet-like cell clots in about 24 hours. The clots probably begin as monolayer syncytia on the lower walls of the test tubes and later contract and thicken. The clots are reversible and in 48–96 hours, cells making up the clots begin to scatter away from each other. One-fourth of the hanging-drop preparations made from the clots and kept at 4°C stayed in good condition for three weeks. Only one type of coelomocyte occurs regularly in Patiria and probably in most other asteroids as well. It is a phagocytic amebocyte. In normal starfish the coelomocytes have bladder-like ectoplasmic extensions; under conditions which induce clotting the coelomocytes become filiform amebocytes; and finally, when present on a clot edge or when applied to a surface such as a coverslip, they develop broad, ruffled or crinkled pseudopodia. Carmine-filled coelomocytes were withdrawn from Patiria inoculated 74 models capable of yielding basic information applicable to many echinoderm groups. Origin and function of the revolving cysts remain obscure. We know some cysts contain phagocytic elements which may be coelomocytes, and the cysts must gain their propellent flagella from one of the flagellated tissues adjacent to the coelomic cavity, perhaps by phagocytes partially ingesting flagellated cells. In vitro , coelomocytes do engulf flagella-bearing cells and the flagella may remain beating for at least some time after the parent cell is incorporated into the phagocyte. Single flagella with the attendant small bits of matter must also be phagocytized as foreign bodies, for they are not normal to the coelomic fluid. If formed by coelomocytes in conjunction with many separate flagellated cells, the cyst-like nature of the completely formed bodies might be induced by the same forces which form the zooids of Volvox and related flagellated colonial protozoans. On the other hand, some uncolored cysts occured in coelomic fluid where nearly all the single coelomocytes had particles of carmine in the cytoplasm, casting doubt on the invariable presence of free coelomocytes in the cysts 1. The occurence of cysts in coelomic fluid just removed from apparently normal Patiria deserves comment. In the process of taking starfish from their environment some tube feet are usually torn and other wounds may occur even when great care is taken. These breaks might release bits of the flagellated epithelium into the coelom where by some process they form into the revolving cysts. It is just as likely that wounds normally acquired would accomplish the same end. Judging from the long life of revolving cysts in hanging drops, one might assume they would last as long or longer in the coelomic cavity since days earlier with carmine, indicating inert foreign material may remain in this species of starfish at least 74 days. Spherical multinucleate macrophages measuring up to 275 microns in diameter occured in the coelomic fluid of carmine-inoculated starfish. They also were present in fresh clots prepared from coelomic fluid of normal starfish. Flagellated cells, minute flagellated corpuscles, and variously-shaped crystal-like bodies were found in coelomic fluid withdrawn from normal starfish. These were probably present adventitiously and were aspirated into the needle from coelomic lining or pyloric caeca. Spherical revolving cysts were found in the coelomic fluid of normal starfish. They sometimes contained coelomocytes as well as flagellated cells but we could not ascertain if they always included coelomocytes. They were not ciliates or abnormally-developing larvae. Sperm were not involved in their formation. At least part of their origin must be in cells or groups of cells from the flagellated coelomic lining. They maintained their integrity and continued revolving up to 73 days in hanging drops, even after all other cells in the drop were lysed or granular, and even in the presence of numerous apparently nonpathogenic bacteria.

An experimental investigation of pattern development in lethal spotting and belted mouse embryos

This investigation is a study of the site of gene action in producing coat spotting in two mutant mice, belted and lethal spotting. An analysis of the pigment-forming potency of skin from the two mutants was made through the use of the technique of intracoelomic grafting. Transplants from potentially pigmented regions of 12 1 2 - and 13 1 2 - day bt bt embryos produced grafts which showed extensive pigment development in the hair and dermis of the graft and in the coelomic lining of the host. Skin isolates taken from the potentially spotted area yielded grafts possessing pigment-free hair. The dermis and coelomic lining were populated by melanocytes in essentially the same number as found in the control group. Embyros used in the lethal spotting series resulted from matings of ls + mice, since ls ls mice usually die between 2 and 3 weeks of age with megacolon. Ten embryos from a group of 45 used in this series produced grafts in which at least some pigment-free hair was found. Those grafts which produced only white hair also failed to develop pigment in the dermis and in the coelomic lining. These embryos were considered to be of the ls ls genotype, and their number corresponds well to the predicted percentage. Transplants of skin from 35 embryos of this series produced grafts which contained pigmented hair only. These embryos are considered to represent ls + and +/+ genotypes. The results of the grafting series clearly indicate that melanoblasts migrate freely throughout all areas of the skin of belted mice and that the genetic block is at the level of the hair follicle. The ls gene appears to act at a different site. An examination of the pigment content of the Harderian gland, the retina and choroid of the eye, the leg musculature, ankle skin, and the membranous labyrinth of the ear was undertaken in normal and spotting mice in an attempt to clarify further the site of gene action. These results substantiate the view that bt acts specifically on the hair follicle. Pigmentation in other tissues of bt bt mice was at the normal level. In contrast to the action of bt, ls reduces the level of pigmentation in the mouse as a whole. The melanocytes in all the tissues studied in ls ls mice, with the exception of those of the retina, were greatly reduced in number or entirely absent. The occurrence of megacolon, the variable nature of the spotting, and the evidence from the descriptive and experimental studies support the hypothesis that the neural crest is abnormal in ls ls mice.

The Occurrence and Morphogenesis of Melanocytes in the Connective Tissues of the PET/MCV Mouse Strain1

ABSTRACT Mice of the PET/MCV strain are characterized by having melanocytes scattered in the connective tissues of most of the body. The ‘internal’ pigment cells generally first appear in fetuses as spherical melanocytes which later produce branches and lay down melanin. By contrast, pigment cells within the skin branch prior to the onset of melanogenesis. Spherical melanocytes of the serosae of the PET/MCV mouse proliferate within the coelomic lining of host chick embryos. However, the degree of proliferation and spread is considerably less than that of melanoblasts under similar conditions. Under the conditions of experimental grafting, both spherical ‘internal’ melanocytes and melanoblasts from the dermis showed a similar pattern of differentiation. The time at which donor melanocytes produce branches within the host chick embryos is in accord with their location and the specific time expected of the host and not of the donor. The data suggest that the branching of a pigment cell is evoked by a morphogenic substance which is produced by certain tissues of the body, particularly by the epidermis. The effect, at least with regard to the mouse and chick, is not species specific, since grafted mouse pigment cells react in keeping with the chick environment.

A Microsporidian Parasite of Anopheles gambiae in Liberia

In connection with a survey of wild Anopholes gambiae for natural infections of malaria, conducted at the Liberian Institute, Harbel, Liberia, West Africa, one of us made some thousands of midgut dissections during the period from June 1955 until April 1956 (Fox, 1957). From time to time a wild mosquito was found to be infected with a microsporidian parasite. The rate of this natural infection was not recorded but was exceedingly low. Colonies of A. gambiae had been maintained at the Institute continuously for the previous 4 years without difficulty. During the summer of 1956 an outbreak of nosematosis occurred, quickly reaching nearly epizootic proportions and spreading to the colony of A. melas. Because infected mosquitoes are not suitable for use in malaria study, the colonies had to be destroyed. A few infected strains were kept for further study of the parasite. The purpose of the present paper is to record the pathology of the microsporidian infection, identified as Nosema stegomyiae, in adult A. gambiae and to present certain observations on nosematosis in mosquitoes. Nosema stegomyiae (Marchoux, Salimbeni and Simond), 1903, has reniform or comma-form spores 4-7 microns long and 2-3 microns wide. It invades the gut, muscles, air sacs, coelomic lining, neural ganglia and ovaries of adult mosquitoes, and was originally observed as a parasite of Aedes. N. stegomyiae Lutz and Splendore, 1908, described as a new species, and N. anophelis Kudo, 1924, are synonyms. Two other microsporidia parasitize mosquitoes: Thelohania grassi Nicholson, 1921, had earlier been recorded but not described by Grassi (1901), Ross (1906), Stephens and Christophers (1908); Plistophora culicis Weiser, 1947, with spores 5 microns long and plasmodia 15 microns in diameter, has been found and fully detailed by Garnham (1956) and Canning (1957). Both these species sometimes invade the larvae. One of us (JW) is preparing a monographic revision of the microsporidia.