kette and blown fuse interact genetically during the second fusion step of myogenesis in Drosophila

Drosophila myoblast fusion proceeds in two steps. The first one gives rise to small syncytia, the muscle precursor cells, which then recruit further fusion competent myoblasts to reach the final muscle size. We have identified Kette as an essential component for myoblast fusion. In kette mutants, founder cells and fusion-competent myoblasts are determined correctly and overcome the very first fusion. But then, at the precursor cell stage, fusion is interrupted. At the ultrastructural level, fusion is characterised by cell-cell recognition, alignment, formation of prefusion complexes, electron dense plaques and membrane breakdown. In kette mutants, electron dense plaques of aberrant length accumulate and fusion is interrupted owing to a complete failure of membrane breakdown. Furthermore, we show that kette interacts genetically with blown fuse (blow) which is known to be required to proceed from prefusion complexes to the formation of the electron dense plaques. Interestingly, a surplus of Kette can replace Blow function during myogenesis. We propose a model in which Dumbfounded/Sticks and stones-dependent cell adhesion is mediated over Rolling Pebbles, Myoblast city, Crk, Blown fuse and Kette, and thus induces membrane fusion.     

Ultrastructural studies on the formation of yolk granules in the statoblast of a fresh-water bryozoan,Pectinatella gelatinosa

The formation of protein-carbohydrate yolk in the statoblast of a fresh-water bryozoan, Pectinatella gelatinosa, was studied by electron microscopy. Two types (I and II) of yolk cells were distinguished. The type I yolk cells are mononucleate and comprise a large majority of the yolk cells. The type II yolk cells are small in number; they become multinucleate by fusion of cells at an early stage of vitellogenesis. In both types of yolk cells, electron-dense granules (dense bodies) are formed in Golgi or condensing vacuoles, which are then called yolk granules. For the formation of yolk granules, the following processes are considered: 1. Yolk protein is synthesized in the rough-surfaced endoplasmic reticulum (RER) of the yolk cells. 2. The synthesized protein condenses in the cisternal space of the RER and is packaged into small oval swellings, which are then released from the RER as small vesicles (Golgi vesicles, 300-600 A in diameter). 3. The small vesicles fuse with one another to form condensing vacuoles, or with pre-existing growing yolk granules. 4. In the matrix of the condensing vacuoles or growing yolk granules, electron-dense fibers are fabricated and then arranged in a paracrystalline pattern to form the dense body. 5. After the dense body reaches its full size, excess membrane is removed and eventually the yolk granules come to mature. Toward the end of vitellogenesis of the yolk cells, the cytoplasmic organelles are ingested by autophagosomes derived from multivesicular bodies and disappear.     

Somatic cell-like features of cloned mouse embryos prepared with cultured myoblast nuclei

Cloning by somatic cell nuclear transfer requires silencing of the donor cell gene expression program and the initiation of the embryonic gene expression program (nuclear reprogramming). Failure to silence the donor cell program could lead to altered embryonic phenotypes. Cloned mouse embryos produced using myoblast nuclei fail to thrive in standard embryo culture media but flourish in somatic cell culture media favored by the donor myoblasts themselves, forming blastocysts at a significant rate, with robust morphologies, high total cell number, and a normal allocation of cells to the inner cell mass in most embryos. Myoblast cloned embryos continue expressing the GLUT4 glucose transporter, which is typically expressed in muscle but not in preimplantation stage embryos. Myoblast clones also exhibit precocious enrichment of GLUT1 at the cell surface. Both myoblast and cumulus cell cloned embryos exhibit enhanced rates of glucose uptake. These observations indicate that silencing of the donor cell genome during cloning either is incomplete or occurs progressively over the course of preimplantation development. As a result, cloned embryos initially exhibit many somatic cell-like characteristics. Tetraploid constructs, which possess a transplanted somatic cell genome plus the oocyte-derived chromosomes, exhibit a more embryonic-like pattern of gene expression and culture preference. We conclude that preimplantation stage cloned embryos have profoundly altered characteristics that are donor cell type specific and that exposure of cloned embryos to standard embryo culture conditions may lead to disruptions in basic homeostasis and inhibition of a range of essential processes including further nuclear reprogramming, contributing to cloned embryo demise.     

In Vitro Orientation of Fibroblasts and Myoblasts on Aligned Collagen Film

A collagen film in which the collagen fibers were aligned was prepared and characterized by scanning electron microscopy. Cell orientation on this film was studied in vitro using human fibroblasts and chick embryo myoblasts. Ninety-four percent of innoculated fibroblasts were aligned along the direction of the collagen fiber. The cell orientation was disturbed when cytochalasin B or colchicine was added to the culture medium. The myoblasts showed a similar alignment along the direction of collagen fiber. The scanning electron microscopic observation revealed that none of the cytoplasmic extensions had consistent relationships to the direction of collagen fiber. Myoblast fusion was accelerated on the aligned membrane as compared to a randomly oriented film, suggesting some role of contact guidance in muscle cell differentiation.     

Antibodies to the Golgi complex and the rough endoplasmic reticulum

Rabbits were immunized with membrane fractions from either the Golgi complex or the rough endoplasmic reticulum (RER) by injection into the popliteal lymph nodes. The antisera were then tested by indirect immunofluorescence on tissue culture cells or frozen, thin sections of tissue. There were may unwanted antibodies to cell components other than the RER or the Golgi complex, and these were removed by suitable absorption steps. These steps were carried out until the pattern of fluorescent labeling was that expected for the Golgi complex or RER. Electron microscopic studies, using immunoperoxidase labeling of normal rat kidney (NRK) cells, showed that the anti-Golgi antibodies labeled the stacks of flattened cisternae that comprise the central feature of the Golgi complex, many of the smooth vesicles around the stacks, and a few coated vesicles. These antibodies were directed, almost entirely, against a single polypeptide with an apparent molecular weight of 135,000. The endoplasmic reticulum (ER) in NRK cells is an extensive, reticular network that pervades the entire cell cytoplasm and includes the nuclear membrane. The anit-RER antibodies labeled this structure alone at the light and electron microscopic levels. They were largely directed against four polypeptides with apparent molecular weights of 29,000, 58,000, 66,000, and 91,000. Some examples are presented, using immunofluorescence microscopy, where these antibodies have been used to study the Golgi complex and RER under a variety of physiological and experimental condition . For biochemical studies, these antibodies should prove useful in identifying the origin of isolated membranes, particularly those from organelles such as the Golgi complex, which tend to lose their characteristic morphology during isolation.     

Herpes simplex virus 1 envelopment follows two diverse pathways

Herpesvirus envelopment is assumed to follow an uneconomical pathway including primary envelopment at the inner nuclear membrane, de-envelopment at the outer nuclear membrane, and reenvelopment at the trans-Golgi network. In contrast to the hypothesis of de-envelopment by fusion of the primary envelope with the outer nuclear membrane, virions were demonstrated to be transported from the perinuclear space to rough endoplasmic reticulum (RER) cisternae. Here we show by high-resolution microscopy that herpes simplex virus 1 envelopment follows two diverse pathways. First, nuclear envelopment includes budding of capsids at the inner nuclear membrane into the perinuclear space whereby tegument and a thick electron dense envelope are acquired. The substance responsible for the dense envelope is speculated to enable intraluminal transportation of virions via RER into Golgi cisternae. Within Golgi cisternae, virions are packaged into transport vacuoles containing one or several virions. Second, for cytoplasmic envelopment, capsids gain direct access from the nucleus to the cytoplasm via impaired nuclear pores. Cytoplasmic capsids could bud at the outer nuclear membrane, at membranes of RER, Golgi cisternae, and large vacuoles, and at banana-shaped membranous entities that were found to continue into Golgi membranes. Envelopes originating by budding at the outer nuclear membrane and RER membrane also acquire a dense substance. Budding at Golgi stacks, designated wrapping, results in single virions within small vacuoles that contain electron-dense substances between envelope and vacuolar membranes.     

Recent advances in imaging embryonic myoblast fusion in Drosophila

Myoblast fusion in the Drosophila embryos is a complex process that includes changes in cell movement, morphology and behavior over time. The advent of fluorescent proteins (FPs) has made it possible to track and image live cells, to capture the process of myoblast fusion, and to carry out quantitative analysis of myoblasts in real time. By tagging proteins with FPs, it is also possible to monitor the subcellular events that accompany the fusion process. Herein, we discuss the recent progress that has been made in imaging myoblast fusion in Drosophila, reagents that are now available, and microscopy conditions to consider. Using an Actin-FP fusion protein along with a membrane marker to outline the cells, we show the dynamic formation and breakdown of F-actin foci at sites of fusion. We also describe the methods used successfully to show that these foci are primarily if not wholly present in the fusion-competent myoblasts.     

Effect of calcitonin on fibroblasts and collagen formation in rabbits: an ultrastructural and scanning electron microscopic study

Transmission and scanning electron microscopic studies demonstrate the stimulatory effect of synthetic salmon calcitonin on the fine structure of fibroblasts and on collagen formation in cutaneous wounds experimentally induced in rabbits. Long-term administration of calcitonin enhances fibroblast growth and collagen synthesis. The fibroblasts hypertrophy and exhibit a highly developed rough endoplasmic reticulum (RER), several polyribosomes, large nuclei, hypertrophic Golgi complex, and many dense granules and lysosomes. Mitochondria are elongate and ramify; intracellular as well as extracellular synthesis of collagen increases. Fibrils appear tightly packed, in large heaps or spicula, with a characteristic periodicity and striation. Scanning electron micrographs of topography and relationships with collagen fibers and fibrils and cells surface changes demonstrate an extensive network of fine fibrils between collagen fibers, marked ruffling of cell membranes as well as numerous blebs on the cell surface. The latter are significant in collagen formation and egestion.     

Spreading of normal and transformed fibroblasts in dense cultures

Cell spreading in dense cultures of normal mouse embryo fibroblasts and of the two lines of mouse transformed fibroblasts was examined by electron microscopy. The mean number of cell layers in culture and cell population density per unit area of the substrate were detetmined; the mean area of the cell projection on the substratum was found from these data.Normal fibroblasts formed multilayefed sheet in dense culture. The cells in this sheet were well-spread. These cells formed thin lamellae (lamellar cytoplasm) over the surface of other cells and over the intercellular substance. The mean cell area in dense culture was not smaller than that of the cell spread on the substratum in sparse culture.Dense cultures of two transformed lines (M 22 and L) had differing morphologies: cultures of one line (M 22) were multilayered, those of the other line (L) were monolayered. Decreased spreading and almost complete (M 22) or complete (L) absence of lamellar cytoplasm were characteristic of both transformed lines. The mean area of the cell in dense cultures of both lines was several times smaller than that of their normal progenitors.It is concluded that similar reactions leading to the spreading accompanied by the formation of lamellar cytoplasm can be induced by the contact of fibroblast with various surfaces: that of the substratum in sparse culture, that of other cells and of intercellular structures in dense culture. Deficiency of these reactions characteristic for transformed fibroblasts may be responsible for abnormal morphology of their cultures.     

Conversion of dermal fibroblasts to a myogenic lineage is induced by a soluble factor derived from myoblasts

The limb and axial skeletal muscles of mammals originate from somitic dermomyotome, which during early development separates to form two discrete structures, the dermatome and the myotome. The latter cell mass gives rise to the muscle-forming lineage while cells of the dermatome will form the skin dermal fibroblast population of the dorsal regions of the body. It has been generally accepted for some time that myotome-derived myoblasts were the sole source of muscle fibre nuclei, but evidence has recently been presented from several laboratories that fibroblasts can fuse with myoblasts to contribute active nuclei to the resulting myotubes. We report here an investigation into the myogenic capacity of fibroblasts. Confluent monocultures of mouse dermal fibroblasts, muscle fibroblasts, and C2C12 myoblasts each retain their individual phenotype when maintained for periods up to 7 days in culture. We also grew isolated colonies of fibroblasts and myoblasts in an arrangement which allowed free exchange of tissue culture medium between the 2 cell types. We found evidence of the conversion of dermal fibroblasts to a myogenic lineage as measured by the appearance of MyoD-positive cells expressing the muscle-specific intermediate filament desmin. In addition, dermal fibroblast cultures contained multinucleate syncytia positive for MyoD and containing sarcomeric myosin heavy chain. In contrast, muscle-derived fibroblasts showed no evidence of myogenic conversion when maintained in identical culture conditions. We prepared conditioned medium from confluent cultures of C2C12 myoblasts and added this material to confluent monocultures of either dermal or muscle fibroblasts. While muscle fibroblasts showed no phenotypic alterations, cultures of dermal fibroblasts responded to myoblast conditioned medium by converting to a myogenic lineage as judged by expression of MyoD and desmin. We conclude that a proportion of dermal fibroblasts retain a myogenic capacity into stages well beyond their early association with myoblasts in the dermomyotome. © 1996 Wiley-Liss, Inc.     

Modification of mitochondrial metabolism in fibroblasts from mice with a skeletal muscle mutation (muscular dysgenesis). Evidence of embryonic communication between myoblasts and fibroblasts

Muscle development during embryogenesis is a complex process involving many mechanisms. It requires a close communication among the different cellular types of the muscle, especially the fibroblasts and myoblasts. Indeed, any abnormality in one cell type might influence the differentiation of the other. Thus, any disturbance altering the metabolism of the myoblasts might lead to modifications in the fibroblasts. To study this phenomenon, we used the dysgenic mouse (mdg-"muscular dysgenesis") carrying a homozygous recessive lethal mutation expressed only in skeletal muscle cells. First, we found that fibroblasts isolated from such mutant muscle (and not from mutant skin tissue) and grown in culture exhibited an altered metabolism. Secondly, muscle fibroblasts showed a lower capacity for proliferation. We also observed that respiration and ATP synthesis of dysgenic muscle fibroblasts were deficient, while respiratory chain enzymatic activities were normal. Finally, intracellular [Ca2+] levels of dysgenic fibroblasts are 50% of those of normal fibroblasts. These results support the hypothesis that certain characteristics of fibroblasts are determined by the surrounding cellular environment during embryonic organogenesis, and that such modifications are stable when the fibroblasts are isolated in vitro. Since fibroblast differentiation was disrupted permanently, this suggests, in the case of myopathies, that the modified cells, surrounding the muscle tissue, could contribute to the muscle pathology. Synergistic activities of this type should be considered when studying the course of pathologies in different types of muscle diseases.     

EFFECTS OF PUROMYCIN ON THE STRUCTURE OF RAT INTESTINAL EPITHELIAL CELLS DURING FAT ABSORPTION

This report provides information on the morphology of rat intestinal epithelial cells during fat absorption. In addition, the role of protein metabolism in this process has been evaluated by blocking its synthesis with puromycin and studying the fine structure of mucosal cells from rats at various times after fat intubation. The results indicate that SER-derived vesicles, containing fat droplets, migrate from the apical cytoplasm of the absorptive cell and fuse with saccules or vacuoles of the Golgi complex. Arguments are made that the Golgi complex is important in completing chylomicron formation and in providing appropriate enveloping membranes for the chylomicron. Such membranes may be necessary for Golgi vacuoles to fuse with the lateral cell membranes and release chylomicra. Puromycin treatment causes the absorptive cell to accumulate increased quantities of lipid that are devoid of membrane during fat absorption. In addition, puromycin-treated cells contain much less RER and Golgi membranes are strikingly decreased in number. In this paper we discuss the consequences of these abnormalities and suggest that continued protein synthesis by the RER is required in order to generate Golgi membranes. If such membranes are absent the cell's ability to discarge chylomicra is impaired and lipid accumulates.     

Culture of intramural cardiac ganglia of the newborn guinea-pig

Summary The present study describes the ultrastructure of non-neuronal cells and their interrelationships with intracardiac neurones present in cultures dissociated atria and interatrial septum from newborn guinea-pig. When compared with the in situ preparation, most of these features in culture were similar to those observed in situ, but some differences were also apparent. Both mature and immature Schwann cells were observed in culture, and as in situ, the latter were closely associated with intracardiac neurones, whilst the former were more widely separated. The ultrastructure of satellite cells was more variable in culture than in situ: three general types were distinguished on the basis of their 10-nm filament content. This variation could be due to conditions of culture. Interstitial cells were present in culture and closely resembled those described in situ, although there was less space between cultured interstitial cells and their associated cells. Many fibroblasts, some myoblasts and a few mast cells were also found in the culture preparations.     

Immunoelectron microscopical localization of lysosomal beta-galactosidase and its precursor forms in normal and mutant human fibroblasts

Immunoelectron microscopy was performed to study the biosynthesis of lysosomal beta-galactosidase (beta-gal) in normal and mutant human fibroblasts. Using polyclonal and monoclonal antibodies we show in normal cells precursor forms of beta-gal in the rough endoplasmic reticulum (RER) and in the Golgi apparatus throughout the stack of cisternae. In the lysosomes virtually all beta-gal exists as a high molecular weight multimer of mature enzyme. In the autosomal recessive disease GM1-gangliosidosis caused by a beta-gal deficiency and in galactosialidosis, associated with a combined deficiency of lysosomal neuraminidase and beta-gal, precursor forms of the latter enzyme are found in RER, Golgi and some labeling is present at the cell surface. The lysosomes remain unlabeled, indicative for the absence of enzyme molecules in this organelle. In galactosialidosis fibroblasts also no mature beta-gal is found in the lysosomes but in these cells the presence of the monomeric form can be increased by leupeptin (inhibition of proteolysis) whereas addition of a partly purified 32 kDa "protective protein" results in the restoration of high molecular weight beta-gal multimers in the lysosomes.     

Biological and morphological characterization of human neonatal fibroblast cell culture B-HNF-1

In the present study, human neonatal fibroblasts were isolated from a two-month-old human male. The purpose of the present investigation was the analysis of the morphology (light and transmission electron microscopy), karyotype and growth characteristics of the human neonatal fibroblast cell culture B-HNF-1. Moreover, STR typing and mitochondrial DNA amplification and sequencing was also performed. Analysis of chromosomes count showed that B-HNF-1 cell culture is diploid and has normal male karyotype 46, XY, which was stable during cultivation. The transmission electron microscopy demonstrated the ultra-structure of the B-HNF-1 cells; they have typical morphological features of proteosynthesis-active cells. Large number of fibroblasts bearing different shapes and surface characteristics adhered to the substrate with microvilli and filopodia. Our in vitro expanded fibroblasts have a large and irregular nucleus with prevalence of euchromatin. One to three nucleoli are present in each nucleus. The cytoplasm contains a richly developed rough endoplasmic reticulum and prominent Golgi apparatus cisterns. The result of the fragment analysis is a DNA profile defined as multiplex of STR markers and sex determining system. Sequencing analysis of hypervariable segments of control region of mitochondrial DNA showed haplotype that belongs to haplogroup W. In this study, we complexly characterized a new cell culture of human neonatal fibroblasts B-HNF-1 from different points of view. This culture should be used for further biomedical experiments.     

Morphofunctional study of the organization of the peripheral parenchyma of Acoela Oxyposthia praedator

The ultrastructure of undifferentiated cells in the peripheral parenchyma of Oxyposthia praedator was studied, along with the ways of their differentiation. The type I cells (3.5-4.0 microns in diameter) undergo mitotic division, while the type II cells (9 microns in diameter) produce specialized cells of the parenchyma. At the beginning of secretory cell differentiation one cistern of the rough endoplasmic reticulum (RER) is formed by the outer membrane of the nuclear envelope, the formation of other cisternae follows. The Golgi complex is formed simultaneously. The differentiated secretory cells are characterized by the abundance of RER cisternae and Golgi complexes. In the course of differentiation of other cell types RER cisternae are formed by several portions of the nuclear envelope. The Golgi complex appears in cells 12-14 microns long. The differentiation of digestive cells is characterized by autophagy. Autophagosomes are formed by RER cisternae. The consecutive stages of autophagosome formation are described. Using a cytochemical reaction revealing acid phosphatase the process of digestion of the autophagosome content was followed.     

Assembly of the sarcoplasmic reticulum proteins in differentiating rat skeletal muscle cell cultures: localization by immunofluorescence of sarcoplasmic reticulum proteins in differentiating rat skeletal muscle cell cultures

Immunofluorescent staining techniques were used to study the distribution of the Ca(2) + Mg(2+)-dependent ATPase and calsequestrin in primary cultures of differentiating rat skeletal muscle cells, grown for different periods of time under various culture conditions. In mononucleated myoblasts calsequestrin was detected after 45 h in culture whereas the ATPase was not detected until 60 h. After cell fusion began, both proteins could be identified in all multinucleated cells. Myoblasts grown for longer than 60 h in low Ca(2+) medium contained calsequestrin and the ATPase, even though they were unable to fuse. These studies at the cellular level confirm biochemical findings on the biosynthesis of calsequestrin and the ATPase. Immunofluorescent staining of myoblasts showed that calsequestrin first appears in a well-defined region of the cell near one end of the nucleus. At later times, the staining occupied progressively larger regions adjacent to the nucleus and took on a fibrous appearance. This suggests that calsequestrin first accumulates in the Golgi region and then gradually spreads throughout the cell. In contrast, the ATPase appeared to be concentrated in many small patches or foci throughout the cytoplasm and was never confined to one particular region, although some parts of the cell often stained more intensely than others. In multinucleated cells, alternating dark and fluorescent strands parallel to the longitudinal axis of the cells were evident.     

SV40 immortalizes myogenic cells: DNA synthesis and mitosis in differentiating myotubes

Primary skeletal muscle myoblasts have a limited proliferative capacity in cell culture and cease to proliferate after several passages. We examined the effects of several oncogenes on the immortalization and differentiation of primary cultures of rat skeletal muscle myoblasts. Retroviruses containing a SV40 large T antigen (LT) gene very efficiently immortalize myogenic cells. The immortalized cell lines retain a very high differentiation capacity and form, in the appropriate culture conditions, a very dense network of muscle fibers. As in primary culture, cell fusion is associated with the synthesis of large amounts of muscle-specific proteins. However, unlike normal myoblasts (and previously established myogenic cell lines), nuclei in the multinucleated fibers of SV40-immortalized cells synthesize DNA and enter mitosis. Thus, withdrawal from DNA synthesis is not obligatory for cell fusion and biochemical differentiation. Using a retrovirus coding for a temperature-sensitive SV40 LT, myogenic cell lines were produced in which the SV40 LT could be inactivated by a shift from 33 degrees C to 39 degrees C. The inactivation of LT induced massive cell fusion and synthesis of muscle proteins. The nuclei in those fibers did not synthesize DNA, nor did they undergo mitosis. This approach enabled the reproducible establishment of myogenic cell lines from very small populations of myoblasts or single primary myogenic clones. Activated p53 also readily immortalized cells in primary muscle cultures, however the cells of eight out of the nine cell lines isolated had a fibroblastic morphology and could not be induced to form multinucleated fibers.     

Prolactin cells of female rats treated with the carcinogen 7, 12-dimethylbenz(A) anthracene (DMBA) in vivo

Pituitary prolactin (PRL) cell microadenomata developed in females of a strain of Sprague-Dawley rats (SD1) following intragastric treatment with the carcinogenic hydrocarbon 7,12-dimethylbenz(a) anthracene (DMBA). Similar treatment of another strain of Sprague-Dawley rats (SD2) resulted in mammary tumour development but no PRL cell microadenomata. In SD1 strain rats morphologically distinct populations of PRL cells appeared after DMBA treatment, one composed of cells characterised by abundant, organised but very dilated RER and with large hormone storage granules, 500-600 nm in diameter (P1). The other cell type had electron-dense cytoplasm, narrow organised arrays of RER and moderately large, pleomorphic granules (P2). Both cell types appeared active with large Golgi and prominent nucleoli. P2 cells were most numerous 2 months after DMBA treatment but had virtually disappeared at 6 months and microadenomata were common at 8 months. PRL cells of SD2 rats were uniform in morphology, characterised by only moderate accumulations of RER, pleomorphic hormone storage granules, large Golgi and prominent nucleoli, and showed no close resemblance to either P1 or P2 cells of SD1 strain rats. It is possible that the morphological variations which developed in SD1 PRL cells may represent changes in responsiveness to factors controlling PRL cell secretion and proliferation and which may be pertinent to microadenoma development.     

Prostaglandin binding activity and myoblast fusion in aggregates of avian myoblasts

Myoblast aggregates provide a system for studying cell interactions which have several advantages over standard, stationary cultures. In gyrotory rotation, aggregate size can be controlled and is independent of cell migration. In muscle aggregates, fibroblasts are excluded, yet myoblast differentiation and fusion occur in a highly synchronous fashion. Specific PG binding occurs in chick or quail myoblast aggregates: in chick the peak of binding is at 35-36 hr. Aggregation is complete 16 hr before PG binding activity appears. This suggests either that gyrotory aggregation is not identical to myoblast recognition, or that PG binding activity occurs subsequent to myoblast recognition. Myoblast aggregates begin to release PG before 18 hr. The amount detected remains constant until binding begins at 34 hr when PG binding to the aggregates begins. Thus, both the release of PG and PG receptor activity are characteristics of the myoblasts and release of prostaglandin precedes appearance of the binding activity. As a first step in identifying the PG receptor and determining its appearance on the myoblast cell surface, we have prepared antisera against myoblast surfaces which blocks receptor-ligand interaction and have absorbed it against both peripheral and intrinsic membrane fractions. The results indicate that the PG receptor is a myoblast peripheral membrane macromolecule.