摘要:

AbstractThe developmental features of the pancreas are reviewed as an example of cytodifferentiation and organogenesis. Attention is directed to the regulatory characteristics of the specific proteins synthesized and secreted by the endocrine and exocrine cells. The following topics are discussed: (1) number of specific protein species and, inferentially, the number of genes involved in differentiated function. (2) The stringent regulation of the concentration of these specific proteins and the probable restriction of their synthesis to exocrine and endocrine cells. (3) The multiphasic pattern of accumulation of these specific proteins during pancreatic development and the synchronized but noncoordinate regulation of individual protein species. Synthetic rates of specific exocrine proteinsin vitrocorrelate closely with measurements of the accumulation of proteins during development. (4) A model postulating three regulatory transitions. The primary transition (related to organ “determination”) denotes the conversion of a “predifferentiated” cell to the “protodifferentiated” state in which low but significant levels of specific proteins are present. The secondary transition is viewed as an amplification of this specific protein synthesis and is associated with typical pancreatic histogenesis. In the third regulatory transition, the synthesis of specific proteins in the “differentiated state” is modulated by diet, or hormonal states, etc. The third regulatory transition may be similar to some types of “enzyme induction” as studied in multicellular systems. (5) The differentiative fidelity in an organotypic culture system; the role of mesenchymal tissue or a particle fraction derived therefrom in supporting the protodifferentiated state and the secondary regulatory transition. (6) The possible mechanisms of the secondary regulatory transition in exocrine cells. Effects of actinomycin D, bromodeoxyuridine, and other mitotic inhibitors suggest the requirement for a critical cell division prior to the loss of proliferative capacity. (7) The synthesis of pro‐insulin and insulin during primary and secondary regulatory transitions; the possible interrelationships of endocrine and exocrine cells in

ISSN:0021-9541    

DOI:10.1002/jcp.1040720403

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractCytodifferentiation of skeletal muscle has been studied in cell cultures derived from leg muscle of 12‐day chicken embryos. Myogenesis in cell culture closely simulates myogenesisin vivo, but is more highly synchronized. Massive cell fusion occurs in control cultures between the second and third daysin vitro, during which time most of the myoblasts are swept into syncytia. On successive days, the syncytia mature into cross‐striated muscle fibers, and the cultures are progressively overgrown by fibroblastic cells. Myosin‐containing cells can be detected at any time by immunofluorescence, and myosin has been measured by quantitative immunological precipitation as early as 3 daysin vitro, a few hours after fusion. Myosin in the cultures increases over the next few days, and this is reflected in the rate of incorporation of labeled amino acids into immunologically precipitable myosin. Creatine kinase, assayed spectrophotometrically by linked dehydrogenase reactions, shows a similar pattern: measurable early but rapidly increasing in activity after fusion. That this increase in myosin and creatine kinase is strictly a function of the multinuclear cells is demonstrated by experiments in which the mononuclear cell population has been drastically reduced by treatment with 5‐fluorodeoxyuridine shortly after fusion. Myosin synthesis has not been detectable in cells prevented from fusing by growth in 5‐bromo‐deoxyuridine, but low levels of creatine kinase have been demonstrated. Newly formed muscle fibers incorporate precursors into RNA at lower rates than do mononuclear cells. The relationship of this change in RNA synthesis to the formation of muscle proteins rema

ISSN:0021-9541    

DOI:10.1002/jcp.1040720404

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractIt is becoming increasingly evident that thein vitroinduction of vertebral chondrogenesis may not be due to the acquisition of a new metabolic pattern, but to the stabilization of one that is already existing. The sclerotome region of embryonic chick somites will undergo chondrogenesisin vitroas a response to various stimuli, the most effective being the embryonic notochord. Before stimulation, the somites have all of the metabolic machinery necessary for chondrogenesis, as evidenced by the fact that they can synthesize chondromucoprotein. They cannot, however, accumulate matrix except under special circumstances,e.g., when they are stimulated by an “inducer” such as the notochord (induced cartilage) or when they are permitted to express their chondrogenic bias under suitable culture conditions, such as in enriched nutrient medium (spontaneous cartilage).A study of chondroitin sulfate synthesis in these tissues has shown that the embryonic somites utilize glucosamine differently when compared to cartilage tissue. Analysis of the metabolic steps between glucosamine and UDP‐N‐acetylgalactosamine indicates the possibility of a metabolic “block,” which would prevent the efficient transformation ofN‐acetylglucosamine‐1‐P to UDP‐

ISSN:0021-9541    

DOI:10.1002/jcp.1040720405

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractPolycrylamide gel electrophoresis of chicken lens proteins showed 17 crystallins, divided over three groups. Within each group physicochemical heterogeneity was combined with (partial) immunological homogeneity. It is assumed that more than one gene is involved in the synthesis of any crystallin species. During development of the chicken embryo, α‐crystallin was first demonstrated by immunofluorescence in centrally located lens fibers at 3 days. At 8 days the epithelium became positive and the fibers lost some fluorescence. This continued until in 5‐week‐old chickens the lens core was negative. Lens placode cells showed immunofluorescence for δ‐crystallin at 52 hours, mainly in their basal parts. The reaction gradually spread and at 3 days the entire lens was positive. From 8 days on the epithelium reacted progressively weaker, but the fibers remained positive. Five weeks after hatching, epithelium and cortex were negative, while the center still showed strong fluorescence. The behavior of β‐crystallin was intermediate between that of the other two. Immunoelectrophoresis suggested a differential production onset for the components of each single crystallin type. Under normal conditions no crystallins were found outside the lens. Therefore, crystallin synthesis occurs after placode formation has taken place and must be restricted to the lens itself. Autoradiography after3H‐thymidine treatment indicated that all placode cells still replicate, though some already produce crystallins. A generation time of 8 to 10 hours was determined with an M phase of 30 minutes, an S phase of 6 hours, and a G2of 2 ½ hours. During DNA synthesis the nuclei were located in the basal parts of the cells, and for mitosis they migrated to the lumen. Autoradiography after3H‐glucosamine application suggested that the placode cells take active part in the synthesis of the basement membrane interposed between lens rudiment and optic cup. This membrane later becomes the lens capsule, and in mice with the “shrivelled” gene, abnormal masses of anterior epithelial cells also clearly produce extra capsule material. This results in anterior polar cataracts. Several of the above findings are in disagreement with some of the current theories on the regulation of lens differentiation. No substitutes are prese

ISSN:0021-9541    

DOI:10.1002/jcp.1040720406

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractThe process of vitellogenesis in oviparous vertebrates embraces a number of integrated physiological and developmental phenomena. Since this process is readily subjected to experimental control inXenopus laevis, we have been able to undertake a preliminary survey of the pertinent mechanisms operating in this animal.The information at hand is discussed as it relates to (a) the hormonal (estrogen) induction of yolk protein synthesis by the liver, (b) the characterization of the yolk protein produced and its relationship to the proteins of the mature egg, (c) the transport of the yolk protein to the ovary and its specific uptake by the developing oocyte, and (d) the transformation of the accumulated protein into crystalline yolk platelets.

ISSN:0021-9541    

DOI:10.1002/jcp.1040720407

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

ISSN:0021-9541    

DOI:10.1002/jcp.1040720408

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractSinceEuglena gracilisKlebs var.bacillarisPringsheim contains a species of DNA unique to the chloroplast, an important question concerns the extent to which light unblocks the reading of the organelle's template to provide the informational RNA's necessary to construct the plastid proteins. Experiments with32Pilabeling of chloroplast and nonchloroplast RNA's during light‐induced chloroplast development show that both the RNA of the chloroplast and of the rest of the cell become labeled during this process, with the chloroplast RNA's displaying the higher specific activity. The fact that chloroplast RNA is not uniquely labeled indicates that process other than a simple reading of the chloroplast DNA are involved. If we are to preserve the concept of a reasonable degree of chloroplast informational autonomy, we may assume, from this and other data, that the light induction of chloroplast development involves not only the unblocking of chloroplast DNA to make information available, but also a concomitant unblocking of other sites of informational RNA synthesis (e.g., nuclear and mitochondrial DNA's). Such sites external to the developing chloroplast may be concerned with making available the building blocks and energy necessary for the synthesis of chloroplast constituents coded for by the chloroplast DNA. This model leads to the prediction that photosynthesis could be gratuitous for chloroplast development if these nonchloroplast sites were providing most of the building blocks and energy. Experiments are reported which show that chloroplast formation and the acquisition of photosynthetic competence can be achieved under conditions where photosynthesis is completely inhibited for the entire span of development by using the highly selective inhibitor 3, (3,4‐dichlorophenyl) 1, 1‐dimethyl urea (DCMU), in agreement with the proposed model. The fact that more than just the chloroplast responds to the inducing signals for chloroplast differentiation raises the problem of experimental measurement of interaction among cellular organelles. Since chloroplast development is usually carried out in resting cells to avoid complications due to cell division, we discuss the limitations imposed by turnover in such nondividing systems and present evidence that most of the RNA labeling observed, although actinomycin‐D‐sensitive, is due to turnover and/or the utilization of preexisting pools. Evidence obtained with mutants ofEuglenathat form only partial chloroplasts or that lack plastid DNA and plastid‐related structures is reported. Such evidence indicates that the functional proplastid restrains overall RNA labeling in the uninduced cells and suggests that the proplastid might be the source of regulatory metabolic signals in the normal plastid‐con

ISSN:0021-9541    

DOI:10.1002/jcp.1040720409

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractBecause its differentiation can be channeled into different pathways, amphibian gastrula ectoderm is a convenient test system for studying factors that control embryonic differentiation.1Chemical nature of inducing factors: A substance that induces muscle and notochord in ectoderm has been isolated from chick embryos and other sources. The factor is protein in nature (mol. wt. in 6 m urea 25,000‐‐30,000). Neural tissue is induced by a crude ribonucleoprotein fraction. Purified RNA has only a very weak inducing activity. The inducing factors are preferentially located in cytoplasmic particles.2Mechanism of action: Embryonic induction has to be considered as a derepression. Preliminary experiments have shown that a high‐molecular‐weight, water‐soluble substance takes part in the inhibition of mesodermal differentiation. The inhibition of differentiation is released by the inducing factors. A close relationship between differentiation and RNA synthesis has been revealed by experiments with actinomycin D (0.5‐‐2.5 μg/ml), which inhibits RNA synthesis. If RNA synthesis is completely stopped in the gastrula stage, the mesodermal area, which is already determined to differentiate into muscle and notochord, still forms some notochordal cells and myoblasts. The differentiation of neural tissue, however, is completely inhibited. DNA‐RNA hybridization experiments at the saturation level suggest that new messenger RNA species are synthesized if differentiation proceeds. But this does not exclude that the inducing factors exert control primarily at the leve

ISSN:0021-9541    

DOI:10.1002/jcp.1040720410

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractThe biochemical information now available on the cellular slime mold is sufficient to attempt a kinetic model of a portion of the metabolism essential to differentiation. A computer model has been constructed which is consistent with thein vivoandin vitrodata obtained for this system, and which makes specific predictions now being tested in the laboratory.Uridine diphosphoglucose (UDPG) is a precursor of soluble glycogen as well as of the saccharide end products of differentiation, such as cell wall material. The rate of turnover of UDPGin vivois known, and is sufficient to account for the observed accumulation of end products over the period of time normally required during differentiation. The accumulation patterns during development of the various polysaccharides and of UDPG, glucose‐1‐phosphate (G‐1‐P), and uridine triphosphate (UTP) are known.Kmvalues of UDPG synthetase for G‐1‐P and UTP have been determined, as have changes in specific enzyme activity during development. A series of differential equations describing the synthesis and utilization of UDPG have been used to construct a computer model for the conversion of glycogen through G‐1‐P and UDPG to the end products of differentiation. An analysis of this model demonstrates that an increase in UDPG pyrophosphorylase concentrationin vivocannot account for the enhanced rate of synthesis of UDPG nor for the accumulation patterns observed. The major controlling factor in this system is shown to be the availability of G‐1‐P. This analysis reflects upon: (1) the general significance of alterations in the concentration of enzymes during differentiation, and (2) the importance of understanding control mechanisms responsible for the availabilityin vivoof precursors for synthetic pathways necessa

ISSN:0021-9541    

DOI:10.1002/jcp.1040720411

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractExperiments have been carried out to study the properties of ribosomal RNA synthesis and ribosomal assembly in cells having stable messenger RNA templates (adult epithelial cells in stationary phase and fiber cells) and rapidly turning‐over templates (dividing epithelial cells). The data show that 45S and 30S precusor ribosomal RNA is synthesized in epithelial cells in a stationary phase and in fiber cells. These precursor ribosomal RNA's, however, are not chased into 28S and 18S RNA. Similar pulse‐chase experiments with dividing epithelial cells of calf and adult lenses show a significant chase into 28S and 18S RNA. The data indicate that there is a regulation of ribosomal assembly at the step responsible for 28S and 18S RNA formation. The details of this regulation are discus

ISSN:0021-9541    

DOI:10.1002/jcp.1040720412

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY