Early Tailbud Embryos Research Articles

CYTOLOGICAL AND ULTRASTRUCTURAL STUDIES ON MUSCLE DIFFERENTIATION IN THE ASCIDIAN, PEROPHORA ORIENTALIS*,*.

Muscle cell differentiation in the tail of the ascidian, Perophora orientalis, from early tail-bud embryos to swimming larvae, were studied cytologically and ultrastructurally. Myogenic cells did not form multinucleated myotubes, but remained as mononucleated cells. Nucleolar component increased prior to a marked increase in cytoplasmic RNA. Cytoplasmic RNA appeared first around nucleus and later concentrated in the peripheral cytoplasm. The fine filaments measuring 20-30 Å in their thin parts and 30-45 Å in their thick parts in diameter appeared initially, forming loose networks, in the peripheral cytoplasm where ribosome clusters had been concentrated. These filaments were tightly attached by particles of various size and density. These filaments tended to be arranged in parallel as they increased in their size. They seemed to be precursors of both actin and myosin filaments of formed myofibrils. Z band precursors were found as dense patches in association with loosely arranged myofilaments and consisted of particulate and filamentous materials. The myofibrils seemed to grow further by organizing free filaments into bundles and further by aligning bundles of myofilaments at both ends.

Genetic and developmental characteristics of a new color variant, axanthic, in the Mexican axolotl, Ambystoma mexicanum shaw

A new genetic trait, axanthic, is described in the Mexican axolotl with a phenotype completely lacking visible xanthophores and iridophores. Animals received from the colony of Dr. L. E. DeLanney at Ithaca, New York, have been mated among themselves for a total of seven spawnings, in which the axanthic trait was inherited as a simple Mendelian recessive. The symbol for this newly found gene is ax. Homozygous recessive animals appear normal except for their lack of visible xanthophores and iridophores. The results of the spawnings also indicate that the axanthic gene is not linked to the melanoid gene ( m). Its linkage with other known mutants in the axolotl is being tested. Hanging-drop preparations in Niu-Twitty's solution of neural fold explants were made using axanthic and normal embryos. Xanthogenesis was never visible in cultures from axanthic animals, whereas all those made from yellow animals had fully differentiated xanthophores within 9 days after the explants were made. Counts of cells in cultures from axanthic animals showed a complement of axanthic xanthoblasts, that is, otherwise normal xanthophores without pigment. Epidermal vesicles were made by combining flank somatopleure from early tailbud embryos with neural fold from neurulae. Axanthic somatopleure was combined with axanthic and xanthic neural fold (in separate vesicles) and xanthic somatopleure combined with folds from axanthic embryos. Only the vesicles with neural fold from xanthic donors had visible xanthophores. Homotopic reciprocal neural crest transplants were made between axanthic and yellow larvae at midtailbud stages. The grafted regions were obvious after pigment synthesis had begun. Xanthophores were visible in these regions whenever the donor material was derived from a yellow larva. The graft region was devoid of visible xanthophores whenever the donor material was axanthic neural crest, even when the crest was transplanted onto xanthic animals. Chromatographic analyses of the pteridine contents in axanthic as compared to normal animals showed that the axanthic animals were lacking five fluorescent spots found in normal larvae. Two of these spots from normal animals had yellow fluorescence; three had blue fluorescence. The three blue fluorescent compounds were tentatively identified as Ranachrome-3, Hynobius-blue, and 2-amino-4-hydroxypteridine-6-car☐ylic acid. One yellow spot appeared to be sepiapterin. The other could not be identified. The chromatographic results indicate that the axanthic mutant is deficient in these pteridine compounds, which are evidently necessary for the pigmentation of xanthophores. The results from the experimental work offer support to the hypothesis that the axanthic trait, inherited as a simple Mendelian recessive, acts at the level of the xanthoblasts to prevent the accumulation of yellow pigments in these cells. As there are no visible iridophores in these mutants either, it may also prevent the accumulation of pigmentary purines in iridoblasts.

Chromosomal and embryological analyses of nuclear changes occurring in embryos derived from transfers of nuclei between Rana pipiens and Rana sylvatica

Diploid nuclei from normal blastulae of Rana pipiens were transplanted into enucleated eggs of Rana sylvatica. Following 10–12 divisions in the foreign cytoplasm, their nuclear descendants were transferred back into enucleated pipiens eggs. All but one of the embryos derived from these eggs developed abnormally and were arrested at various stages ranging from late blastulae to young tadpoles. These results, like those obtained by Moore (1958a,b, 1960), demonstrate that pipiens nuclei are changed as a consequence of replicating in sylvatica cytoplasm and are no longer capable of promoting normal development when returned to their own type of cytoplasm. The main objective of this investigation was to determine whether the restrictions in the developmental capacity of these nuclei involve cytologically detectable alterations in the chromosome complement. Analysis of metaphase plates from the abnormally developing back-transfer embryos showed that all of these embryos did in fact possess abnormal numbers and types of chromosomes as follows: 1. 1. The chromosomal abnormalities were most pronounced in back-transfer embryos arresting in early stages of development (gastrulae, neurulae, and early tailbud embryos). When compared with the normal pipiens karyotype, the chromosome complements of these embryos were found to possess the following abnormalities: (a) gains and losses of from one to four chromosomes, (b) ring chromosomes, minute chromosomes, and in one case, acentric fragments, and (c) a decrease in the number of large chromosomes coupled with an increase in the number of small ones. The karyotypes were so abnormal that it was not possible to specify which chromosomes were involved in the rearrangements. 2. 2. The chromosomal alterations in back-transfer embryos arresting in later stages of development (late tailbud embryos and abnormal tadpoles) were less extensive and consequently could be analyzed in greater detail. These embryos were aneuploid by one or two chromosomes and, unlike the more deficient embryos described above, did not possess rings or minutes. A detailed karyotype analysis of one clone revealed that the chromosome complements of these embryos lacked one or more specific chromosomes known to be present in the normal diploid set and, at the same time, possessed new structural types normally not found in Rana pipiens. Some of these new types of chromosomes appeared to be the result of a translocation involving a distinctive chromosome (no. 10) of the normal pipiens complement. Karyotypic abnormalities of the types just described can account for the changes in the developmental capacity of pipiens nuclei as a consequence of replicating in sylvatica cytoplasm and for the results obtained by Moore (1960), which show that these changes are irreversible. According to the evidence presented in this report, alterations in the chromosome complements arise during the time that pipiens nuclei are dividing in sylvatica cytoplasm but exactly when and how they are produced remains to be worked out.