Using differential interference contrast optics, combined with cinematography, we have studied the morphological changes that the living, syncytial embryo undergoes from stage 10 through 14 of Drosophila embryogenesis, that is just prior to and during formation of the cellular blastoderm. We have supplemented these studies with data collected from fixed, stained, whole embryos. The following information has been obtained. The average duration of nuclear cycles 10, 11, 12 and 13 is about 9, 10, 12 and 21 min, respectively (25 degrees C). In these four cycles, the duration of that portion of the mitotic period that lacks a discrete nuclear envelope is 3, 3, 3 and 5 min, respectively. The length of nuclear cycle 14 varies in a position-specific manner throughout the embryo, the shortest cycles being of 65 min duration. During nuclear cycles 10 through 13, it is commonly observed in living embryos that the syncytial blastoderm nuclei enter (and leave) mitosis in one of two waves that originate nearly simultaneously from the opposite anterior and posterior poles of the embryo, and terminate in its midregion. From our preparations of quick-frozen embryos, we estimate that these mitotic waves take on average about half a minute to travel over the embryonic surface from pole to equator. The yolk nuclei, which remain in the core of the embryo when the rest of the nuclei migrate to the periphery, divide in synchrony with the migrating nuclei at nuclear cycles 8 and 9, and just after the now peripherally located nuclei at nuclear cycle 10. After cycle 10, these yolk nuclei cease dividing and become polyploid. The syncytial embryo has at least three distinct levels of cytoskeletal organization: structured domains of cytoplasm are organized around each blastoderm nucleus; radially directed tracks orient colchicine-sensitive saltatory transport throughout the peripheral cytoplasm; and a long-range organization of the core of the embryo makes possible coherent movements of the large inner yolk mass in concert with each nuclear cycle. This highly organized cytoplasm may be involved in providing positional information for the important process of nuclear determination that is known to occur during these stages.

Studies of nuclear and cytoplasmic behaviour during the five mitotic cycles that precede gastrulation in Drosophila embryogenesis

Cell-free Formation of RNA Granules: Bound RNAs Identify Features and Components of Cellular Assemblies

A protein component of Drosophila polar granules is encoded by vasa and has extensive sequence similarity to ATP-dependent helicases

Publisher Summary This chapter discusses germ plasm and the differentiation of the germ cell line. Germ plasm is defined as a substance present in the cytoplasm of gametes, which is segregated into specific cells during blastulation and determines that those cells shall become the progenitors of the germ cell line during subsequent development. The composition of germ plasm is discussed. The histochemical evidence for the presence of RNA in polar granules of insects and germinal plasm of amphibians indicates that RNA is present only in eggs and early embryos, while a protein component is present throughout the germ cell cycle. Good morphological and experimental evidence for the presence of germ plasm is available for only a few animal species. The germinal cells of chaetognaths were present in early stages of embryonic development and identified a group of cells at the bottom of the archenteron cavity in gastrulas of Sagitta as the primordium of the germ line. Experimental studies are given to determine the role of germinal plasm in anuran eggs by altering the cytoplasm in which it is contained.

Germ plasm and the differentiation of the germ cell line.

Identification and manipulation of the germ line are important to the study of model organisms. Although zebrafish has recently emerged as a model for vertebrate development, the primordial germ cells (PGCs) in this organism have not been previously described. To identify a molecular marker for the zebrafish PGCs, we cloned the zebrafish homologue of the Drosophila vasa gene, which, in the fly, encodes a germ-cell-specific protein. Northern blotting revealed that zebrafish vasa homologue (vas) transcript is present in embryos just after fertilization, and hence it is probably maternally supplied. Using whole-mount in situ hybridization, we investigated the expression pattern of vas RNA in zebrafish embryos from the 1-cell stage to 10 days of development. Here we present evidence that vas RNA is a germ-cell-specific marker, allowing a description of the zebrafish PGCs for the first time. Furthermore, vas transcript was detected in a novel pattern, localized to the cleavage planes in 2- and 4-cell-stage embryos. During subsequent cleavages, the RNA is segregated as subcellular clumps to a small number of cells that may be the future germ cells. These results suggest new ways in which one might develop techniques for the genetic manipulation of zebrafish. Furthermore, they provide the basis for further studies on this novel RNA localization pattern and on germ-line development in general.

Zebrafish vasa homologue RNA is localized to the cleavage planes of 2- and 4-cell-stage embryos and is expressed in the primordial germ cells

Restoration of fertility in sterilized Drosophila eggs by transplantation of polar cytoplasm

Repair of a genetically-caused defect in oogenesis in Drosophila melanogaster by transplantation of cytoplasm from wild-type eggs and by injection of pyrimidine nucleosides☆

M. Okada

Papers

Restoration of fertility in sterilized Drosophila eggs by transplantation of polar cytoplasm

Repair of a genetically-caused defect in oogenesis in Drosophila melanogaster by transplantation of cytoplasm from wild-type eggs and by injection of pyrimidine nucleosides☆

Chimeric Drosophila adults produced by transplantation of nuclei into specific regions of fertilized eggs.