No abstract

GridPix detectors are thin µTPCs, they provide an excellent vertex determination for internal radioactive sources and they have a 4π angular acceptance. Gridpix detectors are ideal to study anomolies in Internal Pair Conversion signaling elusive light neutral bosons.

Trochoblasts are the first cells to differentiate during the development of spiralian embryos. Differentiation is accompanied by a cell division arrest. In embryos of the limpet Patella vulgata, the participation of cell cycle-regulating factors in trochoblast arrest was analysed as a first step to unravel its cause. We determined the cell cycle phase in which the trochoblasts are arrested by analysing the subcellular locations of mitotic cyclins. The results show that the trochoblasts are most likely arrested in the G2 phase. This was supported by measurement of the DNA content in trochoblast nuclei after the last division. Trochoblasts complete their final division at the sixth mitotic cycle. This mitotic cycle resembles the first postblastoderm cell cycle of Drosophila, in which mitotic activity is controlled by expression of the string gene. As failure of string expression results in cell cycle arrest in the G2 phase, negative regulation of a Patella string homolog could be responsible for trochoblast arrest. Although Stl messengers disappeared from trochoblasts during their final division, expression was observed again 20 min later. Messengers remained present in all trochoblasts at low levels during further development. Thus, expression of the stringlike gene allows the cell cycle arrest of these cells, whereas in Drosophila cells arrested in division lack string messengers.

No abstract

As the first five cleavages of the Patella vulgata embryo are synchronous, they are well suited to determine the mRNA level of cyclin A and B genes in an embryo. During the third and fourth cleavage cycle the quantity of A and B mRNA is regulated in a cell-cycle-dependent way, reaching a high level between cleavages and a lower level just after mitosis. This implies that transcription of the cyclin genes occurs before the overall transcription increases directly after the fifth cleavage. During the first cleavages cyclin A and B mRNA is localized in distinct parts of the cytoplasm. Between two successive cell devisions it is found as a crescent-shaped domain at the peripheral side of the nucleus. At cytokinesis it is present between two separating nuclei and at newly formed cell membranes. At the fifth cleavage this localization disappears. Changes in the expression pattern of cyclin A and B may be expected after the fifth cleavage, when the first cells become arrested in cell division and differentiate. The mechanism causing cell division arrest of these primary trochoblasts is still unknown. Cell division arrest caused by the absence of cyclin A and/or B mRNA could be conditional for further differentiation. However, a decrease in cyclin A and B mRNA level in the trochoblasts is not detectable until 4 h after their last division. Later in development no cyclin A and B mRNA can be detected in these cells, whereas cyclin A and B mRNA is present in other cells of the embryo. Thus, the absence of cyclin A and B mRNA in primary trochoblasts, and in the later differentiating secondary and accessory trochoblasts is not obligatory for cell division arrest or cell differentiation.