Coenocyte

Abstract: The female gametophyte develops from the megaspore formed in the nucellar tissue of the ovule. The megaspore enlarges and after some mitotic divisions a small coenocyte differentiates. In general, a megagametophyte consists, after cytokinesis, of an egg apparatus, a central cell, and antipodal cells.

Abstract: The cereal endosperm develops from a coenocyte to a cellular storage organ through formation of nucleo-cytoplasmic domains and cell wall deposition in the interzones between these domains. During its early stages, the endosperm develops in close contact with nucellus, the sporophytic tissue which gives rise to the megagametophyte. Owing to the positioning of the two tissues deeply within the ovary, neither cell types have been easily accessible for molecular studies. In this paper we report for the first time the cloning of molecular markers for the barley endosperm coenocyte and the nucellus. The novel END1 and NUC1 cDNAs were isolated by differential screening of a cDNA library from 5 DAP (days after pollination) ovaries using a positive probe from hand-dissected embryo sacs with adhering nucellus and testa cell layers, and a negative probe from pericarp. In situ and northern blot hybridization data show that END1 transcripts are asymmetrically distributed in teh endosperm coenocyte limited to an area over the nucellar projection. In the cellular endosperm, END1 transcripts are present in modified aleurone cells and a few layers of ventral starchy endosperm cells. The second clone, NUC1, hybridizes to transcripts in the nucellus before fertilization and in autolyzing nucellus cells after fertilization. At later stages, after the disappearance of nucellus, NUC1 transcripts are present in the nucellar epidermis and in the lateral cells of the nucellar projection. This work provide tools for future elucidation of the genes specifying endosperm histogenesis.

Abstract: Cells show diverse appearances and sizes, ranging from some 30 nanometers up to several meters in length. Besides the classical prokaryotic and eukaryotic cells, there are also very bizarre cells such as the highly reduced symbiotic mitosomes which lack DNA. Other examples of extremely small cells in the nanometre range are the mycosomes and nanobacteria. On the other hand, there are huge eukaryotic cells, the size of which can reach up to several meters. Most of these are multinucleate (coenocytic) due to mitotic divisions not having been followed by cytokinesis. Moreover, cells at all levels of cellular complexity show an inherent tendency to form cell-cell channels. The most conspicuous example is the plant’ supercell’ where all the cells of the plant body are permanently connected via plasmodesmata. In the last year, the first reports of similar cell-cell channels between animal cells have been published. Moreover, fungal cells fuse together into supracellular mycelia, even exchanging their motile nuclei. This phenomenon is also known for plant cells. Intriguingly, transcellularly moving fungal nuclei communicate with their mating partners via pheromone-like signaling mechanisms.

Abstract: Cytokinesis in microsporocytes of moth orchids is unusual in that it occurs simultaneously after meiosis, the cytoplasm does not infurrow in the division planes, and cell plates are deposited in association with centrifugal expansion of phragmoplasts. Microtubules radiating from the nuclear envelopes appear to be of fundamental importance in establishment of division planes. Primary interzonal spindles develop between sister nuclei and interaction of radial microtubules triggers development of secondary interzonal spindles between non-sister nuclei. From three to six or more phragmoplasts, depending upon the arrangement of nuclei in the coenocyte, develop from these postmeiotic arrays. The phragmoplasts consist of co-aligned microtubules and F-actin organized into bundles that are broad proximal to the mid-plane and taper distally. Ultrastructure of the phragmoplast/cell plate reveals that abundant ER is associated with vesicle aggregation and coalescence. Cell plates are deposited in association with phragmoplasts as they expand centrifugally to join the parental wall and/or fuse with one another in the interior of the cell.