PubMed · 14800298
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W H SCHOPFER. 1950. [Morphogenesis and vitamins].. https://pubmed.ncbi.nlm.nih.gov/14800298/
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In the last few years, somatic embryogenesis and organogenesis of white spruce (Picea glauca) have been used as model systems to investigate biochemical and physiological events related to morphogenesis. This has been possible because studies conducted in vitro allow for manipulations of the culture conditions, in which changes in morphogenetic events can be easily related to physiological alterations. De-novo synthesis, salvage, and degradation of both purine and pyrimidine nucleotides are operative at all stages of somatic embryo maturation and germination. Fluctuations in the activity of these pathways delineate important morphogenic events. The early phases of embryo development are accompanied by a decreased salvage activity of purine nucleotides, which reflects a reduction of cell proliferation and the initiation of organized growth. Activities of the salvage enzymes are present throughout the maturation period, and also during the imposition of the drying period, which is required for successful embryo germination. The operative salvage pathway in dried embryos is needed for the enlargement of the nucleotide pool necessary to sustain the reactivation of the overall cellular metabolism at germination, before the reactivation of the de-novo pathway, which is a later event. Manipulations of the culture medium which improve the germination frequency of the embryos also result in increased salvage activity. Similar changes in nucleotide synthesis were also observed during the initiation of shoot development from epicotyl explants of white spruce and cotyledons of radiate pine. Results from these studies can be used for improving growth and development in culture.
Both the physiological and the pathological morphogenetic processes that we can meet in embryogenesis, neogenesis and degenerative dysgenesis present common features: they are ruled by three different kinds of mechanisms, one related to cell migration, the second to cell differentiation and the third to cell proliferation. We deal here with an application to the cambial growth which essentially involves the third type of mechanism. Woody plants produce secondary tissue (secondary xylem and phloem) from a meristematic tissue called vascular cambium, responsible for the radial growth of a tree. This paper focuses on the formation of secondary xylem, considered in two dimensions in a cross-section framework. A new discrete modelling approach is used, based on the cellular scale, in order to attain a more accurate understanding of how the elementary microscopic behaviour of each cell takes part in the macroscopic morphogenesis. The mathematical model essentially uses an occurrence method simulating the main features of radial growth with simple geometric rules, such as Thom's division rule (Thom,1972)to account for the cell proliferation. The study applies to concrete instances in which the changes made in the geometrical cellular patterns of the vascular cambium clearly affect the shape of the tree, as in Pinus radiata (D. Don.).
Myxobacteria build their species-specific fruiting bodies by cell movement and then differentiate spores in specific places within that multicellular structure. New steps in the developmental aggregation of Myxococcus xanthus were discovered through a frame-by-frame analysis of a motion picture. The formation and fate of 18 aggregates were captured in the time-lapse movie. Still photographs of 600 other aggregates were also analyzed. M. xanthus has two engines that propel the gliding of its rod-shaped cells: slime-secreting jets at the rear and retractile pili at the front. The earliest aggregates are stationary masses of cells that look like three-dimensional traffic jams. We propose a model in which both engines stall as the cells' forward progress is blocked by other cells in the traffic jam. We also propose that these blockades are eventually circumvented by the cell's capacity to turn, which is facilitated by the push of slime secretion at the rear of each cell and by the flexibility of the myxobacterial cell wall. Turning by many cells would transform a traffic jam into an elliptical mound, in which the cells are streaming in closed orbits. Pairs of adjacent mounds are observed to coalesce into single larger mounds, probably reflecting the fusion of orbits in the adjacent mounds. Although fruiting bodies are relatively large structures that contain 10(5) cells, no long-range interactions between cells were evident. For aggregation, M. xanthus appears to use local interactions between its cells.