Eutely

Abstract: Several cytological aspects have been considered in tardigrades. Firstly, the cell constancy which is not a true eutely being several mitoses present even after hatching, even though some organs, such epidermis and nervous ganglia, have the same cell number in juveniles and adults. The total number of these cells is species-specific. Then the ultrastructure of cuticle, epidermis, feeding and digestive apparatus, excretory and osmoregulatory organs, muscles, nerve cells, sensory cells and storage cells has been considered. Instead, the ultrastructure of the germ cells has been considered in the chapter on reproduction. With regard to chromosome number and shape, it has been observed that generally there is little difference among the species (n = 5 or n = 6), but several cases of polyploid populations exist, often very similar to diploid populations from a morphological point of view. In most cases the polyploid populations do not have males and reproduce by apomixis. Studies on the genome size have confirmed the presence of polyploid populations, as well as the presence of nuclei with multiple amounts of DNA within the same specimen. The genome size of the tardigrades is always relatively small and does not seem related to phylogenetic lineages. Studies on tardigrade genomes have placed this phylum at the centre of discussions on the evolution of Metazoa and have considered the role of horizontal gene transfer in animal evolution with contrasting results.

Abstract: The larval central nervous system (CNS) of the ascidian Ciona intestinalis (L., 1767) arises from an embryonic neural plate and contains sufficiently few cells, about 330, to enable definitive counts. On the basis of such counts, there is evidence both for cell constancy (eutely) in the larval CNS and for small variations in the overall numbers of cells and among defined cell types within this total. However, evidence for the range of such deviations and the existence of a true phenotypic wild type are lacking. The record of cell lineage, i.e., the mitotic ancestry of each cell, and the fates of some of these cells have recently received increased documentation in both the genus Ciona and Halocynthia roretzi (von Drasche, 1884). Relatively few generations of cells, between 10 and 14, form the entire CNS in C. intestinalis, and cell death does not occur prior to larval hatching. The tiny complement of larval CNS cells can therefore be seen as the product of a small fixed number of determinate cleavages, an...

Abstract: Nematodes are generally considered to have an adult cell number that does not vary among wildtype individuals as a consequence of invariant cell lineages (eutely). However, there is extensive evidence that at least some cell lineages can be variable in nematodes. In a comparative study of 13 free-living nematode species, we have shown that the adult epidermis of most species contained variable numbers of nuclei and that this variance was positively correlated with mean epidermal nuclear number. Here we present simulations of the lateral seam cell lineages of four species and show that variance in cell number is influenced by lineage topology, as well as by the frequency of lineage variants. We show that the epidermal variability of Panagrellus redivivus cannot be accounted for by the complexity of its lineage, but requires higher levels of lineage variability than are found in Caenorhabditis elegans, Oscheius myriophila and Rhabditella octopleura. Our findings suggest that many nematodes may have tissues composed of indeterminate numbers of cells formed from variable lineages and, as such, resemble other metazoans. Les nematodes sont generalement consideres comme ayant un nombre de cellules invariable chez les individus de type sauvage, consequence d’un lignage cellulaire fixe (eutelie). Cependant, il est d’evidence qu’au moins certains des lignages cellulaires peuvent varier chez les nematodes. Dans une etude comparative portant sur 13 especes de nematodes libres, nous avions montre que l’epiderme de la plupart de ces especes comportait un nombre variable de noyaux et que cette variabilite etait correlee positivement avec le nombre de noyaux epidermiques. Nous presentons ici des simulations des lignages cellulaires de la suture laterale de quatre especes et demontrons que le nombre de cellules est influence tant par la topologie du lignage que par la frequence des variants de ce lignage. Nous montrons que la variabilite de l’epiderme de Panagrellus redivivus ne peut etre mise au compte de la complexite de son lignage, mais demande des niveaux eleves de variabilite de ce lignage, tels ceux trouves chez Caenorhabditis elegans, Oscheius myriophila et Rhabditella octopleura. Nos observations suggerent que nombre de nematodes possedent des tissus composes d’un nombre indetermine de cellules derivant de lignages variables et, de ce fait, ressemblent aux autres metazoaires.

Abstract: 1. In the first stages of the larval development, the yolk-sac remains attached to the embryo, and the growth of the latter takes place at the expense of the yolk absorbed. The muscle-cells simply undergo an increase in size. This growth is relatively much more rapid than the growth of the whole embryo. No cell division seems to occur. The muscle cells already formed when the larva hatches simply increase in size. The growth of the embryo is therefore almost entirely dependent on thegrowth (enlargement) of the muscle cells. 2. But as soon as the larva begins feeding, and the yolk absorption is diminished or finished, the development of the muscle tissue is brought about in two ways — cell growth and cell multiplication. With the beginning of the adult life of the brown-trout, there is an appreciable, though gradual fall in the relative cell growth of the muscle cells; although the fish itself is growing quite rapidly to an enormous size. So that the cell growth cannot keep pace, as it were, with the body growth. This difference in the relative growths of the two is made good by the increase in the number of cells, which occurs in an increasing degree as the development proceeds. So that the muscle cells grow in size as well as more cells are formed, either by division of the pre-existing ones, or the proliferation of new cells. And gradually, the phenomenon of cell growth yields to the phenomenon of the cell-multiplication to account for the growth of the fish body. The appearance of muscle cells dividing amitotically becomes more and more common in the latter part of the development.