Syncytium

SYNCYTIUM (from the Greek sync- and cytos-cell) is the joining of many cells into a single common formation of various sizes and shapes; this includes both the connection of neighboring cells by protoplasmic anastomoses of varying widths (syndesm), and their fusion into a single common multinucleated mass (syncytial mass, plasmodium). S. arises either through the destruction of boundaries between individual cells or as a result of cell division that does not go to completion. The term S. was first introduced by Haeckel (1872) in application to the ectoderm of sponges; even earlier, de Bari (1859) proposed the term plasmodium to denote protoplasmic masses of myxomycetes arising from the fusion of amoeboid cells; later the botanist Hanstein (1880) introduced the term symplast, which signified multinucleated formations corresponding to many cells (polyenergids of Sachs). All these terms began to be used in histology by different authors in very different senses, which created the need for their differentiation and more precise definition. Bonnet (1903) proposed using the term S. for formations arising from previously isolated cells through the destruction of their boundaries; the term plasmodium for multinucleated masses resulting from incomplete cell division; and the term symplasm for dying and fusing masses. Studnicka (1911, 1929) defines S. as a multinucleated formation corresponding to several cells, and symplasm as tissue in which cells fuse together. Rubashkin (1931) distinguishes S. as the connection of cells by anastomosing processes and symplast as their fusion into a common protoplasmic mass. The establishment of precise nomenclature is hindered by the existence of numerous transitions between individual forms and often the uncertainty of their origin, therefore many authors use the term S. indiscriminately for all formations (the term syncelium, proposed as a general name, did not catch on). Syncytial forms are found in various tissues and organs, both developed and embryonic. The epithelial syncytium is a continuous protoplasmic sheet without cell boundaries; in this form it has been described several times in invertebrates (hypodermis) and in vertebrates (liver capillaries, glomerulus, Bowman's capsule); in addition, giant multinucleated formations are found in the endothelium of the pleura, pericardium, and Descemet's membrane. In the group of connective tissue, various forms of S. have been described. Fibroblasts and fibrocytes, connecting with processes of varying widths, can form networks that, after tissue irritation, break down into separate cells (Mebes); similar networks are observed in fibroblast cultures. Reticular tissue in the early stages of development also has the character of S. (cytogenic tissue). Another type of syncytium is observed in the rudiments of cartilage tissue, where cells fuse into one mass, losing their boundaries. From isolated formations, polycaryocytes (osteoclasts) and giant cells, which arise in pathological cases (introduction of foreign bodies, various bacteria) through the fusion of free cells, are classified as S. The striated muscle fiber has long been recognized as S; it arises in lower vertebrates (cyclostomes) through the fusion of cells (Maurer), in higher vertebrates through incomplete cell division; embryonic cardiac muscle has a clear character of reticular S. and apparently retains it further. The nervous system in the early stages of development is described as "neurosynchronytium" (His); later, neuroblasts separate from it, while neuroglia, as many authors now recognize, retains the character of S. throughout life. S. is also found in reproductive organs: this includes the follicular epithelium of the testes of both invertebrates and vertebrates; the absence of cell boundaries has also been described in the germ cords of the developing testis. Large plasma masses are formed in the placenta as a result of tissue death. Finally, the trophoblast of the human embryo in the early stages has a clearly expressed plasmodial layer on the surface. Experimentally, it is possible to induce a syncytial state in cleaving eggs (sea urchins) by the action of alcohol and other substances. It should be borne in mind that many cases described as S. must be treated critically; this applies primarily to the absence of cell boundaries in tissues: they have more than once been detected using more advanced techniques. The widespread occurrence of syncytial forms has often been cited as an argument against the cell theory. A number of authors (Sedgwick, Whitman, Rohde, Rubashkin) prove on this basis that "non-cellular" formations predominate in the developed organism and that the isolation of individual cells is caused each time by special circumstances. The opinion was even expressed (Delage, Labbe) that the cell is generally a secondary formation, and that Metazoa originate from multinucleated protozoa similar to syncytium. Under pathological conditions, the breakdown of syncytial formations, the disintegration of S. with the active division of the resulting free forms or their degeneration, is often observed. On the other hand, the fusion of individual cells and the development of plasmodial structures may be noted. The first process is more common.