Stroma

STROMA (from Greek stroma - bedding), a concept denoting the supporting or structural framework of an organ. In this respect, the concept of S. is opposed to the concept of parenchyma (see). Typically, S. consists of a capsule covering the organ externally, and trabeculae extending from it into the organ and forming a kind of skeletal framework. S. is constructed from dense connective tissue, rich in elastic fibers and often containing smooth muscle fibers (see Parenchyma).-S t r o m a of the cell. This term denotes structural formations that determine or fix the shape of the cell. Since the aggregate state of protoplasm is liquid, the cell under the influence of surface tension forces would always have to be spherical. In cases where the cell has some constant form different from spherical, and this form does not depend on the contact of the cell with neighboring tissue elements (cells or intercellular formations), but is determined by intrinsic properties inherent to this cell, then the presence of such a form presupposes the existence of any external or internal skeletal formations, i.e., stroma, giving the cell a specific form. External skeletal formations are represented by the pellicle-plasma membrane, which is the outer layer of protoplasm that has undergone a transition to a gel state. The external pellicle may be reinforced by internal skeletal parts incorporated into it. The denser, thicker, and harder the outer layer of the cell, the more strongly it stabilizes the cell's form. In addition to the pellicle, an external static organelle of the cell may be a membrane, for example, the sarcolemma of a muscle fiber, which is also a colloidal modification of the surface layer of cytoplasm and differs from the pellicle in greater thickness, density, double-contour, and also in that it is sharply demarcated from the cytoplasm. A dense shell developing on one side of the cell is called a cuticle. Sometimes a cell with liquid cytoplasm, regardless of the presence or absence of a pellicle, fixes its specific form with the help of an internal skeleton made of the finest rigid fibrils. These fibrils, usually clearly visible in the living cell due to strong light refraction, should be considered as gelatinized parts of protoplasm (tonofibrils of M. Heidenhain), possessing along with rigidity great elasticity and resilience. Tonofibrils are well developed in the epithelium of the skin, where, passing from cell to cell through intercellular bridges, they form springy systems that give the epidermis great rigidity. Supportive fibrils are particularly strongly developed in infusorians, where they often form complex systems giving the infusorian body a complex and bizarre shape. Investigating the heads of spermatozoa of various animals, N. K. Koltsov discovered that the specific form of these cells is determined by the presence of skeletal supporting threads. Generalizing his observations, Koltsov came to the conclusion that all cells in one form or another have a solid skeleton. Supportive fibrils usually run along the periphery of the cell, singly or in bundles, sometimes passing from one cell to neighboring ones without interruption. Skeletal fibrils similarly form the basis of cilia or flagella. The latter are constructed from a thin axial elastic thread clothed with a layer of protoplasm. In the cells of ciliated epithelium, in addition to the axes of the cilia, skeletal fibrils form within the protoplasm the so-called intracellular thread apparatus (Faserwurzeln), consisting of thin fibrils converging toward the nucleus in the form of a cone. A similar structure (axial skeletal thread clothed with a layer of protoplasm) is also found in the tails of spermatozoa. In addition to supportive tonofibrils, there are also fibrillar formations to which a certain physiological function is attributed (myofibrils, neurofibrils). However, this does not exclude the possibility of them simultaneously performing a static supporting function for the cell containing them.---One can speak of the stroma of the nucleus only in relation to fixed and stained nuclei, since the living nucleus in the vast majority of cases is optically empty and reveals no structures. After fixation (especially with mercuric chloride mixtures), a more or less dense network is found in the nucleus, bearing the name of linin or achromatin and usually considered as the S. of the nucleus. In the nodes of this network, chromatin clumps are deposited upon fixation. In pathology, the concept of stroma and parenchyma is particularly frequently used in the doctrine of tumors (see).