Parenchyma

PARENCHYMA (properly pronounced parenkhima) (from Greek para- around, near and echeo- I pour, fill). At present, this word has lost its meaning as a term, but is still used in descriptive and microscopic anatomy in the same sense as in antiquity, namely - to denote the proper substance of large glands and gland-like organs: liver, spleen, kidneys, lungs, etc., in contrast to the dense capsule (capsula, tunica) covering these organs and the partitions (septa) or trabeculae extending from this capsule in some cases. In modern morphology, the word P. entered with the time of Blumenbach. At present, the word parenchyma is used purely descriptively, and with this name are designated structures that often have nothing in common with each other either morphologically or functionally. This word is a relic of medieval concepts in the modern scientific language, having lost its meaning, and therefore modern morphologists try to avoid using it. Usually the concept of P. is contrasted with the concept of stroma, under which is understood the connective tissue basis of an organ, the interior of which is filled with the pulp-parenchyma. The stroma is built from dense connective tissue rich in elastic fibers; often it contains smooth muscle fibers. Anatomically, the stroma usually breaks down into the capsule surrounding the organ, from which partitions-trabeculae or septa extend into the organ, which often causes the voluminous glandular organ to be divided into lobes and lobules. Through the capsule along the trabeculae into the organ penetrate the blood and lymphatic vessels that nourish it, as well as nerves. Thus, the scheme of the structure of a glandular or gland-like organ is as follows: on the outside, a connective tissue fibrous capsule, from it inward extend similar trabeculae, carrying blood vessels and nerves in themselves, and the space between them and the capsule is filled with the working part of the gland-pulp, or parenchyma. Along these connective tissue partitions, the excretory ducts exit from the organ, carrying away the secretory products of the organ (if we are dealing with an exocrine gland). However, sometimes the opposition of stroma and P. becomes fundamentally impossible - in the case of the spleen or lymph node, both P. and stroma consist of connective tissue, although of different nature. Strictly speaking, P. of different organs cannot be compared with each other, since, as indicated above, with this word are designated structures that have nothing in common with each other. Indeed, for example, the "parenchyma" of the liver, consisting of epithelial glandular tissue, and the "parenchyma" of the spleen, consisting of reticular connective tissue with its derivatives, differ sharply from each other both in origin from different embryonic leaflets (entodermal parenchyma of the liver and mesodermal P. of the spleen), and in the method of embryonic development, and in the prospective potentials of their constituent elements (highly differentiated and specialized glandular cells of the liver, on the one hand, and retaining embryonic character pluripotent reticular cells and lymphocytes of the spleen-on the other), and of course in function. Similarly, there is little in common between the "parenchyma" of other organs, for example, between P. of the ovary and thyroid gland, etc. Often the P. of an organ turns out to be complex, and its constituent parts differ sharply in origin, properties, and functions. Such is, for example, the difference between the connective tissue P. of the cortical part of the thyroid gland and the epithelial P. of its medullary part, or between the P. of the cortical layer of the adrenal gland, originating from the coelomic epithelium, and the P. of its medullary part, having neurogenic origin. However, in both cases this complex P. is in a common capsule. The examples given confirm the indefiniteness of the concept of P. and the undesirability of using this word in the scientific language. Along with the word P., the expression "parenchymatous" is sometimes used to denote formations rich in P. (for example, parenchymatous organs), as well as various changes if they concern mainly P. (for example, parenchymatous degeneration, parenchymatous inflammation).

V. Aleshin.