Myoma

MYOMA, myoma (from Greek mys-muscle), a tumor of muscle tissue. Two types of M. are distinguished: some consist of smooth muscle fibers and are called leiomyomas (leiomyoma), while others consist of elements of striated muscle and are called rhabdomyomas (rhabdomyoma, or myoma striocellulare). Leiomyomas in their pure form, i.e., consisting only of smooth muscle fibers, are rarely encountered (gastrointestinal tract) and are characterized by small size [see vol. XV (pp. 431-432), fig. 5]. The muscle cells in such M. may be connected into bundles, in which rod-shaped, more or less pyknotic nuclei are oriented in one direction, without the participation of interstitial tissue, so that a third cell moves between two cells with its ends. Bundles of smooth muscle cells pass in various directions and planes, and on a microscopic section, there is an alternation of longitudinal bundles with transverse ones, with nuclei cut across having the shape of round dots. The diversity in the direction of muscle bundles distinguishes the tissue of leiomyoma from hypertrophic muscle tissue. Usually, M. along with smooth muscle fibers contains a greater or lesser amount of connective tissue and elastic fibers, and the tumor takes on the character of a fibromyoma (see) (fibromyoma). The most common and typical site of development for fibromyomas is the uterus (see). Leiomyomas are less frequently found in the vagina, ovaries, and broad ligaments. The second, much rarer site for myomas is the digestive tract. Here they occur in the stomach as well as in the intestines, more rarely in the esophagus. In the intestines, they can cause intussusception. Fibromyomas are even rarer in the skin, where they are often multiple. Regarding the histogenesis of leiomyomas and fibromyomas, there are still no definite and positive data. Thus, Klebs and his followers derive the histogenesis of leiomyomas and fibromyomas from connective tissue and the muscular coat of blood vessels. Pozharsky considers this hypothesis incorrect in view of the fact that blood vessels are present in almost all tissues, while tumors occur only in certain organs. According to the hypothesis of Cohnheim, these tumors arise from lost embryonic rudiments; in confirmation of this, the fact is cited that in the composition of uterine fibromyomas, epithelial elements in the form of glandular passages, lined with cuboidal, cylindrical, and even ciliated epithelium and attributed to remnants of Wolffian bodies or Müllerian ducts, are sometimes found. However, this very fact of the presence of epithelium in the composition of tumors, in the opinion of some researchers, speaks against Cohnheim's hypothesis and points to new paths in solving the question. It is becoming clear that epithelial elements can be pinched off in the extra-embryonic period from the epithelium of the uterine mucosa (Recklinghausen) and the serous membrane (Meyer). The above-mentioned epithelial-muscle tumors are called adenomyomas (adenomyoma). The latter can be observed in the uterus and tubes, rarely in the vagina, breast gland, and pylorus of the stomach. If one takes into account that adenomas as well as adenomyomas can develop in this way, it is easy to conclude that myomas also arise in connection with epithelium, and in the gradual development of these tumor forms, epithelial elements either die (Pozharsky) or are completely used up, transforming into the myomatous part of the tumor (Korsitsky). It should be noted, however, that in most myomas and fibromyomas of the uterus, glandular elements cannot be found, and the question of their histogenesis requires further study. Among the causal theories regarding uterine M., mention should be made of the theory of irritation (Virchow), infectious (Korde), and hormonal (Seitz) (see Tumors, Uterus). The growth of leiomyomas is expansive and slow, for which reason they are classified as benign tumors; however, although rarely, malignant myomas (myoma malignum) are encountered. The possibility of malignant transformation of benign leiomyomas and fibromyomas is doubtful. Malignant myomas should be considered not as having developed from benign ones, but as having arisen primarily and independently. Histologically, they differ from benign leiomyomas by the larger size of muscle cells, as well as by pyknotic nuclei. Polymorphism of cells and nuclei is often observed. Such polymorphic malignant myomas are often called myosarcomas. Rhabdomyomas are generally rare. In their pure form, they occur as pale nodules and thus do not resemble muscle tissue externally. The tumors are nodular, lobulated, and soft to the touch. Histologically, the fiber bundles sometimes contain transverse striations, sometimes they are devoid of them. In general, their histological structure is diverse. Sometimes these are long, thin fibers, bulbously swollen at one end, sometimes these are spindle-shaped, oval, and even round cells of the myoblast type. Sometimes there is also longitudinal striation of the protoplasm. Rhabdomyomas are most often observed in the kidneys and other parts of the urinary system; here they have an embryonic origin and are often a component of mixed tumors; rhabdomyomas rarely occur in the heart, where they are multiple, reach the size of a walnut, and consist of spider-like cells with transverse striation in the peripheral parts. Abricosov and other authors believe that these cells are similar to Purkinje cells and that the tumors themselves should be classified as hamartomas (see Hamartoma, hamartia, hamartoblastoma). Very rarely do rhabdomyomas develop in striated voluntary muscle. Most of the above-mentioned types of rhabdomyomas are benign neoplasms; sometimes, however, they exhibit a malignant course and grow like sarcomas (rhabdomyosarcomas). The cells of rhabdomyomas usually contain glycogen.