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Heterotopic striated muscle tissue was found in the myometrium of a 50-year-old Japanese woman with myoma uteri. It is suggested that the tissue originated in a differentiation of stromal cells in the myometrium. Susumu Terada, Department of Obstetrics and Gynecology, Kanazawa University, School of Medicine, Kanazawa (Japan) Introduction Discovery of a wide variety of heterotopic tissues in the endometrium has been described in the literature, including bone [3], cartilage [7], smooth muscle [4], and nerve [7]. However, the occurrence of heterotopic striated muscle in the myometrium is very rare [2]. The present paper reports heterotopic striated muscle in a uterus with myomata. Case History M.C. (No. 88–373), a 50-year-old, self-employed Japanese woman complained that menstruation was profuse, with blood clots. She had had 5 pregnancies with 3 normal deliveries and 2 spontaneous abortions. At age 34 she had a tubal ligation to induce sterility. Myoma uteri was diagnosed 7 or 8 years previously on a checkup for vaginal cancer. She visited a gynecological clinic because of severe lumbago and hypermenorrhea lasting 2 months. The physical examination was normal. Gynecological examination revealed a large uterus with a tumor mass of the size of a goose egg on the left side of the uterine body. Ultrasonography revealed a giant mass with only partial calcification and a myometrial echo in the posterior wall of the uterus which strongly suggested myoma uteri. Laboratory findings were normal. A simple total hysterectomy was done. The uterus weighed about 1,100 g. It contained two large subse-rous tumerous nodules, one intraligamentary and the other pedun-culated. The myoma uteri was composed of smooth muscle cells, and the center of the nodule in the uterine body was degenerated and partially calcified. D ow nl oa de d by : 54 .7 0. 40 .1 1 11 /2 /2 01 7 6: 10 :0 5 P M On microscopic examination, heterotopic striated muscle tissue was detected in the stretched myometrium adjacent to the intraligamentary myoma nodule (fig. 1). The tissue was not observable macroscopically. The cut surface of the muscle tissue was composed of 113 irregularly shaped striated muscle cells ranging from 10 to 30 μm in diameter. The stroma was occupied by connective tissue and capillaries (fig. 2). Discussion The striated muscle tissue which was present in the uterus of this patient was about 1 mm in diameter and was present in the distended myometrium around the myoma. This tissue was mature striated muscle with a normal microscopic appearance. Since the myometrium was otherwise normal, the tissue appears to be heterotopic. Heterotopic tissues are rarely found in the myometrium. Further, although heterotopic bone, cartilage and glial tissue have been identified in the uterus [3,7], heterotopic muscle tissue (smooth or striated) has been reported only rarely [4, 5]. Striated muscle tissue and osteoid tissue in the fetal uterus was reported by Meyer [2]. However, there have been no reports of heterotopic striated muscle tissue alone in the myometrium in the international literature between January 1972 and January 1990. Thus, the present finding of heterotopic striated muscle tissue in the myometrium is considered significant. Several mechanisms might be suggested to explain the presence of heterotopic striated muscle tissue in the uterus. We cannot conclude the mechanism operant in this case, but let us consider the possibilities: The myometrium and stroma of the endometrium originate in the 192 Terada/Tomita/Nakagawa/Akasofu/Mizukami Fig. 1. Heterotopic striated muscle. Endometrium, myome-trium and striated muscle tissue (arrow) in the myometrium adjacent to intraligamentary myoma nodule. Hematoxylin and eosin. × 20. D ow nl oa de d by : 54.70.40.11-11/2/20176:10:05PM Fig. 2. Higher magnification of the striated muscle illustrated in figure 1. The nucleus of striatedmuscle is eccentrically placed. The stroma is occupied by connective tissue and capillaries.Hematoxylin and eosin. ×400.mesenchymal tissue surrounding the primordial utero-vagina. As the mesenchyme develops, themultipotential stem cell becomes a specific stem cell that can differentiate gradually into specifictissue, and further differentiate into progenitor cells with the proliferative faculty ofcharacteristic tissues, finally becoming well-differentiated cells [6]. Therefore, it is possible thata mesoder-mal cell remnant could become differentiated into normal striated muscle.There is a possibility that the tissue is an implantation of striated muscle of fetal origin followinga spontaneous or induced abortion [7]. Although the present patient has a history of abortion, thesmall amount of striated muscle tissue present in the myometrium, the absence of other fetaltissues, and the absence of myometrial wound healing (fibrosis) after curettage make it unlikelythat implantation of fetal tissue occurred in this case. The heterotopic tissue could be an acquiredgraft of pelvic floor muscle (a homograft), but, although there was a history of tubal ligation, anintramyometrial graft of pelvic floor muscle or abdominal wall muscle is unlikely. Meyer hasreported that striated muscle can reach the uterus with the internal cremaster muscle through theround ligament [1]. Such an origin is ruled out here because no component tissues of the roundligament were found in the myometrium other than the isolated striated muscle tissue.ReferencesMeyer, R.: Heterologe Gewebe. Handbuch der speziellen patho-logischen Anatomie undHistologie, Vol. VII/1, p. 49 (Springer, Berlin 1930).Meyer, R.: Heterologe Gewebe ohne Geschwulstbildung im Uterus und in einfachenGeschwulsten des Uterus. Handbuch der speziellen pathologischen Anatomie und Histologie,Vol. VII/1 ‚ p. 406 (Springer, Berlin 1930).Courpas, A.S.; Morris, J.D.; Woodruff, J.D.: Osteoid tissue in utero. Report of 3 cases. Obstet.Gynec. 24: 636–640 (1964). Bird, C.C; Willis, R.A.: The production of smooth muscle by theendometrial stroma of the adult human uterus. J. Path. Bact. 90: 75–81 (1965).Novak, E.R.; Woodruff, J.D.: Gynecologic and obstetric pathology with clinical and endocrinerelations; 6th ed., p. 244 (Saun-ders, Philadelphia 1967).Owen, M.: The origin of bone cells. Int. Rev. Cytol. 28: 213–238 (1970).Newton, C.W. Ill; Abell, M.R.: Iatrogenic fetal implants. Obstet. Gynec. 40:686–691 (1972). Downloadedby: 54.70.40.11-11/2/20176:10:05PM