Cell Tissue Res (2004) 315:223–231 DOI 10.1007/s00441-003-0826-x REGULAR ARTICLE Suzana Guimar¼es Moraes · Mônica Campos Pinheiro · Olga Maria Szymanski Toledo · Paulo Pinto Joazeiro Phenotypic modulation of fibroblastic cells in mice pubic symphysis during pregnancy, partum and postpartum Received: 9 December 2002 / Accepted: 14 October 2003 / Published online: 7 November 2003  Springer-Verlag 2003 Abstract In many species, the cartilaginous pubic sym- physis of the pregnant female is gradually replaced by a fibrous connective tissue, forming a flexible and elastic interpubic ligament. This newly formed ligament is responsible for the separation of the pubic bones, enabling safe delivery of the young. Following labor, the ligament undergoes rapid involution. To our knowledge, no previous work has focused on the phenotypic modulation that is responsible for the changes present at the interpubic ligament throughout the relaxation and closing of the symphysis. The purpose of this study was to investigate the ultrastructural features and immunophe- notype of the peculiar cell type found in the pubic symphysis of cycling, pregnant and postpartum mice. In particular, immunohistochemistry studies were conducted on the expressions of the cytoskeletal proteins desmin, vimentin and a-smooth muscle actin (a-SMA). During pregnancy, the pubic symphysis cells always expressed a-SMA, whereas the expression of vimentin and desmin was transient from early pregnancy to postpartum. Furthermore, the expression patterns of these three cytoskeletal proteins were distinct. Cells present in the medial region of the mouse symphysis in cycling and at D12 displayed ultrastructural features characteristic of a typical fibroblast. In contrast, during the last week of pregnancy and in postpartum these cells acquired ultra- structural features representative of a myofibroblast; for example, a fibronexus and a contractile apparatus were found to be present lying in close contact with the extracellular collagenous and elastic system fibrils. Taken together, these results strongly suggest a contractile function for these cells which might contribute to support of the varying mechanical stresses present during pubic bone movement. Keywords Pubic symphysis · Pregnancy · Immunohistochemistry · Desmin · Vimentin · a-SMA · Mouse (Swiss) Introduction In the last week of mouse pregnancy, the pubic bones separate to enable safe delivery of the young. This separation is attributed to three factors: (a) a progressive reabsorption of the symphyseal surfaces of the pubic bones; (b) the swelling of the cartilaginous matrix; and (c) the formation of a fibrous interpubic ligament. At parturition, this ligament undergoes a rapid and pro- nounced swelling which will transform it into a flaccid structure with a maximum length of 6 mm in mice (Hall 1947; Crelin 1954; Storey 1957; Kroc et al. 1958; Steinetz et al. 1957; Horn 1960). The symphyseal spreading is primarily hormonally regulated by estrogen and relaxin (Sherwood 1994) and occurs in other species, including guinea pigs (Ruth 1937; Talmage 1947a, 1947b; Wahl et al. 1977) and bats (Crelin and Newton 1969; Crelin 1969a). Modest relaxation of the pelvic ligaments also occurs during human pregnancy (Crelin 1969b; Vix and Ryu 1971; Gamble et al. 1986). However, it does not occur in species such as rats (Crelin and Brightman 1957; Ortega et al. 2001) and sheep (Bassett and Phillipps 1955). The formation of the interpubic ligament has been regarded essentially as a primary growth process, where chondrogenic cells released from their lacunae and the resulting fibroblasts arrange themselves, transversely oriented, along the collagen fibers. The resulting trans- formation changes the symphyseal tissue into a new structure that slightly resembles a true ligament (Crelin This work was supported by grants from Coordenaç¼o de Aper- feiçoamento de Pessoal de Nível Superior (CAPES). This paper is part of a thesis submitted to the Department of Histology and Embryology of the Biology Institute State University of Campinas, in partial fulfillment of the requirements for the Master’s degree. S. G. Moraes · P. P. Joazeiro Department of Histology and Embryology, LCI, Institute of Biology, CP 6109, UNICAMP, 13083-970 Campinas, Brazil M. Campos Pinheiro · O. M. S. Toledo Department of Morphology, UNIFESP, S¼o Paulo, Brazil