Contribution to the development of the stapedius muscle structure in humanfetuses

The purpose of this report is to contribute to the knowledge of the morphog enesis of the human stapedius muscle. We have studied the stapedius structu re development in 17 temporal bones from human fetuses. Chronological ages were from 48 days (21 mm) to 29 weeks (270 mm). Samples were fixed in 10% f ormol, decalcified with 2% nitric acid, embedded in Paraplast, sectioned in sequence of 7 mum and were stained with haematoxylin & eosin and Martins' trichrome. Four maturation stages were observed in the development of the s tapedius muscle structure. First, the formation of the 'primordium muscular is' in the mesenchyme of the tympanic cavity (promyoblasts) was observed in a 36-mm embryo. Second, its differentiation into myoblasts, and its next c ellular fusion to form muscle tubes, was seen in a 99-mm foetus. In the thi rd stage, myofibrogenesis phenomena were developed, to synthesize, the myot ube myoblasts, myofilaments, phenomena we observed that took place during t he 14th to 17th weeks of development. The last stage was characterized by t he presence of striated muscle fibres, at first intermingled with myotubes (18th to 26th weeks), until in the 29th week fetuses (270 mm) the skeletal muscle fibre was completely differentiated. We conclude that in the 29th we ek of development (270 mm) the stapedius muscle structure is completely dif ferentiated, although the functional development is not complete until birt h.