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Prenatal Development, Histochemistry and Innervation of Porcine Muscle^{1, 2,}

University of Wisconsin, Madison 53706

Abstract

Prenatal development and innervation of porcine muscle was studied using a variety of techniques for light microscopy. Two types of fetal myofibers were found. Those designated as primary fetal myofibers were characterized by the arrangement of their myofibrils in a subsarcolemmal hollow cylinder, by the location of their nuclei in an axial core of cytoplasm, by their predominance in younger fetuses, by their tendency to lie centrally within the developing fasciculi of older fetuses, and by their metamorphosis to a postnatal type of myofiber construction just before birth. Those designated as secondary fetal myofibers were characterized by the arrangement of their myofibrils in an axial strand, by the location of their nuclei in a subsarcolemmal position, by their increase in numbers in older fetuses, by their tendency to lie peripherally within the developing fasciculi of older fetuses and by having a postnatal type of myofiber construction prenatally. In a series of fetuses of increasing age, a progressive restriction of areas of cholinesterase activity on fetal myofibers was associated with a corresponding simplification and localization of neural components of the neuromuscular junction. With accepted histochemical methods, no prenatal histochemical differentiation between myofibers could be demonstrated conclusively. However, using an unorthodox method of postvital staining with methylene blue, myofibers located centrally within the developing fasciculi of older fetuses were stained in a pattern similar to the postnatal distribution of myofibers with a high content of oxidative enzymes.

¹ Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison, and by Public Health Service Research Grant FD-00107-14.

² Muscle Biology Laboratory manuscript No. 25.

³ Part of this work was done during the tenure of a Research Fellowship of Muscular Dystrophy Associations of America.