Fine structural analysis of extraocular muscle spindles of a two-year-old human infant

PURPOSE. To clarify whether structural peculiarities formerly described in extraocular muscle (EOM) spindles of aged persons are already present in EO M spindles of a 2-year-old infant. METHODS. Distal halves of two EOMs obtained from a 2-year-old multiorgan do nor were immersion fixed and prepared for electron microscopy. The fine str ucture of 10 muscle spindles and of 1 "false spindle" was investigated. RESULTS. Extraocular muscle spindles of an infant 2 years of age had 2- to 4-layered outer capsules, 376 mu m (range, 217-606 mu m) long and 97 mu m ( range, 55-140 mu m) wide. In 10 EOM spindles, 4 to 16 intrafusal muscle fib ers (mean, 7.9) were present. From a total of 79 intrafusal fibers, 43 (54% ) were nuclear chain fibers, and 8 of the 43 exhibited posttraumatic degene rative changes. Thirty-six (46%) intrafusal fibers indistinguishable from e xtrafusal fibers were called anomalous fibers. No nuclear bag fibers were f ound. Each muscle spindle contained a variable number of chain fibers and a t least one anomalous fiber. Sensory nerve terminals were restricted to the 35 normal chain fibers but were absent from damaged chain libers and from anomalous fibers. One "false spindle" without a periaxial space was compose d of three anomalous fibers and one chain fiber, all of them devoid of sens ory terminals. CONCLUSIONS. Most structural particularities of human EOM spindles describe d in aged persons are already found in the infant. They cannot be interpret ed as age-related changes, but rather they represent specific features of h uman EOM spindles.