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 EOM spindles of a 2-year-old infant. METHODS Distal halves of two EOMs obtained from a 2-year-old multiorgan donor were immersion-fixed and prepared for electron microscopy. The fine structure 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 microm (range, 217-606 microm) long and 97 microm (range, 55-140 microm) wide. In 10 EOM spindles, 4 to 16 intrafusal muscle fibers (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 degenerative changes. Thirty-six (46%) intrafusal fibers indistinguishable from extrafusal fibers were called anomalous fibers. No nuclear bag fibers were found. Each muscle spindle contained a variable number of chain fibers and at least one anomalous fiber. Sensory nerve terminals were restricted to the 35 normal chain fibers but were absent from damaged chain fibers and from anomalous fibers. One "false spindle" without a periaxial space was composed of three anomalous fibers and one chain fiber, all of them devoid of sensory terminals. CONCLUSIONS Most structural particularities of human EOM spindles described in aged persons are already found in the infant. They cannot be interpreted as age-related changes, but rather they represent specific features of human EOM spindles.