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Fine structural analysis of extraocular muscle spindles of a two-year-old human infant
R Blumer1, J R Lukas, M Aigner
1Institute of Anatomy, University of Vienna-General Hospital, Austria.
Insights
Structural peculiarities in infant extraocular muscle (EOM) spindles are present at 2 years old. These findings suggest these are inherent features, not age-related changes in human EOM spindles.
Area of Science:
- Ophthalmology
- Muscle Physiology
- Developmental Biology
Background:
- Extraocular muscle (EOM) spindles are proprioceptors crucial for eye movement control.
- Previous studies described unique structural features in EOM spindles of aged individuals.
Purpose of the Study:
- To investigate if the structural peculiarities observed in aged human EOM spindles are also present in a 2-year-old infant.
- To determine if these features represent age-related changes or inherent characteristics of human EOM spindles.
Main Methods:
- Electron microscopy was used to examine distal halves of EOMs from a 2-year-old multiorgan donor.
- The fine structure of 10 EOM muscle spindles and one "false spindle" was analyzed.
Main Results:
- Infant EOM spindles exhibited 2- to 4-layered capsules, with lengths averaging 376 micrometers.
- Spindles contained 4-16 intrafusal fibers, with 46% identified as anomalous fibers lacking sensory innervation.
- No nuclear bag fibers were found; sensory nerve terminals were absent from damaged and anomalous fibers.
Conclusions:
- Most structural particularities previously noted in aged human EOM spindles were present in the 2-year-old infant.
- These findings indicate that these features are specific characteristics of human EOM spindles, not indicative of age-related degeneration.
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.