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Muscle spindles in extraocular muscles of human infants
1Institute of Optometry, Buskerud College, Frogs vei 41, N-3600 Kongsberg, Norway. jan.richard.brunech@hibu.no
Insights
Infant extraocular muscle spindles exhibit atypical features, including incomplete fibers and lack of bag fibers. These findings indicate that unusual muscle spindle morphology is not due to aging.
Area of Science:
- Ophthalmology
- Neuroscience
- Histology
Background:
- Muscle spindles in human extraocular muscles (EOM) have peculiar morphology, raising questions about their proprioceptive function.
- It was hypothesized that these atypical features might be a result of aging.
Purpose of the Study:
- To investigate the morphology of muscle spindles in infant human EOM.
- To determine if the atypical features observed in adult EOM spindles are present in infants, thus clarifying if aging is the cause.
Main Methods:
- Light microscopy was used to examine serial transverse sections of 33 identified muscle spindles from five infant EOM.
- Structures were analyzed for the presence and type of intrafusal fibers, equatorial expansion, and capsule characteristics.
Main Results:
- Infant EOM spindles contained primarily nuclear chain fibers (74.3%), with no unequivocal bag fibers.
- Spindles showed limited equatorial expansion, incomplete inner capsules, and numerous atypical intrafusal fibers with extrafusal features (25.7%).
- A majority of nuclear chain fibers were interrupted or fragmented, with most spindles containing incomplete fibers.
Conclusions:
- The atypical morphology of muscle spindles, including incomplete and fragmented fibers, is present in human infants.
- These findings demonstrate that the peculiar morphology of EOM muscle spindles is not attributable to aging.
Abstract:
The capacity of muscle spindles in adult human extraocular muscles (EOM) to provide effective proprioception was questioned on the grounds of their peculiar morphology. Their appearance could be attributable to ageing and to test this possibility examples of infant muscle spindles have been examined. Forty encapsulated structures from five extraocular muscles removed post mortem from 4 infant patients aged 6 days, 5, 23 and 30 months were examined by means of light microscopy using serial transverse sections. Seven of them were identified as false spindles. The remaining 33 structures, identified as spindles, contained a total of 175 intrafusal fibres varying from 2 to 12 (mean: 6) in each. 130 of these fibres (74.3%) were of nuclear chain type. Unequivocal evidence of bag fibres was not found. Spindles lacked or had a limited equatorial expansion, and the inner capsule was incomplete and irregularly shaped. 45 (25.7%) of the intrafusal fibres had extrafusal features with large diameters, peripherally placed nuclei, no equatorial modification and without associated sensory nerve terminals. Serial sections revealed that a majority of the nuclear chain fibres were interrupted, fragmented or terminated abruptly, and most spindles contained at least one incomplete fibre. These observations show that the atypical features observed in adult human EOM spindles are also present in infants and are therefore not attributable to ageing.