Origin Of Intrafusal Fibers From A Subset Of Primary Myotubes In The Rat
afferents; Anatomy & Morphology; development; Developmental Biology; developmental myosins; expression; intrafusal fibers; isoforms; muscle; muscle spindles; muscle spindles; myosin heavy chains; myosin heavy-chain
S46, a monoclonal antibody (mAb) specific for the SM-1 and SM-2 isoforms of avian slow myosin heavy chains (MHC), was used to study the earliest stages of development of intrafusal fibers in muscle spindles of the rat hindlimb. Spindles formed only in the regions of fetal muscles that contained primary myotubes reactive to mAb S46, such as the axial region of the tibialis anterior muscle. The first intrafusal fiber to form, the nuclear bag, fiber, originated from within the population of S46-reactive primary myotubes. Binding of mAb S46 by myotubes giving rise to the bag, fibers preceded the appearance of encapsulated spindles in the muscles by electron microscopy. However, reactivity to S46 intensified in the myotubes transforming into bag, fibers after the innervation of the fibers by afferents, and dissipated in myotubes differentiating into slow-twitch (type I) extrafusal fibers. Thus, afferents may enhance intrafusal expression of the MHC isoform reactive to mAb S46. The pattern of S46 binding to nuclear bag and chain intrafusal fibers in both developing and adult spindles was the same as that reported for the mAb ALD19, suggesting that both antibodies bind to the same MHC isoform. This isoform is probably a developmental form of slow myosin, because it was transiently expressed during the development of type I extrafusal fibers. The origin of bag, intrafusal and type I extrafusal fibers from a bipotential subpopulation of primary myotubes reactive to mAb S46 correlates with the location of muscle spindles in the slow regions of muscles in adult rat hindlimbs.
Kucera J; Walro J M
Anatomy and Embryology
1995
1995-08
Journal Article or Conference Abstract Publication
<a href="http://doi.org/10.1007/bf00186003" target="_blank" rel="noreferrer noopener">10.1007/bf00186003</a>
Sequences Of Intrafusal Fiber Formation Are Muscle-dependent In Rat Hindlimbs
Anatomy & Morphology; cat; development; Developmental Biology; embryonic-development; expression; innervation; intrafusal fibers; motor; muscle; muscle spindles; myosin heavy chains; myosin heavy-chain; neonatal rats; skeletal-muscle; slow myosin; spindles; tenuissimus muscles
A rat muscle spindle typically contains four intrafusal fibers - one nuclear bag(2), one nuclear bag, and two nuclear chain fibers. We compared the sequence of formation of the three intrafusal fiber types among the tibialis anterior (TA), soleus (SOL) and medial gastrocnemius (RIG) muscles using immunocytochemistry of spindle-specific myosin heavy chain isoforms. Spindles of the TA began to differentiate earlier and acquired the full complement of intrafusal fibers sooner than spindles of the SOL or MG muscles. At the onset of spindle assembly, the intrafusal myotubes expressed myosin heavy chains similar to those expressed by extrafusal myotubes. The first intrafusal myotube then differentiated into the bag, fiber regardless of the muscle. However, the fate of the second-forming intrafusal myotube varied among the muscles studied. It usually differentiated into a chain fiber in the TA, into a bag(1) fiber in the SOL, and into either a bag(1) or a chain in the MG. The fate of the third-forminge was reciprocal to that of the second; i.e. in those spindles in which the bag(1) fiber was second to form, a chain was third, and vice versa. The fourth and last intrafusal myotube gave rise to a chain fiber. The inter- and intramuscular variability in the fate of intrafusal myotubes of the second and third generation argues against the existence of a program intrinsic to the myotubes that would mandate their differentiation along specific paths. Rather, an extrinsic regulatory factor, probably associated with the primary afferent neuron, may govern differentiation of pluripotential myotubes into particular types of intrafusal fiber. The fate of the intrafusal myotubes might then depend on the timing of the regulatory effect of afferents relative to the stage of development of the intrafusal bundle.
Kucera J; Walro J M
Anatomy and Embryology
1994
1994-09
Journal Article or Conference Abstract Publication
<a href="http://doi.org/10.1007/bf00234305" target="_blank" rel="noreferrer noopener">10.1007/bf00234305</a>
Differential-effects Of Neonatal Denervation On Intrafusal Muscle-fibers In The Rat
afferents; Anatomy & Morphology; denervation; Developmental Biology; expression; heavy-chain isoforms; innervation; intrafusal muscle fiber; motor denervation; muscle spindles; myosin heavy chains; neonatal rat; nerve dependence; nerve growth-factor; spindle development; spindles
The response of developing muscle spindles to denervation was studied by sectioning the nerve to the medial gastrocnemius muscle of rats at birth. The denervated spindles were examined daily throughout the first postnatal week for changes in ultrastructure and expression of several isoforms of myosin heavy chain (MHC). Each of the three different types of intrafusal muscle fiber exhibited a different response to denervation. Within 5 days after the nerve section nuclear bag, fibers degenerated completely; nuclear bag1 fibers persisted, but ceased to express the 'spindle-specific' slow-tonic MHC isoform and thereby could not be differentiated from extrafusal fibers; nuclear chain fibers did not form. The capsules of spindles disassembled, hence spindles or their remnants could no longer be identified 1 week after denervation. Neonatal deefferentation has little effect on these features of developing spindles, so removal of afferent innervation is presumably the factor that induces the loss of spindles in denervated muscles. Degeneration of the bag2 fiber, but not bag1 or extrafusal fibers, reflects a greater dependence of the bag2 fiber than the bag1 fiber on afferent innervation for maintenance of its structural integrity. This difference in response of the two types of immature bag fiber to denervation might reflect an origin of the bag2 fibers from a lineage of myogenic cells distinct from that giving rise to bag1 or extrafusal fibers, or a difference in the length of contact with afferents between the two types of bag fiber prior to nerve section.
Kucera J; Walro J M; Reichler J
Anatomy and Embryology
1993
1993-04
Journal Article or Conference Abstract Publication
<a href="http://doi.org/10.1007/bf00185898" target="_blank" rel="noreferrer noopener">10.1007/bf00185898</a>