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1996

The early development of muscle spindle in human foetus

Ital J Anat Embryol. 1996 Jul-Sep;101(3):163-72.

Deriu F, Tedde Piras A, Montella A. Source Institute of Normal Human Anatomy, School of Medicine, University of Sassari, Italy.

Abstract

The normal development of muscle spindles has been carefully described in a variety of species but only a few attempts were made to study the embryological development of the muscle spindle in humans. Most of the studies aimed to define the early development of muscle spindle in human foetuses, stated the begin of this process at the 11th week of intrauterine life, when the formation of a network of very fine nerve fibres with enlargements and ringle around developing muscle cells occur. In the present study we tried to document the probably earliest time of appearance of muscle spindles in the skeletal muscle of human foetuses. To this aim we examined fragments of deltoid and gastrocnemius muscles removed from human foetuses at the 9th and 10th week of gestation by using the light microscope technique. Data collected in muscle specimens at the 9th and 10th week of gestation showed the presence, at this time, of a structure with features clearly different from that of the adjacent muscular areas. This structure consists in a number of flat mesenchymal cells surrounding and forming several layers around a bundle of myoblasts smaller and lesser differentiated than the muscle cells located around and it is in close relationship with nervous fibres. These morphological findings might allow to identify the earliest stage of muscle spindle formation in human species.

PMID 9112824

1995

Origin of intrafusal fibers from a subset of primary myotubes in the rat

Anat Embryol (Berl). 1995 Aug;192(2):149-58.

Kucera J, Walro JM. Source Department of Neurology, Boston University Medical Center, MA 02118-2394, USA.

Abstract

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 bag2 fiber, originated from within the population of S46-reactive primary myotubes. Binding of mAb S46 by myotubes giving rise to the bag2 fibers preceded the appearance of encapsulated spindles in the muscles by electron microscopy. However, reactivity to S46 intensified in the myotubes transforming into bag2 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 bag2 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.

PMID 7486011

1984

Stages in the development of cat muscle spindles

J Embryol Exp Morphol. 1984 Aug;82:177-216.

Milburn A.

Abstract

The structure of developing spindles has been examined in cat peroneal muscles by light and electron microscopy, beginning at the 34- to 38-day foetal stage. By this stage alpha motoneurons have formed end-plates on primary myotubes. Secondary extrafusal myotubes then develop beneath the basal lamina of primary myotubes, and are innervated by motor axons early in their assembly. First-series secondary myotubes separate from primary myotubes prior to the development of subsequent series. The assembly of extrafusal fibres is completed by birth. Intrafusal fibres assemble in a similar manner. At the 34- to 38-day foetal stage developing spindles consist of a single primary myotube containing a small accumulation of myonuclei beneath the terminals of the Ia afferent axon. Simple motor nerve terminals also innervate this myotube, which will ultimately become the bag2 fibre of the mature spindle. Secondary intrafusal myotubes then assemble beneath the basal lamina of the primary bag2 myotube, in the order presumptive bag1, long-chain, intermediate-chain and typical-chain fibres. Their assembly begins at the equator, beneath the sensory terminals, and spreads to the poles. The bag1 and long-chain myotubes separate from the bag2 in the spindle pole prior to the development of the other chain fibres. The assembly of intrafusal fibres is completed by birth. The periaxial space begins to develop in the first postnatal week. The development of tandem spindles containing b2c units is described. The role of sensory and motor innervation in the assembly and differentiation of mammalian intrafusal fibres is discussed.

PMID 6238118

http://dev.biologists.org/content/82/1/177.full.pdf

1982

The number and distribution of muscle spindles in human intrinsic post vertebral muscles

J Anat. 1982 Oct;135(Pt 3):585-99.

Amonoo-Kuofi HS.

Abstract

Serial transverse sections of intrinsic postvertebral muscles of a 21 weeks old human fetus were studied after staining with haematoxylin and eosin, Van Gieson's stain with Weigert's haematoxylin, and Glees' silver impregnation technique. Muscle spindles have been demonstrated throughout the entire length of the intrinsic postvertebral muscles. The total numbers of spindles on the right and left sides were determined. Using a special technique of graphic reconstruction, the locations of spindles within these muscles were plotted. Regional differences in the numbers and distribution of spindles were noted. The significance of these findings has been discussed.

PMID 6218153