The early development of muscle spindle in human foetus

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.