Regression of polyneural innervation in the human psoas muscle

During the early stages of mammalian ontogeny muscle fibres are innervated by more than one axon. This polyneural innervation is replaced by mononeural innervation in the course of development. The regression of polyneural innervation in the psoas muscle in the human is the topic of the present study. Innervation patterns were studied in fetuses from 15 1/2 weeks of post menstrual age (PMA) and in babies until 80 weeks PMA (40 weeks after term age) and compared to data from two adults. Motor endplates were stained by a combined acetylcholinesterase stain. Innervation patterns and motor endplate morphology were studied and the sizes of endplates were measured. As a main result of our study polyneural innervation of the psoas muscle remains at a level of about 2 endings per endplate (range 1-5 terminals) until 18-25 weeks PMA and decreases thereafter. From 52 weeks PMA (12 weeks post term) onwards, muscle fibres are predominantly mononeurally innervated. During development the morphology of the terminal patterns of the nerve endings becomes more complex and the size of endplates increases, implying that the adult pattern of muscle innervation is reached at the age at which a major functional transformation in the neurobehavioural repertoire occurs (i.e. the end of the second and the beginning of the third month.