PECULIARITIES OF STRUCTURAL REAR-RANGEMENT OF NEUROMUSCLE CONNECTIONS OF MASTICATORY AND TEMPORALY MUSCLES IN THE POSTNATAL PERIOD OF ONTOGENESIS

It is known that masticatory loads are one of the main mechanical factors in the morphogenesis of the oral cavity. They contribute to the formation and adaptive rearrangement of organs and tissues of the oral cavity and maxillofacial area in the postnatal period of ontogenesis. Dysfunction of the masticatory muscles is a powerful etiopathogenetic factor in the occurrence of abnormalities in the position of the teeth and the devel-opment of the jaws, leading to anomalies of occlusion. Therefore, the aim of our work was to establish the morphogenesis of neuromuscular connections (NMC) of the masticatory and temporal muscles in the postnatal period of ontogenesis. The material for the study was the temporal and masticatory muscles of 2-month-old and 6-month-old white outbred male rats. Histological, histochemical and electron microscopic research methods were used. Mor-phometry was performed using Image J version 1.47t. Computer data processing was performed using the statistical package STATISTICA 10. In 2-month-old rats, the intramuscular nerve conductors form much thinner bundles, and the number of nerve fibers in them is higher than in adult animals by one and a half to two times. In the area of ramification to the terminal branches, the axons lose the myelin sheath and are surrounded by single nuclei of terminal neurolemocytes and muscle fiber (MF). The area of NMC in 2-month-old animals in the temporal masticatory muscles is 1.6-1.5 times smaller than the corresponding values in adult animals. The pattern of axon branches in 6-month-old rats in the temporal and masticatory muscles is 1.7-1.9 times greater than the corresponding values in 2-month-old rats (in all cases p> 0.05). It should be noted that in 2-month-old animals on the same MF we observed NMCs, formed by terminals, originating from different axons, which may indicate their polyneuron innervation, whereas in 6-month-old animals it is only mononeuronal. A distinctive feature of the structure of the NMC is that in the temporalis muscle the area of the NMC and the pattern of ramification of the terminal branches of axons is probably larger than the corresponding indicators of the masticatory muscle. This pattern is observed in rats of different age groups. Electron microscopically, the components of NMC in young rats are significantly different from those in adult animals. The axoplasm of the terminals contains: single mitochondria with clearly differentiated cristae, a small number of microtubules, neurofilaments and synaptic vesicles. In the masticatory muscle of 2-month-old animals, the area of neuromuscular synapse (NMS) and the number of folds of the synaptic membrane are probably smaller compared to the temporalis muscle, while the sizes of other components of NMS in the two studied muscles probably do not differ. In 6-month-old rats, electron microscopic examination of the NMS showed a greater number of active zones (15-18) with an increased concentration of synaptic vesicles. Thus, the area of NMC and NMS in 2-month-old animals in the temporal and masticatory muscles is probably smaller than in 6-month-old animals. Some MF of 2-month-old animals have polyneuron innervation, whereas in 6-month old - it is only mononeuronal. In the temporomandibular muscle, the area of the NMC and the pattern of axon branches is probably larger than the corresponding indicators of the masticatory muscle, and this pattern is observed in rats of different age groups. The area of the NMS, the number and area of folds of the postsynaptic membrane and the distance between them, the number of synaptic vesicles increase with age, and the synaptoarchitectonics of the NMS in the studied muscles of different age groups of rats does not differ significantly.