Functional conversion from sucking to mastication : An immunoelectron microscopic study concerning the development of peripheral nervous system.

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Mammals go through functional conversion from sucking to mastication during their postnatal development. This functional conversion seems to be closely related to the development and maturation of the peripheral nervous system, particularly in the innervation of the periodontal ligament. In this study, the postnatal development of Ruffini endings, categorized by low-threshold stretch mechanoreceptors, was ultrastructurally investigated in the upper incisors of the rat by means of protein gene product 9.5 (PGP 9.5) immunohistochemistry. The Wistar rats were divided into six groups according to their postnatal days : 1,4 (incisor un-erupted stage), 7-11 (incisor eruption stage), 15 (commencement stage of occlusion between upper and lower incisors), 25 (commencement stage of occlusion between upper and lower first molars), and 80 days (functional occlusion stage).(1) It is notable that mechanoreceptor-like structures being surrounded by two or more Schwann sheaths were recognized at 4days … More after birth. (2) At incisor eruption stage, parts of the axon terminals extended through the slits of Schwann sheaths and formed finger-like projections called axonal spines. These axonal spines developed in number and in length as well as in ramification during the period from incisor eruption to that of occlusal establishment. (3) Various types of small vesicles in axon terminals have decreased in number after the commencement stage of incisor occlusion. (4) The mitochondria in the axon terminals increased in number with time and became longer after the onset of occlusion between the upper and lower incisors. (5) The axonal spines makes no contact with collagen filament, indicating the intervention of an amorphous layr. The amorphous layr thickened and lamellated after the onset of occlusion between incisors. In conclusion, present study has disclosed that the ultrastructural differentiation and maturation of Ruffini endings proceed drastically during the period from incisor eruption to that of occlusal establishment. Less