Organization of the Fibrous Component of Connective Tissue Paraneural Structures in Different Periods of Ontogenesis

Background: The steady increase in road traffic accidents, seasonal injuries leading to damage to the peripheral nerves of the extremities, as well as the military conflicts that are becoming more frequent now actualize research aimed at improving the diagnosis, treatment, and prevention of post-traumatic changes in the nerve trunks of the extremities. The frequency of peripheral nerve injuries varies from 1.5% to 13% of all injuries in peacetime; and during hostilities, it reaches 20%, and disability is 60%. At the same time, specific destructive changes in the nerves of the extremities after their traumatic injury require extraordinary organizational, therapeutic, and functional approaches to their restoration. The purpose of this study was to evaluate the dynamics of changes in the fibrous component of the paraneurium connective tissue structures in different periods of ontogenesis.
Methods and Results: The study consisted of two stages. At the first stage, to study changes in the paraneurium connective tissue of the sciatic nerve (SN) in vivo, ultrasound was performed using an RS85 ultrasound scanner (Samsung Medison, South Korea, 2021) and two linear transducers, LA4-18B and LA2-9A. The thickness of the SN and the surrounding paraneurium were measured, and their structural organization was evaluated. The inclusion criterion was the absence of pathology from the peripheral nervous system.  All subjects were divided into four age groups (15 people in each group): Group 1 (0-11 years), Group 2 (12-25 years), Group 3 (26-40 years), and Group 4 (41-60 years).
The second stage was performed on cadaveric material of the paraneurium connective tissue of the SN of persons of both sexes, of different ages, whose cause of death was not related to diseases or injuries of the nervous system. To determine the qualitative and quantitative ratio of collagen fibers of different degrees of maturity in the connective tissue structures of the paraneurium tissue of the sciatic nerve, polarization microscopy (MicMed-6, Lomo, Russia) was applied, and the basic principle of double refraction, which in combination with Sirius red staining, made it possible to differentiate types I and III collagen. The amount of each collagen type was determined by analyzing the color gamut after Sirius red staining in polarizing light. Fibers containing type I collagen had a red glow, while those containing type III collagen had a green glow. The ratio of collagen types was calculated using the Fiji program (USA, 2022).
US examination revealed the presence of a non-pronounced bilateral asymmetry in the structural organization of the paraneurium and an increase in the thickness of the SN with age, from 0-11 years to the age group of 41-60 years.
Polarization microscopy of micro-preparations of the human sciatic nerve with paraneurium structures made it possible to analyze the density of the fibers of the paraneurium connective tissue and identify types I and III of collagen. At the age of 11, the amount of type I collagen fibers was maximum (69.74±0.41%), and type III collagen fibers amounted to 30.26±0.44%. Further, with increasing age, the amount of collagen III significantly diminished. The ratio of type I to type III collagen increased with patient age, with the highest ratio in the age group 41-60 years.
Conclusion: The revealed dynamics of changes are quite comparable with the age-related features of connective tissue since it is this tissue that makes up the morphological substrate of the paraneurium of the sciatic nerve.