E71 Distribution of cells and extracellular matrix in the normal marginal synovium of the hip joint capsule during the postnatal period
Authors: Fedotchenko Andrii 
Published: August 2023

Abstract

Introduction
The marginal transitional zone is of fundamental importance in joint pathology. It includes the marginal cartilage and marginal synovium. The normal marginal synovium takes part in articular cartilage repair (Adachi et al. 2012). The inflamed marginal synovium, in turn, promotes irreversible damage to the adjacent cartilage and bone (Hitchon & El-Gabalawy, 2011).

Objectives
The morphology of the normal marginal synovium from birth till the 90th day was studied using the hip joints of Wistar rats.

Methods
The collection of joints was carried out on the first, seventh, 14th, 30th, 45th, 60th and 90th day (six rats in each group, 42 in total). Joints were fixed, decalcified and dehydrated. Histologic paraffine sections were stained with Mallory's trichrome and Hart's elastin staining, analyzed at the acetabular labrum by light microscopy (χ100), morphometrical and statistical (P &lt; 0.05) methods with extrapolation to humans (Sengupta, 2013). The obtained data were presented as percentages.

Results
In the marginal synovium, cells and components of the extracellular matrix, which are typical for fibrous connective tissue, could be distinguished. The synovial lining cells are embedded in the modified basement membrane which is a meshwork of collagen and elastic fibers. They form a three-dimensional reticulum in it and adjacent synovial subintima. Vessels of the microcirculatory bed are also widely represented in the marginal synovium. The overall quantity of collagen fibers increased gradually, exceeding on the 90th day (∼18–19 human years) that of newborns by one and a half times (48.92 ± 3.39 vs 72.17 ± 7.77; P = 0.023). The number of arranged collagen fibers increased gradually, exceeding on the 90th day that of newborns almost twofold (37.25 ± 3.21 vs 60.08 ± 5.99; P = 0.008). The area of distribution of disarranged collagen fibers in newborns was almost four and a half times less than the overall quantity of collagen fibers (11.67 ± 1.03 vs 48.92 ± 3.39; P &lt; 0.001) and did not change substantially throughout the experiment. The number of elastic fibers increased till the 30th day (0.63 ± 0.11 vs 18.3 ± 1.25; P &lt; 0.001) and did not undergo significant changes later. In newborns, the area of distribution of ground substance was not considerably higher than the ratio of disarranged collagen fibers (10.42 ± 1.8 vs 11.67 ± 1.03; P = 0.562) and did not undergo substantial changes later. The distribution density of vessels increased till the 30th-45th day (4.5 ± 0.67 vs 7.5 ± 0.82 and 7.7 ± 0.54; P &lt; 0.001; ∼9-14th human years), followed by a gradual decrease from the 60th day (7.05 ± 0.72; ∼16 human years). The maximum number of cells was in newborns. Subsequently, it gradually decreased and was almost three times less on the 90th day than that of newborns (38.01 ± 3.06 vs 12.58 ± 1.53; P &lt; 0.001).

Conclusion
The morphogenesis of the marginal synovium during the postnatal period is accompanied by a gradual decrease in the number of cells, an increase in the area occupied by elastic and arranged collagen fibers with a relatively constant distribution density of disarranged collagen fibers and ground substance together with an undulatory pattern in the distribution of vessels. These peculiarities emphasize the morphological uniqueness of the marginal transitional zone, which forms the innate nonspecific barrier between the joint capsule and the articular cartilage, and thereby provides the necessary anatomical and physiological integrity of the joint as a whole organ. Knowledge about the patterns of morphogenesis of the normal marginal synovium will help to understand joint pathology.

Ethics
The study was approved by the ethics committee of the Zaporizhzhia State Medical University.