Morphological Changes in The Collagen Matrix During the Subcutaneous Implantation of Fibrous Polymer Frame

Background: The major challenge in tissue engineering is to optimize cell isolation, multiplication and differentiation, as well as to construct the matrices or delivery systems thereby promoting the maintenance and coordination of three-dimensional tissue regeneration. One of the important criteria that should be considered when constructing the matrix is its ability to form an optimal scaffold for the transplantation of the cell substrates. Aim of

and differentiation, as well as to construct the matrices or delivery systems thereby promoting the maintenance and coordination of three-dimensional tissue regeneration [2,3]. One of the important criteria that should be considered when constructing the matrix is its ability to form an optimal scaffold in combination with optimal hemodynamics inside the scaffold to transplant the cell substrates [4,5]. Therefore, the aim of our study was to experimentally assess the nature of the development of collagen fibers during all the periods of subcutaneous implantation of the biopolymer fibrous matrix.

Materials and Methods
For the study, the fibrous matrix made of 100% Pure Polylactide (PLA) granules developed by us was used. The matrix was created by the method of phase separation. The average thickness of the fibrous matrix was 30 mm. Fiber diameter ranged from 0.7 μm to 10 μm. The aforementioned matrices were submitted to sterilization by gamma radiation. The scaffolds, hermetically sealed in a sterile double package, were uniformly exposed to electron beam radiation with an energy of 4 mega-electron-volt (MeV) and a pulse duration MeV did not cause nuclear transmutations, i.e., it did not result in the formation of radioactive isotopes and did not create a residual object background radiation. After sterilization, the biopolymer matrices were subcutaneously implanted in laboratory animals.
The study included 20 laboratory animals (rabbits) divided into 2 groups: 10 animals of Group I underwent surgery involving the creation of a subcutaneous pocket and suturing; 10 animals of Group II underwent subcutaneous implantation of the biopolymer matrix into the back area between the shoulder blades. The material samples were collected on the 1st, 2nd, 3rd months, namely the matrix alongside with the surrounding tissues was surgically removed from the animal body. All the animal manipulations were carried out in accordance with the European Convention for the

Protection of Vertebrate Animals Used for Experimental and other
Scientific Purposes [6]. To carry out histological examination, the matrix with the surrounding tissues was dissected in mutually perpendicular directions into 25 segments. Nine segments were used for the study, namely 1 centrally located segment, 4 segments of the paracentral zone, 4 segments of the peripheral zone. These samples were fixed in 10% neutral formalin (Ph-7.0) for 24 hours.
Then, all of the pieces of the organs studied were dehydrated in graded alcohols, defatted in chloroform, then, placed in a mixture of chloroform-paraffin (1:1), paraffin wax (at a temperature of 37°С).
After paraffin processing, specimens were embedded in paraffin wax. Then, 4-6-µm-thick paraffin sections were prepared using a sliding microtome. Preparations were stained with hematoxylin and eosin [7].

Results
Pathomorphological study of implant peripheral zones on the 1 st , 2 nd and 3 rd months demonstrated the development of connective tissue in the inter-fiber spaces of the implanted structure. The analysis of collagen fiber structure showed a change in the thickness of the connective tissue capsule, as well as density and location of the fibers themselves. One month after subcutaneous implantation, connective tissue of the peripheral zone contained mainly elongated fibers arranged loosely and the ground substance found in the spaces between these fibers. Fibrocytes, fibroblasts, macrophages, blood vessels were visualized in the ground substance. In the areas being in close contact with a dense fibrous layer of a polymer implant, there were seen circularly arranged connective tissue fibers with a thickness of 56.18+0.638 µm (Table 1).  thinner as compared to the paracentral zone of the same period but thicker than those 1 month after implantation (Table 1). In

Discussion
Based on the studies conducted, collagen fiber was found to  b) The fibrous matrix constructed by us, due to its hygroscopicity and porosity, creates a kind of the bridge for tissue ingrowth and the formation of a three-dimensional collagen matrix.