In 1977, an article detailing the mechanisms of osteogenesis was published in the scientific journals of Ilizarov’s institute. Many of the concepts discussed in this article are still relevant to our current understanding of tissue repair following an injury. Below is a translation of the article from Russian.
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A. M. KHELIMSKIY.
OSTEOGENIC COLONY FORMATION IN REPARATIVE REGENERATION OF BONE TISSUE
The phenomenon of bone tissue formation during cultivation of osteogenic cells (Friedenstein A. Ya., Lalykina K. S., 1973), discovered in recent years (see the article by R. Ya. Galanova et al. in this collection), served as the basis for us to interpret the pictures of reparative bone formation in a new way. This concept of osteogenesis is presented below.
Bone damage is accompanied by two processes: 1) blood containing a certain amount of colony-forming osteogenic cells constantly circulating in the blood flows into the injury zone; 2) distant and local regulators are formed in the injury zone. Let us conventionally call the first of them a pheromone (from the Latin fero to carry, tonco to persuade), the second an attractant (from the Latin attraho to attract). The pheromone, spreading through the blood or lymph through veins or lymphatic pathways, forces the precursor cells of osteogenesis to leave the bone marrow of various bones of the body into the general blood flow. The attractant remains in the area of injury and attracts colony-forming cells of osteogenesis, giving them a signal to begin column formation. Therefore, bone formation occurs precisely at the site of injury, despite the circulation of precursor cells in the general blood flow. Both the pheromone and the attractant (regardless of their structure, mono- or polycomponent nature, etc.) must first of all be products of bone tissue disintegration, since if they were of a different origin, for example, a product of the destruction of the vascular wall, blood, fibrous tissue, etc., they would not be specific to bone formation and would not attract specifically the stem cells of osteogenesis to this focus.
Thus, if, for example, we inflict a repeated injury some time after bone injury, healing will proceed faster, since the attractant in the new place will attract pre-stimulated pheromone-stimulated cells – precursors of osteogenesis. It is only necessary that the interval between the first and repeated injuries correspond to a certain “incubation period”. We believe that the time of the onset of distraction should be partly dictated by the size of this period in each specific case.
The pheromone, of course, activates column-forming cells not only in distant parts of the body, but also in the bone marrow of the damaged bone. The attractant draws them to the injury zone. The process of osteogenic column formation, the development of a clone of bone-forming cells, begins. Three stages of colony formation can be outlined: Stage 1: formation of colonies from colony-forming cells of the hematoma in places where there are suitable conditions for this; Stage 2: formation of osteogenic colonies from activated local precursor cells in the bone marrow of the damaged bone (the beginning of the endosteal reaction). At this time, there are many small “foci” (osteogenesis colonies) in the regeneration zone. What is a colony during bone formation?. This is the offspring of one stem cell, gradually passing through the stages of differentiation through fibroblasts, preosteoblasts, osteoblasts to osteocytes, and the products of their excretory function, forming the bone matrix. The center of the column is its more mature, more differentiated sections. On the periphery are younger individuals. In the bone marrow, under conditions of stable fixation, the colonies are rounded, on cortical plates or large bone fragments they are spread out in the form of plaques (the more mature part of the colony on the base of the plaque). Often the center of the colony is located near the bone fragment of the attractant localization. Stage 3: formation of colonies from colony-forming cells that have arrived through the blood and penetrate into the damage zone along the supplying bone vessels of the periosteum or endosteum. Perosteal colonies arise in places of preserved blood supply; due to mechanical reasons they also spread out on the cortical plate in the form of plaques. The endosteal regenerate is now composed of three sources: colonies that arose in the hematoma, colonies from local precursor cells; colonies from alien precursor cells of osteogenesis. All these colonies merge as they develop, the distal part of the regenerate (where colony formation usually lags behind due to blood flow conditions) merges with the proximal part. By about day 14 after osteotomy, the colony “matures”, turning into a bone focus surrounded by active osteoblasts.The diameter of the focus is 1-2 mm. The further fate of the formed bone depends on biomechanical conditions: either it will develop as a result of the activity of osteoblasts, or in the absence of load, osteocytes will begin to die, and osteoclasts will resorb the decaying bone.
Although the center of bone formation is the vessel supplier of colony-forming cells, as the colony develops, its center, where bone formation begins, seems to move away from the vessel. As the colony matures, proliferation ceases and bone is replaced, the bone again “approaches” the vessel, now directly adjacent to it. Thus, the type of osteogenesis (desmal or angiogenic) is more appropriately considered as a manifestation of the “packing density” of the vessels, and, consequently, osteogenic colonies.
With a lack of nutrition and energy, osteogenesis is distorted, the colony turns into a chondroid focus.
It is obvious that the activity of the reparative reaction depends on the volumetric level of blood flow in a given area. The more intense the blood supply, the more intensive the delivery of colony-forming cells, nutrition and energy for the colonies. Therefore, metaphyseal injuries heal faster than diaphyseal injuries. The Ilizarov method, which preserves muscle and joint function, i.e. ensures blood and lymph flow, and the
Thus, the transport of pheromone, as well as ensuring the integrity of vessels and osteogenesis colonies in the contact zone (due to stable fixation of fragments), creates optimal conditions for bone regeneration. Bone formation should be considered from this standpoint during distraction. Distraction constantly maintains the release of pheromone and attractant, i.e. constantly maintains the influx of column-forming cells into the stretch zone. The middle growth zone in the distraction regenerate is abundantly supplied with vessels. As the density increases, it moves further and further away from the ends of the fragments. Newly formed osteoid beams in the growth zone are most exposed to the action of stretching, i.e. the formation of beams is combined with their partial disintegration, disintegration of the bone matrix. The attractant is localized here, and the vessels deliver colony-forming cells here. Here the “packing density” of the vessels is high enough to form merging lace-like colonies of bone formation around them, i.e. for the manifestation of the so-called direct angiogenic osteogenesis. At the same time, endosteal and perosteal bone formation in the fragments increasingly subsides. As the experiments of A. A. Shreiner showed, an abundant vascular network develops during distraction. At first, it is located at the ends of the fragments, corresponding to the development of endosteal foci of osteogenesis. As distraction progresses, the zone of abundant blood supply shifts toward the center of the regenerate, toward its growth zone. V. G. Berko’s studies have shown that in the growth zone the number of vessels during distraction increases on average by 1 vessel per day per 1 sq. mm of section area (during femur lengthening in dogs). Accordingly, the space between the vessels decreases. The area of the functional-structural unit of bone formation decreases during distraction from 0.040 to 0.015 mm². From the 21st to 28th day of distraction, the picture corresponding to desmal osteogenesis is replaced by a picture of direct
angiogenic bone formation. These data can be compared with the observations of
K. V. Petrakova, A. A. Tolmacheva, A. Ya. Friedenstein (1963), who showed that the direction of differentiation of the osteogenic cell culture depends on the “packing density”. Thus, when growing the same number of osteogenic cells in vivo in a chamber with an area of 50 mm², reticular tissue was formed, and in a chamber with an area of 5 mm², foci of bone formation appeared.
After distraction, during the period of subsequent fixation, the stretching of the beams ceases, the production of the Pheromone and attractant ceases, the colonies mature and the growth zone is replaced by bone.
If stability is not ensured during the healing of a bone wound, the level of injury is high, there are many colonies, but they are partially destroyed, and excessive but disordered bone formation occurs.
In the places where the spokes are inserted, the pattern of bone formation is naturally similar to that observed in other areas of damage. But if the spoke is under the action of compression or distraction forces, colony formation is induced in a certain direction. Probably, such pathological processes as ossifying myositis, ossification of tendons after injury, etc., are also the result of accidental formation of an attractant in an unusual place and the occurrence of osteogenic colony formation.
As for physiological regeneration and bone remodeling, it seems to us that this is the result of activation or inactivation under the influence of overload or inactivity of osteoblasts remaining on the bone surface (endosteum, periosteum, osteon canals), and not the result of attracting circulating colony-forming cells.