Читать книгу Biological Mechanisms of Tooth Movement - Группа авторов - Страница 42

The fibroblast is the major cell type in all connective tissues, including the PDL. Fibroblasts differentiate from mesenchymal stem cells and are responsible for the synthesis and secretion of all the elements of the ECM, collagen and oxytalan fibers, and proteoglycans. Furthermore, fibroblasts synthesize and secrete a wide variety of regulatory molecules that act as local signals for the adaptation of the tissue to changing conditions (Lekic and McCulloch, 1996; Jiang et al., 2016).

Osteoblasts are large cells (20–30 μm), in the form of a polyhedron, with a basophilic cytoplasm, and with a substantial rough endoplasmic reticulum and Golgi apparatus. They originate from the mesenchymal stem cells of the bone marrow, endosteum, periosteum, and perivascular pericytes (Fernández‐Tresguerres‐Hernández‐Gil et al., 2006a). They differ from fibroblasts because they can express RUNX2, which is essential for the differentiation of mature osteoblasts. RUNX2 is the first transcription factor that is upregulated in pre‐osteoblasts and it is downregulated again in mature osteoblasts (Li et al., 2018). The osteoblasts synthesize and secrete the collagen and noncollagen proteins, such as osteocalcin and osteopontin, that form the organic bone matrix or osteoid material. Furthermore, they express alkaline phosphatase (ALP), which is essential for the mineralization of the osteoid (Lerner et al., 2019). Osteoblasts also produce and secrete hydroxyapatite into the osteoid, forming the strong and well‐organized mineralized matrix of the bone (Hasegawa, 2018). Part of the osteoblasts are buried in the bone matrix as osteocytes, maintaining contact with each other and osteoblasts through extended cellular processes that lie in narrow canals within the bone matrix, the canaliculi (Figure 3.2).

Although osteocytes are relatively inert cells, they are capable of transmission of signals over long distances through the canalicular network (Lerner, 2012). They are considered to be mechanosensory cells that play an important role in the regulation of the activity of osteoblasts and osteoclasts (Burger and Klein‐Nulend, 1999; Klein‐Nulend et al., 2013; Tresguerres et al., 2020) (Figure 3.3)

Osteoclasts are multinucleated cells that are derived from hemopoietic stem cells, and more specifically from extravasated monocytes that can differentiate into macrophages and through fusion into osteoclasts. They are responsible for the resorption of bone. The formation of an effective seal around the resorption compartment is essential, because it enables the formation of an isolated compartment between the bone and the cell, called Howship’s lacuna. The cell membrane of the Howship’s lacuna becomes highly invaginated and forms the so‐called ruffled border, allowing massive secretory and endocytotic activity. This enables the vesicular transcytosis of the mineral and degraded collagen from the ruffled border to the free membrane of the cell, and its release into the extracellular compartment (Roodman, 1993; Duong et al., 2000; Fernández‐Tresguerres‐Hernández‐Gil et al., 2006b; Takahashi et al., 2007). For an elaborate overview on the concerted interplay between osteoblasts, osteocytes, and osteoclasts see Lerner (2012) and Chapter 4 of this book.

Figure 3.2 Photomicrograph of the bone matrix, showing osteocyte lacuna and lacuna–canalicular network.

(Source: Jaap Maltha.)

Figure 3.3 Fluid flow (arrows) after the application of an orthodontic force (F). Apart from a very rapid redistribution of fluid within the PDL, a limited fluid flow in the lacuna‐canalicular system leads to shear stress as a message for the mechanosensory system.

(Source: Jaap Maltha.)

Figure 3.4 The transposition of a dog premolar during the first 5 hours after the application of an orthodontic force. Two phases can be recognized: an initial phase lasting only a few seconds with a very rapid tooth movement, and a second phase in which the rate of tooth movement gradually decreases until it stabilizes.

(Source: Jaap Maltha.)

In addition, cementoblasts are part of the PDL and are derived from the mesenchymal dental follicle. They cover the root surface and are responsible for cementogenesis, through processes more or less comparable with osteogenesis. Some cementoblasts are buried in the cementum matrix to form cementocytes, especially in the more apical areas of the root. Similarly to osteocytes, they show thin cell processes in the ECM. However, they are far less extensive than in bone and do not form an intercellular network (Nanci and Bosshardt, 2006; Yamamoto et al., 2016).

Macrophages are a type of white blood cells that have the same origin as osteoclasts, namely extravasated monocytes. They are essential as phagocytes in defense against pathogenic microorganisms, in clearance of dead or senescent cells, and in removal of cell debris. Furthermore, they promote homeostasis through their trophic, regulatory, and repair functions (Gordon and Martinez‐Pomares, 2017; Gordon and Plüddemann, 2017)