# Static Osteogenesis versus Dynamic Osteogenesis:
## 2 Structural and Functional Differences between SO and DO  
Static and dynamic osteogenesis in various aspects in terms fo their structure, timing, inductive conditioning factors, and meaning. For clarity only we will firstly describe the characteristics of DO (which is classically well known) and then those of SO (which was only recently discovered), even if, when both occur, they take place in a time sequence (first SO and then DO). In particular, with regard to morphology they present consistent differences in: (1) the mobility, arrangement, movement, and polarization of osteoblasts; (2) the morphology and distribution of the resulting osteocytes; and (3) the collagen texture of the bone matrix (Table 1). In DO, bone deposition occurs by means of monostratified laminae in which prismatic osteoblasts are arranged in epithelial-like manner, all polarized in the same side and sharing the same osteogenic direction, i.e., towards the growing osteogenic surface. As is well known, each osteoblast displays an ill-defined euchromatic nucleus and an organellar machinery which is characteristically highly developed and located in polarized secretory cells. A distinctive feature of osteoblastic laminae during DO is their location in an \"asymmetrical\" environment, between the mineralized front (from which they move away as they secrete the preosseous matrix) and the vascular surface (towards which they migrate, thus maintaining a favorable metabolic situation) (Figure 2). With regard to the DO osteocyte morphology, there are dendritic elements displaying an almond-like shape in the cell body, in which cytoplasmic processes occur in an asynchronous and asymmetrical manner (first short \"mineral-facing\" dendrites, then long/slender \"vascular-facing\" ones) [8] in concomitance with, and depending on, the recruitment in the plane of the \"committed\" osteoblasts transforming into osteocytes. Such recruitment, in turn, influences the collagen texture, giving rise mostly to lamellar bone. During DO, gap and adherent  
Figure 1: A diagram showing how the wideness of osteoblast secretory territory (red line) varies during its recruitment (green osteoblast), since the produced osteoid seam (blue bars) stratifies on a larger surface according to the morphological modification from a prismatic to a flattened shape. This fact explains the incorporation of recruited osteoblasts transforming into osteocytes (see text for explanation). junctions are observed between movable osteoblasts inside the lamina, in addition to gap junctions or simple contacts between osteoblasts and DO osteocyte dendrites as well as between osteocytes in cytoplasmic processes [3]. In SO, bone deposition occurs by means of unmovable (i.e., stationary) osteoblasts irregularly arranged in cords that differentiate among the vessels (see below); inside the cords, formed by 2-3 layers of cells, osteoblasts are polarized in different directions \begin{table}
\begin{tabular}{c c c} \hline \hline
**STATIC OSTEGGENESIS** & & **DYNAMIC** \\  & & **OSTEGGENESIS** \\ \hline at the beginning of the process & & \\  & & \\  & & \\  & & \\ \hline cytokines issued by vessels & & \\  & & \\  & & \\  & & \\ \hline \multirow{2}{*}{cords} & **ARRANGEMENTS OF** & \\  & **OSTEOBLAST** & \\ \hline \multirow{2}{*}{various directions} & **POLARIZATION OF** & same direction (i.e. towards \\  & **OSTEOBLASTS** & the pre-existent bone) \\ \hline stationary osteoblasts & **OSTEOBLAST MOTION** & movable osteoblasts \\ \hline haphazardly, often in & & \\  & **DISTRIBUTION OF** & \\  & **OSTEOCYTES** & \\  & & \\  & & \\ \hline mostly woven bone & **TEXTURE** & mostly lamellar bone \\ \hline bad (due to the high & **MECHANICAL** & good (osteocytes located in \\ cellularity and woven texture) & **PROERTIES** & planes and lamellar texture) \\  & & -more suitable to loading \\ \hline \multirow{2}{*}{to provide a preliminary rigid} & & \\  & & \\  & & \\  & & \\  & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main morpho-functional differences between SO and DO.