6. Histology

Studying the behavior of MSCs in vitro has become an urgent need to give more insights on their behavior in vivo and their mechanisms in initiating osteogenesis. Indeed, histological examination of MSCs is one of the main goals for studying their morphology in vitro by light microscope. Although it is a primary step, yet, it is not sufficient to rely on it alone, to detect their behavior during their differentiation process, and as such it has to be accompanied by ultrastructure examination to correlate between their morphology and behavior.

### 6.1. Light microscope

MSCs are characterized by being star-shaped cell with thin long processes [48]. Using hematoxylin and eosin stains, MSCs are characterized by pale cytoplasm, large vesicular nucleus and multiple thin processes (Figure 1).

#### 6.2. Phase contrast microscope

Regardless of the issue of origin, all MSCs share characteristics by consensus definition: they are spindle-shaped and plastic-adherent. In our study, [38] isolated human bone barrow MSCs

Figure 1. Light microscopic picture of the umbilical cord showing MSCs with many thin processes (arrow). Each cell exhibits a vesicular nucleus. Scale bar 50 μm.

revealed that the cells were adherent to the surface of tissue culture plastic flask. Furthermore, the cells were spindle in shape; which is considered as a second important characteristic of mesenchymal cell morphology. Researchers [38] described a population of adherent cells in culture till P5 (Figure 2). Most of the cells exhibited fibroblast-like spindle shape and showed vesicular nuclei with prominent nucleoli. Moreover, in P9, the adherent cells remained attached to the surface with their characteristic spindle shape (Figure 3). The cells exhibited vesicular nucleus, prominent nucleolus and multiple processes [38].
