Due to the existence of muscles, connective tissue, dermal layer, and basement membrane, cells within the mass of the injected area encounter these barriers, inhibiting the distance of migration between the application region and inflammatory site, which has an estimated diameter of 20–30 μm (Fig. 6). High-density cell injection at the topical region is also an obstacle for homing, thus using a vasodilator like heparin, culturing the cells under hypoxic condition, maintaining a lower confluence, or the addition of IL-3, IL-6, IGF-1, tumor necrosis factor alpha (TNF-α), or interferon-gamma (IFN-γ), can be used to increase C-X-C chemokine receptor type 4 (CXCR-4) [38], which is a specific receptor for stromal-derived-factor-1 (SDF-1, also called CXCL12), an MSC chemotactic factor that could improve homing efficiency [39].

Locally injected MSCs decreased sharply in number from the buccal site within a week (Fig. 6a). One day after injection, some 7-AAD-positive cells were detected within the MSC mass at the subcutaneous tissue. From days 3 and 5, MSCs began to undergo apoptosis within the local administration region. At day 7 after injection, the level of MSC apoptosis was approximately 90% (Fig. 6b).

Isolated MSCs were seeded at four concentrations (5 × 10², 5 × 10³, 5 × 10⁴, 5 × 10⁵ MSCs/ml) in adipogenic and osteogenic induction conditions. When the medium was switched to adipogenic and osteogenic differentiation medium, the MSC concentrations were approximately 30, 60, 80, and 100%, respectively. MSCs differentiation into adipocytes and osteoblasts was determined by PPARγ and Runx2 immunofluorescence staining, respectively (Fig. 7 A, B). At a density of 5 × 10⁴ MSCs/ml, adipogenic and osteogenic differentiation were confirmed by increased expression of specific adipogenic markers and Runx2, respectively, using western blotting. Therefore, cells at a suitable cell density within the MSC mass were induced to undergo osteogenic or adipogenic differentiation. However, 3 days after injection, the MSCs began to undergo apoptosis over time in the local administration region. A high ratio of apoptosis occurred immediately after local administration of MSCs, which reduced the amount of viable cells at early stage.