Background: Bone remodeling depends on the dynamic balance between osteoclast-mediated bone resorption and osteoblast-mediated bone formation. Follistatin-like 1 (FSTL1) has been reported as an osteoclast-secreted protein that inhibits osteoclast differentiation, but its direct effects on osteoblast differentiation remain unclear. This study aimed to determine whether FSTL1 regulates osteoblast differentiation and mesenchymal stem cell migration and characterizes its role in osteoclast-osteoblast cellular cross-talk under in vitro conditions. Methods: Bone marrow-derived macrophages (BMMs) and stromal cells (BMSCs) from mice were used to induce osteoclast and osteoblast differentiation, respectively. Chemotaxis was assessed by Transwell migration, and osteoblast differentiation was evaluated in BMSC and MC3T3-E1 cells using staining, qRT-PCR, Western blotting, and proliferation assays. Results: FSTL1 significantly suppressed osteoclast differentiation and resorptive activity, confirmed by TRAP staining and pit assay, respectively. Expression of osteoclast markers such as NFATc1, TRAP, and DC-STAMP was reduced under FSTL1 treatment. In BMSCs, FSTL1 did not affect proliferation but significantly enhanced chemotaxis. Moreover, FSTL1 promoted osteogenic differentiation and mineralization, as demonstrated by increased ALP activity and Alizarin Red S staining. In MC3T3-E1 pre-osteoblasts, FSTL1 increased cell proliferation and mineralization by MTS and Alizarin Red staining. Key osteogenic markers, including Runx2 and osteocalcin, were also upregulated. Conclusions: Osteoclast-derived FSTL1 significantly suppresses osteoclastogenesis and promotes mesenchymal cell chemotaxis and osteogenic differentiation, indicating a role in regulating osteoclast–osteoblast cellular interactions in vitro. Targeting FSTL1 signaling may represent a promising therapeutic strategy for osteoporosis and other disorders of impaired bone remodeling.

Guided bone regeneration (GBR) with conventional collagen membranes may yield limited bone formation under estrogen-deficient conditions. This study evaluated whether a collagen membrane incorporating high-molecular-weight polynucleotide (PN) enhances bone regeneration in osteoporotic rats. PN (approximately 5 mg) was loaded onto a porcine collagen membrane and characterized by attenuated total reflectance Fourier-transform infrared spectroscopy, X-ray diffraction, and an in vitro release assay. Bilateral 5-mm calvarial defects were created in 30 ovariectomized female Sprague–Dawley rats and left unfilled (Control) or covered with a collagen membrane alone (Mem) or a PN-incorporated collagen membrane (PN_Mem) (n = 10 defects per group per time point). Regeneration was assessed at 4 and 8 weeks by micro-computed tomography, Masson’s trichrome staining, and immunohistochemistry for runt-related transcription factor 2 (Runx2) and bone morphogenetic protein-2/4 (BMP-2/4). PN was physically incorporated and released biphasically (approximately 50% within 8 h and 77% within 7 days). Bone volume did not differ among the groups at 4 weeks, whereas at 8 weeks it was greater in PN_Mem (5.98 ± 2.20 mm³) than in Control (1.24 ± 0.67 mm³; p < 0.0001) and Mem (2.52 ± 1.79 mm³; p < 0.001), with more Runx2-positive cells than in both groups and stronger BMP-2/4 immunoreactivity than in Control. PN incorporation enhanced the osteogenic performance of collagen membranes in this osteoporotic calvarial model; validation in clinically relevant alveolar defect models is required.