While Ceacam10-deficiency was not associated with any alterations in structural bone parameters, static histomorphometry demonstrated a decreased trabecular bone volume in 3- and 6-month old Ceacam1-deficient mice. This was indeed an interesting finding, as Ceacam1-deficient mice, under basal conditions, have been reported to display no gross phenotypical abnormalities. As mentioned above, while previous studies have primarily focused on the pathophysiologic functions of CEACAM1 in various in vivo and in vitro disease models, this observation pointed towards a physiologic role of CEACAM1 in the regulation of bone cell activity. Although Ceacam1 was differentially expressed during osteoblastogenesis, the cellular and dynamic histomorphometry failed to detect defective osteoblastogenesis or osteoblast function. Surprisingly, increased osteoclast parameters were found in trabecular bone, indicating accelerated osteoclastogenesis in Ceacam1- deficient mice. To further characterize this effect at the cellular level, we differentiated bone marrow cells into INCB18424 osteoblasts and osteoclasts. In line with our in vivo observations, primary osteoblasts derived from Ceacam1-deficient mice displayed normal matrix mineralization and alkaline phosphatase activity in vitro. In contrast, although no alteration in the numbers of nuclei per osteoclasts could be detected, Ceacam1-deficient bone marrow cells demonstrated an increased osteoclastogenesis when cultured with M-CSF and RANKL. Therefore, it is now possible to conclude that CEACAM1 functions as a negative regulator of osteoclastogenesis in vivo and in vitro. The fact that we could detect increased levels of serum OPG in 6-month old animals is interesting, however does not explain the observed bone phenotype and increased osteoclastogenesis associated with the lack of CEACAM1. This is supported by the finding that 3-month-old mutant animals displayed a low bone mass phenotype despite normal OPG levels. Furthermore, as we failed to detect differences in the expression of Tnfsf11 and Tnfrsf11b in primary osteoblasts derived from Ceacam1-deficient mice, this particular phenomenon is most likely explained by an age-dependent counter regulatory mechanism rather than an intrinsic osteoblast defect. On the molecular level, we could detect differential expression of Ceacam1 not only in bone marrow derived osteoclast progenitors, but also in the pure macrophage cell line RAW264.7, providing a potential explanation for the increased osteoclast formation in Ceacam1-deficient mice. Since the formation of mature osteoclasts primarily depends on RANKL-induced activation of key transcription factors and the subsequent expression of several osteoclast marker genes, we monitored the expression of NF-kb, Nfatc1, Acp5, Tmf7sf4, Tnfrsf11a, cFos, and Calcr during osteoclastogenesis in bone marrow cells derived from WT and Ceacam1-deficient mice. While indicators of mature osteoclasts, including Calcr and Acp5, were found to be expressed at similar or only temporarily elevated levels compared to WT controls, respectively, increased expression of Nfatc1 in Ceacam1-deficient cells was found.