## 3 Estrogen Receptors
### Differential Expression of Estrogen Receptors in Bone  
All estrogen receptors are differently expressed in bone cells. In 1998, [PERSON] et al. reported the expression of mRNA ER\(\alpha\) in the chondrocytes, active osteoblasts, and lining cells on trabecular surfaces of both human (9-15 years of age) and rabbit bones. They were unable to detect mRNA expression in osteocytes or osteoclasts in either species [29]. In 1999, [PERSON] et al. reported the immunodalization of ER\(\alpha\) in the growth plates of rabbits, but not in rats, during sexual maturity [30]. In the same year, [PERSON] et al. reported the expression of ER\(\alpha\) and ER\(\beta\) in total bone marrow, as well as in primary murine marrow stromal cell cultures, at the mRNA, as well as at the protein level [31]. [PERSON] et al. (1999) reported the expression of ER\(\alpha\) in the human pre-osteoclast cell line, TCG 51, but its expression was not observed in the more mature, bone resorptive TGC23 cell line. Neither could they detect expression of ER\(\alpha\) in samples obtained from patients with conditions known to increase osteoclast numbers such as hyperparathyroidismism, CGTB,  
Figure 2: Estrogen Receptors Structure. Estrogen receptors \(\alpha\) and \(\beta\) share similar characteristics. They are formed by 6 domains and have shared homology, being the DNA Binding Domain, the most highly conserved. They both have the D domain where the nuclear localization signal (NLS) is located and a hinge for the protein to fold. In the E/F domain, they have the ligand-binding domain (LBD) and the AF-2 which is ligand-dependent. They differ in the A/B domain, ER\(\alpha\) has an activator factor (AF-1) that is ligand-independent, whereas ER\(\beta\) has a repressor site. Upon binding to Estrogen there is a conformational change and a folding of the receptor at the D domain, bringing the C-terminal close to the N-terminal and in the case of ER\(\alpha\) attracting transcription factors (TF). and Pagetic bone [32]. [PERSON] et al. (1997) showed differential expression of both ERs mRNA during differentiation of human fetal osteoblast-like cell line (SV-FHO). They reported that while ER\(\beta\) mRNA levels increased during osteoblast differentiation, levels of ER\(\alpha\) were kept constant after day 10 of differentiation (the moment of maximal expression of alkaline phosphatase) [33]. In the early years, after ER\(\beta\) discovery and cloning, several groups reported expression of ER\(\beta\) in the bone of several species from rats [34] to humans [35]. More specifically, they reported that in areas of active bone formation or bone remodeling, ER\(\beta\) is highly expressed in the nuclei of osteoclasts, and less expressed in cells of the osteoblastic lineage. Cells in the hypertrophic cartilage are negative for ER\(\beta\), but those close to endochondral osteogenesis are positive [35]. [PERSON] et al. (2001), using human neonatal rib bone, reported that ER\(\beta\) is mainly expressed in trabecular bone with little expression in cortical bone. The opposite being for ER\(\alpha\), which is highly expressed in cortical bone and low expressed in trabecular bone. Most osteocytes in the cortical bone, as well as osteoblasts within the osteons and on the periosteal and endosteal surfaces, showed intense ER\(\alpha\) staining. Osteocytes within the trabecular bone express low levels of ER\(\alpha\), but with the expression more intense in those cells closer to newly mineralizing osteoid [36]. GPER1/GPR30 is present in the growth plate of young humans but its expression declines during puberty [37]. Its expression has been confirmed in bone marrow mesenchymal stem cells [38], hypertrophic chondrocytes [37], osteoblasts, and osteocytes [39]. Just recently, [PERSON] et al. (2020) reported that GPER1/GPR30 mediates bone marrow mesenchymal stem cell proliferation via the cAMP/PKA/CREB pathway [38]. The expression of estrogen receptors is regulated by the methylation state in the promoter region.