Bone is a mineralized connective tissue that exhibits four types of cells: osteoblasts, bone lining cells, osteocytes, and osteoclasts [1, 2]. Bone exerts important functions in the body, such as locomotion, support and protection of soft tissues, calcium and phosphate storage, and harboring of bone marrow [3, 4]. Despite its inert appearance, bone is a highly dynamic organ that is continuously resorbed by osteoclasts and neoformed by osteoblasts. There is evidence that osteocytes act as mechanosensors and orchestrators of this bone remodeling process [5–8]. The function of bone lining cells is not well clear, but these cells seem to play an important role in coupling bone resorption to bone formation [9].
Bone remodeling is a highly complex process by which old bone is replaced by new bone, in a cycle comprised of three phases: (1) initiation of bone resorption by osteoclasts, (2) the transition (or reversal period) from resorption to new bone formation, and (3) the bone formation by osteoblasts [10, 11]. This process occurs due to coordinated actions of osteoclasts, osteoblasts, osteocytes, and bone lining cells which together form the temporary anatomical structure called basic multicellular unit (BMU) [12–14].
Normal bone remodeling is necessary for fracture healing and skeleton adaptation to mechanical use, as well as for calcium homeostasis [15]. On the other hand, an imbalance of bone resorption and formation results in several bone diseases. For example, excessive resorption by osteoclasts without the corresponding amount of nerformed bone by osteoblasts contributes to bone loss and osteoporosis [16], whereas the contrary may result in osteopetrosis [17]. Thus, the equilibrium between bone formation and resorption is necessary and depends on the action of several local and systemic factors including hormones, cytokines, chemokines, and biomechanical stimulation [18–20].
Recent studies have shown that bone influences the activity of other organs and the bone is also influenced by other organs and systems of the body [21], providing new insights and evidencing the complexity and dynamic nature of bone tissue.
In this review we will address the current data about bone cells biology, bone matrix, and the factors that influence the bone remodeling process. Moreover, we will briefly discuss the role of estrogen on bone tissue under physiological and pathological conditions.
