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Christina Lopez
Christina Lopez


What is the skeletal system made of? What does the skeletal system do? At the simplest level, the skeleton is the framework that provides structure to the rest of the body and facilitates movement. The skeletal system includes over 200 bones, cartilage, and ligaments.



How does the skeleton move? Muscles throughout the human body are attached to bones. Nerves around a muscle can signal the muscle to move. When the nervous system sends commands to skeletal muscles, the muscles contract. That contraction produces movement at the joints between bones.

Bones of the appendicular skeleton facilitate movement, while bones of the axial skeleton protect internal organs. All skeletal structures belong to either the appendicular skeleton (girdles and limbs) or to the axial skeleton (skull, vertebral column, and thoracic cage).

An infant skeleton has almost a hundred more bones than the skeleton of an adult. Bone formation begins at about three months gestation and continues after birth into adulthood. An example of several bones that fuse over time into one bone is the sacrum. At birth the sacrum is five vertebrae with discs in between them. The sacrum is fully fused into one bone usually by the fourth decade of life.

The regulation of bone remodeling by an adipocyte-derived hormone implies that bone may exert a feedback control of energy homeostasis. To test this hypothesis we looked for genes expressed in osteoblasts, encoding signaling molecules and affecting energy metabolism. We show here that mice lacking the protein tyrosine phosphatase OST-PTP are hypoglycemic and are protected from obesity and glucose intolerance because of an increase in beta-cell proliferation, insulin secretion, and insulin sensitivity. In contrast, mice lacking the osteoblast-secreted molecule osteocalcin display decreased beta-cell proliferation, glucose intolerance, and insulin resistance. Removing one Osteocalcin allele from OST-PTP-deficient mice corrects their metabolic phenotype. Ex vivo, osteocalcin can stimulate CyclinD1 and Insulin expression in beta-cells and Adiponectin, an insulin-sensitizing adipokine, in adipocytes; in vivo osteocalcin can improve glucose tolerance. By revealing that the skeleton exerts an endocrine regulation of sugar homeostasis this study expands the biological importance of this organ and our understanding of energy metabolism.

It is from the discovery of leptin and the central nervous system as a regulator of bone remodeling that the presence of autonomic nerves within the skeleton transitioned from a mere histological observation to the mechanism whereby neurons of the central nervous system communicate with cells of the bone microenvironment and regulate bone homeostasis. This shift in paradigm sparked new preclinical and clinical investigations aimed at defining the contribution of sympathetic, parasympathetic, and sensory nerves to the process of bone development, bone mass accrual, bone remodeling, and cancer metastasis. The aim of this article is to review the data that led to the current understanding of the interactions between the autonomic and skeletal systems and to present a critical appraisal of the literature, bringing forth a schema that can put into physiological and clinical context the main genetic and pharmacological observations pointing to the existence of an autonomic control of skeletal homeostasis. The different types of nerves found in the skeleton, their functional interactions with bone cells, their impact on bone development, bone mass accrual and remodeling, and the possible clinical or pathophysiological relevance of these findings are discussed.

The appendicular skeleton is one of two major bone groups in the body, the other being the axial skeleton. The appendicular skeleton is comprised of the upper and lower extremities, which include the shoulder girdle and pelvis. The shoulder girdle and pelvis provide connection points between the appendicular skeleton and the axial skeleton to where mechanical loads transfer. Of the 206 bones in the adult human body, a total of 126 bones form the appendicular skeleton. The bones that contribute to the appendicular skeleton include the bones of the hands, feet, upper extremity, lower extremity, shoulder girdle, and pelvic bones.[1]

A single upper extremity includes 14 phalanges (proximal, intermediate, and distal), five metacarpals, eight carpal bones, two forearm bones (radius and ulna), the humerus, and the shoulder girdle (scapula and clavicle).[2] A single lower extremity contains 14 phalanges (proximal, intermediate, and distal), five metatarsals, seven tarsal bones, two leg bones (fibula, tibia), the femur, and the hip bone or coxal bone (ilium, ischium, and pubis).[3][4] These bones articulate with each other and are joined by a multitude of ligaments, cartilage, and tendons to form the appendicular skeleton. There are also bony prominences and protuberances that serve as muscle attachment sites on the surfaces of these bones. The appendicular skeleton is structured for a greater range of motion and locomotion generation when compared to the axial skeleton.[5]

There are two bilateral joints where the appendicular skeleton directly articulates with the axial skeleton. The first of these articulations is the sternoclavicular joint, where the sternum of the axial skeleton articulates with the clavicle of the appendicular skeleton. The sternoclavicular joint is a synovial joint.[8] The second point where the appendicular skeleton directly articulates with the axial skeleton is the sacroiliac joint, where the sacrum articulates with the ilium. The sacroiliac joint is both a synovial joint and a syndesmosis. The connection between the sacrum and the ilium is important to transfer the load of the axial skeleton to the lower limb of the appendicular skeleton.[9]

The thoracoscapular articulation is a second articulation between the upper limb of the appendicular skeleton and the axial skeleton. This articulation is not an actual joint and does not have a synovial membrane. The thoracoscapular articulation forms between the anterior surface of the scapula and the posterior ribs 2-7.[10]

The bones of the foot function to form a base where the skeleton contacts the ground while standing. During the gait cycle, the articulations between the bones of the foot combined with the fascia and ligaments allow for deformation of the arches, which create spring-like properties in the foot that are utilized during walking and running.[11]

The appendicular skeleton first appears as limb buds near the end of the first month of embryogenesis. There are two upper limb buds and two lower limb buds. These form when the lateral plate mesoderm grows outwards. As these limb buds grow outwards, chondrification forms hyaline cartilage around the sixth week and continues cartilage growth the limb buds. This chondrification continues rapidly in a proximal to distal manner.[12] Around the tenth week, the ossification of the cartilage begins.[13] Ossification continues post-birth with secondary and ultimately complete ossification that is ongoing until around 20 years of age.[14]

The blood supply to the lower extremity of the appendicular skeleton originates from the common iliac arteries, which are the terminal branches of the descending aorta. The common iliac artery branches into the internal and external iliac arteries, supplying all the structures of the pelvis and the lower extremities.[15] The external iliac artery continues into the lower extremity to become the femoral artery as it passes under the inguinal ligament.[16] A major branch of the femoral artery is the deep femoral artery. The deep femoral artery supplies blood to the femur. The medial femoral circumflex artery and lateral femoral circumflex artery are early branches of the deep femoral artery that vascularize the hip joint.[16] The femoral artery continues posteriorly to the knee as the popliteal artery, then continues into the lower leg where it divides into the anterior tibial artery and the posterior tibial artery. The posterior tibial artery then bifurcates into the posterior tibial and fibular arteries, which distally contribute to the vasculature of the foot.[17][18][19]

The blood supply to the upper extremity of the appendicular skeleton comes from the subclavian artery. The subclavian artery is a branch of the brachiocephalic trunk on the right or a branch directly off the aortic arch on the left. The clavicle receives vascular supply from the suprascapular artery, thoracoacromial artery, and the internal thoracic artery.[20] The subclavian artery becomes the axillary artery after the lateral edge of the first rib. It then becomes the brachial artery after passing the inferior border of the teres minor muscle. The brachial artery bifurcates near the elbow into the radial and ulnar arteries, which distally contribute to the vasculature of the hands.[21][22]

The appendicular skeleton is clinically relevant in many areas of medicine. External forces applied to the appendicular skeleton from traumas can lead to fractured bones. In the upper limb, the most common fracture is a distal radius and ulna fracture. The second most common fracture site is the phalanges and metacarpals of the hand.[38] Repetitive smaller forces acting on the appendicular skeleton can also lead to stress fractures. A study of lower extremity stress fractures in the United States military found that the tibia and fibular were the most common location of stress fractures.[39]

Bones of the appendicular skeleton can also be a primary site for malignancy, such as multiple myeloma or osteosarcoma.[40][41] The joints of the appendicular skeleton are also susceptible to a wide variety of pathologies, including osteoarthritis, rheumatoid arthritis, and gout, to name a few. The bones of the appendicular skeleton often undergo imaging through various modalities, including X-ray, computed tomography scan, and magnetic resonance imaging. The imaging technique selected is dependent on the pathology that is being imaged.[42] 350c69d7ab


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