The skeletal system is comprised of bones, bone marrow, ligaments, and cartilage. The skeletal system is responsible for a number of functions. Most prominently, the bones bear the weight of the body, provide attachment points for muscles, and are responsible for locomotion. Bones also serve as stores for calcium and phosphorus. Red bone marrow is responsible for producing erythrocytes and the cells of the immune system, while yellow bone marrow plays a key role in fat storage.

The bones can be broken into two main categories: the axial skeleton and the appendicular skeleton. The bones that make up the axial skeleton are the vertebral column, thoracic cage, hyoid bone, and the skull. The appendicular skeleton is made of the upper and lower limbs, the pelvic girdle, and the pectoral girdle.

Cartilage serves to provide cushioning between bones, and ligaments are essential for maintaining stability.

Functions of the skeletal system include support, protection, mineral storage, and hematopoiesis. The structure of bones and their coordinated function with muscles, ligaments, and tendons allow for movement and support of the body. They also provide protection of vital organs, such as the rib cage surrounding the heart and pelvis protecting reproductive organs. Calcium and phosphate are stored in the hydroxyapatite matrix of the bone and hematopoiesis, the generation of red blood cells, takes place in red bone marrow, which houses hematopoietic stem cells.

Several systems coordinate thermoregulation, but ultimately this function is performed by the muscles. Muscles are responsible for most ATP usage in the body. Break down of ATP releases heat, which is why we shiver when we are cold. Regulation of blood flow and actions of the integumentary system also contribute to thermoregulation in other manners.

The correct answer is osteons. An osteon (or haversian system) is the fundamental unit of most compact bone. Osteons are formed during the process of bone remodeling, but are not one of the three types of cells involved in bone homeostasis.

Bone homeostasis involves the development, growth and remodeling of bones. The three cell types responsible for these processes are osteoblasts, osteocytes and osteoclasts. Each cell type refers to a different process:

• osteoblasts = bone forming cells
• osteocytes = mature bone cells
• osteoclasts = break down and reabsorb bone

All of these layers make up hyaline cartilage. The superficial layer is distinguished by the parallel (relative to the articular surface) arrangement of it's collagen fibers. The middle zone is characterized by the oblique orientation of its fibers. The deep zone has collagen fibers that are oriented perpendicularly to the articular surface. The calcified layer connects directly to the underlying (subchondral) bone.