An endoskeleton is an internal support structure of an animal. Three phyla of animals possess true endoskeletons of various complexity: Chordata, Echinodermata and Porifera. An endoskeleton allows the body to move and gives the body structure and shape. A true endoskeleton is derived from mesodermal tissue. Such a skeleton is present in echinoderms and chordates. The poriferan ‘skeleton’ consists of microscopic calcareous or siliceous spicules or a spongin network. In arthropods, the endoskeleton is derived from the epidermis, and is continuous with and an extension of the exoskeleton, although internalized. It is composed of various internal rigid support rods (e.g., the tentorium), the membranous tracheae and the anterior and posterior portions of the digestive tract. All of these are shed, along with the exoskeleton proper, during ecdysis (moulting). However, the endoskeleton of spiders is partly derived from mesodermal tissue and therefore comparable to vertebrate cartilage.
Bone is a relatively hard and lightweight composite material, formed mostly of calcium phosphate in the chemical arrangement termed calcium hydroxyapatite. It has relatively high compressive strength but poor tensile strength. While bone is essentially brittle, it does have a degree of significant elasticity contributed by its organic components (chiefly collagen). Bone has an internal mesh-like structure, the density of which may vary at different points.
Bone can be either compact or cancellous (spongy). Cortical (outer layer) bone is compact; the two terms are often used interchangeably. Cortical bone accounts for 80% of the total bone mass of the adult skeleton. Because of its high density it accounts for approximately 10% of the total surface area. Cancellous bone is trabecular (has an open, meshwork or sponge-like structure). It has a relatively high surface area (about 10 times that of cortical bone) and it accounts for approximately 20% of the total bone mass.
Bone can also be either woven or lamellar. Woven bone forms quickly during periods of repair or rapid growth. Woven bone is unique in its ability to form spontaneously without a pre-existing strucure. It is so called because of its primitive, disorganized appearance. Its collagen fibres, which are few, are randomly oriented, accounting for its low strength. Lamellar bone forms slowly during periods of repair or typical growth. It does not form spontaneously. Lamellar bone forms only on calcified hyaline cartilage or bone. It is so called because its structure, observed microscopically, consists of a uniform system of layers, or lamellae. In contrast to woven bone, lamellar bone’s collagen fibres, which are many, are oriented in parallel, accounting for its greater strength. Woven bone often forms initially and, as either growth or repair continues, it is replaced by lamellar bone. This process is known as “bony substitution.”
Long bones are tubular in structure (e.g. the tibia). The central shaft of a long bone is called the diaphysis, and has a hollow middle—the medullary cavity filled with bone marrow. Surrounding the medullary cavity is a thin layer of cancellous bone that also contains marrow. The extremities of the bone are called the epiphyses and are mostly cancellous bone covered by a relatively thin layer of compact bone. In children, long bones are filled with red marrow, which is gradually replaced with yellow marrow as the child ages.