endoskeletons

An endoskeleton is an internal framework that supports the body from within. In many animals, the endoskeleton consists of bone and/or cartilage, providing shape, structural support, and a surface for muscle attachment. Endoskeletons are characteristic of vertebrates—such as fishes, amphibians, reptiles, birds, and mammals—and are also found in echinoderms, whose internal calcareous plates form a skeletal system beneath the skin.

Structure and composition vary among groups. In vertebrates, bones are primarily made of mineralized tissue (hydroxyapatite) reinforced by collagen, with cartilage present in joints and developing areas. The endoskeleton grows with the organism and remains inside the body, undergoing ongoing remodeling and repair. Joints and connective tissues enable movement, while marrow within certain bones produces blood cells and stores minerals.

Development commonly involves bone formation through ossification. Intramembranous ossification forms flat bones directly from connective tissue, while endochondral ossification builds long bones from a cartilage model that is gradually replaced by bone. In some lineages, cartilage remains predominant in the skeleton, as seen in sharks and other cartilaginous fishes, where a cartilage-based endoskeleton persists rather than true calcified bone.

Diversity and function vary by taxon. Vertebrate endoskeletons provide rigid support, protect vital organs, anchor muscles, and enable a wide range of movements. In echinoderms, the internal ossicles are embedded in a connective tissue matrix and contribute to a flexible yet sturdier internal framework. Endoskeletons contrast with exoskeletons, which form external shells, and with hydrostatic skeletons, which rely on fluid pressure for support.

In humans and other animals, the endoskeleton also serves metabolic roles, including mineral storage and, in many species, hematopoiesis from bone marrow.