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Skeletal system

Three increasingly-magnified photos of a histology slide featuring human bone.

Bone marrow

The average adult contains 2-3 liters of bone marrow, a primary lymphoid organ located in the core of long bones. Bone marrow is composed of myeloid tissue (tissue capable of hematopoiesis) and has three major functions: production and support of stem cells; B cell maturation; and antibody production.

Bone marrow has a definite architecture, with environments containing unique factors to guide differentiation of each cell type. The myeloid tissue promotes differentiation into different cell types based on signals from outside the barrow marrow. Bone marrow also contains memory B cells, similar to lymph node germinal centers.

Bone development

There are three kinds of bone growth: endochondral ossification (for long bone growth); intramembranous ossification (for flat, irregular bone growth); and remodeling (role of osteoblasts and osteoclasts).

Endochondral ossificationOccurring in fetal development, this is the development of a cartilage model into a skeletal system.
Intramembranous ossificationOccurring in fetal development, this is the development of bone without a cartilage model.
RemodelingThis is the body's own system of removing bone where it is not needed and adding bone where necessary. This can be a response to injury or exercise.
Intramembraneous ossification involves the replacement of sheet-like connective tissue membranes with bony tissue.

Bones formed in this manner are called intramembraneous bones. They include certain flat bones of the skull and some of the irregular bones. The future bones are first formed as connective tissue membranes.

Osteoblasts migrate to the membranes and deposit bony matrix around themselves. When the osteoblasts are surrounded by the matrix they are called osteocytes.

Endochondral ossification involves the replacement of hyaline cartilage with bony tissue.

In this process, the future bones are first formed as hyaline cartilage models. During the third month after conception, the perichondrium that surrounds the hyaline cartilage "models" become infiltrated with blood vessels and osteoblasts and change into a periosteum.

The osteoblasts form a collar of compact bone around the diaphysis. At the same time, the cartilage in the center of the diaphysis begins to disintegrate.

Articulations (Joints)

There are three structural/functional classifications of joints: fibrous/immovable joints; cartilaginous/slightly movable joints; synovial/freely movable joints.

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