Spongy and Compact Bones

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Spongy Bone

bone that consists of spicules and trabaculae. Provides strength with minimal weight. Spaces are filled with red bone marrow.

Trabaculae

Thin plates of bone in spongy bone that develop along bone's lines of stress.

Spicules

Sliver os bone in spongy bone.

Bone Marrow

soft tissue that occupies the marrow cavity of a long bone and small spaces amid the trabeculae of spongy bone

Red marrow (myeloid tissue, hemopoietic tissue)

produces blood cells. In nearly every bone in a child. In adults, found in skull, vertebrae, ribs, sternum, part of pelvic girdle, and proximal heads of humerus and femur.

Yellow Marrow

found in adults. Most red marrow turns into this fatty marrow. No longer produces blood.

Ossification (Osteogenesis)

the formation of bone by either intramembanous ossification, or endochonral ossification

Intramembranous ossification

produces flat bones of skull and clavicle. Such bones develop within a fibrous sheet similar to the dermis of the skin

Endochondral ossification

process in which the bone is preceded by a hyaline cartilage model that becomes replaced by osseous tissue. Begins around 6th week of fetal development and continues into 20's. Most bones develop this way including vertebrae, ribs, sternum, scapula, pelvic girdle, and bones of limbs

Intramembranous stage 1

Some of the embryonic connective tissue (mesenchyme) condenses into a layer of soft tissue with a dense supply of blood capillaries. The mesenchymal cells enlarge and differentiate into osteogenic cells, and regions of mesenchyme become a network of soft sheets called trabeculae.

Intramembranous stage 2

Osteogenic cells gather on trabeculae and differentiate into osteoblasts. These cells deposit osteoid tissue. As trabeculae grow thicker, calcium phosphate is deposited in the matrix. Some osteoblasts become trapped in the matrix and are now osteocytes. Mesenchyme close to the surface of a trabeculae remains uncalcified, but becomes denser and more fibrous, forming a periosteum.

Intramembranous stage 3

Osteoblasts continue to deposit minerals, producing a honeycomb of bone trabeculae. Some trabeculae persist as permanent spongy bone, while osteoclasts resorb and remodel others to form a marrow cavity in the middle of the bone.

Intramembranous stage 4

Trabeculae at the surface continue to calcify until the spaces between them are filled in, converting the spongy bone to compact bone. This process gives rise to the sandwich like arrangement typical of mature flat bones

Osteoid tissue

organic matrix deposited by osteoblasts that is a soft collagenous tissue similar to bone except for a lack of minerals

Endochondral stage 1

Mesenchyme develops into a body of hyaline cartilage, covered with a fibrous perichondrium, in the location of a future bone For a time, the perichondrium produces chondrocytes and the cartilage model grows in thickness.

Endochondral stage 2

Eventually, the perichondrium stops producing chondrocytes and begins producing osteoblasts. These deposit a thin collar of bone around the middle of the cartilage model, encircling it like a apkin ring and providing physical reinforcment.

Endochondral stage 2B

Former perichondrium is now considered a periosteum. Meanwhile, chondrocytes in the middle of the model enlarge and the matrix between their lacunae is reduced to thin walls. Walls of matrix between the lacunae calcify and block nutrients from reaching the chondrocytes. The cells die and their lacunae merge into a single cavity in th emiddle of the model.

Endochondral stage 3

Blood vessels penetrate the bony collar and invade the primary ossification center. As the center of the model is hollowed out and filled with blood and stem cells, it becomes the primary marrow cavity. Various stem cells introduced with the blood give rise to osteoblasts and osteoclasts. Osteoblasts line the cavity, begin depositing osteoid tissue, and calcify it to form a temporary network of bony trabeculae.

Endochondral stage 3b

As the bony collar under the periosteum thickens and elongates, a wave of cartilage death progresses toward the ends of the bone. Osteoclasts in the marrow cavity follow this wave, dissolving calcified cartilage remnants and enlarging the marrow cavity of the diaphysis. Soon, chondrocyte enlargement and death occur in the epiphysis of the model as well, creating a secondary ossification center.

Endochondral stage 4

Secondary ossification center becomes holowed out by the same process as the diaphysis, generating a secondary marrow cavity in the epiphysis. This cavity expands outward from the center, in all directions. In bones with two secondary ossification centers, one center lags behind the other in development, so at birth there is a secondary marrow cavity at one end while chondrocyte growth has just begun at the other. The joints of the limbs are still cartilaginous at birth.

Endochondral stage 5

During infancy and childhood, the epiphyses fill with spongy bone. Cartilage is then limited to the articular cartilage covering each joint surface, and to an epiphyseal plate which persists through adolescence and serves as a growth zone for bone elongation.

Endochondral stage 6

By late teens to early twenties, all remaining cartilage in the epiphyseal plate is generally consumed and the gap between the epiphysis and diaphysis closes. The primary and secondary marrow cavities then unite into a single cavity, and the bone can no longer grow in length.

metaphysis

region of transition from cartilage to bone at each end of the primary marrow cavity in endochondral ossification

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