Bones develop through a process called ossification, also known as osteogenesis. in human infants and fetuses, the bone develops utilizing two different methods of ossification; intramembranous ossification and endochondral ossification.
Intramembranous ossification is responsible for the development of many of the flat bones found in the skull and in the clavicle. Some of these bones develop with a sheet of fibers, similar to the dermis of the skin, and because of this, they may be referred to as dermal bones. There are several stages involved in the process of intramembranous ossification.
The mesenchyme, a layer of embryonic tissue, develops into a layer of soft tissue, rich with blood vessels. The cells found within the mesenchyme enalarge, becoming osteogenic cells and areas of the soft tissue develops into trabeculae, or thin plates of osseous tissue.
The osteogenic cells then congregate on the trabeculae and develop into osteoblasts. Once the ostegenic cells have transformed into osteoblasts, the osteoblasts begin depositing the bone matrix, also known as osteoid tissue, This osteoid tissue is very similar to fully formed bones, but lacks the mineralization of fully formed bones. Some osteoblasts become trapped within the matrix of bone they have laid and develop into osteocytes. The mesenchyme surrounding the matrix remains uncalcified, however, it begins to develop into the thick fibrous coating that will surround the bones, known as the periosteum. The osteoblasts continue depositing the matrix and forming more trabeculae. This trabeculae may remain in place and form into spongy bone, or may be remodeled or broken down forming a space for the medullary cavity, or the central canal of the bone where the marrow is located. Trabeculae located closer to the surface of the bone continues to calcify until the spaces between the plates have been filled in with bone. The spongy bone has then been converted into compact bone.
Endochondral ossification takes place while a fetus is still within the womb. A fetus originally develops a skeletal model composed of hyaline cartilage. This process begins during the sixth week of a fetus’ gestation and continues into the individual’s twenties. The majority of the bones within the body are developed this way.
The mesenchyme develops into a “bone” made of hyaline cartilage, covered with a fibrous substance known as the perichondrium. The perichondrium is responsible for producing chondrocytes as the cartilage grows in length and width. The perichondrium begins to produce osteoblasts and ceases the production of chondrocytes. These osteoblasts then begin depositing a thin layer of bone around the hyaline cartilage. The perichondrium is now referred to as the periiosteum. Chondrocyctes still remain within the center of the hyaline cartilage “bone,” and begin to enlarge. The site of chondrocyte enlargement is known as the primary ossification center. The matrix between the cavities, known as lacunae, that contain the chondrocytes, becomes thicker, cutting off nutrients to the chondrocytes. Eventually the chondrocytes die and the lacunae they were contained within merge together, forming the central cavity of the bone.
Blood vessels begin to penetrate the bone and make their way into the primary ossification center. This center is hollowed out and begins to fill with stem cells. This central cavity then becomes the primary marrow cavity. Cells found within this cavity are responsible for the development of cells such as osteoblasts (responsible for the building of bone) and osteoclasts (responsible for the breakdown of bone). The osteoblasts then begin lining the interior wall of the central canal, forming layer of trabeculae. As the bone under the periosteum thickens, the cartilage beneath dies and the osteoclasts break down the dead cartilage, thus expanding the marrow cavity. The transitional area of the bone where bone tissue merges into cartilage, is known as the mataphysis. The secondary marrow cavity is developed within the epiphysis, just as the primary ossification canter develops within the diaphysis.
The bones then begin filling with spongy bone during infancy and childhood. The cartilage decreases in presence and is limited to the articular joins and the epiphyseal plate, which allows for the elongation and growth of the bone. The epiphyseal plate is present until alte childhood and adolecence. By adulthood, the carilage within the epiphyseal plate is no longer present and the space between the diaphysis and epiphysis merges together forming a single cavity. At this point, the bone cannot grow any further.
Saladin, Kenneth S.. Anatomy & physiology: the unity of form and function. 5th ed. Dubuque: McGraw-Hill, 2010. Print.
What is Intramembranous Ossification?
Steps of Endochondral Ossification