Bone Formation

BONE FORMATION Bone is formed either by direct ossification of embryonic connective tissue (intramembranous ossification) or by replacement of hyaline cartilage (intracartilaginous or endochondral ossification). Intramembranous ossification takes place in the so-called membrane bones of the skull, while endochondral ossification is characteristic of the bones of the trunk and extremities. . Intramembranous bone formation. !xamine the section of fetal head on slide B35 ( #!) and compare
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   BONE FORMATION   Bone is formed either by direct ossification of embryonic connective tissue ( intramembranous ossification ) or by replacement of hyaline cartilage ( intracartilaginous or endochondral   ossification ). Intramembranous ossification takes place in the so-called membrane bones of the skull, while endochondral ossification is characteristic of the bones of the trunk and extremities. . Intramembranous bone formation . !xamine the section of fetal head on slide B35  ( #!) and compare it to $igure % below. Irregularly shaped trabeculae of spongy bone are present within the mesenchyme lateral and dorsal to the developing nasal chambers. &ocate the osseous tissue (staining pale red) and note that it is called spongy bone or cancellous bone and is characteri'ed by many spaces between the trabeculae. steoblasts line the trabecular surfaces. ome of these cells have become entrapped within lacunae in the newly deposited bone matrix and have thus become osteocytes. In some sections, one may observe resorption of bone from the ends of the trabeculae or occupying shallow excavations in their surface ( Howship's lacunae).      Figure 20  Intramembranous ssification. Taken from:   !artner and Hiatt #olor $e%tboo& of Histolog ,  p. 122, Figure 7-12.   There are several events, which take place in intramembranous bone formation. The events are show in Figure 20 above and listed as follows:  a. Increased (ascularit of tissue . b. cti(e proliferation of mesenchmal cells . *he mesenchymal cells give rise to osteogenic cells, which develop into osteoblasts. c. *steoblasts begin to la down osteoid. steoid is the organic part of bone without the inorganic constituent.   d. *steoblasts either retreat or become entrapped as osteoctes  in the osteoid. e. $he osteoid calcifies to form spicules of spong bone . *he spicules unite to form trabeculae. *he inorganic salts carried in by the blood vessels supposedly bring about calcification. *he salts are deposited in an orderly fashion as fine crystals ( hdro%apatite crstals ) intimately associated with the collagenous fibers. *hese crystals are only visible with the electron microscope. f. Bone remodeling occurs . +eriosteum and compact bone are formed. .  Intracartilaginous +endochondral) bone formation . *his type of ossification involves the replacement of a cartilaginous model by bone and is best observed in long bones, such as the humerus or femur. !vents of endochondral ossification can be seen in $igure  below and include the following a.   ,rimar ossification center. *he first change indicative of beginning ossification takes place about the center of the future bone shaft. ere the cartilage cells hypertrophy and the cartilage matrix becomes calcified. ubseuently, part of the calcified matrix disintegrates, opening cavities that communicate with the connective tissue and vessels at the surface. b. Bone collar. *he bone collar forms concurrently with the primary ossification center. /ells of the perichondrium begin to form bone. *he bone collar holds together the shaft, which has been weakened by the disintegration of the cartilage. *he connective tissue about the bone collar, previously a perichondrium, is now called periosteum. c.   ,eriosteal buds.  *hese are connective tissue buds or 0sprouts0 containing mesenchymal cells (which give rise to osteogenic cells) and blood vessels, which grow from the periosteum to reach the primary ossification center. steoblasts attach to spicules of calcified cartilage in the primary ossification center and begin to produce osteoid. *hus, bone is formed and the process continues toward both epiphyses while this is occurring, the cartilage  outside the primary ossification center increases in si'e by interstitial and appositional growth. d.   -econdar ossification centers . 1bout the time of birth, a secondary ossification center appears in each end (epiphysis) of long bones. +eriosteal buds carry mesenchyme and blood vessels in and the process is similar to that occurring in a primary ossification center. *he cartilage between the primary and secondary ossification canters is called the epiphyseal plate, and it continues to form new cartilage, which is replaced by bone, a process that results in an increase in length of the bone. 2rowth continues until the individual is about  years old or until the cartilage in the plate is replaced by bone. *he point of union of the primary and secondary ossification centers is called the epiphyseal line. Figure 2  +renatal long bone development. Taken from:   -te(ens and /owe Human Histolog ,  p. 246, Figure 13.24.  !xamine the section on slide 3 , fetal finger, longitudinal section, human (*richrome), which shows portions of three developing bones and their primary ossification centers. $ind the collar and, if present, the osteoid  and osteoblasts  on its outer surface. steoblasts in this location are engaged in periosteal bone formation, a type of intramembranous ossification, which is responsible for the growth in thickness of long bones. 3emember there are bones in the body, which are formed exclusively by intramembranous ossification, but there is 1* bone in the body, which is the product of endochondral ossification alone. 4uclei, erythrocytes and bone matrix are red or orange5 collagen and cartilage matrix are blue. Identify the fibrous  and cellular   layer of the periosteum and note there is no sharp boundary between the fibrous periosteum and the surrounding connective tissue. 6any trabeculae are present in the marrow cavity.
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