The bony armor of the earliest jawless fish was dermal bone; so are shark scales, shoulder blades, and the roof of your skull. Vertebrae, ribs, appendages, and the jaw are endoskeletal bone. The vertebrate skull is actually a complex structure of both endoskeletal and dermal bone.
a. Primary ossification center. The first change indicative of beginning ossification takes place about the center of the future bone shaft. Here the cartilage cells hypertrophy and the cartilage matrix becomes calcified. Subsequently, part of the calcified matrix disintegrates, opening cavities that communicate with the connective tissue and vessels at the surface. b.Bone collar. The bone collar forms concurrently with the primary ossification center. Cells of the perichondrium begin to form bone. The bone collar holds together the shaft, which has been weakened by the disintegration of the cartilage. The connective tissue about the bone collar, previously a perichondrium, is now called periosteum. c. Periosteal buds. These are connective tissue buds or &quot;sprouts&quot; containing mesenchymal cells (which give rise to osteogenic cells) and blood vessels, which grow from the periosteum to reach the primary ossification center. Osteoblasts attach to spicules of calcified cartilage in the primary ossification center and begin to produce osteoid. Thus, bone is formed and the process continues toward both epiphyses while this is occurring, the cartilage outside the primary ossification center increases in size by interstitial and appositional growth. d. Secondary ossification centers. About the time of birth, a secondary ossification center appears in each end (epiphysis) of long bones. Periosteal buds carry mesenchyme and blood vessels in and the process is similar to that occurring in a primary ossification center. The 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. Growth continues until the individual is about 21 years old or until the cartilage in the plate is replaced by bone. The point of union of the primary and secondary ossification centers is called the epiphyseal line.
1. Zone of reserve cartilage. This is typical hyaline cartilage and is a large zone in this preparation. 2. Zone of cell proliferation (ZP). The cartilage cells are small and tend to be arranged in columns, which run parallel to the long axis of the cartilage. This arrangement is indicative of their intense mitotic activity. 3. Zone of cell and lacunar maturation and hypertrophy enlargement (ZH). Chondrocytes and lacunae are larger than in the previous zone. The chondrocytes increase in size and resorb some their lacunar walls, enlarging them to such an extent that some of the lacunae become confluent. 4. Zone of calcification (ZC). This is a small zone having a slightly darker appearance than the preceding zone due to the basophilic staining of the calcified cartilage. The chondrocytes die in this zone. 5. Zone of cartilage removal and bone deposition. Osseous elements are present among the pieces of calcified cartilage.
Introduction to Verterbrate Bones
The Bones Lectured by Bien Nillos, MD Comparative Vertebrate Anatomy
<ul><li>Dermal bone consists of bony structures (plates and scales) that develop in the skin. </li></ul><ul><li>Dermal bone does not form from cartilage first and then calcify, but endoskeletal bone does; in fact, in cartilaginous fishes it may never form true bone. </li></ul>VERTEBRATE BONES DERMAL BONE ENDOSKELETON
<ul><li>All vertebrates have cartilage in addition to bone, or instead of bone. </li></ul>
Function of the Bones <ul><li>1. Support. Provide a hard framework. </li></ul><ul><li>2. Protection of many vital organs. </li></ul><ul><li>3. Movement - act as levers with skeletal muscles moving them. Joints control possible movements. </li></ul><ul><li>4. Mineral storage. </li></ul><ul><li>5. Blood cell formation – hematopoiesis </li></ul>
Types of Bones <ul><li>According to Development </li></ul><ul><ul><li>Membranous bones </li></ul></ul><ul><ul><li>Cartilaginous bones </li></ul></ul><ul><ul><li>Membro-cartilaginous bones </li></ul></ul><ul><ul><li>Ossification is the formation of bone by the activity of osteoblasts and osteoclasts and the addition of minerals and salts. Calcium compounds must be present for ossification to take place. Osteoblasts do not make these minerals, but must take them from the blood and deposit them in the bone. By the time we are born, many of the bones have been at least partly ossified. </li></ul></ul>
BONE FORMATION <ul><li>Formed either by direct ossification of embryonic connective tissue (intramembranous ossification) or by replacement of hyaline cartilage (intracartilaginous or endochondral ossification). </li></ul><ul><li>Intramembranous ossification takes place in the so-called membrane bones of the skull </li></ul><ul><li>Endochondral ossification is characteristic of the bones of the trunk and extremities. </li></ul>
Intramembranous Ossification <ul><li>a. Increased vascularity of tissue. </li></ul><ul><li>b. Active proliferation of mesenchymal cells. The mesenchymal cells give rise to osteogenic cells, which develop into osteoblasts. </li></ul><ul><li>c. Osteoblasts begin to lay down osteoid. Osteoid is the organic part of bone without the inorganic constituent. </li></ul><ul><li>d. Osteoblasts either retreat or become entrapped as osteocytes in the osteoid. </li></ul><ul><li>e. The osteoid calcifies to form spicules of spongy bone. The spicules unite to form trabeculae. The inorganic salts carried in by the blood vessels supposedly bring about calcification. The salts are deposited in an orderly fashion as fine crystals (hydroxyapatite crystals) intimately associated with the collagenous fibers. These crystals are only visible with the electron microscope. </li></ul><ul><li>f. Bone remodeling occurs. Periosteum and compact bone are formed. </li></ul>
Endochondral Ossification <ul><li>This 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 </li></ul><ul><li>Primary ossification center. </li></ul><ul><li>Bone collar </li></ul><ul><li>Periosteal buds </li></ul><ul><li>Secondary ossification centers </li></ul>
5 zones of Cartilage <ul><li>Zone of reserve cartilage. </li></ul><ul><li>Zone of cell proliferation </li></ul><ul><li>Zone of cell and lacunar maturation and hypertrophy enlargement </li></ul><ul><li>Zone of calcification </li></ul><ul><li>Zone of cartilage removal and bone deposition </li></ul>
Types of Bone <ul><li>On the basis of Structure </li></ul><ul><ul><li>Compact </li></ul></ul><ul><ul><li>Spongy </li></ul></ul>
<ul><ul><li>Compact or cortical bone, is made up many rod-like units called osteons or Haversian systems which run longitudinally within the bone. Haversian systems have a central Haversian canal which carries blood and lymphatic vessels and nerve branches. </li></ul></ul>
<ul><li>The osteon, or Haversian system, is the fundamental functional unit of much compact bone </li></ul><ul><li>Each osteon consists of concentric layers, or lamellae, of compact bone tissue that surround a central canal, the Haversian canal. The Haversian canal contains the bone's nerve and blood supplies. The boundary of an osteon is the cement line </li></ul>Osteons are separated from each other by interstitial lamellae between systems. Some of the osteoblasts develop into osteocytes, each living within its own small space, or lacuna. Osteocytes make contact with the cytoplasmic processes of their counterparts via a network of small canals, or canaliculi. This network facilitates the exchange of nutrients and metabolic waste.
<ul><li>Spongy or cancellous bone consists of a lattice of thin threads of bone called trabeculae and is less dense than compact bone. The orientation of the trabeculae is affected by the mechanical stress to which the bone is exposed . </li></ul>