The Development (Embryology) of Bone

Bone Categories

I. SPONGY OR CANCELLOUS BONE

a. contains many large spaces filled with red marrow.

b. makes up most of the bone tissue of short, flat, and irregularly shaped bones and most of the epiphyses of long bones.

c. provides some support and a storage area for marrow.

II. COMPACT, OR DENSE DONE

a. contains few spaces.

b. is deposited in a layer over the spongy bone tissue.

c. layer of compact bone is thicker in the DIAPHYSIS (shaft of the bone) than the EPIPHYSES (ends of the bone).

d. provides protection and considerable support.

COMPACT BONE

-Compact bone has a concentric ring microscopic structure, whereas spongy bone does not.

-Blood vessels and nerves from the PERIOSTEUM and ENDOSTEUM penetrate the compact bone through VOLKMANN'S CANALS. The blood vessels of these canals connect with blood vessels and nerves of the MEDULLARY CAVITY and those of the HAVERSIAN CANALS. The Haversian canals run longitudinally through the bone. Around them are LAMELLAE concentric rings of hard, calcified, intercellular substance. Between the lamellae are small spaces called LACUNAE where osteocytes are found. OSTEOCYTES are mature osteoblasts that have lost their ability to produce new bone tissue. Radiating in all directions from the lacunae are minute canals called CANALICULI which contain slender processes of osteocytes. The canaliculi connect with other lacunae and, eventually, with Haversian canals. Thus an intricate network is formed throughout the bone. This branching network of canaliculi provides numerous routes so that nutrients can reach osteocytes and wastes can be removed. Each Haversian canal, with its surrounding lamellae, lacunae, osteocytes, and canaliculi, is called an HAVERSIAN SYSTEM or OSTEON. Haversian systems are characteristic of adult bone. The areas between Haversian systems contain INTERSTITIAL LAMELLAE. These also possess lacunae with osteocytes and canaliculi, but their lamellae are usually not connected to the Haversian systems.

 

SPONGY BONE

In contrast to compact bone, spongy bone does not contain true Haversian systems. It consists of an irregular latticework of thin plates of bone called TRABECULAE. The spaces between the trabeculae of some bones are filled with marrow. The cells of the red marrow are responsible for producing blood cells. Within trabeculae lie lacunae, which contain osteocytes. Blood vessels from the periosteum penetrate through to the spongy bone, and osteocytes in the trabeculae are nourished directly from the blood circulating through.the marrow cavities.

 

OSSIFICATION
OSSIFICATION or OSTEOGENESIS-the process by which bone forms in the body.

The "skeleton" of a human embryo is composed of either fibrous membranes or hyaline cartilage. Both are shaped like bones and provide the medium for ossification. Ossification begins around the 6th or 7th week of embryonic life and continues throughout adulthood.

Two kinds of bone formation occur:

A. INTRAMEMBRANOUS OSSIFICATION

B. ENDOCHONDRAL OSSIFICATION

These two kinds of ossification do not lead to differences in the structure of mature bones. They simply indicate different methods a bone formation. Both mechanisms involve the replacement of a preexisting connective tissue with bone.

The first stage in the development of bone is the migration of mesenchymal cells(embryonic connective tissue cells) into the area where bone formation is about to begin. These cells increase in number and size. In some skeletal structures where capillaries are lacking they become chondroblasts. The CHONDROBLASTS will be responsible for cartilage formation. The osteoblasts will form bone tissue by intramembranous or endochondral ossification.

A.INTRAMEMBRANOUS OSSIFICATION

Of the two types of bone formation, this is the simpler and more direct process. The flat bones of the roof of the skull, parts of the mandible, and probably part of the clavicles are formed in this way. The essentials of the process are as follows:

1. Osteoblasts formed from mesenchymal cells cluster in the fibrous membrane. The site of such a cluster is called a CENTER OF OSSIFICATION.

2.The OSTEOBLASTS then secrete intercellular substances partly composed of collagenous fibers that form a framework, or MATRIX in which calcium salts are quickly deposited. The deposition of calcium salts is called CALCIFICATION.

3. When a cluster of osteoblasts is completely surrounded by the calcified matrix it is called a TRABECULAE. As trabeculae form in nearby ossification centers, they fuse into the open latticework characteristic of spongy bone.

4.With the formation of successive layers of bone, some osteoblasts become trapped in the lacunae. The entrapped osteoblasts lose their ability to form bone and are called osteocytes.

5.The spaces between the trabeculae fill with red marrow. The original connective tissue that surrounds the growing mass of bone then becomes the PERIOSTEUM. The ossified area has now become true spongy bone.

6.Eventually, the surface layers of the spongy bone will be reconstructed into compact bone. Much of this newly formed bone will be destroyed and reconstructed so the bone may reach its final adult size and shape.

 

B. ENDOCHONDRAL OSSIFICATION

The replacement of cartilage by bone. Most bones of the body, including parts of the skull are formed in this way. Since this type of ossification is best observed in a long bone, we will investigate the tibia.

 

1. Early in embryonic life, a cartilage model or template of the future bone is laid down. This model is covered by a membrane called the PERCHONDRIUM.

2. Midway along the shaft of this model a blood vessel penetrates the peirichondrium, stimulating cells in the internal layer of the perichondrium to enlarge and become osteoblasts.

3. The osteoblasts begin to form a collar of compact bone around the middle of the diaphysis of the cartilage model. Once the perichondrium starts to form bone, it is called the PERIOSTEUM.

4. Simultaneously with the appearance of the bone collar and the penetration of blood vessels changes occur in the cartilage in the center of the diaphysis. In this area, called the PRIMARY OSSIFICATION CENTER, the cartilage cells hypertrophy (increase in size)-probably because they accumulate glycogen for energy and produce enzymes that catalyze future chemical reactions.

5. When the hypertrophied cells burst, there is a change in extracellular pH to a more alkaline pH causing the intercellular substance to become calcified, that is, minerals are deposited within it. Once the cartilage becomes calcified, nutritive materials required by the cartilage cells can no longer diffuse through the intercellular substance and this may cause the cartilage cells to die.

6. Then the intercellular substance begins to degenerate leaving large cavities in the cartilage model. The blood vessels grow along the spaces where cartilage cells were previously located and enlarge the cavities further. Gradually, these spaces in the middle of the shaft join with each other, and the marrow cavity is formed.

7. As these developmental changes are occurring, the osteoblasts of the periosteum deposit successive layers of bone on the outer surface so that the collar thickens, becoming thickest in the diaphysis. The cartilage model continues to grow at its ends, steadily increasing its length.

8. Eventually, blood vessels enter the epiphyses and SECONDARY OSSIFICATION CENTERS appear in the epiphyses and also lay down spongy bone. In the tibia one secondary ossification center develops in the proximal epiphysis soon after birth. The other center develops in the distal epiphysis during the child's second year.

9. After two secondary ossification centers have formed, bone tissue has completely replaced cartilage except in two regions. Cartilage continues to cover the articular surfaces of the epiphyses where it is called ARTICULAR CARTILAGE. It also remains as a plate between the epiphysis and diaphysis, where it is called the EPIPHYSEAL PLATE.

10. The epiphyseal plate allows the diaphysis of the bone to increase in length until early adulthood. As the child. grows, cartilage cells are produced by mitosis on the epiphyseal side of the plate. Cartilage cells are then destroyed and the cartilage is replaced by bone on the diaphyseal side of the plate. In this way, the thickness of the epiphyseal plate remains fairly constant, but the bone on the diaphyseal side increases in length.

11. Growth in diameter occurs along with growth in length. In this process, the bone lining of the marrow cavity is destroyed so that the cavity increases in diameter. At the same time, osteoblasts from the periosteum add new osseus tissue around the outer surface of the bone. Initially, diaphyseal and epiphyseal ossification produce only, spongy bone. Later, by reconstruction, the outer region of spongy bone is reorganized into compact bone.

-Bone growth ceases at age l8 in females and age 20 in males.

-The clavicle is the last bone to stop growing.

-Ossification of all bones is usually completed by age 25.

-Bone shares with skin the feature of replacing itself throughout life.