This lecture on development of bone explains the basic concepts of the topic, including the types, steps and cells involved in the process.
Outline of Lecture
DEVELOPMENT OF BONE
INTRODUCTION
According to the embryological origin, there are 2 types of bone development.
INTRAMEMBRANOUS
Bone develops within a layer of connective tissue.
Does not involve removal and replacement of cartilage.
Examples:
Flat bones of the skull
Frontal & parietal bones
Parts of the occipital & temporal bones
Maxilla & Mandible
INTRACARTILAGINOUS
Cartilage forms a framework for bone deposition.
Cartilage is eventually replaced by bone.
Examples includes:
Bones of the base of skull
Bones of face
Bones of axial skeleton
CELL TYPES IN OSTEOGENESIS
DIFFERENT TYPES OF CELLS IN SITES OF BONE FORMATION
- OSTEOGENIC OR OSTEOPROGENITOR CELLS
- OSTEOBLASTS
- OSTEOCYTES
- OSTEOCLASTS
- INTRAMEMBRANOUS
STEPS IN INTRAMEMBRANOUS BONE FORMATION
In the locations where bone is to be laid down, the mesenchyme becomes richly vascularized.
Active proliferation of mesenchymal cells takes place.
Within the primitive connective tissue bed, some of the cells show structural changes & identified as osteoprogenitor cells & osteoblasts.
Between these cells thin bars of dense intercellular substance appears.
Non calcified matrix or osteoid.
CALCIFICATION OF THE MATRIX
Calcification of bone is dependant upon:
Availability of adequate amount of minerals
Requisite chemical & physical conditions
SEEDING PHENOMENON
In the process of calcification, minerals are deposited as minute crystals of hydroxyapatite. This process is initiated by a “seeding phenomenon”.
2 forms of seeding phenomenon has been described:
Accomplished by fibrils of type I collagen.
Hydroxyapatite crystals appear first on the surface of the collagen.
Matrix vesicles are involved.
Arise from osteoblasts.
Contains calcium & phosphate.
RICKETS
Vitamin D deficiency
Uncalcified osteoid at the growing ends of bones.
SCURVY
Deficiency in vitamin C.
Essential for osteoblasts to form collagen capable of normal cross-linkage.
INTRACARTILAGINOUS
An embryonal type of hyaline cartilage precedes the formation of bone.
Shape of bone corresponds closely to that of the prior cartilage.
During development, the cartilage is replaced by bone, except at the joint surfaces.
This replacement is not completely accomplished, however, until the bone has achieved its full size & growth has ceased.
During development, not only does the supporting function have to be maintained but there is continual increase in length & diameter as well.
STEPS IN INTRACARTILAGINOUS BONE FORMATION
Initially, a hyaline cartilage model with the general shape of the bone is formed.
Appearance of a cuff of bone around the cartilage model.
Chondrocytes in the mid region of the cartilage model become hypertrophic.
The calcified cartilage matrix inhibits diffusion of nutrients, causing death of the chondrocytes.
Periosteal cells migrate into the cavity along with growing blood vessels.
ZONES OF THE EPIPHYSEAL CARTILAGE
Several zones can be distinguished in the cartilage.
- ZONE OF RESERVE CARTILAGE
- ZONE OF CELL PROLIFERATION OR MULTIPLICATION
- ZONE OF CELL MATURATION & HYPERTROPHY
- ZONE OF CARTILAGE CALCIFICATION
- ZONE OF CARTILAGE REMOVAL & BONE DEPOSITION (ZONE OF PROVISIONAL OSSIFICATION)
ZONE OF RESERVE CARTILAGE
No cellular proliferation or active matrix production.
Long prior to the formation of the secondary centers of ossification.
Becomes relatively short after secondary centers form.
ZONE OF CELL PROLIFERATION OR MULTIPLICATION
Cartilage cells undergo division.
Organize into distinct columns.
These cells are larger than those in the reserve zone & actively produce matrix.
ZONE OF CELL MATURATION & HYPERTROPHY
Greatly enlarged cartilage cells.
Cytoplasm of these cells contains considerable amounts of glycogen & alkaline phosphatase.
The matrix is compressed into linear bands.
ZONE OF CARTILAGE CALCIFICATION
Zone of variable length.
Enlarged cells begin to degenerate. Prior to degeneration, hypertrophied chondrocytes release membrane-enclosed vesicles.
Matrix becomes calcified.
ZONE OF CARTILAGE REMOVAL & BONE DEPOSITION (ZONE OF PROVISIONAL OSSIFICATION)
Zone nearest the diaphysis.
Calcified cartilage is in direct contact with the connective tissue of the marrow cavity.
Blood vessels & C.T. invade the region.
They form a series of spearheads, leaving the calcified cartilage as longitudinal spicules, at least as seen in L.S.
In a C.S. of the bone, the cartilage appears as a honeycomb because the invading vessels & C.T. have migrated into the sites previously occupied by the cartilage cells.
Bone deposition occurs on the cartilage spicules
Formation of primary spongy bone.
Shortly after birth, a secondary ossification center develops in the upper epiphysis.
Later, a similar ossification center forms at the lower end of the bone.
Remaining cartilages from the original model
Cartilage of the epiphyseal plate is responsible for maintaining the growth process.
For a bone to retain proper proportions & its unique shape, both external & internal remodelling must occur as the bone grows in length.
Thickness of the epiphyseal plate remains relatively constant during the growth.
The amount of new cartilage produced equals the amount resorbed. (zone of proliferation = zone of resorption)
The resorbed cartilage is replaced by spongy bone.
LENGTHENING OF BONE
Actual lengthening of bone occurs when new cartilage matrix is produced at the epiphyseal plate.
INCREASE IN WIDTH & DIAMETER
Bone increases in width or diameter when appositional growth of new bone occurs between the cortical laellae & the periosteum.
DEVELOPMENT OF SHORT BONES
Similar to that of the epiphysis of long bones.
Ossification begins in the center of the cartilage.
Extends in all directions.
DEVELOPMENT OF HAVERSIAN SYSTEM
DEVELOPMENT
Osteons typically develop in preexisting compact bone.
A tunnel is bored through compact bone by osteoclasts.
PARTS OF BONE REMODELLING UNIT
Osteoporosis
Disease of Bone Growth & Calcium Metabolism
Bone reabsorption exceeds deposition.
Osteoclasts mobilize Ca++ to plasma.
Factors: Inadequate Ca++ intake, genes, hormones, smoking.
REMODELLING OF BONE
Remodelling consists of preferential resorption of bone in some areas & deposition of bone in other areas.
Resorption is associated with the appearance of osteclasts.
Any reconstruction of bone occurs in response to local mechanical stresses to which the bone is subjected.
BONE RESORPTION
BONE REMODELLING
BIOLOGICAL MINERALIZATION
FRACTURES & BONE REPAIR
References:
Text & Atlas Of Histology By MICHAEL H. ROSS 5th Edition
BAILEY’S Textbook Of Microscopic Anatomy 18th Edition
Textbook Of Histology By LEESON, LEESON, PAPARO 5th edition
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