Department of Histology and Embryology, P. J. Šafárik University, Medical Faculty, Košice

Osteogenesis : Sylabus for foreign students

Author: Prof. MUDr. Eva Mechírová, CSc.

Revised by: Doc. MVDr. Iveta Domoráková, PhD.

Two processes of bone tissue formation or osteogenesis observed in the embryo are: 1. intramembranous bone tissue formation, in which bone tissue is laid down directly in primitive connective tissue or mesenchyme and 2. endochondral bone tissue formation in which bone tissue replaces a preexisting hyaline cartilage. The mechanism of bone matrix deposition in both ossifications is essentially the same. During both ossifications the expression of specific genes is required – these genes allow differentiation of osteoblasts during intramembranous ossification and hyperthrophy of chondrocytes during endochondral ossification.

Intramembranous ossification

The first evidence of intramembranous ossification is seen around the 8th week of embryonic development in the mesenchyme. By intramembranous ossification develop the flat bones of the skull – frontal, occipital, temporal, part of the mandible, maxilla and part of the clavicle.

mesenchym mesenchymal primary ee blastema bone tissue 1. Condensation of mesenchyme = several primary ossification centers give rise (mesenchymal blastema), highly vascularized. 2. Mesenchymal cells differentiate directly to osteoblasts. 3. Osteoblasts - synthesis of new bone matrix (primary - woven bone tissue), collagen fibers are randomly oriented – woven bone. Calcification of woven bone follows. 4. Osteoblasts are surrounded by calcified bone matrix – change to osteocytes. 5. Newly formed islands of the bone tissue - spicules, trabeculae. 6. Fusion of the spicules – gives the bone a spongy structure. 7. Cavities between spicules – filled with bone marrow (mesenchymal origin). 8. Several ossification centers fuse and fully replace former mesenchymal tissue by anastomosing trabeculae of bone tissue- primary, woven bone. 9. Conversion of woven bone tissue to lamellar – paralelly oriented collagen fibers in lamellae of secondary bone tissue. Endochondral ossification 1. Bone is developed in hyaline cartilage model. 2. Development of short and long bones. 3. In the middle of cartilaginous diaphysis, on the surface, a collar of bone tissue gives rise by intramembranous ossification. From this time the perichondrium is called periosteum. 4. One primary ossification center occurs in the middle of future diaphysis (3rd month prenatal) - hypertrophy of chondrocytes, disintegration of the cartilage and replacement by bone tissue. Between new bone trabeculae the bone marrow cavity is formed. Ossification continues longitudinally toward both epiphyses. 5. Two secondary ossification centra in both epiphysis occur later - hypertrophy of chondrocytes, calcification and replacement of the cartilage by bone goes radially – formation of spongy bone tissue in epiphysis. 6. Cartilage on future articular surfaces persists through adult life. 7. Between epiphysis and diaphysis growing plate (hyaline cartilage) remains to the age 18-20 – responsible for growing of the bone in lenght.

Epiphyseal plate – growing plate

1.Resting zone – hyaline cartilage with chondrocytes. 2.Proliferative zone – active zone showing numerous mitosis (chondrocytes arranged in isogenous columns, elongation of the cartilage). 3.Hypertrophic cartilage zone – accumulation of glycogen in the cytoplasm of chondrocytes that become hypertrophic, secretion of alkaline phosphatase and vascular endothelial growth factor. Chondrocytes also send instruction to perichondrial cells to become osteoblasts to form the bone collar arround the diaphysis. 4.Hypertrophic calcified cartilage zone – calcification of the territorial and interterritorial matrix, degeneration of chondrocytes – cell death – apoptosis. 5.Line of erosion – blood vessels from the perichondrium invade the spaces formerly occupied by dead hypertrophic chondrocytes. Chondroclasts, osteoprogenitor and hematopoietic cells reach the core of calcified cartilage through the perivascular conne- ctive tissue surrounding the newly formed blood vessels. Chondroclasts degrade dead chondrocytes and territorial matrix but interterritorial matrix is preserved as remnants of calcified cartilage matrix and serves for deposition of osteoid by differentiated osteoblasts. 6.Osteoid zone – deposition of bone nonmineralized matrix at the surface of calcified cartilagineous cores (remnants of calcified interterritorial matrix) and formation of trabecular bone. 7.Ossiform zone – primary mineralized woven bone tissue formation 8.Zone of resorption – resorption of woven bone tissue by osteoclasts and deposition of new bone matrix by osteoblasts. By this way primary woven bone tissue is replaced by secondary – lamellar bone tissue.

Results of ossification in long bones Formation of the long bone by 2 processes:

A. Results of endochondral ossification 1. growing of the long bone in lenght 2. formation of bone marrow cavity – firstly filled with bone spicules, later spicules are resorbed by osteoclasts and marrow cavity is formed.

B. Results of intramembranous ossification is thickening of the bone collar (diaphysis) Primary woven bone tissue is replaced by secondary lamellar bone tissue with Haversian systems.