A Practical Approach, Second Edition=Ronald D. Ho.pdf

972 DEVELOPMENTAL REPRODUCTIVE TOXICOLOGY: A PRACTICAL APPROACH, SECOND EDITIONEpiphysisGrowth plateCancellousboneMetaphysisCortical boneEndosteumCiaphysisPenosteumFused growthplateFigure 2 (6) Schematic diagram of a tibia. The interior of a typical long bone showing the growing proximalend with a growth plate and a distal end with the epiphysis fused to the metaphysis.the shaft adjacent to the endosteum (6). Finally, in mature compact bone circular rings of lamellaeare arranged concentrically around longitudinal vascular channels called Haversian canals. Thevascular channel and the concentric lamellae form an osteon or Haversian system. The outer limitsof osteons are defined by distinct borders called cement lines. Osteons communicate with oneanother, the periosteum, and the bone marrow via oblique channels called Volkman’s canals (3, 6).Compact and cancellous bone also contain regularly spaced small cavities called lacunae thateach contain an osteocyte (Figure 1). Extending from each lacuna in a 3 dimensional fashion arebranching tubular passages called canaliculi that connect with adjacent lacuna (6, 3).2.1 Long BonesThe cylindrical shaft of a long bone is called the diaphysis (Figure 2). The diaphysis is made upof a thick wall of compact bone surrounding a central medullary cavity made up of cancellousbone (3). The bulbous ends of long bones in growing humans and animals are called the epiphyses.In mature long bones, the epiphysis is primarily composed of cancellous bone covered by a thinlayer of compact bone. In addition, articular cartilage can be found covering the compact bone atthe uppermost portion of the epiphysis. During postnatal bone growth, epiphyses are separatedfrom the diaphysis by a cartilaginous epiphyseal plate that is connected to the diaphysis by an areaof spongy bone called the metaphysis. Growing bones increase in length in the growth zone thatis formed by the epiphyseal growth plate and the metaphysis.In general, the external surfaces of bone are covered by a layer of fibrous connective tissuecalled periosteum that has the ability to produce bone (Figure 3). The periosteum is made up of 2layers, an outer dense fibrous layer, and an inner vascular cellular layer (4). The inner layer containsosteoblasts and osteoclasts. The periosteum is anchored to bone by bundles of collagen fibers calledSharpey’s fibers.The inner cavities of bone are lined with a layer of osteogenic connective tissue called endosteum(Figure 3). Endosteum is made up of a single layer of flattened osteoprogenitor cells and vascular© 2006 by Taylor & Francis Group, LLC