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

994 DEVELOPMENTAL REPRODUCTIVE TOXICOLOGY: A PRACTICAL APPROACH, SECOND EDITIONCuttingConeReversalZoneClosing ConeOABCDEBFGI II III IVGGGABEFDOForming ResorptionCavityResorption CavityForming HaversianSystemCompletedHaversian SystemFigure 5 (6) Diagram showing a longitudinal section through a cortical remodeling unit with correspondingtransverse sections below. A. Multinucleated osteoclasts in Howship’s lacunae advancing longitudinally fromright to left and radially to enlarge a resorption cavity. B. Perivascular spindle-shaped precursor cells. C. Capillaryloop. D. Mononuclear cells lining reversal zone. E. Osteoblasts apposing bone centripetally in radial closure andits perivascular precursor cells. F. Flattened cells lining Haversian canal of completed Haversian system.Transverse sections at different stages of development: (I) resorption cavities lined with osteoclasts; (II) completedresorption cavities lined by mononuclear cells, the reversal zone; (III) forming Haversian system or osteons linedwith osteoblasts that had recently apposed three lamellae; and (IV) completed Haversian system with flattenedbone cells lining canal. Cement line (G); osteoid (stippled) between osteoblast (O) and mineralized bone.As reported by Hert et al., (91), the diaphysis of long bones in adult humans and large adultmammals (longer living) such as dogs and sheep is composed of Haversian, secondary bone thatreplaces the original primary bone. However, the compact bone of smaller adult mammals such asmice and rats is not made up of Haversian systems (87). In fact, Bellino (92) notes that the rabbitis the smallest species known to undergo Haversian bone remodeling.Jowsey (93) reports that secondary osteons are not present in all species with the same degreeof frequency that is seen in mature human compact bone. He states that in some animals, primarybone is replaced at an early age, while in others, primary bone persists for quite some time. Forexample, it was noted that in the 2-year-old human, cortical bone of the femur consists mostly ofsecondary osteons whereas the cortical bone in the femur of a 7-year-old rabbit only has “occasional”osteons (93).When the tibia and fibula of rats, hamsters, guinea pigs, and rabbits were investigated at agesranging from 6 months to 5 years, it was noted that in varying degrees, primary bone is present incompact bone throughout the life of each of these animals (87). Specifically, in guinea pigs andrats, secondary bone in the form of osteons was found in the compact bone of only a few “veryold” animals. In the adult rabbit, however, a greater degree of “Haversian transformation” i.e.remodeling of primary osteons to secondary osteons was seen to occur throughout the life of theanimal (87).Enlow (94) also noted that there was a characteristic absence of Haversian systems in thecompact bone of many vertebrate species, specifically naming the white rat. However, Enlow (94)reported that the appearance of primary osteons is quite common in the young growing dog and© 2006 by Taylor & Francis Group, LLC