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

POSTNATAL DEVELOPMENTAL MILESTONES 983Table 7 Appearance and Fusion of OssificationCenters in the Rabbit (41, 42)Bone Age at Appearance Age at FusionHumerusProximal Epiphysis 1 day 32 weeksDistal Epiphysis 1 day 32 weeksFemurProximal Epiphysis 1–5 days 16 weeksDistal Epiphysis 1 day 32 weekstrochlea begins to fuse with those of the medial and lateral condyles (36). By 8 to 11 months, theepiphyseal growth plate in the distal region of the femur fuses with the diaphysis (36, 25).3.3.3 MandibleDuring prenatal development, the mandible of the dog, is formed by intramembranous ossification.During postnatal development, endochondral bone formation occurs at the condyle; however,elsewhere in the mandible, bone growth occurs by the apposition of bone along the mandibularsurfaces (39).The rate at which bone is added to the mandible is most rapid during the early postnatal period.After dogs reach about 40 to 60 days of age, the rate of bone formation in the mandible plateaus(39, 40). By the age of 6 to 7 months, even though bone apposition and resorption continues inthe mandible, the volume of the mandible remains constant because of a balance between the twoprocesses (39).During postnatal development, the mandible increases in overall size, length, width, and height.The increase in the length that occurs in the mandible of the dog is primarily due to bone formationon the rear portions of both the ramus and the mandibular body (39). The mandibular ramusincreases in width as more bone is deposited at the caudal border of the mandible than is beingresorbed at the rostral border. Resorption at the rostral border of the mandible occurs to makesroom for eruption of the molars. The height of the mandible increases by apposition of bone onthe ventral surface of the mandible, and by growth of alveolar bone (39). A change in the mandibularangle occurs as bone is laid down on the caudo-ventral portion of the mandible (39). Growth ofthe mandible in a downward and backward direction is caused by endochondral bone formation atthe mandibular condyle (39).3.4 RabbitA summary of the appearance of secondary ossification centers and the fusion of the epiphyses ofthe humerus and femur is provided in Table 7.3.4.1 HumerusWhen maturation of secondary ossification centers was studied in the Japanese white rabbit, nosignificant differences between male and female rabbits were noted (41). As early as postnatal Day1, secondary ossification centers were present in the proximal and distal epiphyses of the humerus(41). By 1 week after birth, all secondary ossification centers had appeared in the long bones withthe exception of those in the proximal epiphysis of the fibula, which appeared at 2 weeks of age (41).At 32 weeks of age, the proximal and distal epiphyses of the humerus fused with the diaphyses (41).© 2006 by Taylor & Francis Group, LLC