title: The cn/cn dwarf mouse. Histomorphometric, ultrastructural, and radiographic study in mutants corresponding to human acromesomelic dysplasia Maroteaux type (AMDM).
abstract: BACKGROUND
  The cn/cn dwarf mouse is caused by a loss-of-function mutation in the natriuretic peptide receptor 2 (NPR-2) gene which helps positively regulate endochondral longitudinal bone growth. The gene mutation corresponds to that in the human skeletal dysplasia Acromesomelic Dysplasia Maroteaux type (AMDM). This study assesses histomorphometric, ultrastructural and radiographic correlates of the growth abnormality.
  
  
  METHODS
  Ten litters of cn/cn and cn/+littermates at ages ranging from 2.5 to 6.5 weeks were studied by skeletal radiographs, histomorphometry and physeal ultrastructure. Skeletal radiographs were done on 2 cn/cn and 2 cn/+littermates at 5 weeks of age. Humeral, femoral, and tibial lengths were measured from 34 intact bones (17 cn/cn, 17 cn/+) at 2.5 to 6.5 weeks. Growth plate histomorphometry in 50 bones (26 cn/cn and 24 cn/+) determined the hypertrophic zone/entire physeal cartilage ratios in 204 sections (87 cn/+, 117 cn/cn) at 3 time periods (2.5-3, 4-4.5, and 6-6.5 weeks). Electron microscopy assessed 6 cn/cn and 6 cn/+age and site-matched physeal cartilage.
  
  
  RESULTS
  Cn/cn mice were two thirds the size of the cn/+. Cn/cn bones were normal in shape or only minimally deformed except for the radius with mid-diaphyseal bowing. Length ratios of cn/cn humeri, femurs, and tibias were a mean of 0.65 (± 0.03, n = 34, 17 ratios) compared to cn/+bones. The main physeal abnormality was a markedly shortened hypertrophic zone with the ratio of hypertrophic zone to entire physis 0.17 (± 0.063) in the cn/cn and 0.30 (± 0.052) in the cn/+mice. Ratio assessments were similar comparing humeral, femoral, and tibial growth plates as were ratios from each of the 3 time periods. Ultrastructural assessments from the resting zone to the lower hypertrophic zone-metaphyseal junction showed no specific individual cell abnormalities in cn/cn compared to cn/+physes.
  
  
  CONCLUSIONS
  The disorder causes a shortened physeal hypertrophic zone but normal ultrastructure of cn/cn chondrocytes points to abnormality primarily affecting the hypertrophic zone rather than a structural cell or matrix synthesis problem.
authors.affiliation: Department of Orthopaedic Surgery, Orthopaedic Research Laboratory, Boston Children's Hospital, 300 Longwood Avenue, Boston, MA 02115, USA. Frederic.shapiro@childrens.harvard.edu.
authors.forename: Frederic
authors.initials: F
authors.lastname: Shapiro
authors.forename: Lauren
authors.initials: L
authors.lastname: Barone
authors.forename: Andrew
authors.initials: A
authors.lastname: Johnson
authors.forename: Evelyn
authors.initials: E
authors.lastname: Flynn
mesh_terms: D000818:Animals; D001835:Body Weight; D001848:Bone Diseases, Developmental / Q000000981:diagnostic imaging* / Q000473:pathology*; D001842:Bone and Bones / Q000000981:diagnostic imaging / Q000473:pathology* / Q000648:ultrastructure*; D019902:Chondrocytes / Q000648:ultrastructure; D004195:Disease Models, Animal; D004838:Epiphyses / Q000473:pathology / Q000648:ultrastructure; D005269:Femur / Q000000981:diagnostic imaging / Q000473:pathology / Q000648:ultrastructure; D006132:Growth Plate / Q000473:pathology / Q000648:ultrastructure; D006801:Humans; D006811:Humerus / Q000000981:diagnostic imaging / Q000473:pathology / Q000648:ultrastructure; D051379:Mice; D008817:Mice, Mutant Strains; D011859:Radiography; D013977:Tibia / Q000000981:diagnostic imaging / Q000473:pathology / Q000648:ultrastructure