We are investigating mice treated with the chemical mutagen N-ethyl-N-nitrosourea (ENU) to establish models for hereditary bone disorders such as osteoporosis, osteoarthritis and skeletal dysplasias. We report on one such mouse model designated Longpockets (Lpk). Mice were kept in accordance with national welfare guidelines and project license restrictions. Lpk/+ mice are viable and fertile, present with short humeri by two weeks of birth and have abnormal vertebrae and disorganised growth plates. These features are consistent with spondyloepiphyseal dysplasia congenita (SEDC). However, homozygous Lpk/Lpk mice die perinatally. These abnormalities resulted from a missense mutation in Col2a1 altering a conserved serine (Ser) at position 1386 to a proline (Pro) residue. Histology, electron microscopy and micro-CT analyses were undertaken to further characterise the phenotype and to delineate the morphological differences between the viable and lethal phenotypes. Both Lpk/Lpk and Lpk/+ embryonic cartilage had reduced safranin-o staining in the extracellular matrix compared to wild type. In addition, chondrocytes in Lpk/Lpk embryonic cartilage grew horizontally to the direction of bone growth, compared to Lpk/+ and wild type which had vertical columns of proliferating chondrocytes. Longitudinal studies revealed growth plate disorganisation in Lpk/+ by 26 days after birth, and abnormal deposition of collagen fibrils in both Lpk/Lpk and Lpk/+ by embryonic day 16.5. Micro-CT scanning at 12 weeks revealed decreased bone mineral density, and total bone volume in the humeri and femora of Lpk/+ compared to wild type mice. Femoral trabecular volume was unchanged whereas humeral trabecular bone was absent in Lpk/+ mice. Furthermore, the elbow and knee joints from Lpk/+ mice had abnormal bone deposition and erosions respectively, consistent with secondary osteoarthritis. Thus, the Col2a1 Ser1386Pro mutation in Lpk appears to have a dominant negative effect on collagen secretion resulting primarily in loss of growth plate polarity, organisation and underlying architectural support.