J. Anat. (1994) 184, pp. 447-456, with 9 figures Printed in Great Britain Review Cellular aspects of the development of diarthrodial joints and articular cartilage* CHARLES W. ARCHER', HEATHER MORRISON' AND ANDREW A. PITSILLIDES2 IDepartment of Anatomy, University of Wales College of Cardif and I Department of Rheumatology, University College London, UK (Received 21 October 1993) ABSTRACT There have been many reports on the histological development of mammalian diarthrodial or synovial joints. While these are useful for comparative purposes, they tell us little of the cellular basis of joint morphogenesis which must underlie a number of morphogenetic defects. The process of joint morphogenesis is complex and can be subdivided into a number of facets and this report will focus on 2 of them. First, the process of joint cavitation in the chick metatarsophalangeal joint, where we propose that the selective secretion of hyaluronan into the presumptive cavity plays a central role. Secondly, the development of articular cartilage where we have used the South American opossum Monodelphis domestica as a model for mammalian development. Like most marsupials, the young are born at a much earlier developmental stage than eutherian mammals. Using antibodies which detect proliferating chondrocytes and those synthesising insulin-like growth factors 1 and 2 and insulin-like growth factor 1 binding protein, we report that the majority of growth (as assessed by these indicators) appears appositional. Key words: South American opossum; Monodelphis domestica; collagen; keratan sulphate; hyaluronan; insulin-like growth factors. INTRODUCTION Vertebrate joints are classified on the basis of their most characteristic features into 3 main categories (Nomina Anatomica, 1950): fibrous (e.g. sutures of the skull), cartilaginous (e.g. intervertebral disc) and synovial (e.g. the knee). The last category, also known as diarthrodial joints, are freely movable and are susceptible to a number of disorders, particularly degenerative diseases such as osteoarthrosis (osteo- arthritis). The development of joints has been of interest because of the lack of understanding of the processes which underlie problems such as congenital hip dislocation, ankyloses of joints as in clubfoot or brachydactyly in interphalangeal joints. While the reasons for studying joint development in relation to malformation are obvious, there are good reasons for studying the same processes in relation to degenerative diseases. A feature of these diseases is a repair response manifest by renewed cell division and upregulated matrix synthesis which resemble some aspects of development (Archer, 1994). The mechanism whereby the skeletal elements are specified and laid down in a temporospatial pattern has been extensively studied largely in relation to, and within, the conceptual framework of positional in- formation (Wolpert, 1989). There are a number of regulatory genes which have been identified recently and appear to control the skeletal pattern in the developing limb (reviewed by Izpisua-Belmont & Duboule, 1992). In phenomenonological terms, the vast majority of work on limb development has been carried out on the chicken embryo. This is because the embryos are readily accessible to experimental manipulation through holes cut in the shell. Additionally, because * Based on a presentation at a Symposium on the Biology of Joints during the Summer Meeting of the Anatomical Society of Great Britain and Ireland, July 1993. Correspondence to Professor C. W. Archer, Department of Anatomy, University of Wales College of Cardiff, PO Box 900, Cardiff CFl 3YF, UK. 31-2 447