Podoplanin is expressed by a sub-population of human foetal rib and knee joint rudiment chondrocytes

The aim of this study was to determine if podoplanin was expressed by rudiment chondrocytes in human foetal cartilages. Podoplanin was immunolocalised in first trimester human foetal rib and knee joint rudiments to a sub-population of chondrocytes deep in the rib rudiments, tibial and femoral growth plates and cells associated with the cartilage canals of the foetal knee joint rudiments. Lymphatic vessels in the loose stromal tissues surrounding the developing rudiments were also demonstrated on the same histology slides using antipodoplanin (MAb D2-40) and anti-LYVE-1 and differentiated from CD-31 positive blood vessels confirming the discriminative capability of the antibody preparations used. The D2-40 positive rib and knee rudiment chondrocytes were not stained with antibodies to LYVE-1, CD-31 or CD-34 however perlecan was a prominent pericellular proteoglycan around these cells confirming their chondrogenic phenotype. Discernable differences were evident between the surface and deep rudiment chondrocytes in terms of their antigen reactivities detected with MAb D2-40 or antiperlecan antibodies. Binding of the cytoplasmic tail of PDPN to the ERM proteins ezrin, radixin and moeisin may result in changes in cytoskeletal organisation which alter the phenotype of this central population of rudiment cells. This may contribute to morphological changes in the rudiment cartilages which lead to establishment of the primary ossification centres and is consistent with their roles as transient developmental scaffolds during tissue development.