DEVELOPMENT, MORPHOLOGY AND HISTOCHEMISTRY OF THE EARLY SYNOVIAL TISSUE IN HUMAN FOETUSES

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Abstract
Histo-chemical studies of the development of the synovial tissue in the knee joint were performed on 50 human embryos and fetuses. The following histochemical reactions were used: 1. metachromatic staining with toluidine blue, 2. Alcian blue-PAS and Alcian blue-van Gieson, 3. PAS staining, 4. gallocyanin-chromalum for nucleic acids, 5. Bennett''s mercury orange method for sulphydryl groups, 6. Millon reaction for tyrosin, 7. ninhydrin-Schiff for a-amino acids, 8. a modified coupling azo dye method for alkaline phosphatase, 9. naphthol-AS-BI phosphate method for acid phosphatase, and 10. the naphthol-AS-acetate method for esterase. The synovial tissue is laid down as the peripheral part of the articular interzone and is derived like the latter from the original skeletal balsterna. During further development, the synovial tissue is the only part of the interzone which becomes vascularized, and in this process it assumes a looser structure. Even before the vasculari-zation, there are formed in the synovial tissue acid mucopolysaccharides which, according to the histochemical reactions, appear to consist of chondroitin sulphate A or C (possibly both). After vascularization there are also PAS positive substances in the intercellular substance of the synovial tissue. Simultaneously with the vascularization the synovial tissue is invaded by numerous histiocytes which are seen, after the formation of the joint cavity, also within the cavity. These cells are never observed in the intermediate layer of the interzone where the joint cavity is later formed. At the 80 mm stage, some time after the formation of the joint cavity (35-46 mm crown-rump), the first scattered mast cells are visible, first in a perivascular situation and later more scattered in the synovial tissue, but even in the later stages they are seen only exceptionally among the superficial cells of the synovial tissue. At 145 mm crown-rump length there is a small accumulation of fat cells in the infrapatellar tissue, while at 80 mm this tissue shows scattered, delicate, collagenous fibres. Owing to the skeleto-blastemal origin, the term synovialocyte is suggested, instead of fibrocyte, to designate the original synovial cell. The following sequence of development is apparent. 1. undifferentiated skeleto-blastemal cells, 2 mucopolysaccharide-producing synovialocyte (chondroitin sulphate), 3. fibre-producing synovialocyte. The first synovia appears at the time of cavity formation and seems to consist of chondroitin sulphate A or C (possibly both). In the later stages too the synovia consists of chondroitin sulphate, presumably formed by the synovialo-cytes, but also contains a PAS positive material (glycoprotein ?). From the earliest stages the synovia contains histiocytes. It is apparent that within the first 20 menstrual weeks there is no histochemical evidence that a production of hyaluronic acid occurs in the synovial tissue. The hypothesis is advanced that the later production of hyaluronic acid is presumably due to the f ibroblasts in the adventitia of the synovial vessels, as these cells are apparently the only synovial component having this property. By the histochemical reactions used in the present study it was not possible to demonstrate any difference between the superficial cells of the synovial tissue and the cell situated deeper in this tissue. Even at an early stage of development, the synovial tissue attains extremely rich vascularization which increases steadily through the subsequent stages.

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