To assess the contribution of apoptosis to the mechanism of synovial joint cavitation, and to clarify morphological cellular changes during cavitation, we investigated the development of the rat knee joint by light and electron microscopy, TUNEL methods, and electrophoresis of DNA fragments. Although cavitation occurred within the interzone, which consists of 2 outer and a middle layer termed the intermediate zone, no morphological or biochemical signs of cell death, in particular apoptosis, were seen in the interzone at any embryonic stage. Microscopic and ultrastructural alterations affecting cell differentiation were clearly observed in the interzone, i.e. mesenchymal cells gradually showed elongation, cytoplasmic vacuolation and pyknosis in the intermediate zone where the elongated cells were arranged in parallel in some strata. Some of these cells were further flattened into spindle cells and the number of strata decreased to 2. The rest of the cells were incorporated secondarily into the outer layers, becoming chondroblasts. Collagen fibrils were arranged in a network structure in the outer layers, which obviously differed from the directional pattern parallel to the long axis of elongated cells in the intermediate zone. In addition, the density of collagen fibrils was higher in the outer layers than in the intermediate zone. During cavitation, the initial separation was detected between the elongated cells in the intermediate zone in paraffin sections at E16n5 and the spindle cells in epoxy sections at E18n5. The spindle cells lining the cavity, namely, the surfaces of the epiphysis and meniscus, finally became chondrocytes. The diminution of proteoglycans and collagen fibrils and the synthesis of hyaluronan in the extracellular matrix are now generally believed to be parts of the mechanism for cavitation based on the concept of ' loss of cohesion '. The microscopic and ultrastructural alterations in the interzone seemed to reflect differences in the arrangement and density of collagen fibrils and the developmental condition of the extracellular matrix between layers. Also it did not seem likely that these alterations inhibit the synthesis of hyaluronan at the presumptive joint line because this synthesis takes place at the plasma membrane. Separation between spindle cells should therefore represent the mechanism for developmentally programmed cavitation. Reorganization of the extracellular matrix is probably necessary for the cellular metamorphoses in the interzone involved in the process of cavitation.

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The Sakhalin Ainu crania were investigated in terms of nonmetric traits for elucidating their populational affinity. MMDs between the Sakhalin Ainu, Hokkaido Ainu, Neolithic Jomon, Amur, East Asian and North American Mongoloids were calculated on the basis of 15 nonmetric traits. Principal coordinate analysis showed that the Sakhalin Ainu are situated intermediately between three major clusters which consist of the Jomon-Hokkaido Ainu, East Asian, and North American Arctic Mongoloids. While the Sakhalin Ainu basically developed from the JomonAinu stock, the results of the analyses suggested that the Sakhalin Ainu had intermarried considerably with northern people and that that genetic influence was beyond our expectation based on traditional craniometry. Ethnic groups of the Amur basin were placed in an intermediate position between the Arctic and East Asian Mongoloids.

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