Extra-cellular Lipid in the Matrix of Human Articular Cartilage.

The presence of fat and other lipids within the cytoplasm of cartilage cells has often been observed. Collins, Ghadially, and Meachim (1965) in a previous paper have described this phenomenon in some detail. They find that intra-cellular lipid is an almost constant feature of the chondrocytes of adult hyaline cartilage in various anatomical situations in man and rodents, that it is more prominent at ages later than infancy and the early growth period, but that it is not necessarily a manifestation of any form of cellular degeneration, since electron microscopy reveals lipid accumulation in cells whose organelles appear perfectly healthy. In the course of that study, the attention of the authors was drawn to the presence of lipid outside the cells, lying in a granular form within the matrix in some areas of certain cartilages. The occurrence of extra-cellular lipid in cartilage is much less well known than that of the intra-cellular lipids, and only a few previous observations have been recorded. Montagna (1949) speaks of "clouds of very fine sudanophilic particles visible only at high magnification" and of "a faint sudanophilic band in the new matrix just under the perichondrium" in adult tracheal cartilage. Barnett, Cochrane, and Palfrey (1963) observed small clusters of "myelin bodies" together with electron-dense granules interspersed among the fibres of the middle zone of ageing rabbit cartilage; it is possible that this may be a different phenomenon from that noted by Montagna. Extra-cellular lipid has been observed by the present authors in a small number of costal and bronchial cartilages in man and on occasion in rabbit articular cartilage, but these studies have been concentrated upon human articular cartilage where extra-cellular lipid can quite often be discovered in the superficial zones and where it seemed that it might possibly have pathological significance. Methods Articular cartilage was collected at necropsy from the head of the humerus in young subjects and from the central area of the head of the humerus, from near the articular edge of a femoral condyle, and from the patellar groove of the femur in adults. These specimens were prepared for examination under a binocular stereomicroscope and, as frozen sections, under the light microscope. In addition, fresh cartilage was obtained from the femoral condyle in two patients aged 58 and 82 years in the operating theatre prior to mid-thigh amputation of the lower limb; this material was prepared for electron microscopy as well as for sectioning on the freezing microtome. The ages of the persons whose cartilage was examined ranged from birth to 82 years. In the juvenile group there were four premature babies dying shortly after birth, two full-term babies dying at the age of 4 and 5 months, two girls aged 8 years, and one youth aged 16 years. The adult patients were aged 28 to 82 years. Many of the samples were free from any osteo-arthritic lesion of cartilage, while others showed superficial or deep fibrillation in the area taken for examination. Stereomicroscopy.-Samples of adult cartilage, either fixed in formol-calcium or unfixed, were prepared as follows. A block of cartilage was divided into thin tangential slices by a series of cuts made at increasing depths parallel to the articular surface. By this means a superficial slice and two or more deeper slices were obtained from the same area of non-fibrillated samples. Deeply fibrillated samples were studied in a single thin tangential slice taken at their roughened surface. The slices were rinsed briefly in 70 per cent. isopropyl alcohol, stained for 10 minutes in a filtered solution of oil red 0, rinsed again in 70 per cent. isopropyl alcohol, and then placed in formol saline until they were examined under a low-power binocular microscope (x 12-5 to x 50) with incident light. Light Microscopy.-Tissue blocks were generally prepared by cutting out a cartilage sample from above the level of its calcified zone. By this method the need for preliminary decalcification is avoided. The blocks were