Ultrastructural examination of tissue in a patient with alkaptonuric arthropathy reveals a distinct pattern of binding of ochronotic pigment.

SIR, we report a female with known alkaptonuria (AKU) undergoing routine hip replacement surgery due to alkaptonuric arthropathy. AKU is caused by a deficiency in the enzyme that breaks down homogentisic acid (HGA), resulting in elevated circulation of HGA levels in the body. HGA is deposited as a polymerized pigment in collagenous connective tissues, predominantly in the weight-bearing joints [1]. It has also been shown to affect other non-joint tissues [2, 3]. Ligamentous capsule was processed routinely for histology and electron microscopy. Macroscopically, ochronotic pigment was clearly visible alongside non-pigmented regions. Haematoxylin and eosin (H&E) staining showed the presence of extracellular pigmentation associated with the collagen fibres and also intracellular pigmentation within fibroblasts. This novel intracellular pigmentation appeared as numerous individual granules located within the cytoplasm of single fibroblasts (Fig. 1A). Extracellular pigmentation appeared as two distinct types: a granular deposition, similar to that within the cells, but also more homogeneous pigmentation that completely encrusted the collagen fibres. Ultrastructural analysis revealed that collagen fibres in transverse section had numerous electron-dense granules of ochronotic pigment associated with them. These granules varied in position and distribution among the fibres. Some had single granules of pigment located within the cross-section of