The fibrocytes and the fibres of Hirudo medicinalis have been studied with the electron microscope. The fibrocyte can be shown by other methods to contain triglyceride droplets, phospholipid droplets, and mitochondria, all of which can be recognized in the electron micrographs. In addition, a ‘vesicular’ component has been observed just below the cell surface.

The collagenous ‘cortex’ of the fibre is tubular, and can be shown to contain a long process of the cell which secretes it. There are many small fibrils about 250 Å in diameter comprising the cortex of the fibre; they lie along the fibre axis, and often are associated in pairs or groups of three or four. These fibrils sometimes show a poorly defined periodicity of 300 Å.

It seems that the fibrils ‘shred off’ from the surface of the fibrocyte. The process shows some resemblance to that described in the chick by other authors.

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