Ultrastructural and immunocytochemical study on normal human palmar aponeuroses

Human palmar aponeurosis can be affected by a fibrotic process whose aetiopathology is unknown. As the organization of that normal tissue has not been completely investigated, the aim of the present study was to define the ultrastructure of the aponeurosis in order to better understand its biology and behaviour in pathology.

Bioptic samples from normal subjects of different ages were analysed by optical and electron microscopy and by immunocytochemistry.

The aponeurotic branches consisted of thick, almost parallel collagen bundles containing columns of prominent cells, characterized by long cytoplasmic projections. Cells did not change in number and distribution with age and appeared longer and slighter in the old than in the young subjects. They exhibited plasma membrane almost completely decorated by pinocytic vesicles, intracytoplasmic bundles of thin filaments with zonal thickenings close to the cell membrane, and well-developed subcellular structures. Cells expressed smooth muscle cell alpha-actin, as revealed by immunostaining. The external surface of the plasma membrane was underlined by a discontinuous basement membrane-like structure and by a thick coat of interwoven filaments, highly positive to hyaluronan-recognizing antibodies. Immunocytochemical analyses revealed that collagen fibrils were positive for collagen types I, III, and VI and that elastin fiber composition was rather complex.

Independently of the age, normal palmar aponeurotic cells show peculiar morphological features and peculiar cell-matrix interactions, very likely mediated by hyaluronan. These findings indicate that normal aponeurotic cells cannot be regarded as typical tenocytes and suggest the need for a better definition of their phenotype in order to understand their behaviour in pathological processes.