The type IV Oligodendrocyte : experimental studies on chicken white matter

Sammanfattning: In mammals, islet amyloid polypeptide (IAPP) is co-produced with insulin in pancreatic ß-cells. In the chicken, the expression of IAPP in the brain is more than 10-fold higher than in the pancreas. We made the fortuitous finding that a polyclonal rabbit antiserum raised against chicken IAPP did not recognise the immunogen, but labelled a subpopulation of oligodendroglia! cells in chicken white matter. The hitherto unknown antigen was called T4-O (Type 4 Oligodendrocyte) since it was localised to the Schwann cell-like Type IV oligodendrocyte of Del Rio-Hortega (1928). This formed a starting point for the present thesis, which is centred on the Type IV oligodendrocyte in chicken white matter.Biochemical analysis of chicken spinal cord showed that the T4-O molecule is a protein with a molecular weight of approximately 100 kDa and an isoelectric point of about 4. Further characterisation has not yet been possible.Immunohistochemical studies on frozen sections revealed that the white matter oligodendrocytes exhibit subpopulations expressing T4-O immunoreactivity strongly, weakly or not at all. Strongly T4-O immunoreactive (IR) oligodendrocytes are co-localised with thick myelinated fibres in the ventral (VF) and lateral funiculi of the spinal cord. A corresponding T4-O immunoreactivity is not found in the fish, the frog, the turtle, the rat and the rabbit.To find out when the T4-0 antigen first appears during development we examined sections from embryonic and post-hatching chicken spinal cords by immunohistochemistry. This showed that the T4-O molecule is first expressed in the VF at embryonic day (E)15, after which the number of IR cells increases with age. Oligodendrocytes cultivated in vitro without or with neurons do not develop a T4-O IR phenotype.These findings called for a closer analysis of the structural development of chicken VF white matter. Electron microscopic (EM) examination revealed a developmental sequence of events principally similar to the development of mammalian white matter, but with a more rapid time course. As seen in the electron microscope the first compact myelin has appeared by E12, when most oligodendrocytes are multipolar. By E15 it seems that these cells have developed a Type IV phenotype, possibly by eliminating some sheaths.Histochemical analysis of Vibratome sections showed that Marchi-positive myelinoid bodies are enriched in white matter areas containing many T4-O IR oligodendrocytes and many large myelinated axons.Examination of the three-dimensional (3D) anatomy of early VF oligodendrocytes in Vibratome slices after 04 labelling or after intracellular injection of a fluorescent dye revealed that these units indeed are Schwann cell-like, with a start length of around 50 µm. We also found that these sheaths expand very rapidly, reaching lengths exceeding 200 µm in three days (E12- E15). The 3D data conformed to our EM evidence that the early oligodendrocytes develop a unipolar Schwann cell-like Type IV anatomy through elimination of some sheaths.To my knowledge the present observations represent the first evidence for an oligodendroglia! heterogeneity in the chicken spinal cord. Differences among oligodendrocytes might, conceivably, explain why inherited disorders of myelin metabolism such as Krabbe's disease, affect some CNS areas more than others.