Dexamethasone and progesterone have been found to accelerate the time of initiation and enhance the rate of myelin synthesis in Schwann cell/neuronal cocultures. The expression of mRNA for cytochrome P450scc (converts cholesterol to pregnenolone), 3␤-hydroxysteroid dehydrogenase (converts pregnenolone to progesterone), and the progesterone receptor were detected and markedly induced during peak myelin formation in the cocultures. The mRNA for the glucocorticoid receptor was detected, but was found to be constituitively expressed. In addition, the specific activity of 3␤-hydroxysteroid dehydrogenase was measured and found to increase by 10-fold. The mRNA for cytochrome P450scc and 3␤-hydroxysteroid dehydrogenase also were found to be induced during the differentiation of O-2A precursor cells to oligodendrocytes. Fibroblast growth factor and plateletderived growth factor were found to have proliferative effects on Schwann cells, but they had no effect on the initiation or the rate of myelin formation. These results demonstrate that myelin-forming cells have inducible enzymes responsible for steroid biosynthesis and suggest a critical role for endogenous steroid hormones in signaling the initiation and enhancing the rate of myelin formation.The myelin sheath is a unique component of the nervous system that functions to maximize the efficiency and velocity of action potentials transmitted through nerve cells. Myelin is an extension of the plasma membrane of Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system. In contrast to myelin formation by oligodendrocytes, Schwann cells develop a one-to-one interaction with an internode rather than forming multiple internodes (1). The neuronal participation in signaling myelination has been examined using Schwann cell/neuronal cocultures, and axonal contact has been found to be essential for peripheral nervous system myelination (2). Schwann cells multiply rapidly when in contact with axons and elongate to either ensheath or myelinate axons of approximately 1 m in diameter or larger. Contrary to the case for Schwann cells, the presence of growth factors and chemically defined media has been found to be sufficient in producing myelin-like sheets in cultured oligodendrocytes, independent of neurons (3-6). The process of myelination has been extensively studied and a number of factors are known to influence the overall process (7); however, the factors that specifically signal the initiation, regulate the rate of formation, and regulate the extent of myelin formation are still unknown.Light microscopy and electron microscopy have provided valuable information concerning the amount of myelin deposited and the number of wraps of compact myelin during development (8, 9). Antibodies have also been used to chart the appearance of various myelin-specific lipids and proteins during the formation of myelin (10, 11). A method has been developed that allows the continuous monitoring of a single internode throughout the myelination proces...

The post-thrombotic syndrome represents a poorly understood and significant vascular health problem. This review focuses on our current understanding of the pathogenesis of post-thrombotic syndrome. We emphasize the cellular and molecular mechanisms that are responsible for the critical components of post-thrombotic syndrome. These include the initiation of deep venous thrombosis, the pathogenesis of elevated venous pressure, and the factors responsible for nonhealing of venous stasis ulcers.

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