The concept of the Schwann cell by Louis Ranvier and his school: The ‘interannular segment’ as a cell unit

The hundredth anniversary of the death of French histologist Louis Ranvier (1835‒1922) is an opportunity to reexamine his elaboration of the first concept of the Schwann cell. A loyal supporter of Theodor Schwann and his discoveries, and an attentive reader of the work of Albert von Kölliker, Ranvier studied the anatomic details of the myelinated nerve fiber with picrocarminate staining. The diffusion of the dye into the nerve fiber at the cut ends and at the sites of the annular constrictions (Ranvier's nodes) set him on the path to defining a new cellular entity surrounding the axon, the "interannular segment," comprising a Schwann nucleus, myelin, and cytoplasm. Ramón y Cajal recognized in 1913 that this concept of the Schwann cell according to Ranvier and his pupil William Vignal had been a brilliant intuition, but it was widely rejected until it was rediscovered using electron microscopy in the 1950s. The article reconstructs the steps of Ranvier and Vignal in building this Schwann cell concept, as well as establishing bridges with the discoveries of the 1950s.