Congenital absence of peripheral myelin: abnormal Schwann cell development causes lethal arthrogryposis multiplex congenita.

We describe a 37-week gestational age infant who presented with lethal arthrogryposis multiplex congenita due to complete absence of peripheral nervous system (PNS) myelin. Schwann cells accomplished successful developmental proliferation, migration, axonal ensheathment, basal lamina production, and subsequent cessation of proliferation, but failed in spiral lengthening and longitudinal growth. Internuclear distance was very short, resulting in marked Schwann cell hypercellularity. No supernumerary Schwann cells (onion bulbs) were found. No PNS myelin proteins (P0, P1, MAG) were detected by immunocytochemical methods, and the Schwann cells adopted many morphologic features characteristic of unmyelinated nerves. The defect appears to be an arrest in Schwann cell differentiation at the stages of mesaxon elongation and longitudinal growth.