The development of the basement membrane of the mouse seminiferous tubule has been studied between birth and the end of the tenth week of postnatal life by means of the electron microscope. The sheath cells which surround the seminiferous tubules are derived from mesenchymal cells, and differentiation of these specialized cells continues for 5 weeks after birth. Between the plasma membranes of the sheath cells and those of the Sertoli cells there are 3 distinct zones. The zone of decreased electron density nearest the seminiferous tubule is thought to be specialized extracellular fluid filtered by the second or intermediate zone, which consists of numerous fine particles, and the term ‘pseudomembrane’ is advanced for this zone. Collagen develops in the broad outer zone next to the sheath cells and fulfils the support requirements of the growing tubule.

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