Pericyte formation occur in -

Haemangiopericytoma is an uncommon tumor arising frompericytes. Pericytes are cells present external to the endothelialcells of capillaries and venules. This is a rare tumorthat can occur at any site and at any age and may vary insize from 1 to 8 cm.Microscopically, the tumor is composed of capillariessurrounded by spindle-shaped pericytes outside thevascular basement membrane forming whorledarrangement. These tumor cells may have a high mitoticrate and areas of necrosis. Silver impregnation stain (i.e.reticulin stain) is employed to confirm the presence ofpericytes outside the basement membrane of capillariesand to distinguish it from haemangioendothelioma Ref: Harsh Mohan - Textbook of Pathology, 6th Edition.page no.414