Pannus is an abnormal layer of fibrovascular tissue or granulation tissue. This material allows for fibrovascular ingrowth within several months. Proliferation of fibrovascular tissue occurs in nodules with a variable lymphoplasmacytic infiltrate. Fibrovascular proliferation can also cause tractional retinal detachment. Histologically, they have a papillary architecture with slender fibrovascular cores and rare basal mitoses. The key disease element in ROP is fibrovascular proliferation. A delicate fibrovascular network can be found between the cells. These cell balls are separated by fibrovascular stroma and surrounded by sustentacular cells. The ARMS cells often clump together and have fibrovascular septae that interrupts the aggregates. True papillary structures (a fibrovascular core surrounded by neoplastic cells).