Which of the following zone is characterized by macrophages and osteoclasts?
(A) Zone of infection (B) Zone of contamination (C) Zone of irritation (D) Zone of stimulation
(C) The reaction of the periradicular tissues to noxious products of tissue necrosis, bacterial products and antigenic agents from the root canal has been described by Fish.
The four zones 

Zone of infection
Zone of contamination
Zone of irritation
Zone of stimulation


Zone of Infection:


This is present in the center of the lesion.
It is characterized by PMNL's.


Zone of Contamination:


It is characterized by round cell inﬁltration.
It demonstrates cellular destruction due to toxins from the central zone or zone of infection. Because of autolysis and death of bone cells, the lacunae were empty.
Lymphocytes are prevalent everywhere.


Zone of Irritation:


This zone is characterized by macrophages and osteoclasts.
The collagen framework was digested by phagocytic cells, the macrophages, while osteoclasts attacked the bone tissue. This opens a gap in the bone all around the center of lesion. That space becomes ﬁlled with PMNL's. It contains cholesterol crystals.
This zone demonstrates much activity preparatory to repair.


Zone of Stimulation: (Peripheral zone)


This zone is characterized by ﬁbroblasts and osteoblasts.
In this zone, the eﬀects of toxins were mild enough to be stimulant which results in laying down of collagen ﬁbers around the zone of irritation that acts both as a wall of defense and as a scaﬀolding on which the osteoblasts built new bone.
This new bone was built in an irregular fashion.