3. Stage of regression (Fig. 1C). Regression is induced by the natural host defence mechanisms (humoral antibody production and cellular immune
defences) and the treatment which augments the normal host response. A line of demarcation develops around the ulcer, which consists of leucocytes that
neutralize and eventually phagocytose the offending organisms and necrotic cellular debris. The digestion of necrotic material may result in initial enlargement of the ulcer. This process may be accompanied by superficial vascularization that increases the humoral and cellular immune response. The ulcer now begins to heal and epithelium starts growing over the edges.
4. Stage of cicatrization (Fig.1D). In this stage healing continues by progressive epithelization which forms a permanent covering. Beneath the epithelium, fibrous tissue is laid down partly by the corneal fibroblasts and partly by the endothelial cells of the new vessels. The stroma thus thickens and fills in under the epithelium, pushing the epithelial surface anteriorly.
The degree of scarring from healing varies. If the ulcer is very superficial and involves the epithelium only, it heals without leaving any opacity behind.
When ulcer involves Bowman's membrane and few superficial stromal lamellae, the resultant scar is calleda 'nebula'. Macula and leucoma result after healing of ulcers involving up to one-third and more than that
of corneal stroma, respectively.