Crescents are defined as "classical histopathological lesions found in severe forms of rapidly progressive glomerulonephritis, also known as crescentic glomerulonephritis (CGN)." Therefore, it is important to understand the mechanism behind the formation of crescents as it will guide the therapy adopted and offer a better management approach to patients with CGN. 

In recent studies, it has been shown that cellular crescents are formed from parietal epithelial cells inhabiting the Bowmans capsule, and their development leads to a reduction in glomerular filtration rate. It has also been noted that cellular crescents may be reversible, but in the case of multilevel growth of PECs, crescents formed may become irreversible in terms of GFR recovery. Different molecular pathways activate the stimulation of PECs, and they also act as therapeutic targets in CGN management. 

Hence, the glomerular crescent is a histomorphological indicator of a rupture of glomerular capillaries, generally occurring due to an intense immune attack including cytotoxic factors, like complement attack complex and extracellular histones, such as necroinflammation. 

Therapeutic intervention can be considered in several upstream mechanisms triggering vascular necrosis, including small vessel vasculitides, immune complex glomerulonephritis, anti-GBM disease, and C3 glomerulonephritis. All these conditions share complement activation; however, the immune mechanism outside the kidney is available for therapeutic intervention. 

The current therapeutic options considered for the treatment of CGN include CD44 and CD9, glucocorticoids, and heparin-binding epidermal growth factor-like growth factor. It has been concluded that the crescent frequently observed in diagnostic kidney biopsy gives a mechanistic indication of the pathophysiology of the underlying disease. However, CGN is no diagnosis, and it only acts as a pointer towards working up a set of differential diagnoses. 

Reference: Lidia A, Renate K, Hans-Joachim A. The glomerular crescent triggers evolution, resolution, and implications for therapy. Current Opinion in Nephrology and Hypertension. 2020; 29: 302-309.