The neem plant is known to accumulate about 25 diterpenoids and 100 different triterpenoids, including the biologically most active azadirachtins. Azadirachtins possess a wide spectrum of therapeutic and powerful pesticidal properties. The terpenoids are synthesized and accumulate within idioblasts known as secretory cells.
Cotyledons also store considerable quantity of neem oil.
The secretory cells differentiate about 40 days after pollination in cotyledons that are 4-6 mm long. Development of a secretory cell commences with cell enlargement, increase in the size the nucleus and vacuole. The cell walls of secretory cells are strengthened by the addition of wall material by the adjacent cells. Small vesicles containing terpenoids accumulate in the cytoplasm. A string of terpenoid vesicles interconnected by narrow regions gives a beaded appearance.