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. The mature vesicles do not possess a boundary membrane. Each vesicle consists of many small droplets, in the range of 50-100 nm in diameter. The vesicles grow by the addition of terpenoids synthesized by ER in the vicinity of the terpenoid vesicles. As the vesicles develop the vacuole fragments into two or more bodies.