Hemocyte reactions and early cellular changes during melanotic tumor formation in Drosophila melanogaster

In Drosophila melanogaster tu bw larvae melanotic tumors form as a result of a cell-mediated immune response involving the encapsulation and melanization by hemocytes of portions of the caudal adipose tissue. The tissue-specific encapsulation response is not due to the disintegration of the basement membrane surrounding the adipose tissue as is reported to be the case in other melanotic mutants. Prior to encapsulation large numbers of hemocytes appear in the circulation and begin to differentiate into flattened cells termed lamellocytes. This transformation occurs at a time when changes are noted within the adipose cells. The localized accumulation of blood cells near intact basement membrane suggests that abnormally developing adipose cells acquire altered molecular surfaces or release substances to which the hemocytes respond. The initial reaction of the hemocytes with the adipose tissue is cell lysis, and this is rapidly followed by the aggregation of numerous additional blood cells which eventually cohere to one another to form a multilayered capsule. What little evidence there is of disintegration of the basement membrane occurs only after hemocytes have lysed at the surface, and other blood cells begin to invade the adipose tissue. Melanization occurs first in the intercellular spaces along the plasma membranes of the lysed cells, and progresses from the innermost layers toward the periphery of the encapsulating cells. Since the changes observed in the hemocytes and adipose cells are precocious, occurring to a lesser degree later in normal development, the initial effect of the genetic mutation in tu bw larvae may be an endocrine dysfunction which causes an asynchronous and abnormal development of the caudal adipose tissue and/or the hematopoietic organs.