The hypotheses of the cause of the myelolipoma include degenerative changes in hyperplastic tumor cells or adenomas of the adrenal glands, metaplasia in primary stem mesenchymal cells of the adrenal cortex and displacement of differentiated bone marrow cells during embryogenesis [7
Other authors suggest metaplasia of reticuloendothelial cells caused by prolonged stress, or development of myelolipoma from blood-bone embolic material [8
]. Many consider myelolipoma to be a choristoma [9
Chromosomal translocations (3;21) (q25;p11) detected in myelolipomas and in benign lipomatous neoplasia seen in patients with acute myelogenous leukemia or myelodysplastic syndrome suggest a bone marrow origin of this tumor, and may indicate that myelolipoma is derived from erroneously transferred erythroid cells [10
Occasionally myelolipomas may be accompanied by endocrine disorders such as Cushing’s disorder, Addison’s disease, Conn’s syndrome, phaechromocytoma, adrenal gland cancer or adenoma, diabetes mellitus or even obesity or hypertension [7
Therefore, some authors emphasize that myelolipoma may be correlated with prolonged excessive steroid production or genome defects of the endocrine glands responsible for multiple endocrine neoplasia type 1.
Histologically, extra-adrenal myelolipoma should be differentiated from mass forming foci of extra-medullary hematopoiesis. The latter has been previously described in the nasal cavity [11
Mass forming extra-medullary hemopoiesis is symptomatic and is associated with myeloproliferative disorder, hemolytic anemia or severe skeletal disease [13
]. Our patient did not have any underlying hematological anormalities.
In contrast to extra-adrenal myelipomas that are well encapsulated, extra-medullary hemopoietic ‘tumors’ lack circumscription and are ill defined.
Microscopically, extra-medullary hemopoietic ‘tumors’ have a predominance of hemopoietic elements with erythroid hyperplasia. Fat is not an enlarged component of the process. Extra-adrenal myelolipomas are composed of a variable proportion of mature adipose tissue and bone marrow cells. The presence of megakaryocytes are considered to be essential for the diagnosis of extra-adrenal myelolipomas [13
]. These lesions can have a predominance of the fat component and a more conspicuous lymphocyte population.
Extra-adrenal myelolipoma is distinct from true bone marrow in that no reticular sinusoids or bone spicules are present. However extra-adrenal myelolipomas containing bone spicules have been reported [14
]. The bone spicules are thought to be the result of osseous metaplasia.
Another challenge on histomorphology is to differentiate extra-adrenal myelolipomas from lesions that contain hemopoietic tissue and adipocytes, such as teratomas. Histology of the latter shows tissue elements from all the three germ layers.