Myelolipoma
Encyclopedia
Myelolipoma is a benign
tumor-like lesion
composed of mature adipose (fat) tissue and haematopoietic
(blood-forming) elements in various proportions.
Myelolipomas can present in the adrenal gland
, or outside of the gland.
have been proposed as to the cause of myelolipoma, the causative process is still not clearly understood. Recent experimental evidence suggests that both the myeloid and lipomatous elements have a monoclonal
origin, which strongly supports the hypothesis that myelolipomas are neoplastic lesions.
Older theories proposing a non-neoplastic origin include the following:
in 0.08% to 0.4% of cases (i.e.: somewhere between 8 per 10,000 and 4 per 1,000 autopsies). They most commonly occur in the adrenal gland, yet only comprise about 3% of all adrenal tumours. They may also occur in other sites, such as the mediastinum
, the liver
and the gastrointestinal tract
.
There is no gender predilection, males and females are affected equally. The peak age range at diagnosis is between 40 and 79 years of age..
. Most do not produce any adrenal hormones. Most are only discovered as a result of investigation for another problem.
When myelolipomas do produce symptoms, it is usually because they have become large, and are pressing on other organs or tissues nearby. Symptoms include pain in the abdomen or flank, blood in the urine, a palpable lump or high blood pressure
.
As they are benign tumors, myelolipomas do not spread to other body parts
. Larger myelolipomas are at risk of localised tissue death
and bleeding, which may cause a retroperitoneal
haemorrhage.
films.
Fine needle aspiration may be performed to obtain cells for microscopic diagnosis
..
.
The typical microscopic features of myelolipomas are shown in the image. There is a mixture of normal adrenal tissue, fat, and a full trilineage maturation of the three major blood-forming
elements: myeloid
(white blood cell forming), erythroid (red blood cell forming), and megakaryocytic
(platelet forming) lines.
Benign
A benign tumor is a tumor that lacks the ability to metastasize. Common examples of benign tumors include moles and uterine fibroids.The term "benign" implies a mild and nonprogressive disease. Indeed, many kinds of benign tumors are harmless to human health...
tumor-like lesion
Lesion
A lesion is any abnormality in the tissue of an organism , usually caused by disease or trauma. Lesion is derived from the Latin word laesio which means injury.- Types :...
composed of mature adipose (fat) tissue and haematopoietic
Haematopoiesis
Haematopoiesis is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells...
(blood-forming) elements in various proportions.
Myelolipomas can present in the adrenal gland
Adrenal gland
In mammals, the adrenal glands are endocrine glands that sit atop the kidneys; in humans, the right suprarenal gland is triangular shaped, while the left suprarenal gland is semilunar shaped...
, or outside of the gland.
Causes
Although several hypothesesHypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
have been proposed as to the cause of myelolipoma, the causative process is still not clearly understood. Recent experimental evidence suggests that both the myeloid and lipomatous elements have a monoclonal
Monoclonal
Monoclonal cells are defined as a group of cells produced from a single ancestral cell by repeated cellular replication. Thus they can be said to form a single "clone". The process of replication can occur in vivo, or may be stimulated in vitro for laboratory manipulations...
origin, which strongly supports the hypothesis that myelolipomas are neoplastic lesions.
Older theories proposing a non-neoplastic origin include the following:
- Adrenal corticalAdrenal cortexSituated along the perimeter of the adrenal gland, the adrenal cortex mediates the stress response through the production of mineralocorticoids and glucocorticoids, including aldosterone and cortisol respectively. It is also a secondary site of androgen synthesis.-Layers:Notably, the reticularis in...
cells, or other cells within the stromaStroma (animal tissue)In animal tissue, stroma refers to the connective, supportive framework of a biological cell, tissue, or organ.The stroma in animal tissue is contrasted with the parenchyma.Examples include:* Stroma of iris...
of the adrenal cortex that are able to differentiateCellular differentiationIn developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...
, may reversibly changeMetaplasiaMetaplasia is the reversible replacement of one differentiated cell type with another mature differentiated cell type. The change from one type of cell to another may generally be a part of normal maturation process or caused by some sort of abnormal stimulus...
into fat or blood-forming cells. This might occur because of the actions of adrenal cortex hormones, or of hormones released by the pituitary glandPituitary glandIn vertebrate anatomy the pituitary gland, or hypophysis, is an endocrine gland about the size of a pea and weighing 0.5 g , in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity covered by a dural fold...
that act on the adrenal glands, such as adrenocorticotropic hormoneAdrenocorticotropic hormoneAdrenocorticotropic hormone , also known as 'corticotropin', 'Adrenocorticotrophic hormone', is a polypeptide tropic hormone produced and secreted by the anterior pituitary gland. It is an important component of the hypothalamic-pituitary-adrenal axis and is often produced in response to biological...
(ACTH). - The blood-forming cells may arise by differentiation of cells within the capillaries of the adrenal gland.
- Myelolipoma simply represents a site of normal blood formation outside the bone marrowBone marrowBone marrow is the flexible tissue found in the interior of bones. In humans, bone marrow in large bones produces new blood cells. On average, bone marrow constitutes 4% of the total body mass of humans; in adults weighing 65 kg , bone marrow accounts for approximately 2.6 kg...
.
Epidemiology
Myelolipomas are rare. They have been reported to be found unexpectedly at autopsyAutopsy
An autopsy—also known as a post-mortem examination, necropsy , autopsia cadaverum, or obduction—is a highly specialized surgical procedure that consists of a thorough examination of a corpse to determine the cause and manner of death and to evaluate any disease or injury that may be present...
in 0.08% to 0.4% of cases (i.e.: somewhere between 8 per 10,000 and 4 per 1,000 autopsies). They most commonly occur in the adrenal gland, yet only comprise about 3% of all adrenal tumours. They may also occur in other sites, such as the mediastinum
Mediastinum
The mediastinum is a non-delineated group of structures in the thorax, surrounded by loose connective tissue. It is the central compartment of the thoracic cavity...
, the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
and the gastrointestinal tract
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....
.
There is no gender predilection, males and females are affected equally. The peak age range at diagnosis is between 40 and 79 years of age..
Symptoms and signs
The majority of myelolipomas are asymptomaticAsymptomatic
In medicine, a disease is considered asymptomatic if a patient is a carrier for a disease or infection but experiences no symptoms. A condition might be asymptomatic if it fails to show the noticeable symptoms with which it is usually associated. Asymptomatic infections are also called subclinical...
. Most do not produce any adrenal hormones. Most are only discovered as a result of investigation for another problem.
When myelolipomas do produce symptoms, it is usually because they have become large, and are pressing on other organs or tissues nearby. Symptoms include pain in the abdomen or flank, blood in the urine, a palpable lump or high blood pressure
Hypertension
Hypertension or high blood pressure is a cardiac chronic medical condition in which the systemic arterial blood pressure is elevated. What that means is that the heart is having to work harder than it should to pump the blood around the body. Blood pressure involves two measurements, systolic and...
.
As they are benign tumors, myelolipomas do not spread to other body parts
Metastasis
Metastasis, or metastatic disease , is the spread of a disease from one organ or part to another non-adjacent organ or part. It was previously thought that only malignant tumor cells and infections have the capacity to metastasize; however, this is being reconsidered due to new research...
. Larger myelolipomas are at risk of localised tissue death
Necrosis
Necrosis is the premature death of cells in living tissue. Necrosis is caused by factors external to the cell or tissue, such as infection, toxins, or trauma. This is in contrast to apoptosis, which is a naturally occurring cause of cellular death...
and bleeding, which may cause a retroperitoneal
Retroperitoneum
The retroperitoneal space is the anatomical space in the abdominal cavity behind the peritoneum. It has no specific delineating anatomical structures...
haemorrhage.
Medical investigation
Most myelolipomas are unexpected findings on CT scans and MRI scans of the abdomen. They may sometimes be seen on a plain X-rayRadiography
Radiography is the use of X-rays to view a non-uniformly composed material such as the human body. By using the physical properties of the ray an image can be developed which displays areas of different density and composition....
films.
Fine needle aspiration may be performed to obtain cells for microscopic diagnosis
Cytopathology
Cytopathology is a branch of pathology that studies and diagnoses diseases on the cellular level. The discipline was founded by Rudolf Virchow in 1858. A common application of cytopathology is the Pap smear, used as a screening tool, to detect precancerous cervical lesions and prevent cervical...
..
Treatment
Small myelolipomas generally do not produce symptoms, and do not require treatment. Ongoing surveillance of these lesions by a doctor is recommended. Surgical excision (removal) is recommended for large myelolipomas because of the risk of bleeding complications.Macroscopic features
Myelolipomas are usually found to occur alone in one adrenal gland, but not both. They can vary widely in size, from as small as a few millimetres to as large as 34 centimeters in diameter. The cut surface has colours varying from yellow to red to mahogany brown, depending on the distribution of fat, blood, and blood-forming cells. The cut surface of larger myelolipomas may contain haemorrhage or infarctionInfarction
In medicine, infarction refers to tissue death that is caused by a local lack of oxygen due to obstruction of the tissue's blood supply. The resulting lesion is referred to as an infarct.-Causes:...
.
Microscopic features
Haematopoiesis
Haematopoiesis is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells...
elements: myeloid
Myeloid
The term myeloid suggests an origin in the bone marrow or spinal cord, or a resemblance to the marrow or spinal cord.In hematopoiesis, the term "myeloid cell" is used to describe any leukocyte that is not a lymphocyte...
(white blood cell forming), erythroid (red blood cell forming), and megakaryocytic
Megakaryocyte
The megakaryocyte is a bone marrow cell responsible for the production of blood thrombocytes , which are necessary for normal blood clotting...
(platelet forming) lines.