Microscopic Findings in Hemangioma
Hemangiomas display dramatically different histologic features depending on their phase of evolution, with early proliferative lesions showing well-defined masses of capillaries lined by plump endothelial cells with numerous mitotic figures, while involuting lesions demonstrate thick, hyalinized basement membranes with apoptotic debris and eventual replacement by fibro-fatty stroma. 1
Early Proliferative Phase Histology
The proliferative phase is characterized by well-circumscribed, unencapsulated masses of capillaries arranged in lobules, separated by delicate fibrous septae. 1
Key microscopic features include:
- Plump endothelial cells with variably enlarged nuclei and abundant clear cytoplasm lining the capillaries 1
- Plump pericytes embedded within a multilaminated basement membrane without associated smooth muscle cells 1
- Numerous normally configured mitotic figures, with widespread expression of cell proliferation markers such as Ki-67 confirming active division of both pericytes and endothelial cells 1
- Lobular arrangement of proliferating capillaries that intermingle nondestructively with surrounding tissues including skeletal muscle fibers, peripheral nerves, salivary glands, and adipocytes 1
- Enlarged draining veins with thick, asymmetric walls due to the high-flow nature of proliferative hemangiomas 1
Involuting Phase Histology
As involution proceeds, mitotic figures wane and apoptotic bodies increase, with lesional capillaries progressively disappearing without evidence of thrombosis or prominent inflammation. 1
Characteristic involutional features include:
- Thick and hyalinized capillary basement membranes containing specks of apoptotic debris 1
- Increased mast cells during early involution 1
- Progressive disappearance of lesional capillaries through apoptosis rather than thrombosis 1
- "Ghost vessels" composed of residual, thickened rinds of basement membrane material containing apoptotic debris without intact endothelial cells 1
- End-stage replacement with loose fibrous or fibro-fatty stroma 1
Immunohistochemical Markers
GLUT1 (glucose transporter-1) immunopositivity is the definitive marker for infantile hemangiomas, distinguishing them from vascular malformations which are GLUT1-negative. 1
- GLUT1 remains positive in residual lesion-type capillaries even during involution, making it useful for confirming diagnosis when histologic features are ambiguous 1
Important Diagnostic Considerations
There is no sharp dividing line between proliferation and involution, and features of both phases typically coexist during much of the natural history. 1
Critical pitfalls to avoid:
- Early proliferative hemangiomas may focally resemble pyogenic granulomas due to their rapidly growing vascular proliferations 1
- Interpretation requires proper clinical context including patient age and lesion behavior, as histologic features change dramatically throughout the natural course 1
- Grossly, proliferative and early involutive lesions appear as well-circumscribed masses with red-to-tan cut surfaces, while later involutive lesions become fibro-fatty and less defined 1