An overview of canine histiocytic disorders - Veterinary Medicine
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An overview of canine histiocytic disorders
The disorders that arise from histiocyte proliferation range from benign, self-resolving lesions to malignant, life-threatening sarcomas and include a newly identified splenic and bone marrow macrophage disorder. These clinicians give you the information you'll need to readily differentiate and manage these disorders.


VETERINARY MEDICINE



A quick guide to histiocytic disorders in dogs
Canine histiocytic proliferative diseases represent a range of disorders with different pathologic features as well as clinical behavior.1,2 At least three well-defined syndromes have been reported in dogs that arise from histiocyte proliferation, including cutaneous histiocytoma, reactive histiocytosis (cutaneous and systemic), and histiocytic sarcoma (localized and disseminated).3 Malignant fibrous histiocytoma was previously grouped with canine histiocytic diseases but is now more appropriately considered a soft tissue sarcoma (see boxed text titled "Difficult to delineate: Malignant fibrous histiocytoma and histiocytic disease").4,5 The dendritic cell lineage of this elusive complex of histiocytic disease syndromes has been recently clarified through the use of immunohistochemical staining and specific monoclonal antibodies (Table 1).1,2,6-9


Difficult to delineate: Malignant fibrous histiocytoma and histiocytic disease
In this article, we focus on how to diagnose the various histiocytic disorders by using routine imaging modalities, cytologic and histologic examination, and immunohistochemistry when needed to accurately distinguish elusive cases. We also discuss the recommended treatment and prognosis for each disease process.

HISTIOCYTE ORIGINS


Figure 1. Adapted from Withrow SJ, Vail D. Histiocytic diseases. In: Small animal clinical oncology. 4th ed. St. Louis, Mo: Saunders Elsevier, 2007;814-823.
Most histiocytes differentiate from CD34+ (CD = cluster of differentiation) stem cell precursors in the bone marrow into macrophages and one of three dendritic cell lineages: epithelial dendritic cells or Langerhans cells in the skin, interstitial dendritic cells in many organs (Figure 1), and interdigitating dendritic cells (antigen-presenting cells located in the T cell zone in peripheral lymphoid organs).3,10 The interdigitating dendritic cells are not yet known to arise in histiocytic disease.

The fate of the CD34+ stem cells is largely influenced by cytokines and specific combinations of them. For example, granulocyte-macrophage colony-stimulating factor (GM-CSF) and macrophage colony-stimulating factor (M-CSF) induce macrophage development from CD34+ stem cells, while GM-CSF, transforming growth factor beta, tumor necrosis factor alpha, and interleukin-4 influence dendritic cell development.3,11

HISTIOCYTOMA

Histiocytomas are benign skin tumors that occur most commonly in young dogs, accounting for 3% to 14% of skin tumors in this species.12


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