**4. Pathology and diagnosis of MCTs**

**3. Cellular biology of mast cells**

80 Canine Medicine - Recent Topics and Advanced Research

**3.1. Inflammatory responses of mast cells**

cells are not fully explored.

**3.2. Heterogeneity of MCTs**

Mast cells play key roles for inflammatory responses through degranulation and cyto‐ kine/chemokine production and secretion [4]. However, proteases, chemical mediators, and cytokines included in mast cell granules are different according to types of mast cells. Heterogeneity among mast cells is important to precisely understand the pathophysiolog‐ ical roles of mast cells in each tissue. Connective tissue‐type mast cells include chemical mediators such as histamine that has strong biologic activity on nerve fibers and blood ves‐ sels. Histamine is known as a pruritogen that induces itch sensation resulting in scratching behavior in humans and animals. Vasoactive effects of histamine initiate inflammation at the affected sites. Scratching behavior stimulates physical degranulation of mast cells leading to exacerbation of swelling and inflammation. Exact roles of mucosal‐type mast cells are not clearly demonstrated. Since numbers of mucosal‐type mast cells are increased in the gut with parasite infection, roles in host reaction against parasite exclusion have been proposed. However, factors and mechanisms that involve in antiparasite effects of mucosal‐type mast

Most MCTs in dogs are consisted with histamine‐rich connective tissue‐type mast cells. Steps responsible for transformation of mast cells are summarized in **Figure 3**. Since recep‐ tors for histamine are expressed in mucosal cells of the stomach, progression of MCTs has been suggested to induce gastric ulcer and serious damage on gastric function. However, the exact and direct association of MCTs with gastric ulcer remains unclear [5, 6]. Connective tissue‐type mast cells contain tryptase and chymase that possess broad protease activities. Particularly, tryptase has been reported to regulate neovascularization. Connective tissue‐ type mast cells can also produce growth factors for vascular endothelial cells; therefore,

**Figure 3.** Malignant transformation of mast cells. For tumor formation, mast cell proliferation must be promoted by activation of ligand‐independent activation on growth factor receptors. Also, acquisition of resistance against apoptosis

pathways and invasive characters may facilitate malignant expansion of MCTs.

## **4.1. Clinical presentation, incidence, and risk factors**

MCTs are characterized by the aberrant proliferation of mast cells, accounting for approxi‐ mately 20% of cutaneous tumors in dogs [8, 9]. They develop in the subcutaneous tis‐ sue and dermis in most cases, and other types of mast cell malignancies such as mast cell leukemia and visceral MCTs are rarely observed. They usually occur in the trunk or limbs and sometimes observed in the head and neck (**Figure 4**) [9, 10]. Because mast cells release proinflammatory mediators, erythema and wheal called Darier's sign are some‐ times observed. However, MCT‐specific symptoms that can distinguish MCTs from other tumors are rare.

Risk factors for MCTs, age, sex, breed, spay/castration, and tumor grading have been reported [8, 11]. Among these, most factors except sex are deeply related to its incidence [11]. Recently, genetic characteristics have been also investigated, showing the high correlation of KIT muta‐ tions with prognosis of dogs with MCTs [12–14]. As Mochizuki et al. [11] presented nice sum‐ mary on each factor except genetic one, we would like to focus on the genetic characteristics of MCTs in the following sections.

**Figure 4.** Representative photograph of MCT‐diagnosed dog. (A) MCT occurred in the left leg in 11‐year‐old female, Shiba. (B) MCT occurred in the left waist in 8‐year‐old female, Pharaoh Hound.

#### **4.2. Diagnosis of MCTs**

Cytological or histological analyses through a fine needle aspiration or biopsy are required for the diagnosis of MCTs. Typically, round‐shape cells with round nuclei and with rich cytosol are observed. Mast cells have abundant cytosolic granules, and specific staining methods with toluidine blue or safranin O can identify them. However, MCTs with undifferentiated more malignant mast cells possess few granules. Confirmation of the swelling of draining lymph nodes and sometimes fine needle aspiration of the lymph node may help to determine the pres‐ ence of metastasis. Patnaik grading is mainly used pathological grading in the veterinary field because it is recognized as a good prognostic marker [15]. There are three pathological grades (grades I, II, and III), and higher grade indicates that the tumor is more malignant [15]. Several analyses have been revealed: the correlation between tumor grading and the c‐*kit* gene mutation, clearly showing that c‐*kit* mutations are more frequently observed in high‐grade tumor [12–14]. Therefore, analyzing c‐*kit* sequence can also be a prognostic marker for MCTs. In addition, ana‐ lyzing c‐*kit* gene is important in terms of selecting proper treatments because several molecular target inhibitors against KIT protein, a receptor encoded in the c‐*kit* gene, are currently available for the treatment of MCTs. Polymerase chain reaction that amplifies exon 11 and intron 11 region using tumor genome enables the detection of internal tandem duplications (ITDs) in the juxta‐ membrane domain, which is the most frequent type of c‐*kit* mutation. Recently, however, whole sequence of c‐*kit* mRNA is more common because the proportion of mutations in other region of KIT domain or other type of mutations in the juxtamembrane domain are not negligible. Recent reduction in the cost for sequencing analysis will probably boost this trend (see Section 5).
