Dermal tumour demonstrates large pools of basophilic mucin separated by thin fibrovascular septa ;

Small islands of epithelial cells are present in the mucinous pool ;

Cells are dense at the periphery; 

Tumour cells are small, cuboidal and bland ;

May have vacuolated cytoplasm;

Arranged in cribriform, papillary, tubular or glandular patterns ;

Almost no mitotic figures are present.

Mucinous carcinomas originate from in situ lesions that may represent, ductal hyperplasia, atypical ductal hyperplasia, or ductal carcinoma in situ  or a combination of the three.

The presence of an in situ component defines the neoplasm as primary cutaneous, but its absence does not exclude the diagnosis.

Cytologic diagnosis of primary mucinous carcinoma of the skin is possible; however, correlation of clinical, radiologic and pathologic features is necessary to arrive at an accurate diagnosis.

Special stain: The mucin is PAS-positive and diastase resistant (consistent with sialomucin). Mucicarmine is also positive.

Immunohistochemistry: 

Tumour is positive for low- molecular weight cytokeratin, CEA, EMA and sometimes S100. 

Some cases demonstrated estrogen and progesterone receptors.

CK20 is negative which might significantly contribute to the differentiation of this tumour from cutaneous metastases from gastrointestinal carcinomas

In all cases of mucinous carcinoma of skin a full clinical assessment is essential.

This tumour must be distinguished from metastatic visceral mucinous carcinoma (breast, colon, stomach, lung, ovary or pancreas).

There should be no evidence of primary tumour at these sites.