Well-circumscribed dermal tumor with a grenz zone between the tumour and the epidermis ;

Variously shaped tumour islands in the dermis ;

May extend into the subcutis ; 

Combination of solid and cystic areas ;

Cystic spaces form as a result of tumour cell degeneration ;

The intervening stroma varies from delicate vascularized  fibrous tissue to dense hyalinized collagen ;  

Minimal epidermal connection may be present ; 

Foci of squamous differentiation; 

Ductal differentiation and intracytoplasmic lumina ; 

Mitotic figures are often present which is of little prognostic significance.

Two types of cells -  One cell type is polyhedral with a rounded nucleus and slightly basophilic cytoplasm. The second cell type is generally round with clear cytoplasm.

Variants:  Clear cell ; Poroid ; Oncocytic ; Epidermoid, and Pigmented. 

Clear cell hidradenomas are characterized by prominent clear cells which is present in less than one third of hidradenomas. Mucus producing cells are found in some cases.  The clear cells contain glycogen and periodic acid-Schiff-positive, diastase-resistant material, but no lipid.

Poroid hidradenomas are well circumscribed , non-encapsulated tumours composed of predominantly poroid cells. 

Cytologically, this variant resembles an eccrine poroma.

Atypical hidradenomas - cytological atypia or infiltrative margins; higher rate of recurrence and malignancy. The significance of atypical nuclear changes in benign eccrine acrospiromas: a clinical and pathological study of 18 cases.