1. Lesion grossly & microscopically symmetric 

1. Lesion asymmetric grossly & microscopically 

2. Small...lesion usually less than 6 mm in breadth

2. Lesion usually (but now not always) greater than 6 mm in breadth

3. Sharp lateral circumscription, demarcation of the intraepidermal melanocytic component (no horizontal spread of individual atypical melanocytes beyond the most peripheral junctional nests)

3. Poor circumscription of the intraepidermal component with horizontal extension of individual atypical melanocytes beyond the bulk of any intradermal component of neoplasm

4. Hyperkeratosis prominent usually

4. Lack of prominent hyperkeratosis favors MM 

5. Hypergranulosis usually

5. Lack of prominent hypergranulosis favors MM

6. Marked epidermal hyperplasia usual

6. Little epidermal hyperplasia favors MM

7. Nests of melanocytes relatively uniform in size and shape

7. Marked variation in size and shape of nests of melanocytes favors MM

8. Nests of melanocytes within the epidermis are elongated and oriented perpendicular, vertcal, to the skin surface.

8. Nests of melanocytes within the epidermis neither elongated nor oriented perpendicular to the skin surface & horizontal favors MM

fascicles not very elongated

fascicles sometimes  very elongated

9. Presence of homogeneous eosinophilic Kamino bodies singly and in nests within epidermis favors SN

9. No homogeneous eosinophilic bodies in nests within epidermis

10. Sharp circumscription of intraepidermal nests of melanocytes from keratinocytes

10. Confluence of nests of melanocytes within the epidermis

11. Clefts between nests of melanocytes and keratinocytes are the rule

11. Few, if any, clefts between melanocytes and keratinocytes as a rule

12. Nests of dermal nevus cells tend to be discrete...no sheets

12. Nests of dermal melanocytes tend to become confluent and form sheets of cells

13. Maturation of nests (decreased size) & melanocytes (decrease in nuclear size) with descent into the dermis...but make sure nuclear cytology looks benign

13. No maturation of melanocytes with descent into the dermis, usually (though cells can get smaller...adjacent nuclei varying 3-4 fold in size is worrisome)

14. Nevus cells at base of lesion commonly arranged as single cells

14. Melanocytes at base of lesion not usually arranged as single cells

15. No pagetoid  melanocytes, usually

15. Pagetoid melanocytes often

16. Mitotic figures near base of lesion rare

16. Mitotic figures near base of lesion favors MM

17. When present, inflammatory-cell infiltrate patchy around blood vessels throughout the lesion, in vertical array

17. When present, inflammatory-cell infiltrate often band-like, lichenoid, beneath the melanocytic component, in horizontal array

18. No evidences of spontaneous regression

18. Foci of spontaneous regression (fibrosis, telangiectases, melanophages) in the thickened papillary dermis common