Here is the reference from our prior live Group case discussion defining how collarettes are formed:

Jacoby RA, Ackerman AB. Is the so-called epidermal collarette formed by epidermal or adnexal epithelium? Am J Dermatopathol. 1982; 4(2):117-24. PMID: 7103013

Key Finding: The study demonstrates that these tissue infoldings are actually formed by epithelial adnexa (adnexal epithelium) rather than the epidermis itself.