Signet ring cells

Signet ring cells are represented by cystocytes, which form numerous layers in the trachea and respiratory tract of birds. They are characterized by large and rounded shapes and the absence of a clearly defined cytoplasmic membrane. Signet ring cell epithelia are found in the tracheal trunks, i.e., in the air ducts of birds, and extend to the surface of the body of birds in the form of cilia. They trap microbes and prevent them from penetrating deep into the mucous membrane.