Ultrastructural immunocytochemistry suggests that periderm granules in embryonic chick epidermis contain beta-defensins

The capability to express an innate antimicrobial action through the production of beta-defensins in the skin of chick embryos has been studied by immunocytochemistry. The immunolabeling of periderm granules present in the initial embryonic epidermis of the chick is obtained with a lizard anti-defensin antibody (Ac-BD-15) that recognizes a homologous peptide in the chick beta-defensin 9. Similar granules, termed honeycomb granules, are also stored in some basophilic granulocytes of adult and hatchling chicks, but did not show immunoreactivity to the antibody. It is unclear whether this morphology indicates different organelles, content or lack of available antigens due to organelle packaging. The remarkable ultrastructural similarity between periderm granules and the honeycomb granules present in basophil granulocytes further indicates that embryonic keratinocytes can synthesize antimicrobial peptides together with corneous proteins. These peptides may participate in the formation of an epidermal microbial barrier during ontogenesis or protect the embryo in case of damage to the amniotic-periderm barrier. These transient organelles, apart from the early keratinization of the epidermis that forms the initial barrier before hatching may also contribute to the formation of an efficient anti-microbial barrier against the penetration of microbes during embryonic life until hatching when the adaptive immunity system is still immature.