Feathers are complex keratinized productions of avian epithelium that are the object of renewed interest as fossil feather forms raise new questions about what really constitutes a feather. Feather morphogenesis has been histologically described (1), but the cytochemistry of cellular processes that produce the form is unknown. This study examined potential roles of cell death and localized division in the growth and sculpting of natal down feathers in White Leghorn chicken embryos.
The primary focus of this work was to determine if areas where cells were described as being "resorbed" or where layers were said to "disintegrate" were in fact being removed by apoptosis (programmed cell death, PCD). PCD is often responsible for removal of cells that have served a purpose and are no longer needed in development (e.g. earliest vertebrate kidney and webs between mammalian digits). Chris Reamer (2) found evidence for PCD in late-stage natal down and periderm, but earlier stages of feather morphogenesis had not been investigated.
We used the TUNEL method (ApopTag®, Serologicals, Inc.) immunocytochemical technique to mark PCD. The method indicated that apoptosis is part of the formation of barbs and barbules, with apoptotic nuclei concentrated in the axial and marginal plates on day 13, as well as in the barbule plate on day 13.5 of development. Markers also suggest that apoptosis remodels the pulp and removes the feather sheath.
We also performed a limited survey of cell proliferation in earliest feather germs to supplement previous investigations that used 3H-thymidine autoradiography on adult feather follicles (1). Embryos that were incubated in ovo with BrdU (Zymed®) and marked with DAB to identify potential localized cell division in days E5 to E9 showed epithelial cells marked on days E5.5 and E6, but did not provide new or conclusive results for proliferation domains in embryonic feather germs. This technique requires further adaptation for use in ovo with chick embryos.
This study has demonstrated that programmed cell death is the major contributor in removing axial and marginal plates of keratinizing barbs and barbules, cells in the pulp spaces, and even the peridermal feather sheath in emerging natal down feathers.
(1) Lucas, A.M., and P.R. Stettenheim. 1972 Avian Anatomy. Integument. USDA Handbook 362. 750 pages.
(2) Reamer, C. 2000 Hamilton College Senior Thesis
Student stipend provided by a Ralph E. Hansmann Summer Science Fellowship
Christine presented this work at the Hamilton College Summer Research Poster Session, 7 November 2003.