Microanatomy of passerine hard‐cornified tissues: Beak and claw structure of the black‐capped chickadee (Poecile atricapillus)

Abstract

The microanatomy of healthy beaks and claws in passerine birds has not been well described in the literature, despite the importance of these structures in avian life. Histological processing of hard-cornified tissues is notoriously challenging and only a few reports on effective techniques have been published. An emerging epizootic of beak deformities among wild birds in Alaska and the Pacific Northwest region of North America recently highlighted the need for additional baseline information about avian hard-cornified structures. In this study, we examine the beak and claw of the Black-capped Chickadee (Poecile atricapillus), a common North American passerine that is affected by what has been described as "avian keratin disorder." We use light and scanning electron microscopy and high-magnification radiography to document the healthy microanatomy of these tissues and identify features of functional importance. We also describe detailed methods for histological processing of avian hard-cornified structures and discuss the utility of special stains. Results from this study will assist in future research on the functional anatomy and pathology of hard-cornified structures and will provide a necessary reference for ongoing investigations of avian keratin disorder in Black-capped Chickadees and other wild passerine species.

Multi-layered bird beaks: a finite-element approach towards the role of keratin in stress dissipation

“…The beak not only consists of the beak bone, but also of a surrounding dermal layer with connective tissue and a cellular layer. The latter consists of living cells that keratinize towards the outer side of the beak, leaving a hard, cornified, dead keratin layer at the outer surface of the beak [6]. Epidermis, dermis and bone are anchored to each other with collagenous Sharpey's fibres (bone -dermis interaction) or numerous tissue interdigitations (dermis -epidermis interaction; A.G., personal observation; in figure 1a).…”

Macroscopic, Histologic, and Ultrastructural Lesions Associated With Avian Keratin Disorder in Black-Capped Chickadees (Poecile atricapillus)

“…8). 32 Individual corneocytes were disarrayed along the juncture between the external and buccal plates in birds with moderate to severe beak overgrowth. There was increased exfoliation of the cornified laminae along external surfaces of the beak in affected birds.…”

“…Twenty-two birds with normal beaks were selected as controls and provided baseline information on normal microanatomy and ultrastructure of the chickadee beak and claw. 32 A subset ( n = 16 affected, n = 14 controls) of the birds used in this study was held in captivity for up to five months at the University of Alaska Fairbanks (UAF) Biological Research and Diagnostics Facility as part of a separate investigation prior to post-mortem examination. 33 All birds that died while in captivity ( n = 9 affected) were necropsied immediately by UAF Veterinary Services personnel.…”

“…32 Images of the head and beak were collected at 20–100× magnification (40kV, 2.8W) using frame averaging ( n = 256) with Dage XiDAT software (Nordson Corporation).…”

“…Tissues were fixed in 10% neutral buffered formalin, processed using an automated tissue processor (Shandon Citadel 2000; GMI Inc., Ramsey, MN), and embedded in paraffin blocks (Reichert-Jung Model 8040 Tissue Embedding Center with vacuum; Reichert Technologies, Buffalo, NY). We used methods described in Van Hemert et al 32 to decalcify and soften beaks and claws. Beak tissues were trimmed for sectioning along either the mid-sagittal ( n = 20 affected, n = 14 controls) or transverse plane ( n = 5 affected, n = 5 controls).…”