The sandwich structure of keratinous layers controls theform and growth orientation of chicken rhinotheca

The sandwich structure of keratinous layers controls the
form and growth orientation of chicken rhinotheca:

Abstract

The upper
beak bone of birds is known to be overlain by the rhinotheca, which
is composed of the horny sheath of keratinous layers. However, the
details of the structure and growth pattern of the rhinotheca are
yet to be understood. In this study, the microstructure of the
rhinotheca from chicken specimens of different growth stages
(ranging from 1 to ~ 80 days old) was analyzed using a
combination of thin section and scanning electron microscopy
observations, and small‐angle X‐ray scattering analysis. We found
that the rhinotheca comprises three different layers – outer,
intermediate, and inner layers – throughout its growth. The outer
layer arises from the proximal portion of the beak bone and covers
the dorsal surface of the rhinotheca, whereas the intermediate and
inner layers originate in the distal portion of the beak bone and
underlie the outer layer. This tri‐layered structure of the
rhinotheca was also observed in wild bird specimens (grey wagtail,
king quail, and brown dipper). On the median plane, micro‐layers
making up the outer and inner layers are bedded nearly parallel to
the rostral bone at the base. However, more distally positioned
micro‐layers of the outer layer are more anteverted distally. The
micro‐layers of the intermediate layer are bedded nearly
perpendicular to those of the outer and inner layers on the median
plane. The growth of micro‐layers in the intermediate layer adds
thickness to the rhinotheca, which causes the difference in profile
between the beak bone and the rhinotheca in the distal portion of
the beak. Moreover, the entire intermediate layer grows distally as
new proximal micro‐layers form. The outer layer is dragged distally
by the intermediate layer as a result of its distal growth, for the
three layers are closely packed to each other at their boundaries.
Furthermore, the occurrence of the intermediate and inner layers in
the distal portion of the rostral bone may be because the distal
end of the beak is frequently used and worn, and the rhinotheca
therefore needs to be replaced more frequently at the distal end.
The rhinotheca structure described here will be an important and
useful factor in the reconstruction of the beaks of birds in
extinct taxa.