Globuli ossei in the Long Limb Bones of Pleurodeles waltl (Amphibia, Urodela, Salamandridae)
Abstract
To date, little is known about the structure
of the cells and the fibrillar matrix of the globuli ossei,
globular structures showing histochemical properties of an
osseous tissue, sometimes found in the resorption front of
the hypertrophied cartilage in many tetrapods, and easily
observed in the long bones of the Urodele Pleurodeles
waltl. Here, we present the results obtained from the
appendicular long bones of metamorphosed juveniles and
subadults using histological and histochemical methods
and transmission electron microscopy. The distal part of
the cone-shaped cartilage contains a heterogeneous cell
population composed of the typical “light” hypertrophic
chondrocytes and scarce “dark” hypertrophic chondrocytes.
The “dark” chondrocytes display ultrastructural character-
istics suggesting that they probably undergo degeneration
through chondroptosis. However, in the hypertrophic, cal-
cified cartilage close to the erosion front by the marrow,
several noninvaded chondrocytic lacunae retained cells
that do not show any morphological characteristics of
degeneration and that cannot be identified as regular
chondrocytes or osteocytes. These modified chondrocytes
that have lost their regular morphology, appear to be
active in the terminal cartilage and synthesize collagen
fibrils of a peculiar diameter intermediate between the
Type I collagen found in bone and the Type II collagen
characteristic of cartilage. It is suggested that the local
occurrence of globuli ossei is linked to a low rate of longitu-
dinal growth as is the case in the long bones of postmeta-
morphic urodeles.