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Abstract
The aim of this study is to evaluate the regulation of the osteopontin (OPN) gene
expression by non-hormonal stimuli, such as calcium flux and mechanical strain during
the daily egg cycle in the oviduct of the laying hen. After the egg enters the eggshell
gland (ESG), the OPN gene is expressed by the epithelium cells in two waves: first
by the basal cells and only then by the apical cells of the epithelium. A reduction
in OPN gene expression was observed 1 h prior to laying. The calbindin gene, which
marks the onset of calcification, was found to be expressed in the glandular epithelium
starting 2 h after OPN gene expression. In addition, the formation of soft shells
was accompanied by a reduction in calbindin, but not in OPN, gene expression. The
application of a mechanical strain comparable to that induced by an egg led to induction
of OPN gene expression at a normally quiescent phase in the cyclical expression of
this gene. The induction of the gene was time- and strain-dependent and temporally
similar to that induced by the entry of the egg into the ESG. In contrast, the calbindin
gene was not affected by mechanical strain. The ESG of the laying hen provides a system
to study the effect of a mechanical strain on matrix protein production in vivo, in
a relevant physiological setting. The finding suggests that, in contrast to calbindin,
OPN gene expression is not regulated by calcium flux but rather by the mechanical
strain imposed by the resident egg.