Calcium reserve assembly: a basic structural unit of the calcium reserve system of the hen egg shell.

A well-defined proper subregion of the mammillary knob is described, consisting of baseplate (BP), calcium reserve body (CRB), and an integral but distinct CRB cover. The BP extends 14 to 20 microns into the outer shell membrane and rises a few microns above its surface. The organic matrix of the BP is a composite of modified membrane fibers and adjacent components. The distinct CRB matrix is attached to the BP and extends outward in attenuated from, much like the fractal aggregates associated with filter "cakes." The three regions comprise a unit designated as the CRA (calcium reserve assembly, or assemblies). The CRA are fully developed and mineralized by 10.5 to 11 h postoviposition of the previous egg. By 13 h postoviposition a new structure, the crown, is evident. The classical mammillary knob consists of the CRA/crown complex. By hatch time the CRA are partially decalcified, except in the air cell region. Consequently, a zone of separation develops in the affected CRA about midway through the CRB. Estimates of the calcium recovered from the egg shell and of the calcium found in the CRA, excluding the CRA in the air cell region, indicate that more than enough calcium is present in the CRA to meet the drawdown of calcium from the egg shell by the developing embryo. It is concluded that the CRA that are available to the action of the chorioallantoic membrane represent the calcium reserve of the hen egg shell.