The calcium reserve/basement membrane complex of the avian eggshell : implications for development of calcium translocation and respiratory functions

Abstract

A well-defined subregion of the mammillary knob of the avian eggshell, designated the calcium reserve assembly (CRA), is identified as a specialized structure housing the calcium reserve for the embryo. Embedded in the fibers of the outer shell membrane, the CRA consists of a baseplate (a composite of modified shell membrane fibers and crystalline accretions) subtending the globose Ca-reserve body (CRB) (a calcified, proteinaceous gel component), and thin CRB-cover. The ontogeny of the CRA is here documented, from initial secretions in the granular isthmus to final capping by a newly-identified structure--the crown-in the uterus. Many eggshell anomalies may be explained as failures in the normal assembling of the components of the CRA, crown, and palisade layer. An empty zone is created beneath the shell by hatch-time, due to the embryo's drawdown of calcium from the CRA. Estimates of calcium recovery from CRA and shell (excluding air space) indicate that the CRA sequester sufficient calcium to supply the embryo's requirement. We have identified a classic basement membrane lining the inner shell membrane (ISM) and verified one of its defining constituents--laminin-by immunocytochemistry. Since, typically, basement membranes are associated with the molecular governance of cytodifferentiation, we propose that the cytodifferentiation of the chorioallantoic membrane (CAM) may be similarly programmed by requisite molecular interactions with the ISM-basement membrane. After adherence between the CAM and ISM-basement membrane (at about day-10 of incubation), an extensive intraectodermal blood sinus develops from capillaries originating in the mesenchyme. External squamous chorionic epithelial cells become transformed into capillary-covering cells having cytoplasmic processes which form a sheath over the blood sinus. The expanded blood sinus cavity is traversed by columns consisting of capillary-covering cell bodies, villus-cavity cells, and basal cells contributed from the inner ectodermal layer. The extensive blood sinus and related structures of the CAM enable proper respiration and calcium retrieval functions to develop. In essence, the two "hemiorgans" (CAM and ISM-basement membrane-CRA complex) become coupled around day-10 of incubation to form a functioning calcium retrieval-respiratory organ.