A morphological and electron-microprobe study of the inorganic composition of the mineralized secretory products of the calciferous gland and chloragogenous.

The two pairs of lobes of the calciferous gland if the earthworm Lumbricus terrestris are specialized oesophageal diverticulae that secrete spherites ranging from 0.5 - 7.0 micrometers in diameter. Correlative transmission and scanning electron microscopy indicated that the spherites (which are predominantly CaCO3) are formed extracellularly in distinctive bays bounded by secretory-cell processes, and are mobilized anteriorly from the gland lumina to the lumen of a non-secretory pouch, where the majority coalesce and undergo phase transformation to concretions 0.5 - 1.0 mm in diameter consisting of a mass of cuboidal crystals with facets up to 40 micrometers. The distribution of Sr(0.1 ml 5% SrCl2 injected into the posterior coelomic cavity) was monitored in the mineralized secretory products of the calciferous glands by X-ray microanalysis of 10 micrometers - thick air-dried cryostat sections in a SEM. Strontium was not detected in chloragosomes at 2h and 24h post-injection. Strontium was transported anteriorly and specifically incorporated into gland spherites (detectable within 2h). This technique of Sr localization afforded sufficient structural and analytical resolutions to provide a confirmation of the sequence of extracellular changes in the gland/pouch system. In addition we were able to distinguish a population of growing spherites from the vast majority of mature spherites; size alone was a singularly poor indication of spherite growth. The major element constituents of the chloragosomes were P, Ca and Zn (Ca: P ranging from 0.4 to 1.0; Zn: P from about 0.05 to 0.45). Analysis of individual spherites showed that Ca was probably bound to P or P-containing matrix components, whilst Zn was probably linked to one or more different but unknown constituents.