Ultrastructural investigation of the mechanism of muscle attachment to the gastropod shell.

The ultrastructure of the muscle-shell attachment was investigated in the land pulmonate snails Helix aspersa, Anguispira alternata, in the freshwater pulmonate Laevipex sp., and in the freshwater prosobranch Pomacea paludosa. In all cases, a collagenous intercellular matrix and a specialized epithelium (tendon cells) intervene between the columellar muscle and the shell. These tendon cells are characterized by hemidesmosomes at both apical and basal ends, connected by thick bundles of microfilaments. The tendon cells do not insert into the shell directly by microvilli, as formerly thought, but by an extensive network of extracellular organic fibers.

A method for the demonstration of pores in calcified eggs of vertebrates and invertebrates.

Determination of pore number, pore size as well as pore canal distribution is important in understanding the biology of calcified eggs among vertebrates as well as invertebrates. Various techniques have been applied to elucidate pore structure of avian egg shells, including fracturing coupled with scanning electron microscopy and differential histochemical staining of the matrix. The best results reported have thus far been from plastic impregnated, etched specimens (Board & Tullett, 1974). The present report is a modification of this latter technique, utilizing the very low viscosity, highly penetrating electron microscopic embedding medium developed by Spurr (1969). This modification is used in this report of the previously undescribed pores from the egg shells of the giant South American land snail, Strophocheilus oblongus. These eggs, which can be up to 50 mm in length, have recently been investigated with respect to the calcium carbonate mineralogy (Tompa, 1976) as well as with respect to the matrix biochemistry Tompa et al., 1977). (Similar resin has been used successfully for the plastic impregnation and ultrathin sectioning of that region of the snail shell where the body muscles insert into the shell comumella (Tompa & Watabe, 1976).) This technique, which can be utilized on avian eggs or on any other calcified material in probing for pores, relies on the high penetrating ability of the resin as well as on its easy commercial availability around the world.

pH and calcium concentration changes in a molluscan egg during development.

During development, the egg albumen of calcified land snail eggs becomes more and more acid, correlated directly with a constant rise in the calcium concentration of this albumen. It is suggested that the developing embryo releases some acid metabolite and the subsequent change in albumen pH aids in embryonic absorption of the CaCO3 (calcite) egg shell, used for making the embryonic body shell or skeleton (CaCO3 in the form of aragonite).

Calcified arteries in a gastropod.

The anterior aorta and major arteries of the land pulmonate snail Anguispira alternata have large calcium deposits in their walls. These deposits occur inside spherule cells, which line the walls of these vessels. The calcium occurs as amorphous calcium carbonate, in the form of intracellular spherules having alternating layers of organic and inorganic material. The spherule cells appear to be degenerating connective tissue cells; they are characterized by large numbers of spherules and by a cytoplasm which is totally empty except for a nucleus, scattered glycogen particles and a few membrane remnants. The injection of 45Ca into the foot of the snail results in rapid incorporation of this calcium into the spherule cells. Although calcium-containing spherules are now known from a wide variety of invertebrate tissues, they have not been previously recorded from arterial walls. The physiologic significance of these deposits is not known.