The size limited penetration of gold particles through insect basal laminae

Abstract

The movement of macromolecules through basal laminae has been examined using colloidal gold as a tracer. The size and charge of the tracer was in the same range as large haemolymph proteins, making it a satisfactory visible model for a study of their penetration through laminae. Three kinds of lamina could be distinguished. Tissues involved in the import and export of haemolymph proteins (the fat body, epidermis and pericardial cells) have basal laminae that allow through gold particles smaller than 15 nm. In the experimental preparations used there was no further size filtration after the lamina. Once particles had crossed the lamina they entered the endocytic pathway without further hindrance. Tissues not known to be involved with the synthesis of large haemolymph proteins, such as silk glands, Malpighian tubules, heart and muscle, had bilayered laminae with an additional less permeable layer inside the outer more permeable one. The outer layer adjacent to the haemolymph resembled the complete filtering laminae of the fat body, epidermis and pericardial cells. The inner layer apposed to the cell membrane was impermeable to the smallest tracer particles (6 nm). The midgut connective tissue sheath prevented particles larger than 6 nm from entering the compartment between it and the midgut basal lamina.