A Specter in the Gills

What am I looking at?

This is an image of the branchial cavity within a fish called a crucian carp. The nuclei of all the cells are blue (1). The actin within all the cells is green (2). Killer T cells are purple (3) and outline the newly discovered lymphoid organ.

Biology in the background

This recently discovered lymphoid organ, called the nemausean lymphoid organ (NELO), is interesting because it provides researchers with new insights into how fish defend themselves from infection. The lymphatic system is a network of interconnected tubes that distribute fluids and immune cells throughout the body. Lymphoid organs are locations where cells of the immune system, such as B cells and T cells, are produced, housed, and/or activated.

The NELO is in the brachial cavity, also known as the gill cavity, which contains the gills. Water constantly flows over the gills to allow the fish to extract oxygen from it. However, the water carries in debris and microorganisms that can be harmful to the gills and the surrounding tissues. Having an organ near the gills that can produce cells to clean up this debris and fight infection is very useful in keeping the fish healthy.

The portion of the branchial cavity visible here is about 4 millimeters wide, or roughly four times smaller than the width of a human thumbnail.

Technique

This image was created using confocal microscopy.