|Black protein thread bundles|
and small granular mucin vesicles
It turns out that a recently published paper in the Journal of Experimental Biology by scientists from Canada and Washington State examined how this slime is produced. The researchers already knew from prior work that the slime glands of the hagfish contain two cell types: one that produces threads of protein filaments and another that makes vesicles containing the glycoprotein mucin (the same protein family secreted by your nasal passages). When a hagfish is threatened, muscles contract to squeeze these two types of cells from the slime glands. The cells rupture and their contents are extruded from the fish, producing a milky, not quite slimy exudate (see image to right). But upon contact with seawater, the protein filaments uncoil and the mucin vesicles rupture, and the thick, clear slime is made.
The authors of the paper wanted to test the hypothesis that contact with seawater leads to the inward diffusion of ions and increasing osmotic pressure in the mucin vesicles, causing them to swell and burst. Hagfish are isotonic with seawater, meaning that they do not reduce their internal solute concentrations compared to seawater as most vertebrates do. Hagfish, like sharks, maintain high concentrations of organic osmolytes to match the osmolarity of seawater. One of these osmolytes in particular, TMAO, is found at high levels in the fluid containing the exuded mucin vesicles. The authors in this JEB paper wanted to characterize the components of this hagfish pre-slime fluid, and determine whether its organic osmolytes, such as TMAO, protect the mucin vesicles from osmotic pressure so that they do not rupture within the slime glands.
As often occurs in science, this was a sensible hypothesis that did not turn out to be correct. While TMAO provided some protection against vesicle rupture, it was not complete, suggesting that some yet unknown mechanism is used to prevent pre-mature slime formation inside the hagfish slime glands.
Paper reference: doi: 10.1242/jeb.038992