Answer to Case 550

Answer to Parasite Case of the Week 550: Ascaris lumbricoides eggs, decorticated

Kudos to everyone who took the time to write in with their answer. All responses were correct! And I really enjoyed hearing about the different layers of of nematode eggs from Old One and Blaine. Here is some of the information they shared in their comments, along with some definitions:

Decorticated - To remove the bark, rind, or husk from; i.e., to remove the outer mammillated layer
Mammillated - Having relatively small protrusions from the exterior, most commonly the surface

This case happened to have both mammillated and decorticated eggs, with a large number of the latter:

When only decorticated eggs are seen, they can be confused for other helminth eggs such as hookworm and Schistosoma japonicum. Fortunately the size and thick shell can be used to reliably differentiate them.

So what are the layers of a nematode egg?  There are several (from outer to inner), as described by Old One and Blaine, with a few comments from my own research:

1. Mucopolysaccharide/protein coat - not present in all nematode eggs; this is an exogenous product of the nematode uterus and what is referred to as the 'mammillated' albuminous layer. It is unique to each species and often stained by the bile salts of its host (yellow, orange, to brown). It is often uniquely textured during its creation. This layer may be friable, hence the decortication seen in this week's specimen. 
2. Vitelline layer - derived from the vitelline membrane of the fertilized oocyte. It envelops the entire egg (including the bipolar plugs of Trichuris eggs). Of note, it can be selectively removed with chemical treatment, with the eggshell remaining intact (and contents remaining viable)
3. Chitinous layer - this is the rigid layer that gives the eggs its shape. 
4. Chondroitin proteoglycan layer - originally thought to be a lipid-rich layer

Layers 2, 3 and 4 are now commonly referred to as the trilaminar outer eggshell that we are well familiar with. Additional inner layers have since been identified, including a lipid-rich layer which serves as a permeability barrier for the developing embryo, and an innermost peri-embryonic layer. 

I had a lot of fun digging through the old and new literature to find all of this information, so thank you Old One and Blaine for the impetus to do so! NIH investigators have published a nice chapter of the structure of the C. elegans shell that is freely available HERE.

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