Q: Why is endodermal sinus tumor named that way? Does it really come from the yolk sac? What’s with the “sinus”? Why would it resemble glomeruli? I looked on the internet and ended up getting more confused. I’d be so grateful if you could explain to me. Please help me!

A: Those are great questions! Sometimes the most confusing part of learning pathology is sorting out the terminology. This is one of those annoying tumors that has two names – it is sometimes called endodermal sinus tumor (its more old-fashioned name), but is more often called yolk sac tumor nowadays.

First things first – what is this yolk sac?

The yolk sac is a membranous pouch attached to the embryo, and plays a role in very early blood circulation. The yolk sac is also the source of the germ cells that will later go on to form the gametes. These cells usually stay put in the ovaries and testes where we would expect them to be, but yolk sac tumor can also occur in other locations, like the brain. Yolk sac tumors usually occur in young children, and only very rarely develop in adults.

The confusion in the name mostly comes from the fact that some yolk sac tumors contain diverse morphology which historically was thought to resemble the kidney as well as…rat placenta. We’ll unpack this below.

Schiller-Duval bodies

First, yolk sac tumors may contain Schiller-Duval bodies, named after two doctors described below. They are not really “bodies”, but rather are layers of endothelial cells surround a capillary, as seen in this image:

Schiller-Duvall body

Photo courtesy of Dr. Shelly Cook

Some yolk sac tumors have Schiller-Duval bodies, but not all of them do. So if you see them, they are pretty much diagnostic of yolk sac tumor, but if you don’t see them you still haven’t ruled it out.

Endodermal sinuses

Getting back the endodermal sinus part. First, no one should feel bad about being confused by this term, since this name has an obscure historical origin from the days of the tumor’s discovery. It turns out that when Dr. Schiller first described this tumor back in 1939, what we now call Schiller-Duval bodies were thought to resemble endothelial structures found in rat placenta, the “endodermal sinuses of Duval”, which are named after another scientist called (you guessed it) Duval.

Glomeruloid appearance

And if two names aren’t bad enough, it could have been even worse. Remember the glomerulus part? Because the tumor with all its Schiller-Duval bodies can sometimes also have a glomeruloid-appearing morphology, Dr. Schiller initially called his discovery “Mesonephroma ovarii” (the term “nephro” refers to nephrons, the filtration structures of the kidney, and yolk sac tumor is most commonly found in the gonads, including the ovaries). That name fell out of favor when it was established that there was not a kidney-related origin to the tumor, and thank goodness we now only have to remember two names!

AFP

No discussion (or diagnostic workup) of yolk sac tumor is complete without considering alpha-fetoprotein (AFP). AFP is a protein produced predominantly by the yolk sac and fetal liver. It is a major component of fetal blood, acting to maintain osmotic pressure, among other not-completely understood roles. The levels of serum AFP play a role in screening for congenital conditions like spina bifida (high AFP) and Down’s syndrome (low AFP), but the important thing here is that AFP levels normally decrease soon after birth, and healthy adults should not have detectable levels of AFP, except in pregnancy where it can be temporarily elevated.

Awesome references

• For more about yolk sac tumors, check out Robbins 9e, page 977.
• For more about the fascinating history of the yolk sac tumor, check out this vintage publication from 1959:

Teilum, G. Endodermal sinus tumors of the ovary and testis. Comparative morphogenesis of the so-called mesonephroma ovarii (schill’r) and extraembryonic (yolk sac-allantoic) structures of the rat’s placenta. Cancer 12, 1092–1105 (1959).

• And for more about the lives and times of Drs. Schiller and Duval, check out this resource:

Aboud, K. A. & Aboud, D. A. Schiller-Duval Bodies and the Scientists Behind them. Gynecology & Obstetrics 4, (2014).

A huge thanks to Michelle Stoffel, MD PhD, PGY2 Pathology Resident at the University of Wisconsin, for yet another great case and informative, easy-to-read post! Check out her other awesome cases here and here. And also a big thank you to Dr. Shelly Cook, who provided the case itself, as well as the photos.