Interactive: A single, 4 micron thick step-section of a very small sea otter fetus. This otter’s mother was found dead on Dec 11, 2000 at Hidden Beach in Aptos, CA. This fetus weighed only 3.9 grams and was only 42 millimeters long from the tip of the tail to its nose. Zoom in and scroll around the image to get a closer look at the tissues and organs!


Sea Otters: Furry on the Outside, Beautiful on the Inside!

By Melissa A. Miller, DVM, Ph.D., Veterinary Pathologist
California Department of Fish & Game, UC Davis

Sea otters are interesting and amazing creatures to watch and enjoy in their coastal environment. Guess what? It turns out that they’re just as amazing and interesting on the inside as they are on the outside! Here in California, scientists perform postmortem examinations on all sea otters that are found dead, so they can monitor population health and learn how to help living sea otters survive and thrive. Although the main goal of these examinations is to determine the cause of death, if we pay close attention, these animals can also teach us a great deal about their past!

For example, did you know the number of pregnancies an otter has had can be determined simply by examining her ovaries? This is because each past pregnancy results in formation of a tiny ovarian scar. The total number of these tiny scars, called corpora albicans (white bodies) corresponds quite accurately with the number of previous pregnancies!

And did you know that all of us (including sea otters) tend to get a little bit grayer on the inside as we get older, in addition to all those gray hairs that we sprout on our heads? This is because we all accumulate a substance called lipofucsin inside our cells over time as we age. Lipofuscin is the intracellular “leftovers” from the normal recycling of cell membranes that occurs throughout our lifetime. Over time lipofuscin builds up as a grey crystalline material in our cells, giving the tissues a slight grey cast. Thus an old otter will commonly have slightly grayer internal organs than a young otter!

Occasionally a pregnant sea otter is presented to us for postmortem examination. This is especially sad, because it means that two lives were lost. However, it makes us try even harder to pinpoint environmental health risks that sea otters are facing, so we can help guide efforts to fix these problems. By examining both the mother and the fetus, we can determine whether the toxins and infectious agents that sea otters are exposed to can pass to the fetus. We can also learn a great deal about how sea otters grow and develop.

When the fetus is very small, the best way to perform a detailed internal examination is by examining tissue sections on a light microscope. To do this, we place the intact fetus in a fixative called formalin, and then embed it in paraffin wax to make the tissue firm enough to cut thin sections for microscopic examination. A series of sequential cuts (called step-sections) are then made along the long axis of the entire fetus, with each cut extending from the head to the tail, similar to slicing a loaf of bread, until the entire fetus has been sectioned.

Just how thin are these sections? In order to see through the tissues on a light microscope, the sections need to be extremely thin, about 4 microns thick. Four microns is only 0.0004 centimeters, or 0.000157 inches! To illustrate just how thin these sections are, if we were to cut a human hair lengthwise at 4 microns thickness, it would take 15 separate knife cuts to pass through a single hair, and over 1400 separate cuts would be required to pass through the thinnest part of a dime!

These thin sections are cut using a metal knife, then placed on glass slides and stained with an H&E (hematoxylin and eosin) stain to enhance visualization of cell structure. The hematoxylin tends to stain intracellular sugars (like glycogen, DNA and RNA) purple, while the eosin stains proteins (like collagen and elastin) pink. Based on these staining characteristics and the tissue structure and organization, veterinary pathologists can identify the internal organs and identify any abnormalities when examining tissues on a microscope.

Zoom in and scroll around the image to get a closer look at the tissues and organs! Even in this very young fetus, many organs, including the liver, kidneys, lungs, heart and brain can be easily recognized. Can you see the hematoxylin (blue or purple) staining of the bones, cartilage and DNA-filled cell nuclei, and the eosin (bright red or pink) staining of the red blood cells and the cell cytoplasm? You can even trace out the front paws, hind flippers and tail. Can you recognize the familiar silhouette of a sea otter?

About is dedicated to raising awareness about California's threatened sea otters and the role research plays in the species recovery and conservation. It's a collaboration of the Monterey Bay Aquarium, U.C. Santa Cruz, U.C. Davis, U.S. Geological Survey, U.S. Fish & Wildlife Service, California Department of Fish and Wildlife, among others. is also home of the world's first HD live stream of southern sea otters in the wild.