Interstitium, Apoplast.

I learned a couple of new words from this fascinating “interactive” NY Times article by Abraham Z. Cooper, a pulmonary and critical care physician and associate professor of medicine (archived versions don’t seem to work, because of the interactivity, but hopefully you can read the text at Facebook):

In 2021, researchers described what they saw when they had examined skin-biopsy samples that included tattoos: The ink particles had traveled deeper than anticipated, through interstitial spaces into the tissue underneath the skin, or the fascia. “That wasn’t supposed to happen,” Neil Theise, a professor of pathology at New York University and a senior author of the paper reporting the results, told me.

The existence of an apparent conduit between skin and the fascia beneath it — two tissue layers not known to connect with each other in this way — broke accepted anatomic boundaries. The researchers also found that the same was true for other previously unknown microscopic connections between organs in the abdomen.

That interstitial spaces exist in and under the skin and between and around the body’s organs had been observed going back more than a century, but they were assumed to exist in isolation from one another, like a patchwork quilt.

Theise and his colleagues published their first observations of these spaces in 2018. Their findings in the 2021 tattoo-ink study implied that the body’s interstitial spaces were parts of a vast interconnected whole — what scientists now call the interstitium. “This is clearly a third bodily system for the circulation of fluids,” in addition to the cardiovascular and lymphatic systems, says Rebecca Wells, a professor of medicine at the University of Pennsylvania and a senior author of the study. The human body suddenly looked less like a patchwork quilt and more like a knitted blanket.

The OED (in a 1900 entry) has interstitium as a synonym of interstice, but in 1993 they added a sense “Anatomy and Zoology. That part of a given region of the body which lies between the principal cells, tissues, etc. of that region” (first citation from 1949). Later in the Times article, we get:

Plants seem to possess their own version of an interstitium, too. It’s called the apoplast, a type of interstitial space that transports water and nutrients outside cell membranes. These and other examples suggest that fluid moving through interstitial spaces might have represented the first circulatory systems to develop in the earliest forms of complex multicellular plant and animal life, hundreds of millions of years ago.

The word apoplast is new enough that it isn’t in the OED; per the Wikipedia article it was coined as far back as 1930, but that was in German, and who knows when it entered English? That’s why we need an OED entry. (Thanks, Bonnie!)