Pycnofibers? Or trichomes?

27 Dec

Pycnofibers and trichomes

Before dinosaurs and birds, other creatures like pterosaurs lived on the Earth. They didn’t have hair or feathers. But they had surface filaments on their bodies that can be seen in their fossils. The cellular structures are called ‘pycnofibers’. They had a single empty cylinder inside each filament. Pycnofibers arose from the epidermal cells, not from under the skin like hairs with their hair follicles. Nor like feathers. 

What amazes me, and which I find amusing, is that plants have these structures, too. They are called ‘trichomes’, and are well documented, including electron micrographs depicting the variety of trichome structures and shapes. Yet, nowhere in the literature that includes ‘pycnofibers’ is any mention of trichomes! Perhaps pycnofibers are the animal equivalent of the plant trichome. 

Trichomes can be inert with no chemical activity, or metabolic. Some are single-celled, some are multi-celled. Some are metabolic, some are secretory. Some are visible with the naked eye, others require a microscope. There is even a ‘trichom-ome’ now; the investigation of all the proteins and pathways involved in producing trichomes and their contents. Yet, the content of some trichomes is only air. And I am pretty sure that some of you have had encounters with trichomes. 

A trichome encounter

Last summer, while conducting a waterfowl breeding survey, I had an intimate

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Trichomes on stinging nettle.

encounter with trichomes. The route for the survey was a walk-through on an unmowed dike dividing two marshes. Much of the vegetation – mustards, and other composite plants- was over my head at 5 feet plus four inches. The route was like pushing through jungle vegetation. One hand held my notebook and pencil and the other hand and arm pushed vegetation aside. 

Three-quarters on my way back to the starting point, my left hand and wrist began burning, like dozens of bites by fire ants. When I reached the vehicle, both hand and wrist were burning, red, and itching like crazy. Good thing I always wear long-sleeved shirts.

The culprit was the trichomes on stinging nettle plants. These tiny hair-like structures are on the stems and undersides of leaves. The thin outer portion of the trichome is silica, and it is very brittle. Shaped like a long hair with a fine point, the tip can break off and penetrate the skin of an animal that brushes up against it. When broken, a complex mixture of chemicals inside the trichome is dumped onto and into the skin. 

A chemical analyses reveals that those trichomes contain neurotransmitters: histamines, acetylcholine, and serotonin. Several acids are also in that mix: formic acid (remember my reference to fire ants?), oxalic and tartaric acids. One or more of these compounds can elicit pain or itching. But the cocktail of all those compounds may be the synergy to induce long-lasting pain, itching and inflammation. 

Over the next week, my hand and wrist swelled so much I had trouble moving my fingers and closing my hand into a fist. I had to switch my watch to the right wrist. I usually held my lower arm up, bent from the elbow. If I didn’t, the wrist and hand throbbed in addition to burning and itching. Twice-daily applications of a dermal corticosteroid somewhat alleviated the symptoms, but not enough. Ice also helped, but I couldn’t keep ice on it 24/7. A phone call to urgent care recommended trying an oral anti-histamine and applying a dermal anti-histamine, with icing once every hour if possible. I added a dose of ibuprofen twice a day. The pain subsided before the itching, which was the hardest symptom to  to ignore. 

Why trichomes?

With that anecdote in mind, you may guess the function of some trichomes and why many plants evolved these dermal structures. Not all trichomes are bad, however, Some are merely hair-like, with pigmentation, and cover the leaves. They are inert; no compounds are inside their trichomes. Their functions may be to trap rain and morning dew to cool the surface of the leaves and stems, or to reflect light away from the surface. Many plants in arid climates have these types of trichomes. 

If you do a Google search on ‘trichome’, the most prevalent result is photos of marijuana trichomes. Trichomes on the stems and leaves of hemp plants, including marijuana, are highly evolved deterrents against herbivory by animals. The three different types of trichomes of Cannabis sativa, or marijuana, are “the very factories that produce the hundreds of known cannabinoids, terpenes, and flavonoids” that the plant is known for. Some trichomes -those that look like tall-stalked, bulbous mushrooms- have higher concentrations of the above compounds and cover specific tissues on the plants: the calyx of the flower buds. When crushed, they exude a sticky, odiferous resin that often repulses animals from eating them, except for humans. 

Why did Mesozoic pterosaurs have pycnofibers? Were their pycnofibers the animal version of plant trichomes? Possibly.  A question is, which evolved first? 

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