The eruption of Mount Vesuvius, a major stratovolcano in southern Italy, in 79 AD, was one of the most catastrophic volcanic eruptions in European history. Millions of tons of molten rock, pulverized pumice and host ash buried the Roman settlements of Pompeii, Herculaneum, Oplontis and Stabiae. The accidental discovery of Pompeii and Herculaneum propelled the development of archaeology as a field of study. [1] Ever since the original excavation in 1738, over 1000 casts of bodies have been recovered in these two Roman cities, along with the remains of buildings and artifacts. [2] With the help of better tools and increasing interdisciplinary studies in archaeology, surprisingly well preserved neurons from a male victim’s skull and spine of the 79 AD eruption in Herculaneum were discovered in a recent study.
The discovery of neuronal tissues from archaeological human remains is very rare, although it’s not unheard of. Most of these discoveries are due to saponification, the process of the formation of a soap-like postmortem product from soft tissues under certain environments. These preserved tissues can reveal environmental and constitutional factors that are useful for forensic studies, but the preservation of fine structures of the tissues is underwhelming. [3] In the recent paleoforensic survey in Herculaneum, however, the scientists discovered glassified brain cells that were preserved due to a different process -- vitrification. Vitrification is a natural process that occurs when a liquid is exposed to heat and then its temperature rapidly drops to below its glass transition temperature. In this case, the vitrification of neurons took place as a result of the rapid cooling of volcanic ash deposit after exposure to the hot ash cloud at a temperature of 500ºC. Under scanning electron microscopy (SEM), a technique that scans a focused electron beam over a surface to create an image, the researchers found neuronal architectures that are comparable to those formed following a high-pressure freezing protocol. This similarity is in support of the theory of the formation of the glassy tissues. [4]
SEM image of brain axons [4]
How did the scientist make sure these tissues are neurons instead of other cell types? This was achieved with a number of techniques, including SEM, artificial neural network and gene expression studies in these tissues. First, energy-dispersive X-ray spectroscopy was used to detect the molecular composition of the vitrified tissue to determine that it mainly consists of organic matter. Next, the scientists investigated the physical features of their region of interests using SEM and measured the diameters of the tubular structures. The mean diameter is similar to those of white matter axons and smaller than those of the blood vessels in the cerebrovascular system. Therefore, they ruled out the possibility of the tissues being blood vessels. The image processing tool based on a neural network detected typical myelin sheath, an electrical insulator wrapped around neurons, further confirmed the cell types of the vitrified tissues. [4, 5] Moreover, the researchers identified the expression data of genes encoding proteins in a sample of vitrified neuronal tissue and found a number of genes that are typically expressed in the human brain tissues, which is in favour of the discovery of human neuronal tissues. [4]
SEM image of spinal cord axons (green), cell bodies (yellow) and sheath-shaped structures (orange) [4]
The unearthing of vitrified 2000-year-old neurons is not only an astonishing discovery itself, but it opens up a new field of biogeoarhaeological investigations on previously undetected evidence buried under volcanic eruptions as well. This study can also provide insights into studying the structures of ancient central nervous systems.
Author: Gracia Gu; MRes in Biochemistry; Linacre College, Oxford
References:
[1] Anon, Archaeology. Available at: http://omeka.wellesley.edu/piranesi-rome/exhibits/show/discovery-of-pompeii-and-hercu/archaeology [Accessed October 18, 2020].
[2] Maiuri, A., 1958. Pompeii. Scientific American, 198(4), pp.68–78.
[3] Ubelaker, D.H. & Zarenko, K.M., 2011. Adipocere: What is known after over two centuries of research. Forensic Science International, 208(1-3), pp.167–172.
[4] Petrone, P. et al., 2020. Preservation of neurons in an AD 79 vitrified human brain. Plos One, 15(10).
[5] Siegel, G.J., Morell, P. & Quarles, R.H., 1999. Myelin Sheath. In Basic neurochemistry molecular, cellular, and medical aspects. Amsterdam: Elsevier.