Approximately 350 million years ago, our evolutionary ancestors and those of all modern vertebrates were simple, soft-bodied creatures living in the ancient oceans. In order to survive and progress in the face of formidable predators, mainly crustaceans at that time, they needed to acquire some form of protection and advantage.
The evolution of dermal armor, such as the sharp spines on armored catfish or the bony diamond-shaped scales called scutes on sturgeons, proved to be a successful strategy. Thousands of fish species developed various patterns of dermal armor, composed of bone and/or dentine, a key component found in human teeth. These protective coatings enabled vertebrates to not only survive but also evolve into new and more sophisticated creatures, ultimately leading to the development of humans.
But how did this armor originate? How did our ancient underwater ancestors evolve to develop this protective coat?
A recent study on sturgeon fish sheds some light on this. According to the research conducted by Jan Stundl, a Marie Sklodowska-Curie postdoctoral scholar in Marianne Bronner’s laboratory at Caltech, a specific population of stem cells known as trunk neural crest cells is responsible for the formation of bony scutes in fish. Neural crest cells are found in all vertebrates, including fish, birds, and humans. These cells become specialized based on their origin, either from the head (cranial) or the spinal cord (trunk) regions. They migrate throughout the developing body of the animal, contributing to the formation of essential structures such as jaws and hearts.
The discovery was made after a previous study from the University of Cambridge in 2017 revealed that trunk neural crest cells give rise to dentine-based dermal armor in a type of fish called the little skate. Building on this knowledge, Stundl and his team speculated that the same population of cells might also be responsible for bone-based armor in various vertebrates.
To investigate this further, they focused their attention on the sturgeon fish, particularly the sterlet sturgeon (Acipenser ruthenus). Sturgeons, well-known for producing the world’s most expensive caviar, still retain many characteristics of their ancient ancestors, making them ideal candidates for evolutionary studies.
The research not only provides insights into the evolutionary history of dermal armor but also highlights the significance of neural crest cells in shaping the development of protective structures in different vertebrates, including us humans.
In their research, Jan Stundl and his team conducted experiments using sturgeon embryos grown at the Research Institute of Fish Culture and Hydrobiology in the Czech Republic. They utilized fluorescent dye to track the migration of the fish’s trunk neural crest cells as they developed within its body. Since sturgeons start developing their bony scutes after a couple of weeks, the researchers kept the growing fish in a darkened lab to avoid disturbing the fluorescent dye with light.
Remarkably, the team observed that the fluorescently labeled trunk neural crest cells were precisely located where the sturgeon’s bony scutes were forming. To gain further evidence, they employed a different technique to highlight the fish’s osteoblasts, which are cells responsible for bone formation. Genetic signatures associated with osteoblast differentiation were detected in the fluorescent cells within the fish’s developing scutes, providing strong confirmation that the trunk neural crest cells indeed give rise to bone-forming cells.
When combined with the 2017 findings regarding the role of neural crest cells in forming dentine-based armor, this research solidifies the conclusion that trunk neural crest cells play a vital role in the development of bony dermal armor, contributing significantly to the evolutionary success of vertebrate fish.
Working with non-model organisms like sturgeon presented some challenges as the tools commonly used for standard lab organisms, such as mice or zebrafish, had limited applicability and required significant adaptation. Despite these obstacles, Stundl emphasizes that studying non-model organisms like sturgeon provides valuable insights, allowing researchers to answer fundamental questions in evolutionary developmental biology with rigor.
Marianne Bronner highlights the significance of studying various animals across the tree of life to infer past evolutionary events. Approaching evolutionary questions from a developmental biology perspective is particularly powerful as many changes leading to diverse cell types occurred through small alterations in embryonic development.
The research paper is titled “Ancient vertebrate dermal armor evolved from trunk neural crest.”