What Sea Squirts Reveal About Nervous Systems
Tunicates, often called "sea squirts," are humans' closest invertebrate relatives in the sea 6 . Their simple appearance belies a complex biology, with nervous systems that hold profound clues about the evolution of our own brain and neurodegenerative diseases like Alzheimer's 6 .
Tunicates share a common ancestor with vertebrates, making them invaluable for evolutionary studies.
Their nervous systems provide insights into conditions like Alzheimer's disease.
The coronal organ is a specialized mechanosensory organ found in tunicate siphons 2 . Its sensory cells are strikingly similar to the hair cells in the inner ear and lateral line system of vertebrates 2 .
This discovery challenges the theory that vertebrate hair cells derived solely from neurogenic placodes, suggesting the evolutionary building blocks were present in the common ancestor of all chordates over 500 million years ago 2 .
Provides clues to the evolutionary origin of hearing
Reveals features of our common ancestor
Traces sensory systems back in deep time
| Finding | Description | Implication for Human Health |
|---|---|---|
| Widespread Genetic Parallels | Identified 428 genes involved in tunicate neurodegeneration that are shared with humans 6 | Conservation of neurodegenerative pathways across 500 million years of evolution |
| Age-Related Neuron Loss | Older colonies showed a nearly 30% reduction in neurons compared to young colonies 6 | Mirrors the age-related vulnerability to neurodegenerative diseases in humans |
| Amyloid Plaque Accumulation | Neurons in aged colonies showed buildup of amyloid precursor protein (APP) 6 | Recapitulates a hallmark feature of Alzheimer's disease pathology in the human brain |
Data based on Stanford Medicine study of Botryllus schlosseri 6
| Research Tool | Function in Tunicate Research | Example of Use |
|---|---|---|
| Morpholinos / siRNA | Gene knock-down; blocks translation of specific mRNA molecules 3 | Used to study the function of a single gene during development due to tunicates' non-duplicated genome |
| Fluorescent Reporter Plasmids | Visualizing specific cells or proteins; can be electroporated into embryos 5 | To label and track specific neuron types, such as cholinergic motor neurons or GABAergic interneurons |
| Voltage-Sensing Fluorescent Proteins | Monitoring neural activity in real-time; detects changes in membrane potential 3 | To study the firing patterns of neurons in the central nervous system and motor ganglion |
| Antibody Staining | Locating specific proteins within tissues (immunohistochemistry) 2 | Used to identify synaptic proteins and map the precise connections of the "connectome" |
| Electron Microscopy | Ultra-high-resolution imaging of cellular structures 2 | Essential for detailing the fine structure of sensory organs like the coronal organ and mapping synaptic networks |
Understanding how the simple neural network controls behavior and studying the adult nervous system's capacity for regeneration 3 .
Using the Botryllus model as a high-throughput, inexpensive system to screen for drugs that can halt neurodegeneration 6 .
Continued study of structures like the coronal organ to understand how complex sensory systems evolved in chordates 2 .
Tunicates, once dismissed as "just little squirts," have firmly taken center stage in some of the most intriguing areas of modern biology. Their simple bodies and privileged evolutionary position make them ideal subjects for deconstructing the complexity of the vertebrate brain.
From revealing the ancient origin of our sensory cells to providing a new model for understanding Alzheimer's disease, the tunicate's unassuming nervous system is proving to be a powerful key to unlocking some of science's biggest questions. As research continues, these marine invertebrates will undoubtedly continue to provide profound insights into where we came from and how we might treat the diseases of our own nervous system in the future.