Writing in the latest edition of Proceedings of the National Academy of Sciences, Professor Anthony Graham and Dr Masataka Okabe suggest that the gills of ancestral marine creatures, which were used to regulate calcium levels, were internalised rather than lost when land-living, four-limbed animals – the tetrapods – evolved.
Many physiological processes such as muscle contraction, blood coagulation and signalling by nerve cells, require specific levels of calcium in the body. In humans, calcium levels are regulated by the parathyroid gland, which secretes parathyroid hormone if the calcium concentration in the blood falls too low. This hormone then causes the release of calcium from bone, and increases its reuptake in the kidney, raising the calcium levels back to normal.
Fish don't have parathyroid glands. Instead they increase their internal calcium concentration by using their gills to take up calcium from the surrounding water.
'As the tetrapod parathyroid gland and the gills of fish both contribute to the regulation of extracellular calcium levels, it is reasonable to suggest that the parathyroid gland evolved from a transformation of the gills when animals made the transition from the aquatic to the terrestrial environment,' said Professor Graham.
'This interpretation would also explain why the parathyroid gland is positioned in the neck. If the gland had emerged from scratch when tetrapods evolved it could, as an endocrine organ, have been placed anywhere in the body and still exert its effect.'
The researchers supported their theory by carrying out experiments that show that the parathyroid glands of mice and chickens and the gills of zebrafish and dogfish contain many similarities.
Both gills and parathyroid gland develop from the same type of tissue in the embryo, called the pharyngeal pouch endoderm; both structures express a gene called Gcm-2, and both need this gene to develop correctly.
Furthermore, the researchers found a gene for parathyroid hormone in fish, and they discovered that this gene is expressed in the gills.
'The parathyroid gland and the gills of fish are related structures and likely share a common evolutionary history,' said Professor Graham. 'Our work will have great resonance to all those people who have seen Haeckels' pictures, which show that we all go through a fish stage in our development. This new research suggests that in fact, our gills are still sitting in our throats – disguised as our parathyroid glands.'
This press release is based on the following article:
The origin of the parathyroid gland
Masataka Okabe and Anthony Graham
Proceedings of the National Academy of Sciences
Professor Anthony Graham
Anthony Graham is Professor of Developmental Biology at the MRC Centre for Developmental Neurobiology, King's College London. His laboratory is interested in the mechanisms and molecules that direct the formation of the vertebrate head. This is an important area of research, which has ramifications outside the immediate field of developmental biology, as there are numerous inherited conditions which result in malformations of the head. His work also seeks to understand of the evolutionary origin of the vertebrates
MRC Centre for Developmental Neurobiology
The MRC Centre for Developmental Neurobiology, funded in partnership by the Medical Research Council and King's College London, was founded in the year 2000 to promote research in this rapidly expanding area of neuroscience. The Centre comprises 23 laboratories and is headed by Professor Andrew Lumsden, FRS. A total of approximately 140 scientists, students, and staff work on related questions concerning the early steps in nervous system development. The overall aim of the Centre is to understand the early events during brain development and, through this, to increase knowledge of the mechanisms that lead to malformation and that limit regenerative processes in the human nervous system.
King's College London
King's College London is one of the oldest and largest colleges of the University of London with some 13,400 undergraduate students and more than 5,000 postgraduates in ten schools of study. It is in the top group of five universities for research earnings and has an annual turnover of over £320 million and research income from grants and contracts in excess of £90 million (2001-2002). King's is a member of the Russell Group, a coalition of the UK's major research-based universities.
Journal
Proceedings of the National Academy of Sciences