Calcium carbonate crystals present in the inner ear play a key role in our sense of balance and movement. Although found in very small amounts, they literally keep us on the straight and narrow.
Pearl1, a calcium carbonate gemstone produced by a marine mollusc, is often used to adorn the body. Its colour and lustre2 have for centuries been used to enhance and complement the body beautiful.
Calcium carbonate otoliths
The utricle and saccule are inner-ear organs that sense gravity and linear movement. It is the presence of small crystals of calcium carbonate called otoliths that allows them to function.
This is a coloured scanning electron micrograph of crystals of calcium carbonate found in these inner-ear organs.
Carbonates in the inner ear
Not all parts of the inner ear in humans are about hearing. Situated in the vestibular organ3 of the inner ear are two large sacs called the utricle4 and saccule5. On the inside walls of these sacs is a bed of specialised sensory cells called hair cells. A gel-like substance sits on top of these cells, and embedded within the gel are crystals of calcium carbonate. The crystals are called otoliths, which literally means ‘ear stones’. During the course of development of the inner ear in the human embryo6, a biomineralisation7 process allows calcium carbonate to be precipitated out as tiny calcite8 crystals.
The utricle and the saccule are the otolith9 organs that sense gravity10 and linear movement in a straight line. The utricle lies in a horizontal position in the head whereas the saccule lies vertically. When the head is tilted to one side, the otoliths in the utricle want to slide ‘downhill’. This moves the gel just enough to bend the sensory hairs, which sends a signal to the brain about the amount of movement. If the person is accelerated forward or back, once again the otoliths move, resulting in a signal being sent to the brain about the extent of this movement. The saccule operates in a similar manner detecting movement up and down as well as nodding the head forward and back. The signals from the otoliths in the saccule and the utricle complement each other and give a combined message about linear motion of the head.
Evidence11 suggests that the human inner ear has evolved12 from a structure very closely related to the saccule and otolith found in modern fish species13. Interestingly fish use otolith for decting sound and for balance - watch this video explanation of how fish detect sound.
Carbonates on the outer ear
Because of its colour, lustre and natural beauty pearls – in particular, pearl earrings – have for centuries been used to enhance, complement and draw attention to the body beautiful.
Expensive pearl
Queen Mary 1st of England owned the La Peregrina pearl (left). The pearl is famous for it’s size and symmetry. In 1969 Richard Burton purchased the pearl for Elizabeth Taylor, who had it made into a necklace with rubies and diamonds (pictured at right). In 2011 the necklace fetched $11 million US dollars at auction.
Of all the gemstones, pearl is the only one produced by a living organism14. Certain types of marine invertebrates15 known as molluscs16 create pearl. They have evolved this adaptation17 to cope with the invasion of parasites18 or foreign objects. The bivalve19 molluscs, such as the oyster Pinctada maxima, are particularly good at producing pearl and are much sought after.
It is through the process of calcium carbonate biomineralisation that pearls are produced:
- If a piece of foreign material is trapped between the mantle20 and the shell of the mollusc, mantle epithelial21 cells slowly surround it, creating a pearl sac.
- Over time, these cells secrete a combination of calcium carbonate and protein22, and a pearl begins to develop.
It has been estimated that only 1 in 1000 oysters grow pearls in this way, making these natural pearls quite rare. Cultured pearls imitate this natural process by surgically placing a ‘bead’ between the shell and the mantle. The bead is the foreign object, and its presence initiates the development of a pearl with the bead at its centre. After a time period of 2–3 years, the cultured pearls that have formed can be harvested and a fresh bead implanted to start the process over again.
It is the combination of calcium carbonate crystals in the aragonite23 form with small amounts of protein and polysaccharide24 that give pearl its colour and lustre. This combination is called nacre25 or mother of pearl.
Scientists are interested in the high strength and fracture resistance26 properties of nacre and are working to replicate biomineralization in the lab to develop materials that might find an application in such diverse areas as human health, electronics27 and non-hydrocarbon-based plastics. Listen to Professor Kate McGrath explain some of the possible long-term benefits of biomineralisation research.
- pearl: A semi-precious gemstone of calcium carbonate produced in the mantle of a living shelled mollusc such as an oyster.
- lustre: The way light interacts with the surface of a mineral. For example, if the mineral has a polished metal appearance, its lustre is described as ‘metallic’. Terms like dull, greasy, waxy, pearly and adamantine (diamond) are used to describe lustre.
- vestibular organ: The structure in the inner ear composed of the utricle, saccule and semicircular canals. Together, they relay information to the brain about movement and sense of balance.
- utricle: A small sensory structure in the inner ear that monitors the tilt of the head by sending nerve impulses to the brain for interpretation.
- saccule: 1. In mammals, a small sensory structure in the inner ear that monitors up and down head movements by sending nerve impulses to the brain for interpretation. 2. In fish, a structure found in the head region that allows fish to sense sound and gravity.
- embryo: The product of a fertilised egg, from the zygote until the foetal stage. The undeveloped plant that forms when the ovule is fertilised.
- biomineralisation: The process by which living organisms produce minerals, often to harden or stiffen existing tissues. Marine invertebrates such as pāua grow a protective shell based on mineral forms of calcium carbonate.
- calcite: The most common and most stable mineral form of calcium carbonate (CaCO3). It is the main component of chalk, limestone and marble.
- otolith: A structure made of calcium carbonate found in the inner ear, especially in fish. Used to detect gravity and/or linear acceleration.
- gravity: The force attracting something towards the centre of Earth (or other large mass, like a moon or planet) – the reason that things fall to Earth.
- evidence: Data, or information, used to prove or disprove something.
- evolve: To develop gradually. Changes in successive generations over long periods of time.
- species: (Abbreviation sp. or spp.) A division used in the Linnean system of classification or taxonomy. A group of living organisms that can interbreed to produce viable offspring.
- organism: A living thing.
- invertebrates: An animal without a backbone, for example butterflies, worms, snails, insects, spiders and aquatic species such as crabs and jellyfish.
- molluscs: A large division of invertebrate (without a backbone) animals. The best-known molluscs are snails, slugs, shellfish, octopuses and squids.
- adaptation: A change in the structure or function of something. In biology, a change in a species, as a result of natural selection. Individuals with a particular feature (adaptation) are more likely to survive and reproduce than individuals without this feature.
- parasite: An organism that lives in or on another organism. Parasites usually cause harm to their host organism.
- bivalve: 1. Any mollusk, of the class Bivalvia, which has a soft body within two hinged-shells. Examples include mussels, oysters and scallops. 2. Having two similar parts hinged together.
- mantle: 1. A layer of the inner Earth between the crust and the core. Varies in temperature from 500 °C to 900 °C. Consists of semi-fluid molten rock. 2. A layer in molluscs that covers the fleshy body. In some molluscs, it secretes a shell (for example, snails), but it doesn’t in others (for example, slugs).
- epithelial: Cells that line the cavities and surfaces of structures throughout the body. The outermost layer of our skin is made from dead squamous epithelial cells.
- protein: Any of a large class of complex compounds that are essential for life. Proteins play a central role in biological processes and form the basis of living tissues. They have distinct and varied three-dimensional structures. Enzymes, antibodies and haemoglobin are examples of proteins.
- aragonite: A mineral form of crystalline calcium carbonate similar to calcite. It is found in the shells of marine invertebrates such as pāua and in living coral reefs.
- polysaccharide: A biopolymer made up of many individual sugar molecules linked together. Examples include cellulose, starch or glycogen.
- nacre: A biomineral produced by various types of mollusc that is made predominantly from calcium carbonate and the carbohydrate chitin. Known as ‘mother of pearl’, it gives the inner shell of the mollusc its characteristic lustrous and iridescent appearance.
- resistance: 1. The opposition to the flow of electric current through a circuit. 2. The ability to withstand harmful or unfavourable conditions, such as an infectious disease.
- electronics: 1. The study of the behaviour and control of electrons. This field has expanded enormously with the discovery of semiconductors. 2. The generic term for hardware used to build electrical components.
