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.
Pearl, a calcium carbonate gemstone produced by a marine mollusc, is often used to adorn the body. Its colour and lustre have for centuries been used to enhance and complement the body beautiful.
Carbonates in the inner ear
Not all parts of the inner ear in humans are about hearing. Situated in the vestibular organ of the inner ear are two large sacs called the utricle and saccule. 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 embryo, a biomineralisation process allows calcium carbonate to be precipitated out as tiny calcite crystals.
The utricle and the saccule are the otolith organs that sense gravity 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.
Evidence suggests that the human inner ear has evolved from a structure very closely related to the saccule and otolith found in modern fish species. 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.
Of all the gemstones, pearl is the only one produced by a living organism. Certain types of marine invertebrates known as molluscs create pearl. They have evolved this adaptation to cope with the invasion of parasites or foreign objects. The bivalve 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 mantle and the shell of the mollusc, mantle epithelial cells slowly surround it, creating a pearl sac.
- Over time, these cells secrete a combination of calcium carbonate and protein, 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 aragonite form with small amounts of protein and polysaccharide that give pearl its colour and lustre. This combination is called nacre or mother of pearl.
Scientists are interested in the high strength and fracture resistance 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, electronics and non-hydrocarbon-based plastics. Listen to Professor Kate McGrath explain some of the possible long-term benefits of biomineralisation research.