Page 47 - 84 human physiolofy part-1

The Integumentary System

Ceruminous glands

Earwax, also known by the medical term cerumen, is a yellowish, waxy substance secreted in the
ear canal of humans and many other mammals. It plays a vital role in the human ear canal, assisting in
cleaning and lubrication, and also provides some protection from bacteria, fungus, and insects. A
comprehensive review of the physiology and pathophysiology of cerumen can be found in Roeser and
Ballachanda. Excess or impacted cerumen can press against the eardrum and/or occlude the external
auditory canal and impair hearing.

Production, composition, and different types

Cerumen is produced in the outer third of the cartilaginous portion of the human ear canal. It is a
mixture of viscous secretions from sebaceous glands and less-viscous ones from modified apocrine
sweat glands.

Two distinct genetically determined types of earwax are distinguished -- the wet-type which is
dominant, and the dry type which is recessive. Asians and Native Americans are more likely to have
the dry type of cerumen (grey and flaky), whereas Caucasians and Africans are more likely to have the
wet type (honey-brown to dark-brown and moist). Cerumen type has been used by anthropologists to
track human migratory patterns, such as those of the Inuit.

The difference in cerumen type has been tracked to a single base change (an single nucleotide
polymorphism) in a gene known as "ATP-binding cassette C11 gene". In addition to affecting cerumen
type, this mutation also reduces sweat production. The researchers conjecture that the reduction in
sweat was beneficial to the ancestors of East Asians and Native Americans who are thought to have
lived in cold climates.

Function

Cleaning. Cleaning of the ear canal occurs as a result of the "conveyor belt" process of epithelial
migration, aided by jaw movement. Cells formed in the center of the tympanic membrane migrate
outwards from the umbo (at a rate equivalent to that of fingernail growth) to the walls of the ear canal,
and accelerate towards the entrance of the ear canal. The cerumen in the canal is also carried outwards,
taking with it any dirt, dust, and particulate matter that may have gathered in the canal. Jaw movement
assists this process by dislodging debris attached to the walls of the ear canal, increasing the likelihood
of its extrusion.

Lubrication. Lubrication prevents desiccation and itching of the skin within the ear canal (known
as asteatosis). The lubricative properties arise from the high lipid content of the sebum produced by the
sebaceous glands. In wet-type cerumen at least, these lipids include cholesterol, squalene, and many
long-chain fatty acids and alcohols.

Antibacterial and antifungal roles. While studies conducted up until the 1960s found little
evidence supporting an antibacterial role for cerumen, more recent studies have found that cerumen
provides some bactericidal protection against some strains of bacteria. Cerumen has been found to be
effective in reducing the viability of a wide range of bacteria (sometimes by up to 99%), including
Haemophilus influenzae, Staphylococcus aureus, and many variants of Escherichia coli. The growth of

Wikibooks | 47