Study supports bi-phasic human eye growth model

Australian researchers have found that there are two distinct phases of eye growth in humans - with prenatal growth being logarithmic and postnatal growth being linear.

Brien Holden Vision Institute and University of New South Wales scientists say the data from the research will be critical in the development of treatments for conditions such as cataract and presbyopia.

Professor Bob Augusteyn, who supervised the study led by Dr Ashik Mohammed, said “We analysed 549 human lenses across an age range of 0–93 years.”

Prof Augusteyn, who was recently made a Member of the Order of Australia (AM) for the impact of his research on growth patterns of lenses in animal species, says the study is the most comprehensive of its kind and is the only one to measure lens dry weights (total solid content) over the whole life span.

“This study shows that postnatally the chemistry of the new lens cells changes. Between birth and late teens, the lens changes shape from nearly round to elliptical,” he added.

Although the lens grows in a linear fashion after birth, the ratio of wet to dry weight changes over time, explained Prof Augusteyn, saying “The different rates of dry and wet weight accumulation result in a slow increase in the percentage of dry weight, which equates to a slow increase in refractive index.”

“The continued growth pushes cells into the centre of the lens where they compact through loss of water until the maximum compaction is reached. This produces the nuclear plateau of constant refractive index and contributes to the loss of lens power which occurs with age and presbyopia.”

The researchers said they also observed differences from birth in lens weight between men and women, supporting the proposition that changes in lens and body weight may be linked. Because of a difference in the prenatal growth rates, male lenses are 4% heavier than female lenses at birth. For the rest of life male and female lens weights increase at the same rate.

“Sexual dimorphism has been observed in other biometric parameters at birth, including head diameter, limb length, and body length,” they add, but so far, “no differences have been detected in ocular parameters.” The findings may be useful in understanding different modes of lens growth, says Prof Augusteyn.

The researchers also say the weights identified in the study can be used to “calculate the volume and density of the lens,” which are “particularly important for models that predict radiation doses to the lens in procedures such as computer aided tomography scans of the head or orbit, especially in the young.”