A recent world first study, undertaken at the Brien Holden Vision Institute, on the weight of the human eye has confirmed two distinct phases to eye growth.
Conducted by Dr Ashik Mohammed, under the supervision of 2019 Order of Australia recipient Professor Robert Augusteyn, the study has produced results influential to the development of treatment for conditions such as cataract and presbyopia.
We can now conclude that there are two distinct phases of lens growth – the first during pre-natal development and the second during post-natal life
The study is the only one to measure lens dry weights (total solid content) over a whole life span. This was achieved by analysing 549 human lenses across an age range of 0-93 years, where previously only 20 weights had been measured for young lenses. “We can now conclude that there are two distinct phases of lens growth – the first during pre-natal development and the second during post-natal life,” said Prof Augusteyn.
“This study, which measured both dry and wet lens weights, demonstrated that prenatally, the lens grows in a rapid logarithmic fashion, while postnatally, growth becomes linear and the chemistry of the new lens cells changes. Between birth and late teens, the lens changes shape from nearly round to elliptical,” he added.
The study also revealed that although the lens grows in a linear fashion after birth, the ratio of wet to dry weight changes over time. Prof Augusteyn said “The different rates of dry and wet weight accumulation result in a slow increase in the percentage of dry weight, which equates into 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.”
As well as this, differences from birth in lens weight between men and women supports 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 per cent heavier than female lenses at birth. Thereafter, male and female lens weights increase at the same rate for the rest of life.
Researches state that, “the lens weight difference is the same as the 4 per cent difference in bodyweight at birth . . . These similarities suggest that regulation of growth lens weight and bodyweight may be through similar mechanisms.”
“Sexual dimorphism has been observed in other biometric parameters at birth, including head diameter, limb length, and body length,” they add, but up until now, “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.”