Michelangelo may have hidden a self-portrait in a sketch of an aristocratic woman poetess friend held in the British Museum, a new study from a Brazilian university says.

The study by art expert Deivos de Campos of Porto Alegre's federal university of health science says the Renaissance genius hid a caricature of himself in the 1525 sketch of Vittoria Colonna.

The small outline of a man hunched over a painting can be seen if you carefully scan the ink lines of the folds in the noblewoman's dress, around the abdomen area, de Campo says.

The expert says the man's shape resembles a self-caricature the artist sketched in 1509 on the side of a sonnet dedicated to his friend and fellow artist Giovanni da Pistoia.

In that first sketch Michelangelo showed himself upright, painting the Sistine Chapel, while in this second sketch he is shown with his body leaning over at an acute angle, as if Michelangelo himself were painting the whole portrait. According to Deivos de Campos, the self-caricature could be a hidden 'signature' by the artist, and it may furnish precious evidence about his build and health at that time, when he was 50.

The alleged discovery is another step forward in the treasure hunt which researchers have pursued over the years to try to find meaningful hidden drawings and resonant symbols in Michelangelo's work.

A year ago, it was de Campos' group that found pagan symbols that allude to the anatomy of the female reproductive system in the Medici Chapels in Florence.

On that occasion, the researchers said Michelangelo carved pagan symbols of the female sex organs in the Medici Chapels in Florence just as he painted them in the Sistine Chapel in the Vatican.

Skulls, shells and spheres allegedly evoke the shapes of the uterus and Fallopian tubes, the team led by de Campos said - the same group that a few months previously found similar allegedly anatomical symbols in the Sistine Chapel, as alleged tokens of female power in a Church that held them back.

The Medici Chapel study focused on three symbols carved beside the tombs of Giuliano and Lorenzo de' Medici: skulls of cattle and rams, spheres connected by cords, and a shell.

According to the Brazilian experts, their shape is a coded reference to that of the uterus and the Fallopian tubes, organs of female reproduction.

The Renaissance genius inserted them into his work with the intention of "representing the capacity for rebirth and regeneration between life and death," said the team.

