At first, it took a long time — up to a day — to make the cells cry. But, with experience and a little prodding, the researchers eventually made them weep in only half an hour.
The tearful cultures, reported in Cell Stem Cell on 16 March, are the first tear-gland ‘organoids’ — three-dimensional assemblages of cells that are designed to resemble miniature versions of organs. Organoids of the glands that produce tears could be used to study and eventually treat disorders that cause dry eyes, including an autoimmune condition called Sjögren’s syndrome.
“It’s very promising,” says ocular pathologist Geeta Vemuganti at the University of Hyderabad in India.
In addition to their role in displaying emotion, tears help to lubricate and protect the eye. Dry eyes can be painful, inflamed and prone to infection.
To study tear production, developmental biologist Hans Clever’s laboratory at the University Medical Center Utrecht in the Netherlands developed a way to grow tear-gland cells as organoids. The group has found ways to grow a menagerie of organoids, including miniature livers, cervical cancers and snake venom glands.
Tear glands, also called lacrimal glands, are a particular challenge to study, says Darlene Dartt, who studies tear production at Massachusetts Eye and Ear in Boston. The glands are located above each eyeball, behind the bony orbit of the eye, making them difficult to biopsy. Samples, when researchers can get them, are often tiny, she says.
Clevers’ lab used their expertise to work out culturing conditions for cells from mouse and human lacrimal glands. To stimulate tear production, they then exposed their organoids to several chemicals, including the neurotransmitter norepinephrine, that relay messages between nerve cells and glands.
Because the organoids lack ducts, ‘tear’ production causes them to swell. “If there had been a little duct, there would have been droplets,” says Clevers. And when the team transplanted the organoids into mice, the assemblages matured and developed duct-like structures containing proteins found in tears.
The team hopes that the cells can be used to study tear glands, and to screen for drugs that affect tear development. Clevers and his colleagues have already used CRISPR genome editing to study tear gland development, and have found that a gene called Pax6 is important in guiding cells to take on a tear-gland identity. Pax6 is a known regulator of eye development: expressing the fly version of Pax6 on the leg of a fruit fly will cause an eye to develop there.
Clevers’ lab is now teaming up with Dutch naturalist and television-show host Freek Vonk, to study structures resembling tear glands in crocodiles. The team hopes to use the organoids to study actual ‘crocodile tears’, which the reptiles use as a way to excrete salt.
Organoids derived from human cells could also eventually provide material for transplants, to replace diseased or damaged tear glands. Clevers’ group and its collaborators have developed salivary gland organelles that will be tested in clinical trials starting this summer for people who suffer from dry mouth, a condition that can cause tooth decay and difficulty in chewing and tasting.
Those salivary-gland trials could serve as a testing ground to work out methods that could then be adapted for future tear-gland transplants, says Dartt. In the meantime, she says, the work that Clevers’ team has done in characterizing tear glands — including creating a detailed cell-by-cell map of the structures and their organoids — has demonstrated that the glands are more heterogeneous than was previously appreciated and could send researchers back to reinterpret old data. “That has implications for a lot of studies.”