Fish reveal cause of altered human facial development

Whether from birth or through events which happen in life, many people have differences in their facial appearance. Worldwide, over one-third of all congenital anomalies relate to the development of a child's head or facial bones -- their craniofacial features -- a common example being having a cleft lip and/or palate. The exact cause of craniofacial differences is not fully understood, but researchers currently think that multiple factors may be involved. This includes genetics, the gestational parent's environment, their diet, some illnesses and certain drugs or chemicals.

Teratogens are substances known to disturb the growth of an embryo or fetus; for example, pregnant people are advised to avoid alcohol and nicotine. Potential teratogens are typically screened for using animals such as rodents and rabbits. But researchers are looking for alternative methods which are quicker, cheaper and reduce the need for testing on mammals.

This is where zebrafish come in. These tiny, 2-5 centimeter freshwater fish grow very quickly, developing as much in a day as a human embryo would in a month. "Zebrafish embryos are transparent and grow outside the mother, so we can monitor the behavior of live cells as they develop," said Toru Kawanishi, project assistant professor at the University of Tokyo's Department of Biological Sciences at the time of the study. Within the past 10 years, several research projects have shown that zebrafish can effectively be used to check for teratogens. However, the exact mechanisms by which teratogens impair or alter typical embryonic development is still being investigated.

The team focused on a specific genetic marker for a group of cells involved in craniofacial development in both mammals and fish. In humans, these are known to become parts of the nose and jaw. "We manipulated the genome of zebrafish embryos and made bone-forming cells fluorescently visible in green. We then treated them with chemicals that are known to cause facial defects in human newborns, and tracked the trajectories of the bone-forming cells throughout embryonic stages," explained Kawanishi.

The team tested five chemicals: valproic acid (used to treat neurological and psychiatric disorders), warfarin (an anticoagulant), salicylic acid (popular in skin ointments), caffeine and methotrexate (used in chemotherapy). They saw that, as expected, all the chemicals tested caused various degrees of craniofacial anomalies 96 hours after fertilization. However, they were surprised by the mechanism which caused this to happen and how quickly it started.

"Bone- and cartilage-forming cells in the head, called cranial neural crest cells (CNCCs), generally move a long distance from where they are first formed around the back of the neck, to their intended destinations such as the jaw or nose," explained Kawanishi. "We were surprised that regardless of how each chemical acts on cells molecularly, impaired migration of bone-forming cells in early development was responsible for the onset of facial malformation for all the five chemicals. We could see signs of this within just 24 hours, at a point where zebrafish and mammalian embryos share very similar morphological and molecular characteristics."

The results indicate the potential existence of a general mechanism by which teratogenic chemicals limit movement of CNCCs early on in embryos, causing the development of facial differences. The researchers extrapolate that facial differences caused by other substances might also follow the same mechanism. "We will aim to reveal the molecular mechanism underlying the impaired cell migration, to understand why different chemicals lead to the shared defects in cell migration," said Kawanishi. The team proposes using this zebrafish-based system as another way to test for cross-species teratogens, so that parents and medical practitioners can be made aware to limit or avoid them.