It’s another busy week of EcoDevo, and even though the campus was closed I still had to give a lecture on endocrine disruptors. I started by laying out Wilson’s Principles of Teratology…wait, what? You don’t know them? I guess I’d better explain them to the internet at large.

These principles are a bit like Koch’s Principles, only for teratology — you better know them if you want to figure out the causes of various problems at birth, and you do: about 3% of all human births express a defect serious enough for concern. Here’s the list:

1. Susceptibility to teratogenesis depends on the genotype of the conceptus and the manner in which this interacts with adverse environmental factors.
2. Susceptibility to teratogenesis varies with the developmental stage at the time of exposure to an adverse influence. There are critical periods of susceptibility to agents and organ systems affected by these agents.
3. Teratogenic agents act in specific ways on developing cells and tissues to initiate sequences of abnormal developmental events.
4. The access of adverse influences to developing tissues depends on the nature of the influence. Several factors affect the ability of a teratogen to contact a developing conceptus, such as the nature of the agent itself, route and degree of maternal exposure, rate of placental transfer and systemic absorption, and composition of the maternal and embryonic/fetal genotypes.
5. There are four manifestations of deviant development (death, malformation, growth retardation and functional defect).
6. Manifestations of deviant development increase in frequency and degree as dosage increases from the No Observable Adverse Effect Level (NOAEL) to a dose producing 100% lethality (LD100).

The first two tell you what is tricky about teratology. There are multiple variables that affect the response: genetic variability in the conceptus (and, I would suggest, maternal variations), and also timing is critical. A drug might do terrible things to an embryo at 4 weeks, but at 3 months the fetus shrugs it off.

Ultimately, though, the teratogen is having some specific effect (3) on a developing tissue. We just have to figure out what it is, while keeping in mind that that effect might be hiding in a maze of genetics (1) and time (2).

Another complication is that in us mammals the embryo is sheltered deep inside the mother, who has defense mechanisms. The agent has to somehow get in (4). A complication within a complication: sometimes the teratogenic agent is harmless until Mom chemically modifies it as part of her defense, and instead creates a more potent poison.

#5 is just listing the terrible outcomes of screwing with development.

#6 I do not trust. It’s saying the effect is going to follow a common sense increase with increasing dosage, but even that isn’t always true. There is a phenomenon called the inverted-U response where the effect increases with dosage, then plateaus, and then drops off at high concentrations. We’re dealing with complex regulatory phenomena with multiple molecular actors that may have unpredictable interactions. There are teratogens that do terrible things to embryos at low concentrations, but do nothing at ridiculously high concentrations — as if the high dose triggers effective defense mechanisms that the low dose sidesteps.

I had to review these principles in class yesterday, because although I’d also discussed them earlier in the semester, we are currently dealing with teratogens of monstrous subtlety, these compounds that mimic our own normal developmental signals, the same signals our bodies use to assemble critical organ systems. It’s as if some joker were placing inappropriate traffic signals along a busy highway — most would do no harm, but some may totally confuse travelers who then end up detouring up into the kidneys rather than down the genitals, as they preferred, or they end up crashing into the thyroid.

Unfortunately, in this case the responsible jokers are mainly gigantic megacorporations who are spewing these dangerous signals all over the countryside…and then we get to wait until the people swimming in them try to have children, and then the teratologists get to say “death, malformation, growth retardation and functional defect”.

In case you were wondering, Wilson didn’t come up with his list first — a 19th century scientist named Gabriel Madeleine Camille Dareste did it first. No, not first. Lots of people have been documenting these developmental problems as long as there’s been writing, like on this Chaldean tablet:

When a woman gives birth to an infant:
With the ears of a lion There will be a powerful king
That wants the right ear The days of the king will be prolonged
That wants both ears There will be mourning in the country
Whose ears are both deformed The country will perish and the enemy rejoice
That has no mouth The mistress of the house will die
Whose nostrils are absent The country will be in affliction and the house of the man will be ruined
That has no tongue The house of the man will be ruined
That has no right hand The country will be convulsed by an earthquake
That has no fingers The town will have no births
That has the heart open with no skin The country will suffer from calamities
That has no penis The master of the house shall be enriched by the harvest of his field
Whose anus is closed The country shall suffer from want of nourishment
Whose right foot is absent His house will be ruined and there will be abundance in that of the neighbor
That has no feet The canals of the country will be cut and the house ruined
If a queen gives birth to:
An infant with teeth already cut The days of the king will be prolonged
A son and a daughter at the same time The land will be enlarged
An infant with the face of a lion The king will not have a rival
An infant with 6 toes on both feet The king shall rule the enemies’ country

Nowadays we’re more interested in causes than imagined consequences, I hope.