Saturday, June 17, 2006

How to grow big breasts

Greg Beck reminds us of one of the world's simple truths: "It’s not the tits that make the woman; it’s the woman that makes the tits."

This is, of course, only partly true. Barring surgical interventions, breast size and density are obviously under substantial genetic control, which means that a woman's mother and father play a big role as well. A literature search on the topic tends to catch a lot of research on heritability of breast cancer and the genetics of Parus major, a European bird known as the "great tit." We'll call that a case of semantic matching gone wrong.

It is known that body fat distribution is highly heritable (various studies put it between 0.2 and 0.5 on a scale of 0 to 1). There's an extensive literature on the genetics of cow udders, and shape and size are highly heritable.

There is a lot of insight to be gathered from a 1999 paper by Feng Chen and Mario Capecchi, showing that the Hox genes, normally associated with embryological developmental pathways, also control the development of breast tissue in pregnant mice.

Hox genes don't actually code for any proteins. They are regulatory genes, turning other genes on and off at the right time for the organism to grow the right part at the right place. Mess with a fly's Hox genes and you can get it to grow a nicely formed leg where its antenna should be.

In mammals, functions of particular genes are harder to parse out, they work together in interacting networks of genes. In a developing embryo, they help establish the major body axes and where all the parts wind up in the body. Hoxa9, Hoxb9, and Hoxd9 are known to be involved in the development of the entire mouse thorax, and mice mutated at all three genes show distinct skeletal abnormalities. The animals have to walk on the backs or sides of their paws because various joints in the forelegs don't match up.

But that's not what Chen and Capecchi looked at. They looked at how the offspring of mice with mutations at these three genes survived. Regardless of the offspring's genotype, some maternal genotypes had all their pups die, and it turns out that they weren't producing milk. Put the pups with a different mother, and they survived just fine.

When they looked at the mammary tissue under the microscope, they saw no difference in virgin mice with or without the mutations. When they looked at mice that had given birth, the difference was massive.

It appears that these three Hox genes work together to promote the growth of mammary tissue in adult mice.

One of the neat things about evolution and common descent is that closely related species tend to share genes and gene functions. While I don't know that anyone has tried to assess these genes expression in humans, we'd expect a similar pattern.

Humans go through a similar growth of mammary size during pregnancy, indeed I'm told that tabloids don't look for growing bellies on celebrities, but look for an expanding bosom first.

Whether these same genes would be involved in the growth of the breasts during puberty is harder to say. Most of the mass of the breast is not glandular tissue, but fatty tissue. And these genes seem to be involved more with gland growth during pregnancy than with growth of adipose tissue.

Chen, Feng and Mario R. Capecchi (1999) "Paralogous mouse Hox genes, Hoxa9, Hoxb9, and Hoxd9, function together to control development of the mammary gland in response to pregnancy," Proc. Nat. Acad. Sci., 96(1):541-546.