Secrets of the Thymus

One of the most baffling glands in the body is the thymus. It lies just below the neck and behind the top of the breastbone, and in all the centuries that man has been studying physiology, its purpose has been unclear. It has hitherto fallen to butchers, marketing the thymus of the lamb and calf as the "neck sweetbread," to give the gland its only obvious usefulness. Now a British cancer researcher, Dr.

Jacques F.A.P. Miller, has found what seems to be the secret of the thymus.

Shaped like a double pendant, the thymus in newborn babies weighs, on the average, one-third of an ounce. In two months it doubles in size, and in a twelve-year-old child it weighs an ounce or more.

Then it shrinks, to two-thirds of an ounce at 20, and to baby-size in old age.

Thymectomized Mice. Dr. Miller's research, as he reported it to the New York Academy of Sciences, was done with mice.

From some newborn mice he cut out the pinhead-size thymuses. For three to four months, these mice seemed to get along as well as their unoperated litter mates.

But after that, 70% of the thymecto-mized group became lethargic, and wasted away, with ruffled fur and a hunched posture. They developed diarrhea and died within three weeks. Yet if thymus removal was postponed until the mice were a week or more old, they rarely developed these disorders--and never, if the operation was done after three weeks of age.

Dr. Miller therefore studied the growth of the mice operated on at birth. He found that their spleens were only half normal size, and that most of the lymphoid tissues (in which protective white cells are made) were badly degenerated.

The mice could not resist infection from bacteria. Their graft-rejection mechanism was severely impaired, and as a result their skins accepted grafts not only from unrelated strains of mice but even from a different species, the rat.

Proxy Immunity. To cross-check his findings, Dr. Miller took some newborn mice, removed their thymuses, and a week later grafted in new thymuses from mice of a different strain. These animals grew up to be healthy, but had a striking peculiarity. They accepted skin grafts from mice of the strain whose thymus glands they carried, while rejecting, in the normal way, other foreign tissues. Dr. Miller called it "immunological reactions by proxy." Despite the differences between man and mouse, the thymus gland probably plays much the same role in both species.

Dr. Miller's work suggests that the human thymus, in the first weeks of life, produces the basic cells that are then distributed to other white-cell factories, in lymph nodes and the spleen, where cells can be mass-produced at short notice to protect the body against invading microbes or foreign tissue. Once the master cells have been distributed, the thymus seems to have done its main job. In adult life, and even in later childhood, the gland can be removed with little apparent effect. Perhaps it eventually becomes use less, despite its vital early role.

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