THE BIOLOGY BOOK

Progesterone

1929

With the discovery of estrogen and its effects on female reproductive function during the 1920s, many scientists believed that it was the only female sex hormone. But not all researchers were satisfied; doubts had been sowed in 1897. In that year, John Beard theorized that the corpus luteum was an “organ of pregnancy,” perhaps even essential for the maintenance of pregnancy. The corpus luteum or “yellow body” is what is left of a follicle after ovulation has occurred.

In 1900, the German researcher Gustav Born observed that the corpus luteum was absent from the ovaries of monotremes, such as the platypus of Australia and New Guinea—the only mammals that feed their babies with milk but lay eggs and lack a placenta. On this basis, Born concluded that the corpus luteum was required for the development of the placenta. Furthermore, he speculated that the corpus luteum released an internal secretion that prepared the uterine mucosa (outer layer) for the anticipated arrival of a fertilized egg and its implantation in the uterine wall. After Born’s death, his student Ludwig Fraenkel continued this work and, in 1903, showed that destruction of the corpus luteum in pregnant rabbits caused an abortion. Finally, in 1929, George Corner and Willard Allen established that an abortion that normally occurs after removal of the corpus luteum in pregnant rabbits could be prevented by administration of an extract from the corpus luteum. This extract, purified in 1933, was called progestin (progesterone).

THE PREGNANCY HORMONE. After ovulation, the corpus luteum springs into action secreting progesterone in anticipation of the potential fertilization of the released egg and ensuing pregnancy. Progesterone stimulates the development of a thick lining of blood vessels in the uterine wall that is needed to sustain the growth of a developing fetus. The corpus luteum continues to produce progesterone for about ten weeks, after which time the placenta assumes this responsibility. Throughout pregnancy, progesterone quiets the uterus and prevents contractions that could lead to a miscarriage. If the egg is not fertilized, the corpus luteum degenerates and no longer secretes progesterone, and a new menstrual cycle ensues.

SEE ALSO: Placenta (1651), Ovaries and Female Reproduction (1900), Secretin: The First Hormone (1902).

In 1803, Nicholai Argunov painted the portrait of a very pregnant Praskovia Kovalyova (1768–1803), who, born into a serf family, became one of the finest opera singers of late-eighteenth-century Russia. Unfortunately, she died weeks after giving birth to her first child.