Classics revisited: C. J. van der Horst on pregnancy and menstruation in elephant shrews.

BACKGROUND: Menstruation occurs only in higher primates, some bats, the spiny mouse and the elephant shrew. Our knowledge of the latter species is due to work by C. J. van der Horst. FINDINGS: Changes in the uterine stroma are initially similar in fertile and infertile cycles and are confined to a small area. In pregnant animals, the presence of the conceptus causes further development to an implantation chamber. In infertile cycles an outgrowth of highly glandular stroma (a polyp) appears. With decline of the corpora lutea it is shed in a process equivalent to menstruation. Van der Horst described the further development of the placenta and a decidua pseudocapsularis in pregnant animals. In addition he built a unique collection that has thrown light on embryonic development and placentation in other South African mammals. CONCLUSIONS: The changes in endometrial stromal cells during the menstrual cycle appear similar between primates and the elephant shrew and deserve to be studied at the molecular level.