Meiosis and fertilization are fundamental and fascinating developmental events, yet they remain incompletely understood despite a century of study. In most sexually reproducing animals, oocytes arrest in meiotic prophase and resume meiosis (meiotic maturation) in response to sperm or somatic cell signals. Proper chromosome segregation during meiosis is critical for the viability of the fertilized egg. In humans, dysregulation of the meiotic process is a major cause of miscarriage, infertility, and Down Syndrome. In many species, hormones trigger signal transduction cascades that regulate meiosis and promote oocyte meiotic maturation and ovulation. Short-range contact-dependent signals between oocytes and somatic cells of the gonad also regulate meiotic maturation. Hormonal dysfunction in humans may play a role in the etiology of meiotic defects because their frequency is increased in both pregnancies of women over forty and early adolescent pregnancies.
