Acute inflammatory lesions of the placenta consist of diffuse infiltration of neutrophils at different sites in the organ. These lesions include acute chorioamnionitis, funisitis, and chorionic vasculitis, and represent a host response (maternal or fetal) to a chemotactic gradient in the amniotic cavity. While acute chorioamnionitis is evidence of a maternal host response, funisitis and chorionic vasculitis represent fetal inflammatory responses. Intra-amniotic infection has been generally considered to be the cause of acute histologic chorioamnionitis and funisitis; however, recent evidence indicates that “sterile” intra-amniotic inflammation, which occurs in the absence of demonstrable microorganisms but can be induced by “danger signals”, is frequently associated with these lesions. In the context of intra-amniotic infection, chemokines (such as interleukin-8 and granulocyte chemotactic protein) establish a gradient favoring the migration of neutrophils from maternal or fetal circulation into the chorioamniotic membranes or umbilical cord, respectively. Danger signals released during the course of cellular stress or cell death can also induce the release of neutrophil chemokines. The prevalence of chorioamnionitis is a function of gestational age at birth, and is present in 3-5% of placentas delivered at term, but in 94% of placentas delivered between 21-24 weeks of gestation. The frequency is higher in patients with spontaneous labor, preterm labor, clinical chorioamnionitis (preterm or term), or ruptured membranes. Funisitis and chorionic vasculitis are the hallmarks for the fetal inflammatory response syndrome, a condition characterized by an elevation in fetal plasma concentrations of interleukin-6, associated with the impending onset of preterm labor, a higher rate of neonatal morbidity (after adjustment for gestational age), and multi-organ fetal involvement. This syndrome is the counterpart of the systemic inflammatory response syndrome in adults; however, in fetuses, it is a risk factor for short- and long-term complications (i.e. neonatal sepsis, bronchopulmonary dysplasia, periventricular leukomalacia, and cerebral palsy). This article reviews the definition, pathogenesis, grading and staging, and clinical significance of the most common lesions in placental pathology. Illustrations of the lesions and diagrams of the mechanisms of disease are provided.
