Pregnancy-related involution of the thymus in the viviparous lizard, Chalcides ocellatus

The thymus gland of the adult viviparous lizard, Chalcides ocellatus (Reptilia, Sauria, Scincidae) undergoes conspicuous morphological changes in relation to gestational period. During pregnancy, the thymus is depleted of thymocytes and undergoes changes coming to consist primarily of numerous degenerative cysts. Thymic involution occurs early during pregnancy (starting at phase II) and reaching its highest level at parturition (phase V). These observations correlate with the circulating level of blood progesterone, measured by radioimmunoassay, and suggest a possible relationship between thymic involution and the activity of sex hormones.