The development of thyroid and adrenal function in the fetal and newborn guinea pigs

Abstract

There was a linear increase in fetal body weight from 27 days to full term. The thyroid gland of the fetal guinea pig is relatively large, follicles are present at 35 days following conception and progressively increase in number and size with increasing fetal age, colloid content increases proportionately. Thyroxine (T4) and Triiodothyronine (T3) are present in fetal plasma. Fetal plasma T4 concentration was greater than that of the mother between 50 and 62 days of gestation while fetal plasma T3 concentration was less than that of the mother throughout gestation.

Fetal adrenal gland activity increased towards the end of gestation. The plasma cortisol levels measured in pregnant guinea pigs after anaesthesia were higher than those in non-anaesthetised animals. In the fetuses this difference was only seen within 3 to 4 days of birth.

The pituitary in the fetus displayed signs of cytological differentiation at the beginning of the second half of intrauterine life. After 60 days until full term the pituitary gland showed a rapid increase in weight and an increase in the number and size of basophil and acidophil cells.

After birth neonatal plasma T4, T3 and cortisol levels increased rapidly to reach their highest values at about 24 hours. The rate of plasma T4 concentration decline in neonatal animals was 27.7 hours while the plasma T4 half-life in adult male guinea pigs was 23.4 hours.

Blocking the secretion of thyroid hormones by administering PTU to pregnant guinea pigs resulted in massive growth of the fetal thyroid and impairment of bone development. The maternal thyroid was little affected by the treatment. In these animals there was a clear decrease in fetal plasma T4, T3 and cortisol concentrations at 50 and 60 days of intrauterine life as well as in the plasma of one hour old guinea pigs.

Daily administration of dexamethasone (10 mg) from 55 days of pregnancy for 6 days resulted in a depression of fetal body and organ weight and a depression of maternal and fetal plasma T4, T3 and cortisol concentrations but an increase in the fetal plasma T3/T4 concentration ratio suggesting enhanced ß deiodination of T4.

Maternal thyroidectomy at 39 days of gestation resulted in a decrease in fetal plasma T4, T3 and maternal T4 concentrations when measured at 60 days of gestation. No changes were observed in maternal plasma T3 or cortisol or in fetal plasma cortisol. Large thyroid follicles and areas of papillary infolding of the columnar epithelium were seen in the thyroid glands of the fetuses from thyroidectomised mothers.

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