Morphological development of the thyroid gland and serum T4-concentration in the intact and decapitated pig fetus.
The morphological and functional development of the fetal pig thyroid gland between 50 and 110 days post coitum have been examined in the normal pig fetus and after fetal decapitation at 42 days postcoitum. Body length and body weight developed at the same rate, comparing control and decapitated animals. Thyroid gland weight increased between 50 days and 110 days from 1.3 +/- 0.5 mg (SD) to 130.0 +/- 35.0 mg in control fetuses and from 0.9 +/- 0.2 mg to 113.4 +/- 23.0 mg in decapitated fetuses. A number of histomorphometrical parameters in thyroid tissue were measured. No significant differences in follicular epithelial height were observed between decapitated and control animals at 75 and 110 days. In control animals follicles increased both in size and number. In decapitated animals the follicles increased strikingly in number, but only slightly in size. Although colloid was formed in glands of decapitated animals, it was much less than in control fetuses. The gland of decapitated animals of 110 days resembled histologically the gland of much younger control animals (60 to 75 days). Gland development after fetal decapitation at 42 days may represent autonomous development when Thyroid Stimulating Hormone (TSH) is depleted. Serum thyroxine concentration (T4) was determined by radioimmunoassay and increased in control animals from 0.06 +/- 0.01 micrograms/100 ml to 4.18 +/- 0.87 micrograms/100 ml at 110 days, the greatest rate of increase being observed between 64 and 90 days. In decapitated fetuses serum T4 remained very low, namely less than 0.20 micrograms/100 ml. It is very unlikely that any significant transfer of T4 from mother to fetus or from one fetus to another occurred. Both the rise in serum T4 and the enlargement of the follicles may be TSH dependent events in fetal pig thyroid gland development, whereby the sudden rise is serum T4 precedes the greatest rate of increase in follicle size.
D000818:Animals; D005260:Female; D005865:Gestational Age; D009929:Organ Size; D011247:Pregnancy; D011863:Radioimmunoassay; D013552:Swine / Q000502:physiology*; D013961:Thyroid Gland / Q000033:anatomy & histology / Q000196:embryology*; D013972:Thyrotropin / Q000502:physiology*; D013974:Thyroxine / Q000097:blood*
D013972:Thyrotropin; D013974:Thyroxine