Hypothalamic neurosecretion and metamorphosis inXenopus laevis

Summary The frontal brain was removed from premetamorphic larvae ofXenopus laevis. Part of the animals was kept in a 0.01% propylthiouracil (PTU) solution for 21, 30 or 90 days, together with unoperated controls. Others were reared in tap water for the same length of time. The hypothalamus, the pituitary and the thyroids were studied in sagittal sections stained with aldehyde fuchsin or pseudoisocyanine (PIC).Operated tadpoles lacking the telencephalon only, could not be distinguished from unoperated controls. Extirpation of the preoptic region of the hypothalamus lowered the activity of the thyrotropic (TSH) cells and of the thyroids, and blocked metamorphosis. In animals with intact PIC-positive cells in the dorsal preoptic region, PTU induced a degranulation of these cells, stimulated the TSH cells and caused a strong hypertrophy of the thyroids. It is concluded that these neurosecretory cells in the dorsal preoptic region produce a thyrotropin-releasing factor (TRF), indispensable for the endocrine regulation of metamorphosis, and that the thyroid hormones have a negative feed-back influence upon the TRF cells.In part of the animals lacking the rostral hypothalamus, new PIC-positive cells were observed to develop immediately behind the preoptic region. In these animals PTU was somewhat less effective than in the animals with an intact dorsal preoptic region. This means either that the newly differentiated PIC-positive cells have a TRF function or that the thyroids can exert a direct negative feed-back upon the TSH cells. The latter possibility is supported by the fact that even in the absence of any neurosecretory cell PTU stimulated the thyroids.Blocking of thyroid hormone production by PTU inhibited the differentiation of the hypothalamus, of the median eminence and of the portal vessels. It may be deducted that during metamorphosis the general morphogenetic effect of the thyroid hormones stimulates the differentiation of the structures necessary for the augmentation of the TSH activity.The author thanks Prof. Dr.P.G.W.J. van Oordt for his active interest and helpful advices, MissTineke Aafjes for technical assistance and Mr.H. van Kooten for making the diagrams and photographs.     

Hypothalamic neurosecretion and metamorphosis in Xenopus laevis

Summary Tadpoles of Xenopus laevis were treated with propylthiouracil from the second half of prometamorphosis. Sagittal sections of the head region were stained a.o. with pseudoisocyanine. The goitrogen caused a degranulation of neurosecretory cells in the dorsal part of the preoptic region of the hypothalamus, suppressed the development of ventral neurosecretory cells and of the outer zone of the median eminence, stimulated the thyrotropic cells in the adenohypophysis, caused a hypertrophy of the thyroids, and impaired metamorphosis. Returning the animals to tap water had reciprocal effects and restored the normal activity of the hypothalamus, adenohypophysis and thyroid glands. It is concluded that thyroid hormones exert a morphogenetic influence upon hypothalamic centres and the outer zone of the median eminence and that a negative feed back relation exists between the thyroids on the one hand and the dorsal neurosecretory cells and the thyrotropic cells on the other.The author thanks Prof. Dr. P. G. W. J. van Oordt for his active interest and helpful advice, and Miss Tineke Aafjes for technical assistance.     

The thyrotropin secreting basophils in the adenohypophysis of Anoptichthys jordani

Summary In order to identify the TSH cells in the pituitary gland of Anoptichthys jordani, the development of the adenohypophysial cell types was studied in relation with the differentiation of the thyroids. In addition experiments were performed in young and adult animals with propylthiouracil (PTU) and thyroxine. At the time of the first thyroid development two meso-adenohypophysial cell types in the pituitary gland of Anoptichthys jordani are distinctly differentiated: orangeophilic cells and Alcian Blue (AB)-positive basophils. Because thyroxine and PTU could evoke changes in activity of the latter but had no effect on the orangeophils, it is concluded that the angular AB-positive basophils in the meso-adenohypophysis of Anoptichthys are responsible for the TSH production. PTU and thyroxine were unable to affect TSH cell activity in adult animals.This study was partly supported by a grant from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.).I wish to thank Prof. Dr. P. G. W. J. van Oordt for his constant interest and helpful suggestions. I am indebted also to Miss M. I. Willems and Miss Tjitske van Soelen, who both gave conscientious technical assistance and to Mr. E. van der Vlist for making the photographs.     

The identification of the thyrotropic cells in the adenohypophysis of the cichlid fish Cichlasoma biocellatum and the role of these cells and of the thyroid in osmoregulation

Summary In order to identify the thyrotropic cells in the pituitary gland of Cichlasoma biocellatum, adult animals were treated with thyroxine. Periodic acid Schiff (PAS)- and aldehyde fuchsin (AF)-positive, and Alcian blue (AB)-negative basophils in the meso-adenohypophysis as well as the thyroid were strongly inactivated by such treatment. Since other cell types in the adenohypophysis were not affected, it is concluded that the PAS-positive and AB-negative basophils in the meso-adenohypophysis are the thyrotrops. In normal animals the thyrotrops and the thyroid are very active. Propylthiouracil did not cause a further activation of TSH cells. Keeping the animals in 25% sea-water caused a strong inactivation of the prolactin cells, the thyrotrops and the thyroid. In deionized water the prolactin cells were stimulated, but the thyroid was less active than in controls. Injections of ovine prolactin did not enhance the activity of the thyroid in salt water animals. It is suggested that in Cichlasoma biocellatum prolactin does not have a direct stimulatory effect on the thyroid.Thanks are due to Prof. Dr. P. G. W. J. van Oordt for his continual interest and valuable suggestions, and to Dr. L. Boomgaart for correcting the english. We are indebted also to Messrs. H. van Kooten and E. van der Vlist for making the photographs. The National Institutes of Health, Bethesda, Maryland (U.S.A.) kindly presented the ovine prolactin.     

The reaction of the preoptic nucleus of Xenopus laevis tadpoles to osmotic stimulation

Summary The development of the preoptic nucleus of Xenopus laevis tadpoles during metamorphosis was studied and the effect of osmotic stimulation on this process investigated. The development of this region was not affected by treatment for one or more days in hypertonic media. It was found that at the end of metamorphosis the neurosecretory cells in the preoptic nucleus are localized in three regions: the rostro-dorsal, the caudo-dorsal and the ventral region. After osmotic stimulation only the neurosecretory cells of the caudo-dorsal region appeared to have reacted, as indicated by their loss of neurosecretory (PIC positive) material. It is concluded that the cells of this region may be involved in the synthesis of the posterior lobe hormones.The author thanks Prof. Dr. J. C. van de Kamer and Dr. F. C. G. van de Veerdonk for their interest and many helpful discussions, Dr. L. Boomgaart and Dr. A. P. van Overbeeke for correcting the English text and Miss C. M. G. van Bemmel for technical assistance.     

Hypophysiotropic Centers in the Brain of Amphibians and Fish

The subject is the localization of three different hypophysiotropiccenters in the brain of amphibians and fish. The thyrotropic hormone-releasing hormone (TRH) in Xenapus mayoriginate from the dorsal magno-cellular neurons of the preopticnucleus. This hypothesis is based on correlative changes betweenthese cells and alterations in thyroid activity during metamorphosis.Experimental data are in support of a functional relationshipbetween certain preoptic neurons and the thyrotropic activityof the pituitary. The MSH inhibiting activity of the hypothalamus is effectedby means of an aminergic innervation of the pars intermediain amphibians, teleosts and elasmobranchs. In amphibians theaminergic fibers originate from the caudal part of the paraventricularorgan (PVO); in elasmobranchs probably from the nucleus mediushypothalamicus(NMI); in teleosts the origin still has to beinvestigated. Two centers producing gonadotropic hormone-releasing hormone(GRH) have been demonstrated. Lesion experiments lead to thehypothesis that GRH is produced in the caudal hypothalamus,i.e., in the nucleus infundibularis ventralis of amphibiansand in the nucleus lateralis tuberis of fishes. ImmunoHuorescencestudies indicate in both groups the presence of neurons, infront of the preoptic area in the telencephalon, and these neuronsare immuno-reactive with anti-mammalian LH-RH.     

Comparative analysis of thyroxine distribution in ascidian larvae

The ascidian endostyle is a mucus-secreting pharyngeal organ, it has iodine-concentrating activity and the biosynthesis of thyroid hormones has been well documented. According to our recent findings, ascidians possess thyroid hormones, which are localized in mesenchymal cells. We have studied the presence and localization of l-thyroxine (T₄) in Ascidia malaca (Traustedt), Ascidiella aspersa (Müller), Phallusia mamillata (Cuvier) and Ciona intestinalis (Linnaeus) larvae and its involvement in metamorphosis. In vivo treatment of swimming larvae with exogenous T₄ and thiourea (a thyroid hormone synthesis inhibitor), demonstrate the presence of T4 during larval development. These results were confirmed by in vitro experiments utilizing dot blotting, radioimmunoassay and immunoperoxidase staining. The hormone was localized in mesenchymal cells of all four ascidians, spread out in the body cavity, under the adhesive papillae and around the intestine. The presence of TH in mesenchymal cells could be related to blood cells, musculature and heart tissue differentiation. The results suggest that this hormone could be involved in the control of metamorphosis.     

Xenopus laevis tadpole limb regeneration in vivo and in vitro: thyroxine directly promotes blastemal cell proliferation and morphogenesis

Regeneration in hindlimbs of Xenopus laevis larvae which were amputated at stage 53 and 55 through the tarsalia region is promoted by thyroxine (T4), while propyl-thiouracil (PTU) inhibits regeneration when compared to controls. In this paper, by in vivo and in vitro experiments, we demonstrate that the promoting effect of T4 on the regenerative processes of larval X. laevis hindlimbs is a direct effect of this hormone on the blastemal cells. By contrast, the inhibitory effect of PTU on the regenerative process is not due to a direct effect on blastemal cells or to a general toxic effect on the treated larvae, but is related to hypothyroidism induced by the drug. We find that: (i) an increase in blastemal cell proliferation is observed not only in blastemata of T4-treated larvae, but also in blastemata cultured in vitro in a medium supplemented with T4; (ii) the renegerative process is accelerated not only in larvae reared in T4 but also in larvae submitted to a combined treatment of T4 and PTU; (iii) inhibition of cell proliferation is observed in blastemata of PTU-reared larvae but not in blastemata cultured in vitro in a medium supplemented with PTU. Experiments on thyroidless larvae (which were submitted to transplantation of hindlimbs from larvae at stages 53 and 55 followed by amputation of their own right hindlimb and the transplanted limbs) have shown that without thyroid hormone the regenerative process is arrested at cone stage and the promoting effect of T4 treatment is dependent on limb stage and amputation level.     

Comparison of potential protective effects of melatonin, indole-3-propionic acid, and propylthiouracil against lipid peroxidation caused by potassium bromate in the thyroid gland

Potassium bromate (KBrO3) is a prooxidant and carcinogen, inducing thyroid tumors. Melatonin and indole-3-propionic acid (IPA) are effective antioxidants. Some antioxidative effects of propylthiouracil (PTU)--a thyrostatic drug--have been found. The aim of the study was to compare protective effects of melatonin, IPA, and PTU against lipid peroxidation in the thyroids, collected from rats treated with KBrO3, and in homogenates of porcine thyroids, incubated in the presence of KBrO3. Wistar rats were administered KBrO3 (110 mg/kg b.w., i.p., on the 10th day of the experiment) and/or melatonin, or IPA (0.0645 mmol/kg b.w., i.p., twice daily, for 10 days), or PTU (0.025% solution in drinking water, for 10 days). Homogenates of porcine thyroids were incubated for 30 min in the presence of KBrO3 (5 mM) plus one of the antioxidants: melatonin (0.01, 0.1, 0.5, 1.0, 5.0, 7.5 mM), or IPA (0.01, 0.1, 0.5, 1.0, 5.0, 7.5, 10.0 mM), or PTU (0.01, 0.1, 0.5, 1.0, 5.0, 7.5, 10.0 mM). The level of lipid peroxidation products (MDA + 4-HDA) was measured spectrophotometrically in thyroid homogenates. In vivo pretreatment with either melatonin or with IPA or with PTU decreased lipid peroxidation caused by KBrO3--injections in rat thyroid gland. Under in vitro conditions, PTU (5.0, 7.5, and 10.0 mM), but neither melatonin nor IPA, reduced KBrO3-related lipid peroxidation in the homogenates of porcine thyroids. In conclusion, melatonin and IPA may be of great value as protective agents under conditions of exposure to KBrO3.     

The role of the thyroid in the development of Mauthner's neuron

Summary Although the normal postmetamorphic atrophy of the giant neurons of Mauthner in anuran amphibians has been attributed to the direct or indirect action of thyroid hormone, the administration of exogenous thyroxin to Rana pipiens larvae appears to facilitate growth of the neuron rather than inhibit it. In the present investigation, Rana pipiens embryos were surgically thyroidectomized. In the absence of the thyroid, metamorphosis never occurred, and hind limb development was arrested at larval stages VI–VIII. However, these animals continued their growth and often exceed their controls in overall size. Mauthner cells of thyroidectomized larvae have nuclei of significantly smaller size than their controls. The observations are consonant with the view that thyroid hormones play an important positive role in the growth and maintenance of Mauthner's cell. It is further indicated that the growth of Mauthner's neuron is more dependent upon the titer of thyroid hormone in the blood than it is upon age, brain size, tail motor influences, or the state of the lateral line system.Dedicated to Professor Berta Scharrer on her 60th birthday. This study was supported by Grants NB-04555, IGS-98 and 5 T 1-GM-102 from the U.S.P.H.S.I am pleased to acknowledge the excellent technical assistance of Mrs. Celia Grubman.     

Galanin: presence and distribution in the brain and pituitary of Rhinella arenarum (Amphibia: Anura) during development

The immunohistochemical distribution of galanin (Gal) in the brain and pituitary of Rhinella arenarum was studied during development. Gal-immunoreactivity was first observed in the brain just after hatching in anterior preoptic area, infundibular area, median eminence and pars distalis of the pituitary as well as in the olfactory epithelium. At the beginning of prometamorphosis new Gal-immunoreactive (ir) cells were observed in the olfactory nerve and bulb. Later in prometamorphosis new Gal-ir cells were observed in the telencephalon, suprachiasmatic nucleus, rostral rhombencephalon and in the pars nervosa of the pituitary. The most numerous accumulations of Gal-ir neurons throughout the larval development were observed in the ventral hyphothalamus where numerous Gal-ir cells of cerebrospinal fluid-contacting type were found. During metamorphic climax and soon after we did not detect Gal-ir neurons in the pallium, medial or pretectal dorsal thalamus. In the median eminence and pars distalis of the pituitary many Gal-ir fibers were found during development indicating that Gal may play a role in the modulation of hypophyseal secretion. Furthermore, the distribution of Gal-ir elements observed throughout larvae development indicates that galaninergic system maturation continues until sexual maturity.     

T3 regulates E-cadherin,and β- and α-catenin expression in the stomach during the metamorphosis of the toad Rhinella arenarum

Abstract

The metamorphosis of Rhinella arenarum was induced precociously for 5 days, then blocked for 3 months to evaluate the role of thyroid hormones as modulators of morphoregulatory molecules such as E-cadherin, and β- and α-catenin during epithelium remodeling. We then performed an immunohistochemical and morphometric study of these molecules in the larval stomach. We show that 3,5,3′-triiodothyronine exerts a positive regulatory effect on E-cadherin and β- and α-catenin expression in stomach epithelium. This suggests continuous synthesis of E-cadherin, and β- and α-catenin; synthesis essentially is thyroid hormone-independent during premetamorphosis and early prometamorphosis, but it becomes thyroid hormone-dependent during metamorphic climax.     

Fine structure of the Paraventricular organ of Xenopus laevis tadpoles

Summary The ultrastructure of the Paraventricular organ in the hypothalamus of Xenopus laevis tadpoles is described. It appeares that the Paraventricular organ of this anuran species is homologous with the Organon vasculosum hypothalami or the Paraventricular organ of other vertebrates.The Paraventricular organ of Xenopus laevis is characterized by an ependymal lining with only few cilia and by two types of nerve cells. Both types of nerve cells have ventricular processes, protruding into the lumen of the third ventricle and forming a network. The protrusions bear cilia of the 8+1 pattern. It has been possible to distinguish both types of nerve cells on account of their dense-core vesicles. A secretory function of both cell types is suggested.In a region close to the Paraventricular organ, another granulated type of nerve cell has been observed. A relationship between these cells and the preoptic nucleus is discussed.The author thanks Prof. Dr. P. G. W. J. van Oordt for his helpful comments and criticism, Mr. H. van Kooten for photographic assistance and Mr. F. Dijk for technical assistance.     

Differential Transmembrane Diffusion of Triiodothyronine and Thyroxine in Liposomes: Regulation by Lipid Composition

How thyroid hormones move across biological or model membranes is a subject of controversy. The passage of the 3,5,3′triiodo l-thyronine and 3,5,3′,5′ tetraiodo l-thyronine across model membranes was evaluated by the addition of the hormones to liposomes containing 2,4,6-trinitrobenzene sulfonic acid. Results indicate that hormones can react with an amino-reactive compound pre-encapsulated into phosphatidylcholine liposomes. The transversal motions of thyroid hormones were characterized by using physiological concentration levels of (¹²⁵I) 3,5,3′triiodo l-thyronine and (¹²⁵I) 3,5,3′,5′ tetraiodo l-thyronine. The hormone distribution between the two monolayers was time-dependent and kinetic data were fitted to a single exponential. Results obtained show that 3,5,3′ triiodo l-thyronine can permeate phospholipid membranes and the diffusion time increases in the gel and liquid-ordered phase. On the contrary, 3,5,3′, 5′ tetraiodo l-thyronine could not diffuse the liposomal membrane from dimyristoyl and dipalmitoyl phosphatidylcholine in gel phase and egg yolk phosphatidylcholine:cholesterol in the liquid-ordered phase. Our results in the liquid-ordered phase suggest that diffusion movement of thyroid hormones across cell membranes depends on the amount of cholesterol in the bilayer. Received: 1 June 1998/Revised: 14 October 1998     

The influence of phenylthiourea on encapsulation,melanization, and survival in larvae of the mosquito Aedes aegypti parasitized by the nematode Neoaplectana carpocapsae

Aedes aegypti larvae, treated with phenylthiourea (PTU) and untreated, were exposed to concentrations of 1000–4000 Neoaplectana carpocapsae juveniles/ml for 1 hr. During the first 9 hr after exposure, the PTU-treated larvae had a significantly lower mortality than the untreated larvae. The rate of encapsulation or melanization of N. carpocapsae juveniles within the hemocoels of the mosquito larvae was investigated by gross dissections and histological techniques. The amount of melanization occurring in the PTU-treated larvae was found to be significantly reduced. Possible explanations were given for the observed differences in mortality. One possibility was that PTU may inactivate the phenol-oxidase system in mosquito larvae, thereby inhibiting the formation of toxic melanin intermediates. Another possibility is that PTU may inactivate a toxin emitted by the nematode or in some way have an effect on its associated bacterium, Achromobacter nematophilus.     

Leptin,NPY, Melatonin and Zinc Levels in Experimental Hypothyroidism and Hyperthyroidism: The Relation to Zinc

Since zinc mediates the effects of many hormones or is found in the structure of numerous hormone receptors, zinc deficiency leads to various functional impairments in the hormone balance. And also thyroid hormones have important activity on metabolism and feeding. NPY and leptin are affective on food intake and regulation of appetite. The present study is conducted to determine how zinc supplementation and deficiency affect thyroid hormones (free and total T3 and T4), melatonin, leptin, and NPY levels in thyroid dysfunction in rats. The experiment groups in the study were formed as follows: Control (C); Hypothyroidism (PTU); Hypothyroidism+Zinc (PTU+Zn); Hypothyroidism+Zinc deficient; Hyperthyroidism (H); Hyperthyroidism+Zinc (H+Zn); and Hyperthyroidism+Zinc deficient. Thyroid hormone parameters (FT₃, FT₄, TT₃, and TT₄) were found to be reduced in hypothyroidism groups and elevated in the hyperthyroidism groups. Melatonin values increased in hyperthyroidism and decreased in hypothyroidism. Leptin and NPY levels both increased in hypo- and hyperthyroidism. Zinc levels, on the other hand, decreased in hypothyroidism and increased in hyperthyroidism. Zinc supplementation, particularly when thyroid function is impaired, has been demonstrated to markedly prevent these changes.     

Protective effect of an antithyroid compound against γ-radiation-induced damage in human colon cancer cells

We have previously reported the radioprotective effect of propylthiouracil (PTU) on thyroid cells. The aim of the present study was to analyze whether tumor cells and normal cells demonstrate the same response to PTU. Human colon carcinoma cells were irradiated with γ-irradiation with or without PTU. We evaluated the clonogenic survival, intracellular reactive oxygen species levels, catalase, superoxide dismutase and glutathione peroxidase activities, and apoptosis by nuclear cell morphology and caspase-3 activity assays. Cyclic AMP (cAMP) levels were measured by radioimmunoassay. PTU treatment increased surviving cell fraction at 2 Gy (SF2) from 56.9 ± 3.6 in controls to 75.0 ± 3.5 (p < 0.05) and diminished radiation-induced apoptosis. In addition, we observed that the level of antioxidant enzymes’ activity was increased in cells treated with PTU. Moreover, pretreatment with PTU increased intracellular levels of cAMP. Forskolin (p < 0.01) and dibutyryl cAMP (p < 0.05) mimicked the effect of PTU on SF2. Co-treatment with H89, an inhibitor of protein kinase A, abolished the radioprotective effect of PTU. PTU reduces the toxicity of ionizing radiation by increasing cAMP levels and also possibly through a reduction in apoptosis levels and in radiation-induced oxidative stress damage. We therefore conclude that PTU protects both normal and cancer cells during exposure to radiation in conditions mimicking the radiotherapy.     

Role of Amino Acid-catabolizing Enzymes in Urea Synthesis for Rats Treated with the Thyroid Hormone

The purpose of the present study was to determine whether the regulation of urea synthesis was mediated through the supply of nitrogen by amino acid-catabolizing enzymes and whether the concentration of acetylCoA would control the N-acetylglutamate concentration when the thyroid status was manipulated. Experiments were conducted on three groups of rats, each being given 6-propyl-2-thiouracil (PTU, thyroid inhibitor) without a triiodothyronine (T₃) treatment, or PTU + T₃, or neither PTU nor T₃ (control), respectively. The urinary excretion of urea, the liver concentration of N-acetylglutamate, and the hepatic activities of serine dehydratase, threonine dehydratase, alanine transaminase (GPT) and aspartate transaminase (GOT) in rats given PTU + T₃ were significantly lower than those in rats given PTU alone. The activity of glutamate dehydrogenase and the concentrations of free amino acids and acetylCoA in the liver of the PTU + T3-treated group were significantly higher than those in the group treated with PTU alone. These results suggest that the higher activity of amino acid-catabolizing enzymes in the hypothyroid (with PTU alone) rats is likely to stimulate urea synthesis.     

Solid cell nests of the human thyroid in early stages of postnatal life. Systematic autopsy study

The anatomical position and prevalence of solid cell nests (SCN) of the thyroid in early stages of postnatal life have not yet been clearly determined. In order to find out about these unsettled questions a systematic search of these ultimobranchial nests from 92 autopsied thyroids from neonates, children and young adults was undertaken. SCN were present in 61% of the patients; they were mainly located in the middle third of the lateral thyroid lobes, and placed along a central to paracentral and slightly dorsal longitudinal axis. These findings, as compared with our previous observations made in older adult thyroids, further demonstrate that there exist a constant anatomical position and prevalence of SCN in postnatal life. The significantly higher frequency of SCN in males (68%) than in females (48%) (p less than 0.01) found in a study that was further extended to 192 thyroids at early and late stages of postnatal life, is a question that requires further investigation.     

THYROID PROLIFERATIVE POTENTIAL AS A FUNCTION OF AGE

The effect of a goitrogenic stimulus on thyroid weight and thyroid cell ³HTdR labeling of Sprague-Dawley rats varying from 2 to 40 weeks of age was determined. Propylthiouracil ad libitum in drinking water produced a spurt in follicle cell labeling index and thyroid weight evident after 24 hr for all age groups. The increase in labeling index reached a peak at 5–7 days and then decreased to a level a few times greater than that of the normal unstimulated thyroid. The tritiated thymidine labeling index for thyroid follicle cells and the effect of PTU thereon was determined for August male rats of 3 days to 12 weeks of age. In the older rats, the follicle cell labeling index rose to 5–6% after 4–5 days of PTU treatment and then slowly fell to about 1%, For the unstimulated control rat of comparable age, the labeling index was about 0.1%. At all ages the thyroid showed a rapid response to PTU. Examination of the time sequence of mitotic labeling showed that the DNA synthesis period was 7.5 hr for normal 2-week-old rats and for 10–12-week-old rats that had received PTU for 4 days. There was no second wave of labeled mitoses in either group during the 48-hr interval studied. From the curve of thyroid weight vs time on PTU and from the labeled mitoses curve, inferences regarding the minimum fraction of proliferating follicle cells in the stimulated ‘adult’rat thyroid were made.