Identification of C-cells in normal and goitrous rat thyroid tissues using antiserum to rat thyrocalcitonin and the immunoperoxidase bridge technique

Follicular cell lineage in persistent ultimobranchial remnants of mammals

It has been a subject of much debate whether thyroid follicular cells originate from the ultimobranchial body, in addition to median thyroid primordium. Ultimobranchial remnants are detected in normal dogs, rats, mice, cattle, bison and humans and also in mutant mice such as Eya1 homozygotes, Hox3 paralogs homozygotes, Nkx2.1 heterozygotes and FRS2α^2F/2F. Besides C cells, follicular cell lineages immunoreactive for thyroglobulin are located within these ultimobranchial remnants. In dogs, the C cell complexes, i.e., large cell clusters consisting of C cells and undifferentiated cells, are present together with parathyroid IV and thymus IV in or close to the thyroid lobe. In addition, follicular cells in various stages of differentiation, including follicular cell groups and primitive and minute follicles storing colloid, are intermingled with C cells in some complexes. This review elaborates the transcription factors and signaling molecules involved in folliculogenesis and it is supposed why the follicular cells in the ultimobranchial remnants are sustained in immature stages. Pax8, a transcription factor crucial for the development of follicular cells, is expressed in the fourth pharyngeal pouch and the ultimobranchial body in human embryos. Pax8 expression is also detected in the ultimobranchial remnants of Eya1 and Hes1 null mutant mice. To determine whether the C cells and follicular cells in the ultimobranchial remnants consist of dual lineage cells or are derived from the common precursor, the changes of undifferentiated cells in dog C cell complexes are examined after chronically induced hypercalcemia or antithyroid drug treatment.

C cells evolve at the same rhythm as follicular cells when thyroidal status changes in rats

C cells are primarily known for producing calcitonin, a hypocalcemic and hypophosphatemic hormone. Nevertheless, besides their role in calcium homeostasis, C cells may be involved in the intrathyroidal regulation of follicular cells, suggesting a possible interrelationship between the two endocrine populations. If this premise is true, massive changes induced by different agents in the activity of follicular cells may also affect calcitonin-producing cells. To investigate the behaviour of C cells in those circumstances, we have experimentally induced two opposite functional thyroid states. We hyperstimulated the follicular cells using a goitrogen (propylthiouracil), and we suppressed thyroid hormone synthesis by oral administration of thyroxine. In both scenarios, we measured T(4), TSH, calcitonin, and calcium serum levels. We also completely sectioned the thyroid gland, specifically immunostained the C cells, and rigorously quantified this endocrine population. In hypothyroid rats, not only follicular cells but also C cells displayed hyperplastic and hypertrophic changes as well as increased calcitonin levels. When exogenous thyroxine was administered to the rats, the opposite effect was noted as a decrease in the number and size of C cells, as well as decreased calcitonin levels. Additionally, we noted that the two cell types maintain the same numerical relation (10 +/- 2.5 follicular cells per C cell), independent of the functional activity of the thyroid gland. Considering that TSH serum levels are increased in hypothyroid rats and decreased in thyroxine-treated rats, we discuss the potential involvement of thyrotropin in the observed results.

Effect of follicular cells on calcitonin gene expression in thyroid parafollicular cells in cell culture

Expression of calcitonin (CT) gene in thyroid parafollicular cells involves alternate formation of CT mRNA or CGRP mRNA. High amounts of CT mRNA are formed only in thyroid gland and formation of CGRP mRNA dominates in the remaining organs. Apart from paracrine and endocrine factors, mRNA formation on the CT gene seems to be affected also by direct contacts with other cells present in the thyroid gland, in which parafollicular cells are located next to follicular cells. The present study aimed at examining whether thyroid follicular cells affect formation of mRNAs for CT and CGRP in parafollicular cells. The studies were performed in cell cultures. A parafollicular cell line (TT cells) and a follicular cell line (F6BTY cells) served as the experimental model. For comparison, co-cultures with fibroblasts, 3T3 cells, and malignant melanoma, MM cells, were also examined. CT gene expression was examined at the level of mRNAs (in situ hybridization and morphometric studies) and at the level of hormones (immunocytochemistry, morphometric studies and radioimmunological estimation of hormone levels in the medium). The immunocytochemical and hybridocytochemical studies, in line with morphometry studies, demonstrated that F6BTY and 3T3 cells were capable of affecting mRNA production for CT and CGRP and that they changed the ratio of CGRP/CT secretion by TT cells, as a sequel of contact between the two cell types and due to mediation of secreted substances. On the other hand, the malignant melanoma MM cells showed no effect on the secretion ratio. Our study seems to indicate that control of mRNA formation from CT gene may involve not only humoral factors but also direct contacts with other cells, which may explain differences in expression of the gene between cells localized in different organs.

Physiologic versus neoplastic C-cell hyperplasia of the thyroid: separation of distinct histologic and biologic entities

BACKGROUND

Although hyperplasia of C-cells has been described in association with various pathologic and physiologic conditions, criteria for its diagnosis are poorly defined. Both neoplastic and physiologic C-cell proliferations have been lumped together under the umbrella designation of C-cell hyperplasia (CCH), creating considerable confusion among clinicians and pathologists.

METHODS

in order to compare the morphologic and immunohistochemical characteristics of the two major types of CCH, we examined thyroid sections of 17 patients with familial forms of C-cell hyperplasia and/or neoplasia and tissue sections of 19 thyroid glands known to have reactive or physiologic CCH (at least 50 C-cells per one low power field, 100X). Hematoxylin and eosin (H & E) stained sections and immunohistochemical stains for calcitonin were assessed in each case.

RESULTS

Physiologic or reactive CCH was not recognized with certainty on H & E stains in any of the cases due to morphologic similarities between C-cells and adjacent follicular cells. Detection of this form of hyperplasia, which was predominantly diffuse, required calcitonin immunostains and quantitative analysis. Conversely, nodular and diffuse neoplastic CCH was easily identified with conventional H & E stains at the periphery of 11/12 (92%) familial medullary thyroid carcinomas (MTC). In the other five cases, neoplastic C-cell hyperplasia was the only pathologic finding on thyroidectomy performed for elevated serum calcitonin levels detected via provocative biochemical screening or identification of the mutated RET proto-oncogene by genetic analysis. The C-cells in this neoplastic form of CCH were large, mildly to moderately atypical, and confined within the basement membrane of thyroid follicles. Moreover, these cells were cytologically indistinguishable from those of invasive MTC cells.

CONCLUSIONS

Physiologic and neoplastic CCH are biologically and morphologically distinct entities. The former cannot be recognized with certainty with conventional stains and requires immunohistochemistry and quantitative analysis for diagnosis. The latter consists of mildly to moderately atypical C-cells that can be identified with H & E stained sections. Consequently, the number of C-cells is of no importance for the diagnosis of neoplastic CCH which is considered to be the precursor (medullary carcinoma in situ) of invasive medullary carcinoma.

Quantitative changes in the frequency and distribution of the C-cell population in the rat thyroid gland with age

The development of calcitonin cells (C-cells) was investigated in rat thyroid glands from birth to 120 days, using an immunoperoxidase technique and a point-counting method. The proportion of C-cells to follicular cells was 4.5% on the day of birth and increased progressively to 10.4% by 120 days. The highest density of C-cells was noted in the mid-region of the lobes along a longitudinal axis. The caudal and cephalic regions of the lobes contained smaller numbers of C-cells. The C-cells tended to be more numerous in the posterior aspects of the lobes. Although the numbers of C-cells in 120-day-old animal were markedly increased as compared to animals at the time of birth, the cell distributions within the glands were similar at all ages.

C-cell hyperplasia in thyroid tissue adjacent to follicular cell tumors

An immunohistochemical study was conducted on the number and distribution of C-cells in the nonneoplastic thyroid tissue adjacent to tumors of follicular cell origin. It consisted of 49 cases, of which 25 were papillary carcinomas, 22 were follicular adenomas, and 2 were follicular carcinomas. Twenty normal adult thyroids from the Broward's Medical Examiner's morgue served as controls. In 17 of the 49 cases (34.6%), there was a statistically significant increase in the number of C-cells in the normal-appearing thyroid tissue adjacent to follicular cell tumors, with at least 50 C-cells in one low power field, while only one of 20 normal thyroids had a similar number of cells. (P = .02; chi 2 = 5.05). In two tumor cases there were more than 100 C-cells in several low power fields with formation of small C-cell nodules similar to those described in the type II Multiple Endocrine Neoplasia Syndrome (MEN). It was concluded that the nonneoplastic thyroid tissue adjacent to 34.6% of tumors with follicular cell phenotypes contains significantly more C-cells than those present in normal adult thyroids. The possible pathogenesis and clinical significance of these findings are discussed.

Evidence of C-cell destruction in the thyroid gland of mice exposed to high 131I doses

The parafollicular cells of the thyroid gland were visualized by means of the Sevier-Munger silver technique in normal mice and in mice receiving 131I in amounts sufficient to completely destroy the thyroid tissue. The destruction of the C-cells was observed in all 131I injected mice, and no histologic signs of recovery were seen during a period of 40 days following the treatment.

Calcitonin cell population and distribution in the thyroid gland of the rat

Calcitonin-containing cells (C cells) were identified in male Wistar white rats using an immunoperoxidase technique. They occupied a central position within the thyroid; very few were found peripherally, inferiorly, and superiorly; and none were present in the isthmus. The number of calcitonin-containing cells present per gram of body weight increased with age up to 70 days and had declined by 100 days. Determining the true total C-cell count through the entire thyroid is a very laborious procedure. However, a simple estimate of this total count can be made; the total number of C cells in every tenth section (6 microns) of thyroid was found to be highly correlated with the weight of the animal expressed as an allometric function. A better estimate can be derived from counts of just three sections: the tenth, twentieth, and thirtieth after the section of greatest cross sectional area.

Alterations of immunoreactive somatostatin in thyroid C cells after induced hypercalcemia, hypocalcemia, and antithyroid drug treatment

In order to elucidate the functional significance of somatostatin in thyroid C cells, the alterations of immunoreactive somatostatin in the cells were investigated under various experimental conditions, i.e., hypercalcemia, hypocalcemia, and antithyroid drug treatment. Guinea pigs and rabbits, in which almost all C cells reveal the intense immunoreaction for somatostatin in addition to calcitonin, were used as experimental animals. After chronically induced hypercalcemia, somatostatin immunoreactivity conspicuously diminished coinciding with the decrease of calcitonin; somatostatin as well as calcitonin was responsive to induced hypercalcemia. After hypocalcemic tetany induced by injection of Escherichia coli L-asparaginase, C cells exhibited very intense immunoreactions for both calcitonin and somatostatin. After chronic treatment of ethylenethiourea, immunoreaction of somatostatin in C cells was the same as that of calcitonin. That is, when immunoreactivity for calcitonin remained unchanged, immunoreactivity for somatostatin was also intensive. However, when immunoreaction of calcitonin became very weak, the reaction of somatostatin was also weak. Thus, in all experimental conditions examined the alterations of immunoreactive somatostatin in C cells completely coincided with those of calcitonin. It seems likely that somatostatin in thyroid C cells exerts the synergistic effect on calcitonin action.

Pituitary immunoreactive calcitonin-like material: lack of evidence for cross-reactivity with pro-opiomelanocortin

In mammals, calcitonin (CT) is synthesized, stored, and secreted by intrathyroidal C cells. Several reports have suggested the presence of immunoreactive CT in the pituitary gland. We have studied the rat pituitary gland using a radioimmunoassay for CT and have also found immunoreactive CT-like material. Assay of extracts of whole rat pituitary glands was performed using a radioimmunoassay for human CT, which gave identical dilution curves with synthetic human CT (hCT), synthetic rat CT (rCT), and mouse and rat thyroid extracts, but not with a variety of other pituitary and hypothalamic peptides. Immunoreactive CT (iCT) content of extracts of whole pituitary glands ranged from 6 to 72 pg/mg wet weight of tissue (60-840 pg/whole pituitary gland), whereas iCT was not measureable (less than 5 pg/mg tissue) in similar extracts of hypothalamus and cerebral cortex. Gel filtration studies of pituitary extracts showed a peak of iCT, which eluted with 125I-rCT and diluted in parallel with rCT. To investigate whether the pituitary iCT was related to pro-opiomelanocortin, extracts of ACTH-producing AtT20/D16 cells from mice, which contain the ACTH precursor in large quantities, were examined and no iCT was found. Immunohistochemical studies of rat pituitary glands with peroxidase-antiperoxidase and immunofluorescent techniques showed positive staining for CT in cells in the pars anterior, but not in the pars intermedia of pars nervosa; this staining was not eliminated when the antiserum was absorbed with CT under conditions that completely obliterated staining of rat thyroid glands. Double staining demonstrated essentially two distinct populations of cells, one positive for CT and another positive for ACTH, with less than 1% of the cells positive for both ACTH and CT. Immunoreactive CT-like material was present in the pituitary glands of rats thyroparathyroidectomized 18 days before they were killed, but was diminished. Biosynthetic labeling in vitro of rat pituitary glands with 3H-leucine showed incorporation into prolactin; there was no incorporation into CT. No in vitro secretion of iCT by whole rat pituitary glands either basally or after high K+ stimulation was observed. We conclude that: (1) a substance that has certain immunologic and size characteristics of CT is present in minute amounts in the pituitary gland of rats; (2) this material is not a part of the ACTH precursor; and (3) positive immunohistochemical staining in pituitary glands may not be specific for authentic CT.

Serum calcitonin, calcium and thyroxine in young and old Zucker fatty rats (fa/fa)

We determined the serum levels of calcitonin (CT), calcium (Ca), and thyroxine (Ti) in lean (?/+) and fatty (fa/fa) male Zucker rats 10 weeks and 10-12 months of age. The most dramatic finding was a high level of serum CT (3.24 +/- 1.18 ng/ml) in young fatties whereas sera from young leans were all below the limit of assay detection (less than 0.120 ng/ml, p less than 0.01). Young fat rats also had elevated levels of both Ca (11.2 +/- 0.2 vs. 9.7 +/- 0.2 mg/dl, p less than 0.001) and Ti (6.7 +/- 0.48 vs. 4.72 +/- 0.28 micrograms/dl, p less than 0.01). In older animals the mean serum level of CT increased further in the fatties and became readily measurable in leans (5.67 +/- 1.94 vs. 1.49 +/- 0.55, p less than 0.01). Thyroid C-cells, identified immunohistochemically, were abundant in both leans and fatties at this age but were substantially more numerous in the fat rats (p less than 0.001). Calcium levels increased somewhat in the older leans, but still remained higher in the fat rats (p less than 0.05). Thyroxine values were essentially the same for old animals of both genotypes (5.07 +/- 0.61 vs. 5.54 +/- 0.88). Age effects were not significant for any measure in the fat animals, but in the leans there were significant age-related increases in CT (p less than 0.02) and serum Ca (p less than 0.05).

Immunocytochemical localization of thyroglobulin in the endostyle of the anadromous sea lamprey, Petromyzon marinus L

Thyroglobulin (TG) was localized in the endostyle of the anadromous sea lamprey, Petromyzon marinus L. by means of the unlabeled antibody peroxidase-antiperoxidase immunocytochemical method. TG was found localized on the apical surface and within the cytoplasm of type 2c and 3 cells and in some type 5 cells. By identifying the cells of the endostyle immunocytochemically it may be possible to study more readily the events of endostylar transformation during metamorphosis.

Light and electron microscopic immunocytochemical localization of thyroglobulin in the thyroid gland of the anadromous sea lamprey, Petromyzon marinus L., during its upstream migration

Antibodies made against thyroglobulin (TG) were used in an immunocytochemical study for the light and electron microscopic localization of TG in the thyroid gland of the anadromous sea lamprey, Petromyzon marinus, during its upstream migration. TG was found in the follicular lumen and in some colloid droplets within the follicular cells. Except for an immunoreactive product observed in a small portion of the interstitial connective tissue, the location of TG in the lamprey was similar to that in the thyroid of the rat.