Cellular and molecular events on the development of mammalian thyroid C cells

Advances and challenges in thyroid cancer: The interplay of genetic modulators, targeted therapies, and AI-driven approaches

Thyroid cancer continues to exhibit a rising incidence globally, predominantly affecting women. Despite stable mortality rates, the unique characteristics of thyroid carcinoma warrant a distinct approach. Differentiated thyroid cancer, comprising most cases, is effectively managed through standard treatments such as thyroidectomy and radioiodine therapy. However, rarer variants, including anaplastic thyroid carcinoma, necessitate specialized interventions, often employing targeted therapies. Although these drugs focus on symptom management, they are not curative. This review delves into the fundamental modulators of thyroid cancers, encompassing genetic, epigenetic, and non-coding RNA factors while exploring their intricate interplay and influence. Epigenetic modifications directly affect the expression of causal genes, while long non-coding RNAs impact the function and expression of micro-RNAs, culminating in tumorigenesis. Additionally, this article provides a concise overview of the advantages and disadvantages associated with pharmacological and non-pharmacological therapeutic interventions in thyroid cancer. Furthermore, with technological advancements, integrating modern software and computing into healthcare and medical practices has become increasingly prevalent. Artificial intelligence and machine learning techniques hold the potential to predict treatment outcomes, analyze data, and develop personalized therapeutic approaches catering to patient specificity. In thyroid cancer, cutting-edge machine learning and deep learning technologies analyze factors such as ultrasonography results for tumor textures and biopsy samples from fine needle aspirations, paving the way for a more accurate and effective therapeutic landscape in the near future.

C-cell differentiation in the wall of an aberrant ultimobranchial sinus in the thyroid gland of an old rat

BACKGROUND

In mammals, the thyroid gland possesses two types of endocrine cells, follicular cells and C cells, which have different functions but share a similar endodermal origin (although from different regions of the primitive pharynx). Specifically, follicular cells derive from the ventral pharyngeal floor, while C cells derive from the fourth pair of pharyngeal pouches through the ultimobranchial bodies (UBBs). Disruptions to human midline thyroid morphogenesis are relatively frequent and known as thyroid dysgenesis, which is the leading cause of congenital hypothyroidism. In contrast, fourth branchial apparatus anomalies are very rare clinical entities.

OBJECTIVES

The aim of this study was to analyze the morphological features and the immunohistochemical pattern of an aberrant ultimobranchial remnant, align with its persistent contribution to the formation of new C cells.

METHODS

The thyroid gland of an old rat was serially sectioned and immunostained for the following markers: calcitonin, thyroglobulin, cytokeratins, PCNA, P63_, E-cadherin, beta-tubulin and CD3.

RESULTS

We detected a spontaneous congenital defect in the organogenesis of the UBB in an old rat, giving rise to an 'ultimobranchial sinus', which was accompanied by thymic tissue and an abscess. The epithelium contained basal/stem cells and contributed to the formation of abundant C cells and scarce follicular cells.

CONCLUSIONS

The ultimobranchial sinus is an exceptional finding for representing the first spontaneous abnormality in the development of UBB reported in rats, and the opportunity to observe sustained C-cell differentiation from stem cells in an old rat. These findings are consistent with a common origin of both C cells and follicular cells from UBB.

Arterial blood supply of the internal neck organs: anatomy, topography, clinical significance in endocrine surgery

BACKGROUND. Current trends of «fast track surgery» give rise to development of new safe techniques of the thyroid and parathyroid surgery, the purpose of which is to minimize the level of postoperative complications, such as vocal cord palsy, hypoparathyroidism, bleeding. In this regard, it is important for the endocrine surgeon to save «dry operating field», which contributes to the clear visualization of such «thin» structures as the recurrent laryngeal nerve, the external branch of the superior laryngeal nerve and the parathyroid glands. Therefore, the key issue of this surgery is to understand the anatomical and topographic features of the blood supply to the internal neck organs (a complex of neck organs consisting of the thyroid and parathyroid glands, larynx, trachea, esophagus).

AIM. To determine the main sources of arterial blood supply of the internal neck organs and their anatomical, topographic features.

MATERIALS AND METHODS. The computed tomography protocols of cervical branches of brachiocephalic arteries were analysed in the study. The thickness of the reconstructed sections in the axial, frontal and sagittal planes was 0.35±0.05 mm. The fact of blood supply was confirmed by the anatomical close of the arterial structure to the internal organ and the presence of intramural arterial branches.

RESULTS. The course of all cervical branches of the subclavian and common carotid artery was traced among 42 patients. It is noted, that only the inferior, superior thyroid arteries and thyroid ima artery supply internal neck organs with the blood. At the same time, the superior thyroid artery was visualized in all angiograms. However, the inferior thyroid artery was absent in 2.4% of cases. The thyroid ima artery was rarely detected (in 4.8% of patients). In 73.2% of cases, the inferior thyroid artery was detected high at the upper third level of the thyroid lobe and then had a descending course. In 23.2% of cases, the artery was formed at the middle third level of the thyroid lobe and was directed horizontally to the gland. Only in 3.6% of cases, the ascending course was determined in the vessel. The inferior thyroid artery was located on the posterior surface of the thyroid lobe, where it formed glandular branches. On the contrary, the branches of the superior thyroid artery were located mainly along the anterolateral surface of the thyroid gland. The average thickness of the inferior thyroid artery was 2.1±0.5 mm, and the superior thyroid artery was 1.6± 0.7 mm.

CONCLUSION. According to the study, arterial blood supply to the internal neck organs is provided mainly by the inferior and superior thyroid arteries. At the same time, the trunk of the inferior thyroid artery is larger than the superior thyroid artery (p=0.032). The inferior thyroid artery forms branches along the posterior surface of the thyroid lobe and from a topographic point of view it is the main source of blood supply to the parathyroid glands. In most cases, the inferior thyroid artery has a descending course, is directed along the posterior surface of the thyroid gland and forms an X-shaped intersection with the recurrent laryngeal nerve.

Comparative Histology of C Thyrocytes in Four Domestic Animal Species: Dog, Pig, Horse, and Cattle

Simple Summary

In this study we have proved that dogs, pigs, cattle, and horses, species belonging to four distinct families, differ in regard to microscopical characteristics of their C thyrocytes. Although the total number of C thyrocyte profiles and their localization within thyroid lobes were comparable, each of the examined species displayed their unique morphological characteristics and distribution pattern. The differences described in our study, easily recognizable microscopically, can be used as a reference material for further studies focused on C thyrocytes biology in physiological and pathological conditions.

Abstract

The number, morphology, and distribution of C thyrocytes within the thyroid gland vary among species; however, studies in domestic animals are limited. In this study we compared the morphology, distribution pattern, and percentage of C thyrocytes in four domestic species: dogs, pigs, horses, and cattle. Eighty thyroid glands, 20 per species, were examined. C thyrocytes were visualized immunohistochemically with anti-calcitonin rabbit polyclonal antibody alone and combined with the periodic acid Schiff method to simultaneously visualize C thyrocytes with the basement membranes of thyroid follicles. C thyrocyte morphology varied considerably between species, from oval- (dogs) and spindle-shaped (pigs) to polymorphic (cattle and horses). Bovine C thyrocytes demonstrated cytoplasmic protrusion. C thyrocytes were located intrafolliculary (all species), epifollicularly (dogs, horses, cattle), or interfolicularly (cattle). Most porcine and bovine C thyrocytes existed individually whereas canine C thyrocytes usually formed clusters. In horses, they tended to form groups of various shapes and sizes or even rims encompassing whole follicles. In all species, the number of C thyrocyte profiles increased from the periphery to the central area of the thyroid lobe. The mean total fraction of C thyrocytes in the superficial, intermediate, and central areas were as follows: 2.55%, 8.43%, and 12.48% in dogs; 3.81%, 7.66%, and 10.79% in pigs; 1.55%, 7.44%, and 8.87% in horses; and 2.62%, 10.75%, and 12.96% in cattle. No statistical differences in the total number of C thyrocyte profiles were observed among species (8.87% in dogs, 8.58% in cattle, 7.98% in pigs, and 5.83% in horses). Our results indicated that the studied species displayed their own morphological characteristics and distribution pattern of C thyrocytes; however, total numbers of C thyrocyte profiles and their localization within the thyroid lobe are comparable.

Pseudodiverticulum of the Cervical Esophagus With Remnant of Branchial Tissues in a Newborn: A Case Report

Congenital pseudodiverticula of the esophagus are very rare. This case report describes the presentation, management and histopathology of a peudodiverticulum of the cervical esophagus in a neonate. The infant presented with respiratory distress and a right neck mass that required surgical excision. Pathology revealed a pseudodiverticulum that contained ectopic thymic, thyroid, and parathyroid tissue within the wall of the lesion. The presence of ectopic tissues of branchial origin and an aberrant right subclavian artery suggest an error in branchial development and neural crest cell migration.

Morphological, histological and ultra-structural studies on the ultimobranchial body of goat (Capra hircus)

Ultimobranchial body (UBB) remnant was considered as an enigma till the last few years, then it was recognized as a necessary organ where it is the origin of the parafollicular cells. The samples were fixed and processed for the histological and electron microscopic examination. Macroscopically, the UBB remnant appeared as a white mass at the end of the cranial one-third of the thyroid lobe. It was composed of solid cell nest, cluster of cells and small thyroid follicles. Transmission electron microscope showed some round cells containing euchromatic nuclei, numerous parafollicular cells with darkly stained granules and paler ones. It also showed some mast cells with heterochromatic nuclei and large darkly stained granules. The parafollicular cells were distributed throughout the thyroid gland but concentrated within the UBB remnant. To our knowledge, our findings represent very unique histological manifestations specially the ultra-structural ones which revealed an original finding about the new clear type of cells suggested to be a UBB remnant and ensure those of the light microscope.

Development of Functional Thyroid C Cell-like Cells from Human Pluripotent Cells in 2D and in 3D Scaffolds

Medullary thyroid carcinoma contributes to about 3–4% of thyroid cancers and affects C cells rather than follicular cells. Thyroid C cell differentiation from human pluripotent stem cells has not been reported. We report the stepwise differentiation of human embryonic stem cells into thyroid C cell-like cells through definitive endoderm and anterior foregut endoderm and ultimobranchial body-like intermediates in monolayer and 3D Matrigel culture conditions. The protocol involved sequential treatment with interferon/transferrin/selenium/pyruvate, foetal bovine serum, and activin A, then IGF-1 (Insulin-like growth factor 1), on the basis of embryonic thyroid developmental sequence. As well as expressing C cell lineage relative to follicular-lineage markers by qPCR (quantitative polymerase chain reaction) and immunolabelling, these cells by ELISA (enzyme-linked immunoassay) exhibited functional properties in vitro of calcitonin storage and release of calcitonin on calcium challenge. This method will contribute to developmental studies of the human thyroid gland and facilitate in vitro modelling of medullary thyroid carcinoma and provide a valuable platform for drug screening.

A Review of the Significance in Measuring Preoperative and Postoperative Carcinoembryonic Antigen (CEA) Values in Patients with Medullary Thyroid Carcinoma (MTC)

Background and Objectives: Medullary thyroid carcinoma (MTC) accounts for 1–2% of all thyroid malignancies, and it originates from parafollicular “C” cells. Carcinoembryonic antigen (CEA) is a tumor marker, mainly for gastrointestinal malignancies. There are references in literature where elevated CEA levels may be the first finding in MTC. The aim of this study is to determine the importance of measuring preoperative and postoperative CEA values in patients with MTC and to define the clinical significance of the correlation between CEA and the origin of C cells. Materials and Methods: The existing and relevant literature was reviewed by searching for articles and specific keywords in the scientific databases of PubMedCentraland Google Scholar (till December 2020). Results: CEA has found its place, especially at the preoperative level, in the diagnostic approach of MTC. Preoperative CEA values >30 ng/mL indicate extra-thyroid disease, while CEA values >100 ng/mL are associated with lymph node involvement and distant metastases. The increase in CEA values preoperatively is associated with larger size of primary tumor, presence of lymph nodes, distant metastases and a poorer prognosis. The clinical significance of CEA values for the surgeon is the optimal planning of surgical treatment. In the recent literature, C cells seem to originate from the endoderm of the primitive anterior gut at the ultimobranchial bodies’ level. Conclusions: Although CEA is not a specific biomarker of the disease in MTC, itsmeasurement is useful in assessing the progression of the disease. The embryonic origin of C cells could explain the increased CEA values in MTC.

Neuroendocrine Tumors of the Thyroid and Their Mimics

This paper will review neuroendocrine lesions of the thyroid and the differential diagnosis with the most significant such tumor of the thyroid, that is, medullary thyroid carcinoma. A brief overview of the understanding of this tumor's identification as a lesion of C cells and its familial and syndromic associations will be presented. Then, a discussion of the various mimics of medullary carcinoma will be given with an approach to the types of tests that can be done to arrive at a correct diagnostic conclusion. This review will focus on practical "tips" for the practicing pathologist.

Selected Topics in the Pathology of the Thyroid and Parathyroid Glands in Children and Adolescents

The goals of this chapter in keeping with the overall general themes of this special edition will be (1) to highlight aspects of development of the thyroid and parathyroid glands with particular focus on the role and contribution of the neural crest (or not) and how this may impact on the pathology that is seen, (2) to emphasize those lesions particularly more commonly arising in the pediatric population that actually generate specimens that the surgical pathologist would encounter, and (3) highlight more in depth specific lesions associated with heritable syndromes or specific gene mutations since the heritable syndromes tends to manifest in the pediatric age group. In this light, the other interesting areas of pediatric thyroid disease including medical thyroid diseases, congenital hypothyroidism, anatomic variants and aberrations of development that lead to structural anomalies will not be emphasized here.

Do You Know the Details of Your PAX8 Antibody? Monoclonal PAX8 (MRQ-50) Is Not Expressed in a Series of 45 Medullary Thyroid Carcinomas

Medullary thyroid carcinomas display cytologic and architectural features that can simulate various primary and metastatic neoplasms. PAX8 immunoexpression in neuroendocrine neoplasms yielded antibody-dependent findings. Since the data regarding the expression profile of monoclonal PAX8 (MRQ-50) antibody is limited in large series of medullary thyroid carcinomas, this study investigated the expression profile of PAX8 (MRQ-50) in a series of 45 medullary thyroid carcinomas. PAX8 (MRQ-50) expression was noted in the thyroid follicular epithelial cells surrounding the tumor and was negative in all medullary thyroid carcinomas. In addition, twenty medullary thyroid carcinomas showed scattered entrapped thyroid follicular epithelial cells at the periphery of the tumor. Entrapped follicular epithelial cells were positive for PAX8 and thyroglobulin, and were negative for monoclonal CEA and calcitonin. A panel approach combining monoclonal antibodies to transcription factors, hormones and cell-specific peptides often assist diagnosticians in the workup of the cellular origin of a neuroendocrine neoplasm. Since PAX8 immunostaining is dependent on the antibody characteristics in neuroendocrine neoplasms, pathologists should be aware of the details of the PAX8 antibody used in a particular case.

Slc:Wistar/ST rats develop unilateral thyroid dysgenesis: A novel animal model of thyroid hemiagenesis

Developmental anomalies of the thyroid gland lead to congenital malformations such as thyroglossal duct cysts and thyroid dysgenesis. However, the pathogenesis of thyroid dysgenesis remains unclear due to the lack of suitable animal models. This study demonstrated that Slc:Wistar/ST rats frequently developed unilateral thyroid dysgenesis, including hemiagenesis, characterized by the absence of one lobe. In Wistar/ST rats, each thyroid lobe was frequently different in size, and approximately 27% and 20% of the rats presented with hemihypoplasia and hemiagenesis of the thyroid gland, respectively. Dysgenesis was predominant on the left side in both sexes, without sex differences. At a young age, thyroid hemiagenesis did not alter body weight. In rats of both sexes with thyroid hemiagenesis, plasma total triiodothyronine and total triiodothyronine levels remained unchanged while plasma thyroid-stimulating hormone levels were significantly elevated in young rats. The remaining thyroid lobes increased in weight, but the follicular epithelial cells appeared normal in terms of their height and proliferating activities. On the side of thyroid dysgenesis, the parathyroid glands were normally localized and were situated at the same location as the contralateral glands. The ultimobranchial body remnants were localized at the level of the thyroid gland along with the cranial thyroid artery and vein, forming cell clusters or cystic structures and containing calcitonin-positive C-cells. In conclusion, Wistar/ST rats developed unilateral thyroid dysgenesis and may be novel and useful animal models for thyroid hemiagenesis in humans and for morphogenesis of pharyngeal pouch-derived organs.

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.

Restricted localization of ultimobranchial body remnants and parafollicular cells in the one-humped camel (Camelus dromedarius)

Parafollicular cells (C-cells) exist within the thyroid glands and display different distributions within the glands among mammalian species. In the one-humped camel (Camelus dromedarius), localization of the C-cells remains under debate. We herein investigated appearance of C-cells and the remnants of the ultimobranchial body, origin of C-cells, in the thyroid glands of one-humped camels. Macroscopically, a white mass was present at one-third the length from the cranial end of the thyroid glands where the cranial thyroid artery entered. In addition, large fossae were frequently found adjacent to the white mass. Histologically, the mass was mainly composed of connective tissues, thyroid follicles, and two types of cell clusters: one was composed of cells with clear cytoplasm and the other was composed of non-keratinized epidermoid cells. The mass and the fossae contained p63-positive cells, indicating that they consisted of ultimobranchial body remnants. Calcitonin was expressed in cells with clear cytoplasm, which were localized just beneath the fossae and in the cell clusters of the white mass. C-cells also resided in both subfollicular and interfollicular spaces adjacent to the white mass, but gradually decreased toward the periphery. C-cells tended to display round shapes in the ultimobranchial body remnants and subfollicular spaces, and spindle shapes in interfollicular spaces. In conclusion, we demonstrated that the ultimobranchial body remnants were limited to the region around the entrance of cranial thyroid artery and vein, and C-cells were mainly concentrated within and around the ultimobranchial body remnants.

Update on Multiple Endocrine Neoplasia Type 2: Focus on Medullary Thyroid Carcinoma

Multiple endocrine neoplasia type 2 (MEN2) is an autosomal dominant hereditary cancer syndrome caused by missense gain-of-function mutations in the RET proto-oncogene on chromosome 10. Specific RET mutations can predispose toward a particular phenotype and clinical course, with strong genotype–phenotype correlations. MEN2 is highly penetrant in medullary thyroid carcinoma (MTC), and it can be associated with bilateral pheochromocytoma and primary hyperparathyroidism. Two different clinical variants of MEN2 are known: MEN2A, which includes the familial subtype, and MEN2B. Treatment includes early thyroidectomy. Recommendations on the timing and extent of surgery are based on the RET mutation risk categories (moderate-, high-, or highest-risk) regarding the age of MTC onset. Early identification of patients with hereditary MTC has improved treatment outcomes. Previously, MTC was diagnosed based on clinical tumors; in contrast, with genetic screening, MTC can be diagnosed at preclinical disease states. This approach has resulted in a high cure rate and a much better prognosis for MTC. However, classification into one of the three RET mutation risk groups for predicting aggressiveness and prognosis has had limited impact. Increasing evidence has shown that patients with RET mutations in different risk classifications exhibit a broad spectrum of MTC aggressiveness during follow-up, with no relevant difference in survival. The specific germline activating mutation of the RET proto-oncogene appears to be the first determinant of the age of MTC onset, but, presumably, different regulatory events determine long-term tumor behavior.

Recent advances in the biology and therapy of medullary thyroid carcinoma

Medullary thyroid cancer (MTC) is a relatively uncommon yet prognostically significant thyroid cancer. Several recent advances in the biology and current or potential treatment of MTC are notable. These include a new understanding of the developmental biology of the thyroid C cell, which heretofore was thought to develop from the neural crest. RET, encoded by the most common driver gene in MTC, has been shown to be a dual function kinase, thus expanding its potential substrate repertoire. Promising new therapeutic developments are occurring; many have recently progressed to clinical development. There are new insights into RET inhibitor therapy for MTC. New strategies are being developed to inhibit the RAS proteins, which are potential therapeutic targets in MTC. Potential emerging immunotherapies for MTC are discussed. However, gaps in our knowledge of the basic biology of the C cell, its transformation to MTC, and the mechanisms of resistance to therapy impede progress; further research in these areas would have a substantial impact on the field.

Development of the thyroid gland

Thyroid hormones are crucial for organismal development and homeostasis. In humans, untreated congenital hypothyroidism due to thyroid agenesis inevitably leads to cretinism, which comprises irreversible brain dysfunction and dwarfism. Elucidating how the thyroid gland - the only source of thyroid hormones in the body - develops is thus key for understanding and treating thyroid dysgenesis, and for generating thyroid cells in vitro that might be used for cell-based therapies. Here, we review the principal mechanisms involved in thyroid organogenesis and functional differentiation, highlighting how the thyroid forerunner evolved from the endostyle in protochordates to the endocrine gland found in vertebrates. New findings on the specification and fate decisions of thyroid progenitors, and the morphogenesis of precursor cells into hormone-producing follicular units, are also discussed.

Morphological and molecular evolution of the ultimobranchial gland of nonmammalian vertebrates, with special reference to the chicken C cells

This review summarizes the current understanding of the nonmammalian ultimobranchial gland from morphological and molecular perspectives. Ultimobranchial anlage of all animal species develops from the last pharyngeal pouch. The genes involved in the development of pharyngeal pouches are well conserved across vertebrates. The ultimobranchial anlage of nonmammalian vertebrates and monotremes does not merge with the thyroid, remaining as an independent organ throughout adulthood. Although C cells of all animal species secrete calcitonin, the shape, cellular components and location of the ultimobranchial gland vary from species to species. Avian ultimobranchial gland is unique in several phylogenic aspects; the organ is located between the vagus and recurrent laryngeal nerves at the upper thorax and is densely innervated by branches emanating from them. In chick embryos, TuJ1-, HNK-1-, and PGP 9.5-immunoreactive cells that originate from the distal vagal (nodose) ganglion, colonize the ultimobranchial anlage and differentiate into C cells; neuronal cells give rise to C cells. Like C cells of mammals, the cells of fishes, amphibians, reptiles, and also a subset of C cells of birds, appear to be derived from the endodermal epithelium forming ultimobranchial anlage. Thus, the avian ultimobranchial C cells may have dual origins, neural progenitors and endodermal epithelium. Developmental Dynamics 246:719-739, 2017. © 2017 Wiley Periodicals, Inc.

Development of Parafollicular Cells and their Relationship with Developing Thyroid Follicles in Human Foetuses

INTRODUCTION

The parafollicular cells or clear (C) cells in man are part of neuroendocrine system under Amine Precursor Uptake and Decarboxylation (APUD) cells. Their role in adults has been reputable but in foetus is still unclear.

AIM

The present study was a baseline study endeavouring to describe the chronological development of the parafollicular cells with particular focus on its correlation with developing human thyroid follicles.

MATERIALS AND METHODS

The study was conducted on 10 aborted foetuses (14-28 weeks), procured from the Department of Obstetrics and Gynaecology, Lok Nayak Hospital, New Delhi, India. Serial sections of foetal thyroid gland were generated, stained with haematoxylin and eosin, and immunohistochemistry using the anticalcitonin antibody and examined qualitatively.

RESULTS

In our study, the parafollicular cells were seen as earlier as by 14^th week. They became morphologically and functionally mature by 16^th week of gestation. The parafollicular cells were getting organized from scattering to parafollicular location then to a more localized area, i.e., intrafollicular along with the follicular development. As the follicles were enlarging, the intrafollicularly located parafollicular cells which was initially present in groups was getting displaced singly between the follicular cells in the same follicle.

CONCLUSION

The sequential development pattern of the parafollicular cells in relation to developing thyroid follicles was established. This immunohistochemical study also concluded that the parafollicular cells might have higher character to play in the early gestational age such as regulation of ossification in the human foetus.

Immunohistochemical profiling of the ultimobranchial remnants in the rat postnatal thyroid gland

Ultimobranchial (UB) remnants are a constant presence in the thyroid throughout rat postnatal life; however, the difficulty in identifying the most immature forms from the surrounding thyroid tissue prompted us to search for a specific marker. With that objective, we applied a panel of antibodies reported to be specific for their human counterpart, solid cell nests (SCNs), using double immunohistochemistry and immunofluorescence. Our results demonstrated that cytokeratin 34βE12 and p63 are highly sensitive markers for the immunohistologic screening of UB-remnants, independently of their maturity or size. Furthermore, rat UB-follicles (UBFs) coincided with human SCNs in the immunohistochemical pattern exhibited by both antigens. In contrast, the pattern displayed for calcitonin and thyroglobulin differs considerably but confirm the hypothesis that rat UB-cells can differentiate into both types of thyroid endocrine cells. This hypothesis agrees with recent findings that thyroid C-cells share an endodermic origin with follicular cells in rodents. We suggest that the persistence of p63-positive undifferentiated cells in UB-remnants may constitute a reservoir of basal/stem cells that persist beyond embryogenesis from which, in certain unknown conditions, differentiated thyroid cells or even unusual tumors may arise.

On the Origin of Cells and Derivation of Thyroid Cancer: C Cell Story Revisited

We will highlight and put into perspective new lineage tracing data from genetic studies in mice indicating that the genuine progenitors to C cells arise in the endoderm germ layer. This overturns the current concept of a neural crest origin of thyroid C cells referred to in every textbook and dedicated paper to this very day. As will become apparent, except for a single experiment, the neural crest theory has little or no support when the evolution and development of calcitonin-producing cells in the entire chordate family are considered. Instead, a unifying origin of all cells of the ultimobranchial bodies reopens questions on the histogenesis of certain thyroid pathologies previously difficult to explain. On this aspect, medullary thyroid cancer shows a stronger connection to gut neuroendocrine tumours than previously recognized. It is envisaged that novel factors implicated in C cell-derived tumour growth and progression will be discovered as the mechanisms that regulate lineage expansion of embryonic C cell precursors from pharyngeal endoderm are uncovered. We will not discuss why C cells go to the bother of burying themselves in the thyroid - this remains a mystery.