Solid cell nests of the thyroid: light microscopy and immunohistochemical profile

Histopathology of C Cells and Medullary Thyroid Carcinoma

C cells are the neuroendocrine cell component of the thyroid gland that embryologically arise from the pharyngeal endoderm. Normal C cells are concentrated in the upper two-thirds of both lateral lobes, appear singly or in small groups dispersed in, among or peripherally to the follicles, and are involved in the production of calcitonin. Reactive C-cell hyperplasia should be differentiated from proliferation of atypical C cells (neoplastic C-cell hyperplasia) which is considered an intraepithelial neoplasia of C cells/medullary carcinoma in situ, a precursor lesion associated to familial medullary thyroid carcinoma (MTC). MTC typically exhibits a lobular and/or trabecular growth pattern with amyloid deposits; however, due to its great histological variability, immunohistochemical positivity for calcitonin, carcinoembryonic antigen, calcitonin-gene-related peptide, insulinoma-associated protein 1, and/or other markers is necessary to confirm diagnosis. Investigation of germline RET proto-oncogene mutation is mandatory to identify familial MTC. Somatic RET mutations or fusions as well as RAS mutations in cytological and/or biopsy samples may represent therapeutic targets. Mixed medullary and follicular-derived cell carcinoma is a heterogeneous group of tumors which needs to be distinguished from collision tumors.

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.

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.

Pitfalls in Challenging Thyroid Tumors: Emphasis on Differential Diagnosis and Ancillary Biomarkers

Thyroid pathology encompasses a heterogenous group of clinicopathological entities including rare and diagnostically challenging neoplasms. The review is focused on morphological, immunohistochemical, and molecular features of rare thyroid neoplasms that can pose diagnostic problems. The tumors are organized based on growth patterns including thyroid neoplasms with predominantly papillary, follicular, solid, and spindle cell growth pattern, as well as neoplasms with distinct cytological characteristics. A special section is also dedicated to rare thyroid tumors with peculiar patterns including thyroid carcinoma with Ewing family tumor elements and intrathyroidal thymic-related neoplasms.

Thyroid Adenoma of Probable Ultimobranchial Body Origin: A Case Report

Solid cell nests are generally believed to represent remnants of the ultimobranchial body, which can be found in the normal thyroid gland, occasionally associated with other branchial pouch remnants such as salivary gland, cartilage, and adipose tissue. We describe the case of a 44-year-old man incidentally found to have a large tumor in the left lobe of the thyroid. The tumor was a circumscribed growth consisting of distinctly lobulated proliferation of solid to cystic epidermoid cell nests and thyroid follicles in a fibromatous stroma, which merged into abundant adipose tissue and focally myxoid matrix. The solid epidermoid cell nests resembled solid cell nests and exhibited a p63+, GATA3+, galectin-3+, TTF1-, PAX8-, thyroglobulin- phenotypes, while the follicles were p63-, GATA3-, galectin-3-, TTF1+, PAX8+, and thyroglobulin+. RAS mutations were not found. This thyroid tumor may represent a hitherto undescribed "ultimobranchial body adenoma" in human.

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.

Solid Cell Nests Within a Parathyroid Gland-Report of an Exceptional Case

The ultimobranchial body (UBB) denotes the cellular mass originating from the fourth branchial pouch, which migrates from the neural crest and infolds within the middle and upper poles of the thyroid lobes, thereby establishing the presence of calcitonin-secreting parafollicular C cells. In various numbers, UBB remnants (entitled "solid cell nests", or SCNs) are found in thyroid glands examined histologically. However, despite the close embryological relation between the UBB and the superior parathyroid glands, intraparathyroidal SCNs have to our knowledge not been previously reported. Here, we describe a patient presenting with a papillary thyroid carcinoma with central and lateral lymph node metastases. Upon postoperative analysis, an unintentionally removed parathyroid gland was observed adjacent to the superior aspect of the right thyroid lobe. Within a 0.6 × 0.5-mm area of the parathyroid gland, solid nests composed of epithelial cells with oval and slightly elongated nuclei were seen. The cells were positive for p40, p63, and GATA3, but negative for PTH. The final diagnosis was a SCN entrapped within the parathyroid gland. Empirically, we have not previously observed SCNs within the parathyroid glands. To our knowledge, our finding thus constitutes a very unusual histological manifestation, and could indicate an underlying aberrancy during embryogenesis given the close anatomical relationship between the UBB and the superior parathyroid glands.

Sporadic minute medullary thyroid carcinoma with a double RET mutation: A case report

We describe a 74-year-old man with a nodular goiter accompanied by an incidental sporadic minute medullary thyroid carcinoma (MTC). Histopathologically, the MTC was a well-defined 1.7 mm tumor in the upper one-third right lobe, with solid cell nests (SCNs) adjacent to the MTC. C-cells were scattered mainly around the SCNs, but C-cell hyperplasia was not evident in the background thyroid. The MTC cell phenotype was immunohistochemically identical to background C-cells, but was completely different from the SCN main cells. Direct DNA analyses of isolated MTC paraffin-embedded specimens revealed two RET proto-oncogene missense point mutations in exon 11 (i.e., C630R and C634W). The non-tumor thyroid tissue did not reveal any mutations. This study reports the smallest case of sporadic MTC with a double RET somatic mutation, substantiating that RET mutations can occur during a very early stage of carcinogenesis. The combined presence of C630R and C634W represent a novel somatic mutation in sporadic MTC. The present case indicates that the sporadic MTC originated from the surrounding C-cells of the SCNs without C-cell hyperplasia and that the SCN main cells may not be able to develop into an MTC.

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.

EWSR1 rearrangement is a frequent event in papillary thyroid carcinoma and in carcinoma of the thyroid with Ewing family tumor elements (CEFTE)

Carcinomas of the thyroid with Ewing family tumor element (CEFTEs) are small-cell thyroid tumors with epithelial differentiation that disclose p63 expression and EWSR1-FLI1 rearrangement, carry a favorable prognosis and may co-exist with papillary thyroid carcinoma (PTC) foci. Two histogenetic hypotheses have been advanced regarding the origin of CEFTEs: arising in PTCs or in solid cell nests (SCN). A total of 3 CEFTEs, 54 PTCs, and 10 SCNs were reviewed, and fluorescence in situ hybridization (FISH) technique was performed in all cases to search for the presence of EWSR1 rearrangements. The three CEFTEs disclosed the EWSR1-FLI1 rearrangement both in the small cell and in the PTC component. Out of the 54 PTC cases, 28 (51.9%) were positive, 20 (37.0%) were negative, and 6 (11.1%) were inconclusive for EWSR1 rearrangement; in two of the positive PTC cases, the EWSR1-FLI1 rearrangement was detected. Classic PTC disclosed more often the EWSR1 rearrangement than other PTC variants (p = 0.031). PTCs with EWSR1 rearrangement disclosed a lower percentage of nuclei with EWSR1 polysomy than those without EWSR1 rearrangement (p = 0.001). Out of the 10 SCNs, 7 (70.0%) were negative and 3 (30.0%) were inconclusive for the EWSR1 rearrangement. Monosomic nuclei were more frequent (mean of 44.3%) in SCNs than in PTCs (p < 0.001). The presence of the EWSR1-FLI1 rearrangement in PTC component of all studied CEFTEs and the existence of the EWSR1 rearrangement in some PTCs favor the origin of CEFTE from PTC. The high frequency of EWSR1 rearrangements in PTC may represent a new diagnostic marker of these tumors.

ICRP Publication 131: Stem Cell Biology with Respect to Carcinogenesis Aspects of Radiological Protection

This report provides a review of stem cells/progenitor cells and their responses to ionising radiation in relation to issues relevant to stochastic effects of radiation that form a major part of the International Commission on Radiological Protection's system of radiological protection. Current information on stem cell characteristics, maintenance and renewal, evolution with age, location in stem cell 'niches', and radiosensitivity to acute and protracted exposures is presented in a series of substantial reviews as annexes concerning haematopoietic tissue, mammary gland, thyroid, digestive tract, lung, skin, and bone. This foundation of knowledge of stem cells is used in the main text of the report to provide a biological insight into issues such as the linear-no-threshold (LNT) model, cancer risk among tissues, dose-rate effects, and changes in the risk of radiation carcinogenesis by age at exposure and attained age. Knowledge of the biology and associated radiation biology of stem cells and progenitor cells is more developed in tissues that renew fairly rapidly, such as haematopoietic tissue, intestinal mucosa, and epidermis, although all the tissues considered here possess stem cell populations. Important features of stem cell maintenance, renewal, and response are the microenvironmental signals operating in the niche residence, for which a well-defined spatial location has been identified in some tissues. The identity of the target cell for carcinogenesis continues to point to the more primitive stem cell population that is mostly quiescent, and hence able to accumulate the protracted sequence of mutations necessary to result in malignancy. In addition, there is some potential for daughter progenitor cells to be target cells in particular cases, such as in haematopoietic tissue and in skin. Several biological processes could contribute to protecting stem cells from mutation accumulation: (a) accurate DNA repair; (b) rapidly induced death of injured stem cells; (c) retention of the DNA parental template strand during divisions in some tissue systems, so that mutations are passed to the daughter differentiating cells and not retained in the parental cell; and (d) stem cell competition, whereby undamaged stem cells outcompete damaged stem cells for residence in the niche. DNA repair mainly occurs within a few days of irradiation, while stem cell competition requires weeks or many months depending on the tissue type. The aforementioned processes may contribute to the differences in carcinogenic radiation risk values between tissues, and may help to explain why a rapidly replicating tissue such as small intestine is less prone to such risk. The processes also provide a mechanistic insight relevant to the LNT model, and the relative and absolute risk models. The radiobiological knowledge also provides a scientific insight into discussions of the dose and dose-rate effectiveness factor currently used in radiological protection guidelines. In addition, the biological information contributes potential reasons for the age-dependent sensitivity to radiation carcinogenesis, including the effects of in-utero exposure.

Selected Case From the Arkadi M. Rywlin International Pathology Slide Seminar: Benign Warthin Tumor of the Thyroid

We report on an exceedingly rare lesion of the thyroid probably of a branchial cleft origin, which was not published in the world literature before. A 58-year-old woman underwent a total thyroidectomy for bilateral goiter. Grossly, there was one yellowish nodule sized 15 mm in the largest dimension found in the right lobe. Microscopically, the thyroid parenchyma showed signs of Hashimoto thyroiditis. The nodule in the right lobe was composed of a part of solid cell nests appearance, another part resembling a branchial cleft cyst, and a part resembling Warthin tumor. This lesion may belong to the histogenetically similar group of entities in the head and neck region which are derived from branchial cleft derivatives and which, under the inflammatory influence, have the ability to a cystic dilatation and proliferation of the epithelial component. The epithelium can afterwards become papillary and may undergo oncocytic transformation, thus gaining features that impart the resemblance of a Warthin tumor. Club members generally agreed with a submitted diagnosis of benign Warthin tumor of the thyroid.

Branchial cleft-like cysts in Hashimoto's thyroiditis: A case report and literature review

We report an extremely rare case of branchial cleft-like cysts in Hashimoto's thyroiditis. The patient was a 77-year-old man with a growing mass in the anterior neck. Ultrasonography and computed tomography revealed a cystic lesion with septum in the left thyroid and multiple small cystic lesions in the right thyroid. Lymph node swelling of the cervical region, supraclavicular fossa and submandibular region was also observed. Left thyroidectomy and lymph node dissection were performed. Histologically, cysts were lined by stratified squamous epithelium and dense lymphoid tissue having conspicuous follicle formation surrounded the epithelial lining. Solid cell nest (SCN)-like aggregations were seen in the thyroid parenchyma adjacent to the cyst walls and a small number of thyroid follicles were observed in the fibrous wall. Immunohistochemically, it is suggested that both the cyst lining and SCN-like aggregations are originally from thyroid follicles. Although, the exact histogenesis of branchial cleft-like cysts remains unclear, there are probably two different processes for its development, one is of branchial cleft origin and the other is mere squamous metaplasia, while in our case the latter is suggested. Herein, we report our new case and update information about branchial cleft-like cysts that appears in the literature.

Solid cell nests of the thyroid gland: morphological, immunohistochemical and genetic features

AIMS

The correct identification of solid cell nests (SCNs) is an important issue in thyroid pathology because of the spectrum of differential diagnoses of this type of lesion.

METHODS AND RESULTS

Ten cases of 295 consecutive thyroidectomies showed the presence of SCNs at histological examination. The identification of the exact SCN type required the distinction of the cystic and solid pattern; SCNs were usually composed of a mixture of main cells (MCs) and C-cells (CCs). The immunohistochemical calcitonin stain identified CCs easily, both inside SCNs and dispersed in islets at the periphery. For the characterization of MCs, we added the utility of p40 to p63. The use of thyroid transcription factor-1 (TTF-1) helped in their identification, as MCs did not react with this marker; the combination of TTF-1 and p40 or p63 IHC stains was useful for the characterization of cystic SCNs of both types 3 and 4. The negativity of mouse monoclonal mesothelioma antibody (HMBE-1) and a very low proliferative index (MIB-1) supported the diagnosis. [Correction added on 23 November 2015, after online publication: MIB-1 was incorrectly defined, the expanded form was deleted.] We discourage the use of galectin-3 (Gal-3) and cytokeratin-19 (CK-19), as they have an important overlap with papillary thyroid carcinoma. The complete absence of any B-Raf proto-oncogene, serine/threonine kinase (BRAF) mutations is an additional fundamental finding.

CONCLUSIONS

We reviewed the most relevant morphological and immunohistochemical features of SCNs and have provided a genetic analysis of the BRAF gene because of its expanding use in thyroid pathology.

Unusual thyroid tumors: a review of pathologic and molecular diagnosis

The most common thyroid neoplasms are either follicular derived (papillary, follicular and Hürthle cell lesions) or C-cell derived (medullary carcinoma). The diagnosis of these tumors can usually be made at the histologic level, with immunohistochemical stains necessary in some circumstances. Specific molecular mutations have been described that can be diagnostically useful or explain, in part, their pathogenesis, including the well-known Ret/PTC and PPARgamma-PAX8 translocations, point mutations in the Ret, Ras and BRAF genes, and loss of heterozygosity of multiple different tumor suppressor genes. Some unusual tumors of the thyroid gland are more difficult to diagnose. In examining these lesions, the pathologist may use the hematoxylin and eosin-stained morphology, coupled with an analysis of the immunohistochemical staining profiles and possibly analysis of the underlying molecular mutational patterns. These less common thyroid tumors include tall cell and cribriform-morular variants of papillary carcinoma, hyalinizing trabecular tumor, mucoepidermoid and sclerosing mucoepidermoid carcinoma with eosinophilia, poorly differentiated (insular) carcinoma, and undifferentiated (anaplastic) carcinoma. The diagnostic features of these rare tumors, including the histology, immunohistochemical expression profiles and the known molecular mutational profiles of each, are reviewed.

Thyroid regeneration: how stem cells play a role?

Many tissues if not all are thought to contain stem cells that are responsible for regeneration and repair of the tissue after injury. Dysregulation of tissue regeneration may result in various pathological conditions, among which cancer is the most extensively studied. Notably, the so-called cancer stem cells or tumor-initiating cells, have been studied in order to understand the mechanisms of carcinogenesis and/or metastasis. However, the nature of cancer stem cells, let alone normal stem/progenitor cells, particularly those of the thyroid remains elusive. There remains a gap in knowledge between adult thyroid stem/progenitor cells and cancer stem cells of the thyroid, and if and/or how they are related to each other. Understanding of the mechanism for thyroid regeneration and mode of participation of normal adult thyroid stem/progenitor cells in this process will hopefully yield a more complete understanding of the nature of thyroid cancer stem cells, and/or help understand the pathogenesis of other thyroid diseases. This review summarizes the current understanding of adult thyroid stem/progenitor cells, with particular emphasis on how they contribute to thyroid regeneration.

Stem cell antigen 1-positive mesenchymal cells are the origin of follicular cells during thyroid regeneration

Many tissues are thought to contain adult stem/progenitor cells that are responsible for repair of the tissue where they reside upon damage and/or carcinogenesis, conditions when cellular homeostasis becomes uncontrolled. While the presence of stem/progenitor cells of the thyroid has been suggested, how these cells contribute to thyroid regeneration remains unclear. Here we show the origin of thyroid follicular cells and the process of their maturation to become follicular cells during regeneration. By using β-galactosidase (β-gal) reporter mice in conjunction with partial thyroidectomy as a model for thyroid regeneration, and bromodeoxyuridine (BrdU) long label-retaining cell analysis, we demonstrated that stem cell antigen 1 (Sca1) and BrdU-positive, but β-gal and NKX2-1 negative cells were found in the non-follicular mesenchymal area 7 days after partial thyroidectomy. They temporarily co-expressed cytokeratin 14, and were observed in part of follicles by day 35 post-partial thyroidectomy. Sca1, BrdU, β-gal, and NKX2-1-positive cells were found 120 days post-partial thyroidectomy. These results suggested that Sca1 and BrdU positive cells may participate in the formation of new thyroid follicles after partial thyroidectomy. The process of thyroid follicular cell regeneration was recapitulated in ex vivo thyroid slice collagen gel culture studies. These studies will facilitate research on thyroid stem/progenitor cells and their roles in thyroid diseases, particularly thyroid carcinomas.

Postnatal fate of the ultimobranchial remnants in the rat thyroid gland

The ultimobranchial follicles (UBFs) are considered embryonic remnants from the ultimobranchial body (UBB). They are follicular structures that vary in size and appearance depending on the age of the rat. The main objective of this article was to study the progressive changes in shape, size, and frequency of the UBFs in the postnatal rat, from birth to old-age. To accomplish that objective, a systematic morphometric and incidental study of the UBF has been carried out in 110 Wistar rats of different ages and both sexes, divided into three groups: 1) young rats (5-90-day-old); 2) adult rats (6-15-month-old), and 3) old rats (18-24-month-old). The glands were serially sectioned and immunostained for calcitonin at five equidistant levels. According to our results, UBFs were observed in all thyroid glands but a more exhaustive sampling was occasionally necessary in male rats. In young rats, immature UBFs predominantly appeared whereas in adult rats, mature UBFs with cystic appearance and variable luminal content prevailed. We frequently found spontaneous anomalous UBFs in old rats, which we have termed as "ultimobranchial cystadenomata." Additionally, in young rats, UBF areas significantly increased with age and they were larger when compared to that of normal thyroid follicles. Likewise, in adult rats, UBFs were significantly larger than normal thyroid follicles but only in female rats. In general, UBFs in females were also significantly larger than those found in male rats. Finally, all these differences related to UBFs together with a higher incidence in females of UB cystadenomata suggest a sexual dimorphism in regard to the destiny of these embryonic remnants during postnatal thyroid development.

Absence of the BRAF and the GRIM-19 mutations in oncocytic (Hürthle cell) solid cell nests of the thyroid

We report the first case of oncocytic solid cell nests (SCNs), found in the right lobe of the thyroid of a 70-year-old man. Conventional SCNs and 1 papillary microcarcinoma (mPTC) were also found in the left lobe. In the oncocytic SCNs, 80% of the main cells showed oncocytic cytoplasm immunoreactive for porin and proteins of the SDHB and SDHA genes. Positivity for cytokeratin 19, p63, galectin-3, and HBME-1 and negativity for thyroglobulin, thyroperoxidase, vimentin, Oct-4, and α-fetoprotein were found in oncocytic and conventional SCNs. An inverse correlation was found between oncocytic metaplasia and p63. Association with C cells was confirmed at protein and messenger RNA levels in both types of SCNs. No germinal mutation of GRIM-19 was detected. No somatic BRAF mutation was found in any of the SCNs nor in the mPTC. We conclude that SCNs may acquire mitochondrial alterations similar to those seen in follicular and C cells, as well as in their respective tumors.

Clinical review: The emerging cell biology of thyroid stem cells

CONTEXT

Stem cells are undifferentiated cells with the property of self-renewal and give rise to highly specialized cells under appropriate local conditions. The use of stem cells in regenerative medicine holds great promise for the treatment of many diseases, including those of the thyroid gland.

EVIDENCE ACQUISITION

This review focuses on the progress that has been made in thyroid stem cell research including an overview of cellular and molecular events (most of which were drawn from the period 1990-2011) and discusses the remaining problems encountered in their differentiation.

EVIDENCE SYNTHESIS

Protocols for the in vitro differentiation of embryonic stem cells, based on normal developmental processes, have generated thyroid-like cells but without full thyrocyte function. However, agents have been identified, including activin A, insulin, and IGF-I, which are able to stimulate the generation of thyroid-like cells in vitro. In addition, thyroid stem/progenitor cells have been identified within the normal thyroid gland and within thyroid cancers.

CONCLUSIONS

Advances in thyroid stem cell biology are providing not only insight into thyroid development but may offer therapeutic potential in thyroid cancer and future thyroid cell replacement therapy.

Small-cell (basaloid) thyroid carcinoma: a neoplasm with a solid cell nest histogenesis?

In the past, undifferentiated (anaplastic) carcinoma of the thyroid included a small-cell variant with either a diffuse or a compact morphology. After the mid-1980s, with the advance of immunohistochemistry, almost all those rare tumors with the former characteristic were reclassified as low-grade lymphomas and those with the latter characteristic as small-cell variants of medullary carcinoma and poorly differentiated ("insular") carcinoma. Since then, no primary thyroid small-cell carcinoma has been reported in the literature, with the exception of a case of a small-cell (neuroendocrine) carcinoma, an exceedingly rare neoplasm akin to medullary carcinoma, with expression of neuroendocrine markers but lacking calcitonin immunoreactivity. Here, the authors report a primary small-cell carcinoma of the thyroid displaying a basaloid appearance and lacking any signs of neuroendocrine or C-cell differentiation.

Inmunohistochemical profile of solid cell nest of thyroid gland

It is widely held that solid cell nests (SCN) of the thyroid are ultimobranchial body remnants. SCNs are composed of main cells and C cells. It has been suggested that main cells might be pluripotent cells contributing to the histogenesis of C cells and follicular cells, as well as to the formation of certain thyroid tumors. The present study sought to analyze the immunohistochemical profile of SCN and to investigate the potential stem cell role of SCN main cells. Tissue sections from ten cases of nodular hyperplasia (non-tumor goiter) with SCNs were retrieved from the files of the Hospital Infanta Luisa (Seville, Spain). Parathormone (PTH), calcitonin (CT), thyroglobulin (TG), thyroid transcription factor (TTF-1), galectin 3 (GAL3), cytokeratin 19 (CK 19), p63, bcl-2, OCT4, and SALL4 expression were evaluated by immunohistochemistry. Patient clinical data were collected, and tissue sections were stained with hematoxylin-eosin for histological examination. Most cells stained negative for PTH, CT, TG, and TTF-1. Some cells staining positive for TTF-1 and CT required discussion. However, bcl-2, p63, GAL3, and CK 19 protein expression was detected in main cells. OCT4 protein expression was detected in only two cases, and SALL4 expression in none. Positive staining for bcl-2 and p63, and negative staining for PTH, CT, and TG in SCN main cells are both consistent with the widely accepted minimalist definition of stem cells, thus supporting the hypothesis that they may play a stem cell role in the thyroid gland, although further research will be required into stem cell markers. Furthermore, p63 and GAL-3 staining provides a much more sensitive means of detecting SCNs than staining for carcinoembryonic antigen, calcitonin, or other markers; this may help to distinguish SCNs from their mimics.

Clinical and pathological implications of concurrent autoimmune thyroid disorders and papillary thyroid cancer

Cooccurrences of chronic lymphocytic thyroiditis (CLT) and thyroid cancer (DTC) have been repeatedly reported. Both CLT and DTC, mainly papillary thyroid carcinoma (PTC), share some epidemiological and molecular features. In fact, thyroid lymphocytic inflammatory reaction has been observed in association with PTC at variable frequency, although the precise relationship between the two diseases is still debated. It also remains a matter of debate whether the association with a CLT or even an autoimmune disorder could influence the prognosis of PTC. A better understanding about clinical implications of autoimmunity in concurrent thyroid cancer could raise new insights of thyroid cancer immunotherapy. In addition, elucidating the molecular mechanisms involved in autoimmune disease and concurrent cancer allowed us to identify new therapeutic strategies against thyroid cancer. The objective of this article was to review recent literature on the association of these disorders and its potential significance.

Tumor-in-Tumor of the Thyroid With Basaloid Differentiation: A Lesion With a Solid Cell Nest Neoplastic Component?

This study describes an encapsulated thyroid tumor having 3 different concentric appearances in a 70-year-old man. The most peripheral neoplasm showed features of a microfollicular adenoma; the intermediate lesion displayed typical features of the follicular variant of papillary thyroid carcinoma (FVPTC); and the main, central neoplasm, showed a basaloid configuration. At variance with the other neoplasms, the latter component exhibited high mitotic activity. Necrosis, angio-invasion, or capsular invasions were not observed. The centrally located neoplasm was immunoreactive for several cytokeratins, p63, carcinoembryonic antigen, and galectin-3, and negative for thyroid transcription factor 1, thyroglobulin, calcitonin, CD5, and CK20, featuring a phenotype similar to that of thyroid solid cell nests. A N-RAS mutation was found both in the basaloid and in the FVPTC components. The clinicopathological and immunohistochemical data ruled out the alternative possibilities of intrathyroidal metastasis and tumor from ectopic (thymic, parathyroid, or salivary gland) tissues.

Development of thyroid gland and ultimobranchial body cyst is independent of p63

The ultimobranchial body (UBB) and thyroid primordium are the origins of the thyroid gland that fuse around embryonic day 14.5 of mouse gestation, ultimately giving rise to calcitonin-producing C cells and thyroglobulin-producing follicular cells, respectively. A homeodomain transcription factor NKX2-1 is expressed both in the UBB and the thyroid primordium, and is critical for development of the thyroid gland. In this study, the role of p63 in development of UBB and the thyroid gland was analyzed by histological, immunohistochemical, and electron microscopic analyses using mice with various combinations of Nkx2-1 and p63 wild-type, heterozygous, and null alleles. In the absence of p63, a normal thyroid gland develops, as revealed by expression of thyroglobulin and calcitonin, thus showing that p63 is not required for thyroid development. However, in mice carrying the Nkx2-1-null allele, the UBB remains as a cystic vesicular structure and/or in nested patterns consisting of p63-positive cells surrounding the vesicle and undifferentiated immature cells with occasional cilia lying inside. The cystic UBB was present even in the Nkx2-1;p63 double-null mice. The structure and p63 expression pattern of the UBB cyst strikingly resemble the solid cell nest. These results show that in the absence of NKX2-1, UBB becomes cystic independent of p63, which is likely the origin of SCN.

Thyroid-type solid cell nests in struma ovarii

Solid cell nests (SCNs) of the thyroid are single or multiple foci of solid and/or cystic clusters of squamoid cells (main cells) with a minor proportion of C-cells, found in the normal thyroid. The SCNs have also been reported in the heart as an ultimobranchial heterotopia. Here, the authors describe a case of thyroid-type SCNs associated with struma ovarii. Main cells were positive for simple and stratified epithelial-type cytokeratins, carcinoembryonic antigen, carbohydrate antigen 19.9, p63, bcl-2, and galectin-3. The neuroendocrine cell population was positive for chromogranin A and synaptophysin but negative for calcitonin, suggesting a common ancestor cell capable of dual differentiation toward thyroid follicular cells and hindgut-type endocrine cells. The existence of thyroid-type SCNs in struma ovarii could be easily understood by considering the struma ovarii as a teratoma; at the same time, these findings also support the idea of a close histogenetic link between the main cells of SCNs and thyroid tissue.

Thyroid stem cells and cancer

BACKGROUND

Thyroid gland development and function are essential for life, and recent findings indicate the presence of stem/progenitor cells within the thyroid gland as a potential source of tissue regeneration and cancer formation.

SUMMARY

This review summarizes the current knowledge on early differentiation of thyroid cells from embryonic stem cells and highlights exciting concepts and recent novel findings on adult thyroid stem/progenitor cells in the normal thyroid gland and in thyroid cancer. Other potential sources and markers of stem/progenitor cells in the thyroid include bone marrow, microchimerism, and embryological remnant-derived multifocal solid cell nests. Finally, we discuss new therapeutic strategies that target thyroid cancer stem cells.

CONCLUSIONS

Thyroid stem/progenitor cell populations are present in the normal and diseased thyroid gland. Advances in normal and cancer thyroid stem cell biology will be essential for future targeted therapies.

Solid cell nests in Hashimoto's thyroiditis sharing features with papillary thyroid microcarcinoma

Solid cell nests (SCN) associated with Hashimoto's thyroiditis may show some atypical nuclear features including prominent nuclear grooves, enlarged overlapping nuclei and nuclear clearing. These features are sometimes mistaken for papillary thyroid microcarcinomas especially when the SCN are numerous. We reviewed SCN associated with Hashimoto's thyroiditis in 12 patients selected from 1,420 archival routinely processed formalin-fixed, paraffin-embedded thyroid specimens of Hashimoto's thyroiditis in which there was more than ten SCN per slide. In addition to the atypical nuclear features, there was a distinct eosinophilic basement membrane surrounding the SCN. Immunohistochemical analysis showed that the SCN were strongly positive for p63, stained weakly for TTF-1 and were negative for thyroglobulin, HBME-1, and calcitonin. This was compared to papillary thyroid microcarcinomas which were strongly positive for thyroglobulin, TTF-1, HBME-1, and variably positive for p63, while calcitonin and chromogranin were negative. These histological and immunophenotypic features can be used to distinguish SCN from papillary thyroid microcarcinomas associated with Hashimoto's thyroiditis.

[Mucoepidermoid carcinoma of the thyroid: a case report and literature review]

The mucoepidermoid carcinoma is a neoplasia that usually occurs at salivary glands, breast, pancreas and gastrointestinal tract. The primary occurrence on thyroid gland is rare and only 33 cases were previously published. Although the majority of cases of mucoepidermoid carcinoma of the thyroid (MECT) show a benign evolution, this paper describes a patient with an aggressive tumor. A literature review over clinical, histopathological, immunohistochemical features and histogenetic origin was discussed.

Origin of the ultimobranchial body cyst: T/ebp/Nkx2.1 expression is required for development and fusion of the ultimobranchial body to the thyroid

The ultimobranchial body (UBB) is an outpocketing of the fourth pharyngeal pouch that fuses with the thyroid diverticulum, giving rise to calcitonin-producing C-cells. In this study, we demonstrate that the UBB is composed of two types of cells: one expressing T/ebp/Nkx2.1 and the other expressing p63. The former cell type, accounting for a majority of the UBB, requires T/ebp/Nkx2.1 for their survival. In contrast, the p63-positive cells, even in the absence of T/ebp/Nkx2.1 expression, can proliferate and give rise to a vesicular structure that is lined by a monolayer of p63-negative cells, surrounded by a cluster and/or single layer of p63-positive cells, displaying the basal/stem cell phenotype. T/ebp/Nkx2.1 haploinsufficiency causes abnormal fusion of the UBB with the thyroid diverticulum, which stays as a cluster of C-cells around the vesicular structure, similar to the one observed in mice null for T/ebp/Nkx2.1 expression. These results demonstrate that T/ebp/Nkx2.1 plays a role in the survival of UBB cells, their dissemination into the thyroid diverticulum, and the formation of UBB-derived vesicular structure.

Pitfalls in thyroid tumour pathology

This review provides an itemized listing of major diagnostic pitfalls in the field of thyroid tumour pathology, emphasizing the features that the authors have found most useful in their recognition and avoidance.

Complex Choristoma of the Gyrus Rectus

We report 2 cases of an unusual central nervous system lesion involving the gyrus rectus and characterized by well-differentiated epithelial and mesenchymal components. One patient was a 30-year-old woman, and the other was a 42-year-old man. Both presented with seizures. Radiologically, the lesions were solid, with ill-defined margins and increased signal on T2-weighted MRI. Both were contrast enhancing. There was no significant mass effect or edema. Histopathologically, the epithelial component was characterized by a tubule-forming, stratified epithelium. The lumina of the tubules were lined, in some areas, by a distinct layer of mature keratinocytes. Smooth muscle actin-immunoreactive myoepithelial cells were present in the basal layer, indicating glandular differentiation. Some lumina were cystically dilated and contained anucleate squames and/or PAS-positive material. In some areas, the epithelial structures were separated by reactive glial tissue resembling pilocytic astrocytoma, with prominent, piloid processes, Rosenthal fibers, and gemistocytic astrocytes. In other areas, glial nuclei formed small clusters reminiscent of subependymoma, but no glioma was identified in either case. Tissue resembling mature adipose was present focally. No mitotic figures were identified. Follow-up interval of up to 2 years showed no evidence of recurrent tumor. By location and histologic features, this lesion appears to represent a rare, distinct clinicopathologic entity, perhaps derived from misplaced cranial neural crest cells.

Branchial-like cysts of the thyroid associated with solid cell nests

Presented herein is the case of a 65-year-old man with a 20 year history of thyroid hypofunction. On sonography a cystic lesion 4 x 4 x 5 cm in largest diameter was found, replacing most of the right lobe of the thyroid gland. Microscopically, the lesion was composed of labyrinth-like cystic structures (LCS) lined by a few layers of benign-appearing squamous cells and filled by mucinous material. Adjacent to the cyst walls were discontinuous patches of a lymphoid tissue, composed of haloed centrocyte-like cells or germinal centers mostly depleted of germinal cells. Additionally, there were numerous squamous cell nests equivalent to solid cell nests (SCN), all of which were surrounded by a similar-looking lymphoid tissue. Rare SCN were thus cystically changed and contained a small amount of mucus. The SCN communicated with the LCS: the former represented the most distal outpouchings of the latter. The epithelial structures were surrounded by a loose collagenous adipocytic stroma with plump fibroblasts, which resembled the stroma often seen in lateral neck cysts associated with structures such as cartilage, accessory salivary gland tissues, cysts and accessory thyroid and thymus. Immunohistochemically, all lesional elements were negative for calcitonin and thyroglobulin. The results of the paper suggest that branchial cleft-like cyst have an origin in the ultimobranchial body.

Telomerase expression and proliferative activity suggest a stem cell role for thyroid solid cell nests

Solid cell nests of the human thyroid gland are composed of main cells and C cells. In order to investigate the putative stem cell nature of the role for solid cell nests, we evaluated the histological features, and the immunohistochemical expression of p63, bcl-2, telomerase catalytic subunit, and two proliferative markers (Ki-67 and minichromosome maintenance protein 2), in a series of 24 cases of solid cell nests. Proliferative indices were determined in (a) solid cell nests, (b) thyroid follicular cells in the vicinity of solid cell nests within a low-power field, and (c) distant thyroid tissue, at a distance of at least three low-power fields from solid cell nests. In 15 cases of solid cell nests (62.5%), mixed follicles were observed; papillary formations were observed in four cases (16.6%), and ciliated cells were observed in the lining of microcysts associated with two cases (8.3%). Salivary gland-type tissue, cartilage islands, adipose and fibrous tissues, and small nerves were also associated with some cases of solid cell nests. We observed that the main cells of the solid cell nests express consistently telomerase, although at lower levels than p63, and show strong cytoplasmic immunoreactivity for bcl-2, which is associated with an increased differentiation potential. We also observed that despite their relative low proliferative index, main cells of the solid cell nests display higher proliferation than follicular cells in the vicinity and follicular cells in more distant thyroid tissue. We conclude that main cells of the solid cell nests apparently harbor the minimal properties of a stem cell phenotype (capacity for both self-renewal, conferred by telomerase activity, and differentiation to one or more than one type of specialized cells, given by the high expression of p63 and bcl-2) and may thus represent a pool of stem cells of the adult thyroid.

Immunohistochemical detection of p53 homolog p63 in solid cell nests, papillary thyroid carcinoma, and hashimoto's thyroiditis: A stem cell hypothesis of papillary carcinoma oncogenesis

Most models suggest that the cell of origin of papillary carcinoma is the mature thyroid follicular epithelial cell. In a recent study, p63 was detected in papillary carcinoma, Hashimoto's thyroiditis, and in squamoid aggregates and solid cell nests (SCNs), embryonic remnants found sporadically in the fully developed thyroid. In the present study, the relationship between solid cell nests and papillary carcinoma was investigated further. Four-micrometer sections from 88 routinely fixed and processed archival thyroidectomy specimens were pretreated with citric acid pH 6.0 for antigen retrieval, then incubated overnight with anti-p63 monoclonal antibody 4A4. Slides were stained with a streptavidin-biotin kit and diaminobenzidine as chromogen and were counterstained with hematoxylin. Squamoid aggregates or SCNs were noted in 21 specimens. Several morphologic variants of SCNs were found, all of which displayed p63 positivity. These included undifferentiated SCNs and those displaying commitment toward squamoid and ciliated glandular differentiation. Small, morphologically inconspicuous aggregates of p63-positive cells were commonly found in Hashimoto's thyroiditis. Commitment of p63-positive undifferentiated cells toward thyroid follicular epithelial differentiation was occasionally noted. One SCN variant, also associated with Hashimoto's thyroiditis, was a floretlike arrangement of p63-positive cells with fusiform nuclei. p63 staining was strong and uniform in some SCNs, but in other SCNs it was compartmentalized and homologous to stem cell-staining patterns in normal squamous or bronchial epithelia. Stem cell-like staining, associated with compartmentalized p63 staining or p63-positive undifferentiated cells, was noted in 7 of 27 papillary carcinomas. p63 immunostaining is a highly sensitive means of detecting SCNs. p63 expression patterns in SCNs and a subset of papillary carcinomas are closely homologous to stem cell-associated p63 staining patterns that have been described elsewhere in squamous and bronchial epithelia. We propose a stem-cell-associated model of papillary carcinoma oncogenesis that suggests that (1) p63-positive embryonal remnants rather than mature follicular cells are the cells of origin of a subset of papillary carcinomas; (2) these p63-positive cells are pluripotent and may stay undifferentiated or undergo benign squamoid or glandular maturation, may undergo thyroid follicular epithelial differentiation, may undergo oncogenic change leading to papillary carcinoma, or may trigger an immune reaction, resulting in lymphoid infiltration and Hashimoto's thyroiditis; and (3) Hashimoto's thyroiditis and papillary carcinoma may therefore be linked etiologically, because both disorders may be initiated by the same population of pluripotent p63-positive embryonal stem cell remnants.

p63 expression in solid cell nests of the thyroid: further evidence for a stem cell origin

Solid cell nests of the thyroid are embryonic remnants of endodermal origin that may be difficult to distinguish from squamous metaplasia, metastatic squamous carcinoma, papillary microcarcinoma, medullary carcinoma, and C-cell hyperplasia. These embryonic structures are composed of main cells and C-cells; cystic structures and mixed follicles are sometimes observed intermingled with solid cell nests. Recently, p63, a p53 homologue that is consistently expressed in basal/stem cells of stratified epithelia and plays a major role in triggering the differentiation of some specific cell lineages, has been characterized. We evaluated the immunohistochemical expression of p63, cytokeratins (CAM 5.2, AE1/AE3, 34betaE12, 7, and 20), carcinoembryonic antigen, thyroid transcription factor 1 (TTF-1), thyroglobulin, and calcitonin using the streptavidin-biotin-peroxidase complex technique in 6 bona fide solid cell nests. We observed that main cells of solid cell nests are strongly decorated by p63, while C-cells and all other thyroid structures were consistently negative. Moreover, main cells expressed carcinoembryonic antigen and all cytokeratins but cytokeratin 20 and lacked TTF-1, thyroglobulin and calcitonin. In contrast to this, C-cells of solid cell nests were immunoreactive for calcitonin, CAM 5.2, AE1/AE3, and cytokeratin 7; focal immunoreactivity for TTF-1 was also observed in some C-cells. We conclude that main cells of the solid cell nests display a basal/stem cell phenotype (p63 and basal cytokeratin positivity), whereas C-cells show features of parafollicular differentiation. We conclude, furthermore, that p63 antibodies may help in distinguishing solid cell nests from their mimics.

Immunocytochemical study of parafollicular (C) cells of the thyroid in some wild rodents

Studies were done on 3 wild species of rodents: field voles (Microtus agrestis, Linnaeus 1761), bank voles (Clethrionomys glareolus, Schreber 1780), and forest mice (Apodemus flavicollis, Melchior 1834). Immunocytochemical reactions were used to detect calcitonin (CT), calcitonin gene-related peptide (CGRP), neuron-specific enolase (NSE), chromogranin A (CgA) in the thyroid parafollicular (C) cells in all species examined. Antisera to human CT, rat CGRP, bovine CgA, rat NSE and human NSE give probably a positive reaction in all C cells in the rodents examined. However, in the NSE determining reaction, much feebler positive C cells were observed. Individual variation in respect of shape and distribution of C cells was observed in all species. In forest mice the C cells resembled in shape the C cells previously described in mouse and rat. In field voles and bank voles the C cells were assembled into small groups more often than in forest mice. Antibodies (anti-human CT, anti-rat CGRP, anti-bovine CgA, anti-rat NSE and anti-human NSE) used by us were good markers of C cells in the thyroids of the 3 species of free-living rodents examined.

Intrathyroidal lymphoepithelial cyst. A report of two cases not associated with Hashimoto's thyroiditis

Two cases of intrathyroidal lymphoepithelial cyst are described. Both of them were solitary, one being found incidentally in a patient operated on for a multinodular goiter, the other being clinically obvious as a cold nodule. They exhibited features of cysts of branchial cleft origin, i.e. squamous cell lining epithelium and abundant lymphoid tissue with reactive germinal centers. The thyroid gland parenchyma showed a discrete lymphoid infiltration consistent with the diagnosis of focal lymphocytic thyroiditis. In the first case a single epidermoid solid cell nest was found. The histogenesis of intrathyroidal lymphoepithelial cysts remains unclear, but their origin from cystically degenerated ultimobranchial body remnants (solid cell nests) seems to be most probable. This assumption is supported by a similar immunohistochemical profile of solid cell nests and epithelial cells lining the cysts and also by the presence of one solid cell nest in the proximity to the cyst in one of our cases.

Immunocytochemical study of parafollicular cells of the thyroid and ultimobranchial remnants of the European bison

The aim of the present study was to compare parafollicular cells in the bison thyroid and its ultimobranchial remnants. The thyroid of 26 European bisons was fixed in Bouin's fluid, 5 microns thick paraffin sections were stained with hematoxylin and eosin. Azan or silver Grimelius methods. For immunocytochemical analysis specific rabbit antisera were used against human calcitonin (CT), human calcitonin gene-related peptide (CGRP), bovine (b) or rat (r) neuron-specific enolase (NSE), human synthetic somatostatin (ST), and porcine chromogranin. Strongly positive reactions in the majority of parafollicular cells were observed after application of antisera against CT, CGRP, bNSE and rNSE only. ST-immunopositive cells were found in small numbers. Immunopositive parafollicular cells were also present outside typical structures of the thyroid within persistent ultimobranchial remnants. In persistent ultimobranchial bodies, parafollicular cells were frequently observed in groups between ultimobranchial follicles in form of solid cell nests. Many of these cells did not react with any of the antisera used and showed features of immature cells. It is concluded that histomorphologic analysis and immunocytochemical examination reveals a heterogeneous population of parafollicular cells in the bison thyroid, and this heterogeneity was particularly clear in persistent ultimobranchial bodies.

Bilateral Thyroid and Ultimobranchial Medullary Carcinoma

The ultimobranchial bodies in human embryos develop from the fourth and fifth branchial pouch complexes along with thymic and parathyroid tissue. They become incorporated within the lateral thyroid lobes and are believed to be involved in the development of C-cells. We report a case of an unusual bilateral thyroid and neck prelaryngeal medullary carcinoma in a 23-year-old male patient who belongs to a multiple endocrine neoplasia type 2a (MEN type 2a) family with thyroid tumors and pheochromocytomas. The medullary carcinoma was located in an abnormal cystic structure that seems to be a remnant of the ultimobranchial body (UBB) in the neck. Within the contralateral thyroid lobe, the medullary carcinoma was associated with C-cell hyperplasia.