Volume 52 (5): 641-652, 2004 Copyright ©The Histochemical Society, Inc. The Role of Hoxa3 Gene in Parathyroid Gland Organogenesis of the Mouse
Department of Anatomy, Kitasato University School of Medicine (YK,YA,TN), Sagamihara, Kanagawa, and Department of Cell and Developmental Biology, Graduate School of Biostudies, Kyoto University (OC), Kyoto, Japan Correspondence to: Yoko Kameda, Dept. of Anatomy, Kitasato University School of Medicine, Sagamihara, Kanagawa 228-8555, Japan. E-mail: kameda{at}med.kitasato-u.ac.jp
Mice with a targeted deletion of the Hoxa3 gene have defects of derivatives of the third branchial arch and pouch. To address the role of the Hoxa3 gene in parathyroid organogenesis, we examined the third pharyngeal pouch development by immunohistochemistry (IHC) using the secretory protein (SP)-1/chromogranin A antiserum, which recognizes the parathyroid from its initial formation onward. At embryonic day (E) 11.5, the SP-1/chromogranin A-immunoreactive primary rudiment of the parathyroid appeared in the cranial region of the third pharyngeal pouch of wild-type embryos. In Hoxa3-null mutants, the third pharyngeal pouch was normally formed but failed to differentiate into the parathyroid rudiment, showing no immmunoreactivity for SP-1/chromogranin A. Classic studies using chickquail chimeras have demonstrated that the ectomesenchymal neural crest cells are required for proper development of the pharyngeal pouch-derived organs, including the thymus and parathyroid glands. To visualize the migration and development of mesenchymal neural crest cells in Hoxa3 mutants, the heterozygotes were crossed with connexin43lacZ transgenic mice in which ß-galactosidase expression was specific to the neural crest cells. In Hoxa3 homozygotes and in wild types, ectomesenchymal neural crest cells densely populated the pharyngeal arches, including the third one, and surrounded the third pouch epithelium. These results indicate that lack of the Hoxa3 gene affects the intrinsic ability of the third pharyngeal pouch to form the parathyroid rudiment and has no detectable effect on the migration of neural crest cells. (J Histochem Cytochem 52:641651, 2004)
Key Words: Hoxa3 parathyroid thymus third pharyngeal pouch SP-1/chromogranin A connexin43lacZ transgenic mice neural crest cells apoptosis
THE PARATHYROID GLANDS synthesize and secrete parathyroid hormone (PTH), which is essential for regulation of serum calcium concentration. The parathyroid and thymus primordia develop from the third pharyngeal pouch in mice (Cordier and Haumont 1980
Chromogranin A, the major secretory protein of adrenal chromaffin cells, is a member of the granin/secretogranin family of acidic glycoproteins that participate in the storage and secretion of peptide hormones and are expressed in many endocrine and neuroendocrine cells (O'Connor et al. 1983
Hoxa3 belongs to the Hox family of transcription factors that play multiple roles in the segmental processes of anteroposterior patterning (Krumlauf 1994
It has been demonstrated by studies of chickquail chimeras that the mesenchymal components of thymus lobes and parathyroid glands of avian species are derived from the ectomesenchymal neural crest cells (Le Lièvre and Le Douarin 1975 The present study first clarified the organogenesis of parathyroid glands in wild-type mouse embryos using the SP-1/chromogranin A antiserum as a specific marker for parathyroid cells. Furthermore, we visualized in Cx43lacZ transgenic mice whether or not mesenchymal neural crest cells enter the parathyroid and thymus primordia to control the growth of these organs. The main purpose of this study was to examine the etiology of the parathyroid deficiency in the Hoxa3-null mutants. Migrations of neural crest cells into the pharyngeal arches and apoptosis of the third pouch endoderm were estimated in Hoxa3 homozygous mutants in comparison with wild-types.
Animals Targeted disruption of the Hoxa3 gene, production of chimeric mice, and determination of genotype by polymerase chain reaction have been reported previously (Chisaka and Capecchi 1991 For histological study, the specimens were fixed in Bouin's solution or 8% paraformaldehyde (PFA) in phosphate buffer (PB) for 2448 hr, embedded in paraffin, and then serially sectioned in the frontal or sagittal plane at a thickness of 5 µm. Selected sections were stained with hematoxylineosin to help determine morphological orientation.
TUNEL Assay
Immunohistochemistry
To determine the etiology of late-term absence of the parathyroid gland in Hoxa3-null mutant mice, we examined the differentiation and development of the third pharyngeal pouch using the SP-1/chromogranin A antiserum as a specific marker for the parathyroid rudiment. The fusion of the endoderm of the third pharyngeal pouch and the ectoderm of the third cleft occurs by E 9.5 (Cordier and Haumont 1980
At E 11.5, the third pharyngeal pouch was separated from the pharynx in many wild-type embryos, forming an elongated follicular structure (Figure 2A) . A bulge of the primary parathyroid rudiment was detected in the cranial region of the third pharyngeal pouch. The rudiment was composed of multiple layers of cuboidal cells immunoreactive for SP-1/chromogranin A (Figure 2B). On the other hand, in Hoxa3 homozygotes at E 11.5, the third pharyngeal pouch was still attached to the pharynx and retained the fissure-like structure (Figure 2C). The third pouch endoderm consisted of high columnar cells and there was no aggregation of cuboidal cells immunoreactive for SP-1/chromogranin A (Figure 2D).
In addition to the differentiation of the third pharyngeal pouch, the Hoxa3 gene appears to play roles in the development of the third pharyngeal arch. In E 11.5 wild-types, the third pharyngeal cleft fused with the second one and the third pharyngeal arch merged into the fourth one (Figure 3A) . Therefore, the third pharyngeal arch of wild-types became indistinct at this stage. In contrast, Hoxa3 homozygous mutants at E 11.5 still had the distinct third pharyngeal arch partitioned by the third and second pharyngeal pouches and clefts, showing the delayed differentiation of the third arch (Figure 3C).
The migration of ectomesenchymal neural crest cells into the pharyngeal arches was examined by immunostaining with the monoclonal anti-ß-galactosidase antibody (Figures 3B and 3D). The ß-galactosidase-positive neural crest cells densely populated the third pharyngeal arch in Hoxa3 homozygotes as well as wild-types at E 11.5 (Figures 3B and 3D). In both wild-type and Hoxa3 mutant embryos, the neural crest cells surrounded the parathyroid/thymus primordia or the third pharyngeal pouch but never invaded them. Subsequently, the time course of parathyroid/thymus primordia development was determined in wild-type embryos. At E 12.0, the thymus rudiment originating from the third pharyngeal pouch became clear and extended in a caudal direction. The thymus still displayed a follicle structure in which the epithelium had many mitotic figures. The parathyroid rudiment was attached to the cranial part of the thymus (Figure 4A) . At E 12.5, the thymus rudiment was significantly increased in size and formed a solid cell cluster. It began to move towards the anterior thoracic cavity. The parathyroid rudiment was localized on the top of the thymus (Figure 4B). At E 13.5, owing to a caudal migration of the thymus, the parathyroid rudiment contiguous with the top of the thymus was situated at the side of the thyroid and ultimobranchial glands (Figure 4C). At E 14.5, the parathyroid joined with the lateral side of the thyroid gland and was completely separated from the thymus, showing a normal position in mature mice (Figure 4D).
In Hoxa3-null mutants at E 12.0, differentiation of the third pharyngeal pouch did not occur and SP-1/chromogranin A-immunoreactive parathyroid cells were never detected in the third pouch, remaining as rudimentary tissue. Because the separate second and third clefts were still distinct in the mutants, the third pharyngeal arch was discriminated clearly even at E 12.0 (Figure 5A) . A small thymus rudiment sometimes appeared in the ectodermal epithelium of the third cleft (Figure 5A). On and after E 12.5, the third pharyngeal pouch disappeared. Therefore, no parathyroid rudiment was formed in the Hoxa3-null mutants (Figure 5B). The thymus lobes were rarely observed in the E 12.514.5 null mutants, although their size was far smaller than controls.
TUNEL assay revealed that high levels of apoptosis occurred in the pharyngeal pouch endoderm, including the second, third, and fourth pouches, in wild-types at E 11.5. In Hoxa3-null mutants at E 11.512.0, there was increased apoptosis in the third pouch relative to age-matched control embryos (Figures 6A and 6B) . Apoptotic cells frequently aggregated in large clusters in the pouch epithelium (Figures 6C and 6D).
Chromogranin A immunoreactivity is present in virtually all enteroendocrine cells (PortelaGomes et al. 1997
Hoxa3 mRNA is expressed in the third pharyngeal pouch endoderm (Manley and Capecchi 1995
Some candidates for signaling molecules involved in the parathyroid organogenesis have been reported. Glial cells missing (Gcm) 2, a mouse homologue of Drosophila Gcm, is the transcription factor demonstrated in the parathyroid gland in late stages of fetal development (Kim et al. 1998
In human 22q11 deletion syndromes, hypoplastic or aplastic thymus and parathyroid glands are characteristic phenotypic abnormalities (Scambler 2000
In Hoxa3 homozygotes crossed with Cx43lacZ transgenic mice, ectomesenchymal neural crest cells immunoreactive for ß-galactosidase densely populated the pharyngeal arches as in wild-type embryos. The migration of mesenchymal neural crest cells into the third arch was not disturbed in the Hoxa3 homozygotes. In avian species, neural crest cells invade the thymus and parathyroid glands and give rise to glandular connective tissue (Le Lièvre and Le Douarin 1975
The endodermal epithelia of the pharyngeal pouches in both wild-type and homozygous embryos showed many apoptotic cells. The differentiation and growth of the pharyngeal pouch-derived organs appear to be controlled by apoptosis. In particular, increased apoptotic cells were detected in the third pouch epithelium showing delayed development in the E 11.512.0 mutants. Therefore, the third pouch endoderm is unable to differentiate in the absence of the Hoxa3 gene and is eventually lost by apoptosis. Although a parathyroid rudiment was never detected, rudimentary thymus lobes were rarely encountered in the null mutants at E 12.514.5. The thymus is formed by the third pouch endoderm fused with the ectodermal cells of the third cleft (Cordier and Haumont 1980
In contrast to the third pharyngeal pouch, the development and differentiation of the fourth pouch were normal in Hoxa3-null mutants. The bilateral ultimobranchial bodies derived from the fourth pharyngeal pouch were always formed and joined with the thyroid glands at E 13.5. At E 14.5, the C-cells immunoreactive for PGP 9.5 began to disperse within the thyroid parenchyma in the Hoxa3 mutants as well as wild-types (Kameda et al. unpublished data). In both wild-type and mutant mice, however, SP-1/chromogranin A immunoreactivity appeared in the C-cells at late stages of fetal development. Furthermore, the appearance of calcitonin immunoreactivity in the C-cells was very late, i.e., at birth. In accordance with our data, Xu et al. (2002)
Received for publication September 15, 2003; accepted December 31, 2003
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