The development of the human hyoid-larynx complex revisited.

OBJECTIVES/HYPOTHESIS: The hyoid-larynx complex is highly prone to anatomical variation. The etiology of anatomical variants such as Eagle's syndrome and the aberrant hyoid apparatus can be explained from embryonic development. Modern textbooks state that the hyoid bone body develops from the second and third pharyngeal arch cartilages, and that thyroid cartilage derives from the fourth and sixth arch cartilages. This description, however, is incompatible with various anatomical variants, and it is unclear whether it was based on observations in human embryos or on comparative embryology. STUDY DESIGN: 14 human embryos from the Carnegie collection between Carnegie stage 17 and 23 (42-60 days) were selected based on their histological quality. METHODS: Histological sections of the selected embryos were examined. Three-dimensional models were prepared in an interactive format. These anatomical models provide crucial spatial information and facilitate interpretation. RESULTS: We observed a less-complicated development of the hyoid-larynx complex than is currently described in textbooks. The body of the hyoid bone originates from a single growth center, without overt contributions from second and third pharyngeal arch cartilages. The fourth and sixth arch cartilages were not detected in human embryos; the thyroid and cricoid cartilages develop as mesenchymal condensations in the neck region. CONCLUSIONS: Despite new research techniques, theories about hyoid-larynx complex development from the beginning of the 20th century have not been refuted properly and can still be found in modern literature. Based on observations in human embryos, we propose a new and relatively simple description of the development of the hyoid-larynx complex to facilitate better understanding of the etiology of anatomical variants. LEVEL OF EVIDENCE: NA Laryngoscope, 1829-1834, 2018.

The anterior commissure of the human larynx revisited.

BACKGROUND: The anterior commissure (AC) of the human larynx is usually understood as an area of the glottis anteriorly situated between the two vocal folds inserting to the thyroid cartilage (TC). The pattern of spread of AC carcinoma could hypothesize that AC could include other structures of the ventral larynx as developmental anatomy could demonstrate. METHODS: Ten adult larynx specimens from donation program cadavers and 15 selected fetal specimens (from 11 to 34 weeks of amenorrhoea) were studied using microdissection and histological serial sections stained with hematoxylin-eosin and reticuline. RESULTS: In adult specimens, internal perichondrium of the TC was easily detached from the entire lateral lamina but not from an intermediate lamina superiorly marked by the superior thyroid notch. On this median band of TC is inserted the ventral connective tissue of the three levels of the larynx: the ventral part of the vocal folds with the anterior macula flava, the Broyle's epiglottic ligament, and the subglottic part of the conoid ligament. In young fetuses (11-12 weeks), intermediate lamina of TC joined at the glottic level but not at the supraglottic level; in fetuses at 22-25 weeks, a meshwork of bundles of connective fibers inserted to the intermediate lamina of TC. In fetuses at 33-34 weeks, organization is practically identical to adult specimens. CONCLUSION: According to the adult anatomical features and the organogenesis, the AC of the human larynx could be anatomically defined ventrally as being made up of the intermediate lamina of TC beneath the superior thyroid notch and dorsally the ventral insertions of vocal folds with macula flava, supraglottic Broyle's ligament, and subglottic conoid ligament leading to a "developmental" AC definition which could better explain specific spreading of AC carcinoma.

Changes of laryngeal parameters during intrauterine life.

OBJECTIVE: Increased care of fetal and neonatal airways has led to advances in neonatal medicine. The early diagnosis and treatment of respiratory diseases require a detailed knowledge of fetal airway anatomy and development. The aim of this study was to determine the anatomical development of the thyroid and cricoid cartilages and their structural variability during fetal life. MATERIAL AND METHODS: The study was performed on the thyroid and cricoid cartilages of 55 human fetal larynges of both sexes, between the ages of 13 and 27 weeks of intrauterine life. Numerous measurements of the thyroid and cricoid cartilages were performed. RESULTS: Correlations between the obtained results were calculated in relation to the crown-rump (C-R) length of human fetuses and to sex. The structural variability of the thyroid and cricoid cartilages of human male and female fetuses in subsequent weeks of intrauterine life was observed. In both genders a correlation between laryngeal size and fetal crown-rump length, regardless of sex, was found. The thyroid cartilage presents a sexual dimorphism. CONCLUSIONS: The results of this study can be useful in the analysis of prenatal examinations, and in planning the treatment of airway emergencies.

Ceratocricoid muscle: an embryological and anatomical study.

This study aimed to document the prevalence and morphology of the ceratocricoid muscle in a large sample of fetuses and adults and to explain its possible origin in a sample of embryos. Forty-five embryos, thirty-four fetuses, and ninety human larynges from adults with no known laryngeal pathology were studied. The muscle was observed in 23% of the fetal sample and in 14% of the adult sample. No significant differences were observed in the laterality in any of the groups. The ceratocricoid muscle is attached between the cricoid lamina and the inferior horn of the thyroid cartilage and also into the capsule of the cricothyroid joint. The muscle is innervated by several branches (between one and three) from the anterior division of the recurrent laryngeal nerve as it courses behind the cricothyroid joint. The ceratocricoid muscle develops from tissue within the mesenchymal bridge which connects the external and internal laryngeal sphincters or rings from embryonic stages 15-20. The close relationship of the ceratocricoid muscle to the recurrent laryngeal nerve could mean that it can exert pressure on this nerve. This may be a possible explanation for the causation of certain idiopathic recurrent laryngeal nerve palsies.

Different expression of 25-kDa heat-shock protein (Hsp25) in Meckel's cartilage compared with other cartilages in the mouse.

The 25-kDa heat-shock protein (Hsp25) is expressed in the cartilage of the growth plate and suggested to function in chondrocyte differentiation and degeneration. Using immunohistochemistry, we examined the temporal and spatial occurrence of Hsp25 in Meckel's cartilage in embryonic mice mandibles, and in other types of cartilage in both embryonic and adult mice. In adults, Hsp25 immunoreactivity was detected in the hypertrophic chondrocytes located in growth plates of long bones and in non-osteogenic laryngeal and tracheal cartilages. No chondrocytes in the resting or proliferating phase exhibited Hsp25 immunoreactivity. In the embryonic mandibles, resting and proliferating chondrocytes in the anterior and intermediate portions of Meckel's cartilage showed Hsp25 immunoreactivity from the 12th day of gestation (E12) through E15, whereas those in the posterior portion showed little or no immunoreactivity. After E16, the overall Hsp25 immunoreactivity in Meckel's cartilage substantially reduced in intensity, and little or no immunoreactivity was detected in the hypertrophic chondrocytes located in the degenerating portions of Meckel's cartilage. The antisense oligonucleotide for Hsp25 mRNA applied to the culture media of the mandibular explants from E10 embryos caused significant inhibition of the development of the anterior and middle portions of Meckel's cartilage. These results suggested that Hsp25 is essential for the development of Meckel's cartilage and plays different roles in Meckel's cartilage from those in the permanent cartilages and the cartilages undergoing endochondral ossification.

Immunolocalization of type X collagen before and after mineralization of human thyroid cartilage.

In this study the distribution of type X collagen in thyroid cartilages of various ages is described. Fetal and juvenile thyroid cartilage was negative for type X collagen, but showed a strong staining reaction for type II collagen. Type X collagen and calcium deposition were first detected in thyroid cartilage of 18-to 21-year-old adults. Type X collagen was restricted to large chondrocytes near or in mineralized cartilage, confirming the notion that type X collagen precedes mineralization. From these observations it was concluded that chondrocytes in thyroid cartilage undergo differentiation steps that are similar, but much slower, compared to cells in growth plate and sternal cartilage. Some type X collagen-positive areas also showed staining for type I collagen, suggesting that there is a further differentiation of chondrocytes to cells which are characterized by the simultaneous synthesis of type X and I collagen. However, a dedifferentiation process during aging of thyroid cartilage where cells switch from synthesis of type II to type I collagen cannot be excluded.

Estudo quantitativo do desenvolvimento da cartilagem tireóide fetal humana: I - 9ª à 38ª semanas pós-concepçäo / Quantitative study of the human fetal thyroid cartilage development: I - 9th to 38th weeks post-conception

O presente estudo quantificou o crescimento da cartilagem tireóide durante o período fetal humano, aplicando o método alométrico Log Y = k LogX + Logb. Os dados lineares (altura e largura) e pondera foram correlacionados com a idade gestacional (spc). Foi verificado que a cartilagem tireóide apresenta um crescimento alométrico negativo para todos os parâmetros analisados

Estudo quantitativo do desenvolvimento da cartilagem tireóide fetal humana: II - 9ª à 38ª semanas pós-concepçäo / Quantitative study of the human fetal thyroid cartilage development: II - 9th to 38th weeks post-conception

O presente estudo quantificou o crescimento da cartilagem tireóide durante o período fetal humano, aplicando o método alométrico Log Y = k LogX + Logb. Os dados lineares (altura e largura máximas) foram correlacionados com o comprimento vértex-cóccix (mm). Foi verificado que o crescimento da cartilagem tireóide apresenta alometria negativa

Estudo quantitativo do desenvolvimento da cartilagem tireóide fetal humana: III - 9ª à 38ª semanas pós-concepçäo / Quantitative study of the human fetal thyroid cartilage development: III - 9th to 38th weeks post-conception

Para o estudo quantitativo do crescimento da cartilagem tireóide durante o período fetal humano, aplicamos o método alométrico Log Y = k Log X + Logb. O peso da cartilagem tireóide (em gramas) foi correlacionado com o peso do feto (em gramas). Nossos resultados indicaram alometria negativa para o seu crescimento