Surgical anatomy of the thyroid and parathyroid glands.

This article describes the anatomy and embryology of the thyroid and parathyroid glands and the recurrent laryngeal nerve, discussing how the anatomy affects function and dysfunction of the glands.

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.

Synaptology and ultrastructural characteristics of laryngeal cricothyroid and posterior cricoarytenoid motoneurons in the nucleus ambiguus of the rat.

The laryngeal motoneurons innervating the cricothyroid muscle (CT) are located in the semicompact formation just ventral to the rostral part of the compact formation of the nucleus ambiguus. The motoneurons innervating the posterior cricoarytenoid muscle (PCA) are located in the loose formation. We retrogradely labeled the CT and the PCA motoneurons using cholera toxin subunit B-conjugated horseradish peroxidase, and determined the ultrastructure and synaptic organization of these neurons. The CT and the PCA motoneurons had the appearance of alpha-motoneurons, i.e., large, oval or polygonal cells containing well-developed organelles and a prominent spherical nucleus. Two kinds of neurons were recognized among the PCA motoneurons. The one (PCA-A) was significantly smaller than the other (PCA-B). The average number of axosomatic terminals in a section was significantly largest in the PCA-B (56.6), smaller in the PCA-A (36.0), and smallest in the CT (32.3) neurons. Most of the axosomatic terminals (64.7%) contained pleomorphic vesicles and made symmetric synaptic contacts (Gray's type II) with the PCA-A neurons, while more than 60% contained round vesicles with asymmetric synaptic contacts (Gray's type I) in the CT (69.5%) and the PCA-B (60.6%) neurons. A few terminals associated with subsurface cisterns were present on all laryngeal motoneurons. These results indicated that the CT motoneurons may receive mostly excitatory terminals, whereas the PCA muscle may be regulated by neurons having many inhibitory terminals, and neurons having many excitatory terminals.

Nonrecurrent laryngeal nerves: anatomic considerations during thyroid and parathyroid surgery.

PURPOSE: In head and neck surgery, damage to the recurrent laryngeal nerve (RLN) during thyroid surgery is the most common iatrogenic cause of vocal cord paralysis. Identification of the RLNs and meticulous surgical technique can significantly decrease the incidence of this complication. Nonrecurrent RLNs (NRRLNs) are exceedingly rare. Surgeons need to be aware of their position to avoid damage to them. MATERIALS AND METHODS: A retrospective review of 513 RLN exposures over a 7-year period was performed. RESULTS: Two NRRLNs were encountered, for an incidence of 0.39%. CONCLUSION: NRRLNs are rare. Awareness of their existence will prevent the surgeon from accidentally severing one if it is encountered during routine thyroid or parathyroid surgery.

Variations in the formation of the cardiac plexus--a study in human foetuses.

The origin and distribution of nerves forming the cardiac plexus and the subdivision of this plexus were studied in six human foetuses (2 male, 4 female) of gestational ages 30 to 40 weeks. The cardiac plexus was not divided into superficial and deep parts in any foetus. True plexiform arrangement of nerves forming the cardiac plexus was seen only after the nerves reached the walls of the heart. The sympathetic trunks, vagi, recurrent laryngeal nerves and phrenic nerves of both sides contributed to the cardiac plexus. The cervical sympathetic trunk showed only two ganglia bilaterally in one foetus; this has not been reported before. In one foetus on the right side, the middle cervical sympathetic cardiac branch joined the recurrent laryngeal and the phrenic nerves which has not been reported earlier. The sympathetic pathways to the heart were found to be very variable; no two foetuses showed the same arrangement. Awareness of these variations in the nerves forming the cardiac plexus would enhance the success of sympathectomy to augment cardiac blood flow or to relieve the severity of cardiac pain.

Naturally occurring motoneuron cell death in rat upper respiratory tract motor nuclei: a histological, fast DiI and immunocytochemical study of the nucleus ambiguus.

The mammalian upper respiratory tract (URT) serves as the common modality for aspects of respiration, deglutition, and vocalization. Although these actions are dependent on coordinated and specific neuromuscular control, little is known about the development of URT control centers. As such, this study investigated the occurrence of naturally occurring motoneuron cell death (MCD) in the nucleus ambiguus (NA) of a developmental series of rats. Standard histological techniques were used to count motoneurons in the ventrolateral brainstem where the mature NA is found. In addition, the neural tracer, fast DiI, was used to determine whether motoneurons were still migrating into the region of the NA during the period that cell counts were first taken. Furthermore, to elucidate the potential effect of inadvertently counting large interneurons on the assessment of motoneuron numbers, an antibody to gamma-aminobutyric acid (GABA) was used. The results of this study have, for the first time, demonstrated that MCD occurs in a URT-related motor nucleus. Approximately a 50% cell death was observed during the prenatal development of NA, with no further loss seen postnatally. The fast DiI studies showed that by embryonic day 17, NA was fully formed, suggesting that motoneuron migration from the basal plate was complete. In addition, use of the GABA antibody showed a lack of inhibitory interneurons within the NA. The finding of MCD in the NA helps define a critical period in the formation of URT neuromuscular control. As the course of MCD is modifiable by epigenetic signals, insult to the organism during this prenatal period may compromise future URT control.

[Non-recurrent inferior laryngeal nerve]. / Sul nervo laringeo inferiore non ricorrente.

The Authors describe a case of non recurrent inferior laryngeal nerve located on the right side observed during anatomical studies of the innervation of the larynx. The description is accompanied by photographic documentation which shows the anomalous course of the nerve and its abnormal relationship with the right subclavian artery, also anomalous. The embryological nature of such anatomical variations is discussed followed by considerations concerning possible consequences with regard to clinical aspects and thyroid surgery. Furthermore, the Authors indicate clinical signs that may suggest the presence of such anomaly so as to avoid anatomical or functional damage of the nerve which, although anomalous, plays an important role in coordinating breathing and phonation.

[The non-recurrent inferior laryngeal nerve. An important rarity in thyroid gland surgery]. / Der Nervus laryngeus inferior non-recurrens. Eine wichtige Rarität in der Schilddrüsenchirurgie.

The occurrence of non-recurrent inferior laryngeal nerve is based on a rare embryologic vascular malformation of the aortic arches. Based on three recent cases, we discuss embryology, clinical presentation and operative technique.

Ultrastructure of the ganglion on human internal laryngeal nerve.

There is now definite evidence for the presence of a macroscopic ganglion on the human internal laryngeal nerve, with the distribution of its post-ganglionic fibres to the glands in the saccule and to the glands at the root of epiglottis in the vicinity of the opening of the saccule. This ganglion could be identified as early as 14 weeks in human foetal larynx, which contains immature neurons. Seven ganglia, dissected from human laryngectomy specimens and resected for carcinoma larynx, were studied by electron microscopy. Ultrastructurally, the neurons and the synaptic terminals had both small, round, luscent vesicles and dense core vesicles. Symmetrical, asymmetrical and electrical synaptic complexes were noted. A few neurons revealed degenerative changes suggestive of axotomy. The location of the ganglion on the internal laryngeal nerve, a branch of nervus vagus, and ultrastructural demonstration of large and small dense core vesicles and small luscent vesicles in the neurons of this ganglion, lead us to believe that the ganglion is parasympathetic in nature.

Cardiac neural crest contribution to the pulmonary artery and sixth aortic arch artery complex in chick embryos aged 6 to 18 days.

Previous studies of cardiac neural crest (CNC) migration in early chick embryos demonstrated CNC cells in the media of pharyngeal arch arteries three, four, and six, and in the most proximal part of the developing pulmonary arteries. The objectives of this study were to learn 1) to what extent the CNC is involved in the later development of the pulmonary arteries, 2) how the CNC cells are distributed in the sixth aortic arch artery including the wall of the ductus arteriosus in the older embryo, and 3) what happens to the CNC as the pulmonary artery/sixth arch complex grows into its adult configuration. Quail-to-chick chimeras were used to study CNC distribution in embryos aged 6 to 18 days. Controls (undisturbed chick embryos) were collected with chimeras. Each was fixed, processed, sectioned, stained with Feulgen-Rossenbeck stain, and analyzed. The results demonstrated that CNC disappeared from the proximal pulmonary arteries by embryonic day 9 and played no further role in pulmonary artery development. With the exception of the endothelium, CNC completely filled the wall of the sixth aortic arch artery as far distally as its junction with the dorsal aorta in younger embryos and with the aorta in older embryos, thus suggesting the possibility of proximodistal migration of CNC along the sixth aortic arch. The ductus wall, filled with CNC, was intimately associated with the recurrent laryngeal nerve, also filled with CNC, thereby strongly suggesting a role for CNC in ductal closure.

Nervo laringeo inferior näo-recorrente / Nonrecurrent laryngeal nerve

Apesar de sua relativa raridade, a ocorrencia do nervo laringeo inferior nao-recorrente e uma anomalia de grande importancia cirurgica, merecendo sempre ser lembrada, pois nestes casos o nervo corre grande risco de lesao em cirurgia das glandulas tiroide e paratiroides. Os autores apresentam sua casuistica de sete casos e tecem comentarios sobre a anatomia e embriologia do nervo laringeo inferior, enfatizando a necessidade da disseccao deste nervo em tireoidectomias

The human larynx at the end of the embryonic period proper. 2. The laryngeal cavity and the innervation of its lining.

The laryngeal cavity was studied in eight serially sectioned embryos of stage 23 and in three early fetuses, and graphic reconstructions were prepared. After the isolation of the tracheal from the pharyngeal cavity during stages 16 through 22, a communication (not necessarily the pharyngotracheal duct) appears again during stage 23. At this time (8 postovulatory weeks) the laryngeal cavity comprises 1) the coronal and parts of the sagittal clefts of the vestibule (uniting later at the laryngeal inlet); 2) the ventricles, which are not yet completely formed; and 3) the subglottic cavity, which appeared already in earlier stages. The characteristic events of stage 23 are the dissolution of the epithelial lamina and the development of the ventricles. The disruption of the epithelial lamina is an active process that comprises rearrangement and growth, but not loss of cells. The ventricles, which begin as solid outgrowths in stage 20, do not represent fifth pharyngeal pouches. They now point toward the middle of the still paired thyroid laminae and are not at the level of the future glottis, which lies more caudally. In the absence of the median part of the soft palate, the nasopharynx communicates widely with the oral cavity. The epithelium of the respiratory tube, including the larynx, resembles that of the pharynx and esophagus in being pseudostratified columnar and showing a clear basement membrane. It is ciliated over that part of the epiglottis that surmounts the arytenoid swellings, and also over the tip and back of the latter. The transitional area between the laryngopharynx and the esophagus is already innervated by the recurrent laryngeal nerve. Nerve fibers have not yet reached the epithelium of the coronal cleft and the ventricles, but fibers are present near the sagittal cleft of the vestibule. The sensory innervation of the pharynx and larynx has been followed and plotted for the first time in an embryo, and previously unrecorded silver-impregnated receptors have been observed.

Effect of hypoxia on the excitability of two cranial reflexes in unanaesthetized fetal sheep.

In fetal sheep acute hypoxia causes a decreased incidence of breathing movements and motor activity, and the excitability of polysynaptic reflexes in the hindlimbs is depressed. To determine whether this inhibitory effect extends to other areas in the fetal CNS, we have studied the effect of hypoxia on two reflexes with cranial pathways. The digastric (jaw opening) reflex was elicited by stimulation of the dental nerve through a pair of stainless steel electrodes implanted into the mandible (4 fetuses). The thyroarytenoid muscle of the larynx was reflexly activated by stimulation of the superior laryngeal nerve by a cuff electrode (4 fetuses). Low level stimulation at 1.5-2 X threshold was repeated at approximately 2 min intervals for 3-4 h; the stimulation did not alter the pattern of electrocortical activity, breathing movements, or cause arousal. The amplitude of the digastric reflex was greatest during low voltage electrocortical activity; conversely, the amplitude of the thyroarytenoid reflex was greatest during high voltage electrocortical activity. Isocapnic hypoxia lasting 30-60 min (16 trials), in which the PaO2 was reduced to 12-14 mmHg, did not reduce the amplitude of either reflex. The reduction of thyroarytenoid reflex amplitude which normally occurred during low voltage electrocortical activity was not present during hypoxia. These experiments show that the inhibitory effects of hypoxia on spinal reflexes, breathing movements and motor activity do not include these cranial pathways.

Role of the vagus nerve and its recurrent laryngeal branch in the development of the human ductus arteriosus.

The reason that the normal ductus arteriosus has a muscular media, contrasting with the elastic lamellar structure of the adjacent great arteries, is unknown. We examined the hypothesis that the anatomic relationship of the ductus arteriosus to the vagus and recurrent laryngeal nerves during early development might be of importance in influencing ductal morphology. Normal human embryos from the Carnegie Embryological Collection and embryos and fetuses from the Hopkins Pathology Collection were studied microscopically, by reconstructions made from serial histologic sections, or by gross dissection. At Carnegie stage 16 the recurrent laryngeal nerves pass medially from the vagus nerve to the laryngeal area and are caudal to the bilaterally symmetric sixth aortic arches. By stage 18 the right sixth aortic arch has disappeared and the left sixth aortic arch is in a more caudal position relative to the larynx. The left vagus nerve and its recurrent laryngeal branch form a sling supporting the distal (or ductus arteriosus component) of the left sixth aortic arch. In subsequent development there is greater relative separation of the larynx and ductus arteriosus. The media of the ductus arteriosus beneath the supporting nerves is thinner and has less elastic fiber formation than the elastic lamellar media of the adjacent aortic arches. The study shows that the vagus and recurrent laryngeal nerves are in a position to provide mechanical support to the ductus arteriosus during its development and that the morphology of the media of the supported ductus arteriosus differs from that of the adjacent unsupported aortic arches. It is suggested that this local mechanical support may be the reason that the normal ductus arteriosus differentiates as a muscular artery and is therefore able to obliterate its lumen in postnatal life. Without such support the ductal media could develop the abundant elastic fibers characteristic of the normal unsupported aorta and pulmonary trunk and become an abnormal, persistently patent ductus arteriosus.

[Information analysis of neurocytes of various peripheral ganglia, and critical periods of their differentiation]. / Informatsionnyi analiz neirotsitov nekotorykh perifericheskikh gangliev i kriticheskie periody ikh differentsirovki.

Calculation of the informatory parameters of entropy and redundancy performed, taking as a base nuclear-cytoplasmic relations of neurocytes in the inferior ganglion of the vagus nerve, in the superior ganglion and in the interorganic ganglia of the respiratory tube, demonstrated certain interconnection of the integral characteristics obtained with some stages of the neuronal morphological differentiation. Periods of an abrupt reduction in the redundant information up to the minimal values which goes in parallel with an increasing entropy on the 2d-4th months of embryogenesis (taking into account heterochronia in the development of various ganglia) are considered as some critical periods in the neuron development when it passes to the stage of growth (the first critical period) and to the stage of maturation (the second critical period). Their existence is regarded in connection with the hierarchical joining of the vascular and neural trophic types of the vegetative ganglia. A supposition is made on a dependence of the specific neuronal differentiation on the epigenetic factors of morphogenesis.

The human larynx at the end of the embryonic period proper. I. The laryngeal and infrahyoid muscles and their innervation.

The larynx at stage 23 (about 27-32 mm C-R, 8 postovulatory weeks) is being studied in detail, and findings on the musculature and its innervation are presented here. Serial sections of six embryos all belonging to the one developmental stage were investigated and compared, and graphic reconstructions were made from five of the specimens. All the infrahyoid and most of the major laryngeal muscles are present, and their innervation follows closely the adult pattern. The ary-epiglottic and thyro-epiglottic apparently may or may not be identifiable. The vocalis is beginning to differentiate and is noted here in the embryonic period proper for the first time. The histological degree of development of the intrinsic laryngeal muscles is not as advanced as that of adjacent muscles in the neck, including the infrahyoid group. Variation within this one stage of development are small but definite. They include differences in the position, level, and shape of the skeletal components, and the precise position and extent of development of some of the muscles.

Embryological considerations of thyroid surgery: developmental anatomy of the thyroid, parathyroids and the recurrent laryngeal nerve.

The surgeon planning a thyroidectomy must be prepared to find variations in three important structures in the neck. He must be prepared to find ectopic thyroid nodules above, below or lateral to the normally located thyroid gland. Any unattached nodule should be considered malignant until proved otherwise. He must identify and preserve parathyroid glands that may not lie in the typical location. He must be prepared to encounter recurrent laryngeal nerves that do not recur. The surgeon who remembers the embryology of the structures in the neck may occasionally be astonished, but never surprised.