Sixth Branchial Arch Research Articles

Fossil evidence for a pharyngeal origin of the vertebrate pectoral girdle

The origin of vertebrate paired appendages is one of the most investigated and debated examples of evolutionary novelty1–7. Paired appendages are widely considered as key innovations that enabled new opportunities for controlled swimming and gill ventilation and were prerequisites for the eventual transition from water to land. The past 150 years of debate8–10 has been shaped by two contentious theories4,5: the ventrolateral fin-fold hypothesis9,10 and the archipterygium hypothesis8. The latter proposes that fins and girdles evolved from an ancestral gill arch. Although studies in animal development have revived interest in this idea11–13, it is apparently unsupported by fossil evidence. Here we present palaeontological support for a pharyngeal basis for the vertebrate shoulder girdle. We use computed tomography scanning to reveal details of the braincase of Kolymaspis sibirica14, an Early Devonian placoderm fish from Siberia, that suggests a pharyngeal component of the shoulder. We combine these findings with refreshed comparative anatomy of placoderms and jawless outgroups to place the origin of the shoulder girdle on the sixth branchial arch. These findings provide a novel framework for understanding the origin of the pectoral girdle. Our evidence clarifies the location of the presumptive head–trunk interface in jawless fishes and explains the constraint on branchial arch number in gnathostomes15. The results revive a key aspect of the archipterygium hypothesis and help reconcile it with the ventrolateral fin-fold model.

Stridor at Birth: Congenital Laryngeal Web

Stridor at Birth: Congenital Laryngeal Web

Equine laryngeal dysplasia as a cause of acute respiratory distress in a neonatal foal

SummaryEquine laryngeal dysplasia (ELD) is a congenital disorder caused by hypoplasia or aplasia of the structures derived from the fourth and possibly sixth branchial arches. The severity of the clinical signs varies widely, and depends on which structures are involved and the extent of the defect. In most cases, affected horses present with abnormal respiratory noise during exercise and poor performance. Manifestation of the disease in foals is very rare. This case report describes an unusual presentation of ELD in a neonatal foal. The foal presented with severe respiratory distress and weakness. Equine laryngeal dysplasia was suspected on endoscopy and later confirmed on post mortem examination of the larynx. To the best of our knowledge, this is the first report describing ELD as a cause of respiratory distress in a neonatal foal.

Immunohistochemical Analysis of the Effects of Cross-innervation of Murine Thyroarytenoid and Sternohyoid Muscles

This work uses cross-innervation of respiratory muscles of different developmental origins to probe myogenic and neurogenic mechanisms regulating their fiber types. The thyroarytenoid (TA) originates from the sixth branchial arch, whereas the sternohyoid (SH) is derived from somitic mesoderm. Immunohistochemical analysis using highly specific monoclonal antibodies to myosin heavy chain (MyHC) isoforms reveals that normal rat SH comprises slow, 2a, 2x, and 2b fibers, as in limb fast muscles, whereas the external division of the TA has only 2b/eo fibers coexpressing 2B and extraocular (EO) MyHCs. Twelve weeks after cross-innervation with the recurrent laryngeal nerve, the SH retained slow and 2a fibers, greatly increased the proportion of 2x fibers, and their 2b fibers failed to express EO MyHC. In the cross-innervated TA, the SH nerve failed to induce slow and 2A MyHC expression and failed to suppress EO MyHC expression in 2b/eo fibers. However, 2x fibers amounting to 4.2% appeared de novo in the external division of the TA. We conclude that although MyHC gene expression in these muscles can be modulated by neural activity, the patterns of response to altered innervation are largely myogenically determined, thus supporting the idea that SH and TA differ in muscle allotype.

Sonic hedgehog is required for the assembly and remodeling of branchial arch blood vessels

Sonic hedgehog (Shh) is a morphogen involved in many developmental processes. Injection of cells (5E1) that produce a Shh-blocking antibody causes an attenuation of the Shh response, and this causes vascular malformations and impaired remodeling characterized by hemorrhages and protrusions of the anterior cardinal vein and outflow tract, delayed fusion of the dorsal aortae, impaired branching of the internal carotid artery, and delayed remodeling of the aortic arches. Distribution of smooth muscle cells in the vessel wall is unchanged. In 5E1-injected embryos, we also observed impaired assembly of endothelial cells into vascular tubes, particularly in the sixth branchial arch, around the anterior cardinal vein and around the dorsal aorta. In 5E1-treated embryos, increased numbers of macrophage-like cells, apoptotic cells, and a decreased level of proliferation were observed in head mesenchyme. Together, these observations show that Shh signaling is required at multiple stages for proper vessel formation and remodeling.

Flow-Induced Asymmetry

Mammalian embryos initially develop symmetrically; then, in response to Nodal signaling and the action of the transcription factor Pitx2, organ asymmetry occurs. For example, during embryonic development, two blood vessels on the right side--the right sixth branchial arch artery and the dorsal aorta--undergo complete regression, resulting in the formation of the asymmetrical aortic arch and the pulmonary trunk (see Snider and Conway). Yashiro et al . determined that although Pitx2 activity was required for regression of the sixth branchial arch artery, which is the first vessel to regress, expression of this gene was not required in the artery itself. Instead, Pitx2 was required for a morphological change, the formation of a spiral structure of the myocardial left outflow tract, a structure through which blood leaves the heart. In animals in which the outflow tract did not spiral, regression of neither the right sixth branchial arch artery nor the right dorsal aorta occurred. Echocardiography showed that after the formation of the spiral structure, the blood flow to the right side vessels destined for regression was reduced, whereas blood flow to the left side vessels was greater than before the morphological change. Furthermore, this change in blood flow was lost in embryos with a promoter-mutated Pitx2 (Pitx ΔASE/ΔASE ), which also lacked a spiral form to the outflow tract and had persistent right vessels. To confirm that decreased blood flow was the signal for regression, the authors ligated the left sixth branchial arch artery prior to the formation of the spiral structure by the outflow tract and found that the left-side vessels (sixth branchial arch artery and dorsal aorta) regressed and the right, unligated vessels persisted. In the persistent left vessels in wild-type animals, there was increased expression of the gene encoding platelet-derived growth factor-A (PDGF-A) and increased phosphorylation of vascular endothelial growth factor receptor 2 (VEGFR2) compared with that in the right vessels after the morphological change. Again, this difference was lost in the Pitx ΔASE/ΔASE embryos. Simultaneous pharmacological inhibition of PDGFR and VEGFR triggered bilateral loss of the sixth branchial arch arteries. Thus, the authors suggest that the Nodal-Pitx2 pathway triggers a morphological change that increases blood flow on the left side and decreases blood flow on the right side, producing an asymmetric signal of growth factor signaling, allowing the side with the higher blood flow to survive and the side with less to regress. P. Snider, S. J. Conway, The power of blood. Nature 450 , 180-181 (2007). [PubMed] K. Yashiro, H. Shiratori, H. Hamada, Haemodynamics determined by a genetic programme govern asymmetric development of the aortic arch. Nature 450 , 285-288 (2007). [PubMed]

Cell autonomous requirement for PDGFRalpha in populations of cranial and cardiac neural crest cells.

Cardiac and cephalic neural crest cells (NCCs) are essential components of the craniofacial and aortic arch mesenchyme. Genetic disruption of the platelet-derived growth factor receptor alpha (PDGFRalpha) results in defects in multiple tissues in the mouse, including neural crest derivatives contributing to the frontonasal process and the aortic arch. Using chimeric analysis, we show that loss of the receptor in NCCs renders them inefficient at contributing to the cranial mesenchyme. Conditional gene ablation in NCCs results in neonatal lethality because of aortic arch defects and a severely cleft palate. The conotruncal defects are first observed at E11.5 and are consistent with aberrant NCC development in the third, fourth and sixth branchial arches, while the bone malformations present in the frontonasal process and skull coincide with defects of NCCs from the first to third branchial arches. Changes in cell proliferation, migration, or survival were not observed in PDGFRalpha NCC conditional embryos, suggesting that the PDGFRalpha may play a role in a later stage of NCC development. Our results demonstrate that the PDGFRalpha plays an essential, cell-autonomous role in the development of cardiac and cephalic NCCs and provides a model for the study of aberrant NCC development.

The special status of the pulmonary arch artery in the branchial arch system of the rat.

A renewed study of the development of the branchial arch system was essential in view of the special morphologic characteristics of the ductus arteriosus, which derives from the pulmonary arch artery or sixth branchial arch artery. In congenital heart disease certain aorto-pulmonary collateral arteries have a marked histological similarity to the ductus arteriosus. To gain a better insight into the development of these vessels, 27 rat embryos, with the number of somites ranging between 19 and 41, were studied. Most embryos were collected after shortterm in vitro-culture, allowing precise staging of age and development. The vascular system of these embryos was injected with Indian ink, to enable easy recognition of even the smallest endothelium-lined vessels. The embryos were serially sectioned (3-5 microns) and reconstructed using a graphic method. The results show that the pulmonary arch artery differs from the other arch arteries in that it is the most cranial vessel of a system of ventral splanchnic arteries, which connects the pulmonary plexus with the dorsal aortae at an early stage. With the exception of the pulmonary arch artery, these connections are transient. The pulmonary arteries develop from the remaining parts of the plexus. It is argued that these connections can persist in the human as aorto-pulmonary collaterals, in certain cases with abnormalities in the pulmonary part of the cardiac outflow tract.

Die Kiemenbogenentwicklung bei Ratte und Maus

The development of the branchial arch region was studied in mouse embryos at the age of 10-13 days after conception [end of embryonal day (ED) 10 to end of ED 13] and in rat embryos at the age oa 1-14 days after conception (end of ED 11 to nd of ED 14) using SEM and serial semithin sections. Special attention was paid to the development of the cervical sinus and the branchial operculum. In both rodents, a typical operculum was not found. Instead, the caudal branchial arch region was remodeled by a rapid growth of the second branchial arch, the retrobranchial ridge and the epipericardial ridge, thus forming a progressively deeper grove in this region, the sinus cervicalis. The epithelium of the fourth and sixth branchial arches participated in the formation of the vagus placode, which in later stages was depressed into a grove dorsally and lost its connection with the surface. Finally, the second branchial arch and the cardiac bulge fused compressing the third branchial arch, the second and third branchial grooves and the aperture of the vagus placode. A typical vesicula cervicalis, lined by ectodermal epithelium and collecting the second, third and fourth branchial grooves and the glossopharyngeal and vagus placodes, was not found. However, single or multiple slit-like lumina may persist within the epithelial remnants of the second and third branchial grooves and the vagus placode.

Site and blood supply of the intertruncal glomera.

The intertruncal glomera have a solely systemic arterial blood supply through branches of the coronary arteries and aorta. There is no supply from the pulmonary circulation to support the concept of a 'glomus pulmonale' representing a true chemoreceptor of the sixth branchial arch. The intertruncal arteries have the histological structure of typical muscular systemic arteries until they come into proximity with glomera. There they show an abrupt change into an elastic structure believed to be associated with a baroreceptor function.

The Skin of the Tadpole of the Common Toad, Bufo bufo bufo (L.), during Metamorphosis

Summary 1 During metamorphosis, the epidermis of the toad tadpole changes from a single-layered to a multi-layered epithelium, and epidermal cells, initially heavily pigmented, lose the pigment granules. Melanophores occur among the epidermal cells at the beginning of the process, but later come to lie internal to the bases of the cells of the stratum gerniinativum. 2 The corium is initially a dense condensation of connective tissue. Following invasion by fibroblasts (hind limb 3-jointed) it differentiates into two layers, the strata spongiosum and compactum. 3 At the beginning of metamorphosis epidermal glands are unicellular. Multicellular glands appear when the anal canal is resorbing. Order of appearance and distribution follow a definite pattern. The development of the two types of gland found is described. 4 The toad tadpole has a functional sixth branchial arch with a hemibranch on the posterior wall of the branchial cavity. 5 Blood vessels reach the skin at the stage when the anal canal is resorbing, but the rete subcutanea begins to form only at the 3-leg, 12 mm. tail stage. Later, capillaries from the rete subcutanea pierce the stratum compactum and the rete subepidermale is formed (4-leg, 5 mm. tail). Finally, epidermal capillaries arise from the rete subepidermale. 6 There is close correspondence between the stages of vascularization of the skin and the response of toad tadpoles to injection with post-pituitary extracts. 7 Developmental processes in the skin of the trunk region are initiated in the field of the occipito-vertebral artery.

An unusual congenital cardiac defect: Failure of appearance of the endocardial cushions, the aortic septum and the distal part of the left sixth branchial arch

An unusual congenital cardiac defect: Failure of appearance of the endocardial cushions, the aortic septum and the distal part of the left sixth branchial arch