Cardiac neural crest
are multipotent cells required for the development of cells, tissues and organ systems.
A subpopulation of neural crest cells are the cardiac neural crest complex. This complex refers to the cells found amongst the midotic placode and somite 3 destined to undergo epithelial-mesenchymal transformation and migration to the heart via pharyngeal arches 3, 4 and 6.
The cardiac neural crest complex plays a vital role in forming connective tissues that aid in outflow septation and modelling of the aortic arch arteries during early development. Ablation of the complex often leads to impaired myocardial functioning similar to symptoms present in DiGeorge syndrome. Consequently, the removal of cardiac crest cells that populate in pharyngeal arches has flow on effects on the thymus, parathyroid and thyroid gland.
Neural crest cells are a group of temporary, multipotent cells that are pinched off during the formation of the neural tube and therefore are found at the dorsal region of the neural tube during development. They are derived from the ectoderm germ layer, but are sometimes called the fourth germ layer because they are so important and give rise to so many other types of cells. They migrate throughout the body and create a large number of differentiated cells such as neurons, glial cells, pigment-containing cells in skin, skeletal tissue cells in the head, and many more.
Cardiac neural crest cells are a type of neural crest cells that migrate to the circumpharyngeal ridge and then into the 3rd, 4th and 6th pharyngeal arches and the cardiac outflow tract.
They extend from the otic placodes to the third somites.
The cardiac neural crest cells have a number of functions including creation of the muscle and connective tissue walls of large arteries; parts of the cardiac septum; parts of the thyroid, parathyroid and thymus glands. They differentiate into melanocytes and neurons and the cartilage and connective tissue of the pharyngeal arches. They may also contribute to the creation of the carotid body, the organ which monitors oxygen in the blood and regulates breathing.
Pathway of the migratory cardiac neural crest cell
Induction
is the differentiation of progenitor cells into their final designation or type. The progenitor cells which will become CNCCs are found in the epiblast about Henson's node. Progenitor cells are brought into the neural folds. Molecules such as Wnt, fibroblast growth factor and bone morphogenetic protein provide signals which induce the progenitor cells to become CNCCs. Little is known about the signal cascade that promotes neural crest induction. However, it is known that an intermediate level of BMP is required: if BMP is too high or too low, the cells do not migrate.Initial migration
After induction, CNCCs lose their cell to cell contacts. This allows them to move through the extracellular matrix and interact with its components. The CNCCs, with the assistance of their filopodia and lamellipodia, leave the neural tube and migrate along a dorsolateral pathway to the circumpharyngeal ridge. Along this pathway, CNCCs link together to form a stream of migrating cells. Cells at the front of the migration stream have a special polygonal shape and proliferate at a faster rate than trailing cells.Development
The cardiac neural crest originates from the region of cells between somite 3 and the midotic placode that migrate towards and into the cardiac outflow tract.The cells migrate from the neural tube to populate pharyngeal arches 3, 4 and 6 with the largest population of the outflow tract originating from those in pharyngeal arches 4.
From here, a subpopulation of cells will develop into the endothelium of the aortic arch arteries while others will migrate into the outflow tract to form the aorticopulmonary and truncal septa. Other ectomesenchymal cells will form the thymus and parathyroid glands.
Epithelial-mesenchymal transition
Prior to migration, during a process known as epithelial-mesenchymal transition, there is a loss of cell contact, remodelling of the cytoskeleton and increased motility and interaction with extracellular components in the matrix. An important step in this process is the suppression of adhesion protein E-cadherin present on epithelial cells to initiate the migration process. This suppression mechanism occurs via the growth factor BMP signalling to turn on a transcriptional repressor Smad-interacting protein 1 and marks the beginning of the epithelial-mesenchymal transition.Early migration
During migration, crest cells destined for pharyngeal arches maintain contact with each other via lamellipodia and filopodia. Short range local contact is maintained with lamellipodia whilst long range non-local contact is maintained with filopodia. During this process, connexin 43 regulates cell interaction by regulating the formation of channels known as gap junctions. Impaired Cx43 function in transgenic mice leads to altered coronary artery patterns and abnormal outflow tracts. Further gap junction signalling is dependent on a cadherin mediated cell adhesion formed during cross talking with p120 catenin signalling.Appropriate outflow tract formation relies on a morphogen concentration gradient set up by fibroblast growth factor secreting cells. Cardiac crest cells furthest away from FGF secreting cells will receive lower concentrations of FGF8 signalling than cells closer to FGF secreting cells. This allows for appropriate formation of the outflow tract. Cells located in rhombomeres 3and 5 undergo programmed cell death under signalling cues from semaphorins. The lack of cells in this region results in the formation of crest-free zones.
The process of migration requires a permissive extracellular matrix. The enzyme arginyltransferase creates this environment by adding an arginyl group onto newly synthesised proteins during post-translational modification. This process aids cells motility and ensures the proteins contained within the actin cytoskeleton is prepped for migration.
Circumpharyngeal ridge
Cell migration towards the circumpharyngeal ridge is forced to paused to allow for the formation of the caudal pharyngeal arches. Little is known about this pausing mechanism, but studies conducted in chicks have uncovered the role of mesoderm expressed factors EphrinB3 and EphrinB4 in forming fibronectin attachments.Caudal pharynx and arch artery condensation
Pharyngeal arches are tissues composed of mesoderm-derived cells enclosed by an external ectoderm and an internal endoderm. Once the caudal pharyngeal arches are formed, cardiac neural crest complexes will migrate towards these and colonise in arches 3, 4 and 6. Cells leading this migration maintain contact with the extracellular matrix and contain filopodia which act as extensions towards the ectodermal pharyngeal arches. A range of secreted factors ensure appropriate directionality of the cells. FGF8 acts as a chemotactic attraction in directing cellular movement towards pharyngeal arch 4.A second signalling pathway that directs crest cell movement are the family of endothelin ligands. Migrating cardiac neural crest cells will populate at the correct pharyngeal arches under signalling guidance from EphrinA and Ephrin B variations. This corresponds with receptor expression at the pharyngeal arches. Pharyngeal arch 3 expresses EphrinA and EphrinB1 receptors and pharyngeal arch 2 expresses EphrinB2 and allows for the binding of EphrinA and EphrinB variations to guide migration of the cardiac neural crest cells.
Aortic arch remodeling
The aortic arch arteries transport blood from the aorta to the head and trunk of the embryo. Normally, early development of the outflow tract begins with a single vessel that forms bilateral symmetrical branches at the aortic sac within pharyngeal arches. This process requires the elongation of the outflow tract as a prerequisite to ensure the correct series of looping and cardiac alignment. The cardiac neural crest complex then colonises in the truncal cushion and is localised to the subendothelial layer prior to spiralisation of the endocardial cushion to form the conotruncal ridges. This later undergoes remodelling to form the left-sided aortic pattern present in adult hearts. The group of cells found in the third aortic arch gives rise to common carotid arteries. Cells found in the fourth aortic arch differentiates to form the distal aortic arch and right subclavian artery, whilst cells in the sixth aortic arch develops into the pulmonary arteries. Cardiac neural crest cells express Hox genes that supports the development of arteries 3, 4 and 6 and the simultaneous regression of arteries 1 and 2. The ablation of Hox genes on cardiac neural crest cells causes defective outflow septation.Ablation of cardiac neural crest complex
Cardiac outflow anomalies
One of the main cardiac outflow anomalies present during cardiac neural crest complex ablation is persistent truncus arteriosus. This arises when the arterial trunk fails to divide and cause the separation of pulmonary artery and aorta. This results in a lack of aorticopulmonary septum as the vessels which would normally disappear during normal development remains and interrupts the carotid vessels. The malformation of the heart and its associated great vessels depends on the extent and location of the cardiac neural crest complex ablation. Complete removal of cardiac neural crests results in persistent truncus arteriosus characterised in most cases by the presence of just one outflow valve and a ventricular septal defect. Mesencephalic neural crest cells interfere with normal development of cardiac outflow septation as its presence leads to persistent truncus arteriosus. However, the addition of trunk neural crest cells results in normal heart development.Other outcomes of cardiac outflow anomalies includes Tetralogy of Fallot, Eisenmenger's complex, transposition of the great vessels and double outlet right ventricle.