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<pubmed limit=5>Chicken embryology</pubmed>
<pubmed limit=5>Chicken development</pubmed>
Skin transcriptome reveals the dynamic changes in the Wnt pathway during integument morphogenesis of chick embryos
PLoS One. 2018 Jan 19;13(1):e0190933. doi: 10.1371/journal.pone.0190933. eCollection 2018.
Gong H1,2, Wang H1, Wang Y1, Bai X1, Liu B1, He J3, Wu J2,4, Qi W3, Zhang W1.
Avian species have a unique integument covered with feathers. Skin morphogenesis is a successive and complex process. To date, most studies have focused on a single developmental point or stage. Fewer studies have focused on whole transcriptomes based on the time-course of embryo integument development. To analyze the global changes in gene expression profiles, we sequenced the transcriptome of chicken embryo skin samples from day 6 to day 21 of incubation and identified 5830 differentially expressed genes (DEGs). Hierarchical clustering showed that E6 to E14 is the critical period of feather follicle morphogenesis. According to Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of the DEGs, two kinds of Wnt signaling pathways (a canonical pathway and a non-canonical pathway) changed during feather follicle and feather morphogenesis. The gene expression level of inhibitors and ligands related to the Wnt signaling pathway varied significantly during embryonic development. The results revealed a staggered phase relationship between the canonical pathway and the non-canonical pathway from E9 to E14. These analyses shed new light on the gene regulatory mechanism and provided fundamental data related to integument morphogenesis of chickens. PMID: 29351308 PMCID: PMC5774689 DOI: 10.1371/journal.pone.0190933
The effect of magnetic resonance imaging on neural tube development in an early chicken embryo model
Childs Nerv Syst. 2018 Feb 1. doi: 10.1007/s00381-018-3734-9. [Epub ahead of print]
Kantarcioglu E1, Kahilogullari G2,3, Zaimoglu M1, Atmis EO4, Peker E5, Yigman Z6, Billur D6, Aydin S6, Erden IM5, Unlü A1.
PURPOSE: We aimed to determine whether varying the magnetic field during magnetic resonance imaging would affect the development of chicken embryos and neural tube defects. METHODS: Following incubation for 24 h, we exposed chicken embryos to varying magnetic fields for 10 min to assess the impact on development. Three magnetic resonance imaging devices were used, and the eggs were divided into four groups: group 1 is exposed to 1 T, group 2 is exposed to 1.5 T, group 3 is exposed to 3 T, and group 4, control group, was not exposed to magnetic field. After MRI exposure, all embryos were again put inside incubator to complete 48 h. "The new technique" was used to open eggs, a stereomicroscope was used for the examination of magnified external morphology, and each embryo was examined according to the Hamburger and Hamilton chicken embryo stages. Embryos who had delayed stages of development are considered growth retarded. Growth retardation criteria do not include small for stage. RESULTS: Compared with embryos not exposed to a magnetic field, there was a statistically significant increase in the incidence of neural tube closure defects and growth retardation in the embryos exposed to magnetic fields (p < 0.05). However, although the incidence of neural tube closure defects was expected to increase as exposure (tesla level) increased, we found a higher rate of defects in the 1.5-T group compared with the 3-T group. By contrast, the highest incidence of growth retardation was in the 3-T group, which was consistent with our expectation that growth retardation would be more likely as tesla level increased. CONCLUSIONS: We therefore conclude that the use of magnetic resonance imaging as a diagnostic tool can result in midline closure defects and growth retardation in chicken embryos. We hypothesize that this may also be true for human embryos exposed to MRI. If a pregnant individual is to take an MRI scan, as for lumbar disc disease or any other any other reason, our results indicate that consideration should be given to an avoidance of MRI during pregnancy. KEYWORDS: Chicken embryo; Growth retardation; Magnetic resonance imaging; Neural tube defects PMID: 29392421 DOI: 10.1007/s00381-018-3734-9
Divergent axial morphogenesis and early shh expression in vertebrate prospective floor plate
Evodevo. 2018 Jan 31;9:4. doi: 10.1186/s13227-017-0090-x. eCollection 2018.
Kremnyov S1,2, Henningfeld K3, Viebahn C4, Tsikolia N4.
BACKGROUND: The notochord has organizer properties and is required for floor plate induction and dorsoventral patterning of the neural tube. This activity has been attributed to sonic hedgehog (shh) signaling, which originates in the notochord, forms a gradient, and autoinduces shh expression in the floor plate. However, reported data are inconsistent and the spatiotemporal development of the relevant shh expression domains has not been studied in detail. We therefore studied the expression dynamics of shh in rabbit, chicken and Xenopus laevis embryos (as well as indian hedgehog and desert hedgehog as possible alternative functional candidates in the chicken). RESULTS: Our analysis reveals a markedly divergent pattern within these vertebrates: whereas in the rabbit shh is first expressed in the notochord and its floor plate domain is then induced during subsequent somitogenesis stages, in the chick embryo shh is expressed in the prospective neuroectoderm prior to the notochord formation and, interestingly, prior to mesoderm immigration. Neither indian hedgehog nor desert hedgehog are expressed in these midline structures although mRNA of both genes was detected in other structures of the early chick embryo. In X. laevis, shh is expressed at the beginning of gastrulation in a distinct area dorsal to the dorsal blastopore lip and adjacent to the prospective neuroectoderm, whereas the floor plate expresses shh at the end of gastrulation. CONCLUSIONS: While shh expression patterns in rabbit and X. laevis embryos are roughly compatible with the classical view of "ventral to dorsal induction" of the floor plate, the early shh expression in the chick floor plate challenges this model. Intriguingly, this alternative sequence of domain induction is related to the asymmetrical morphogenesis of the primitive node and other axial organs in the chick. Our results indicate that the floor plate in X. laevis and chick embryos may be initially induced by planar interaction within the ectoderm or epiblast. Furthermore, we propose that the mode of the floor plate induction adapts to the variant topography of interacting tissues during gastrulation and notochord formation and thereby reveals evolutionary plasticity of early embryonic induction. KEYWORDS: Evo–devo; Gastrulation; Induction; Neural tube; Notochord; Sonic hedgehog; Vertebrates PMID: 29423139 PMCID: PMC5791209 DOI: 10.1186/s13227-017-0090-x
MORN5 Expression during Craniofacial Development and Its Interaction with the BMP and TGFβ Pathways
Front Physiol. 2016 Aug 31;7:378. doi: 10.3389/fphys.2016.00378. eCollection 2016.
Cela P1, Hampl M1, Fu KK2, Kunova Bosakova M3, Krejci P4, Richman JM2, Buchtova M1.
MORN5 (MORN repeat containing 5) is encoded by a locus positioned on chromosome 17 in the chicken genome. The MORN motif is found in multiple copies in several proteins including junctophilins or phosphatidylinositol phosphate kinase family and the MORN proteins themselves are found across the animal and plant kingdoms. MORN5 protein has a characteristic punctate pattern in the cytoplasm in immunofluorescence imaging. Previously, MORN5 was found among differentially expressed genes in a microarray profiling experiment of the chicken embryo head. Here, we provided in situ hybridization to analyse, in detail, the MORN5 expression in chick craniofacial structures. The expression of MORN5 was first observed at stage HH17-18 (E2.5). MORN5 expression gradually appeared on either side of the primitive oral cavity, within the maxillary region. At stage HH20 (E3), prominent expression was localized in the mandibular prominences lateral to the midline. From stage HH20 up to HH29 (E6), there was strong expression in restricted regions of the maxillary and mandibular prominences. The frontonasal mass (in the midline of the face) expressed MORN5, starting at HH27 (E5). The expression was concentrated in the corners or globular processes, which will ultimately fuse with the cranial edges of the maxillary prominences. MORN5 expression was maintained in the fusion zone up to stage HH29. In sections MORN5 expression was localized preferentially in the mesenchyme. Previously, we examined signals that regulate MORN5 expression in the face based on a previous microarray study. Here, we validated the array results with in situ hybridization and QPCR. MORN5 was downregulated 24 h after Noggin and/or RA treatment. We also determined that BMP pathway genes are downstream of MORN5 following siRNA knockdown. Based on these results, we conclude that MORN5 is both regulated by and required for BMP signaling. The restricted expression of MORN5 in the lip fusion zone shown here supports the human genetic data in which MORN5 variants were associated with increased risk of non-syndromic cleft lip with or without cleft palate.
FGF8 coordinates tissue elongation and cell epithelialization during early kidney tubulogenesis
Development. 2015 Jul 1;142(13):2329-37. doi: 10.1242/dev.122408.
Atsuta Y1, Takahashi Y2.
When a tubular structure forms during early embryogenesis, tubular elongation and lumen formation (epithelialization) proceed simultaneously in a spatiotemporally coordinated manner. We here demonstrate, using the Wolffian duct (WD) of early chicken embryos, that this coordination is regulated by the expression of FGF8, which shifts posteriorly during body axis elongation. FGF8 acts as a chemoattractant on the leader cells of the elongating WD and prevents them from epithelialization, whereas static ('rear') cells that receive progressively less FGF8 undergo epithelialization to form a lumen. Thus, FGF8 acts as a binary switch that distinguishes tubular elongation from lumen formation. The posteriorly shifting FGF8 is also known to regulate somite segmentation, suggesting that multiple types of tissue morphogenesis are coordinately regulated by macroscopic changes in body growth. © 2015. Published by The Company of Biologists Ltd. KEYWORDS: Body axis; Chemoattraction; Live imaging; Wolffian duct/nephric duct
Capturing structure and function in an embryonic heart with biophotonic tools
Front Physiol. 2014 Sep 23;5:351. doi: 10.3389/fphys.2014.00351. eCollection 2014.
Karunamuni GH1, Gu S2, Ford MR2, Peterson LM2, Ma P2, Wang YT3, Rollins AM2, Jenkins MW3, Watanabe M1.
Disturbed cardiac function at an early stage of development has been shown to correlate with cellular/molecular, structural as well as functional cardiac anomalies at later stages culminating in the congenital heart defects (CHDs) that present at birth. While our knowledge of cellular and molecular steps in cardiac development is growing rapidly, our understanding of the role of cardiovascular function in the embryo is still in an early phase. One reason for the scanty information in this area is that the tools to study early cardiac function are limited. Recently developed and adapted biophotonic tools may overcome some of the challenges of studying the tiny fragile beating heart. In this chapter, we describe and discuss our experience in developing and implementing biophotonic tools to study the role of function in heart development with emphasis on optical coherence tomography (OCT). OCT can be used for detailed structural and functional studies of the tubular and looping embryo heart under physiological conditions. The same heart can be rapidly and quantitatively phenotyped at early and again at later stages using OCT. When combined with other tools such as optical mapping (OM) and optical pacing (OP), OCT has the potential to reveal in spatial and temporal detail the biophysical changes that can impact mechanotransduction pathways. This information may provide better explanations for the etiology of the CHDs when interwoven with our understanding of morphogenesis and the molecular pathways that have been described to be involved. Future directions for advances in the creation and use of biophotonic tools are discussed. KEYWORDS: avian models; cardiovascular development; congenital heart defects; fetal alcohol syndrome; optical coherence tomography; optical mapping; optical pacing
The involvement of the proamnion in the development of the anterior amnion fold in the chicken
PLoS One. 2014 Mar 19;9(3):e92672. doi: 10.1371/journal.pone.0092672. eCollection 2014.
de Melo Bernardo A, Chuva de Sousa Lopes SM. Author information
The amnion was one of the most important evolutionary novelties in the animal kingdom, allowing independence of water for reproduction and subsequent exploration of terrestrial habitats, and is therefore an important structure to understand evolution. We have studied chicken amniogenesis using ex ovo culture systems and 3D-reconstructions of serially sectioned chicken embryos. We provide evidence for a transient depression of the head in the proamnion, forming a pouch, that positions the extraembryonic membranes dorsal to the head and that is fundamental for the correct formation of the amnion and chorion membranes. When this "sinking" process in the proamnion was blocked, the amnion/chorion did not form, even though the growth of the embryo per se seemed unaffected. Here, we give insight in the role of the proamnion in amniogenesis.
Embryonic development of chicken (Gallus Gallus Domesticus) From 1st to 19th Day-ectodermal structures
Microsc Res Tech. 2013 Dec;76(12):1217-25. doi: 10.1002/jemt.22288. Epub 2013 Sep 5.
Toledo Fonseca E, Menezes De Oliveira Silva F, Alcântara D, Carvalho Cardoso R, Luís Franciolli A, Alberto Palmeira Sarmento C, Fratini P, José Piantino Ferreira A, Maria Angélica Miglino A. Source Department of Surgery, Faculty of Veterinary Medicine and Animal Science, University of São Paulo, São Paulo, Brazil.
Birds occupy a prominent place in the Brazilian economy not only in the poultry industry but also as an animal model in many areas of scientific research. Thus the aim of this study was to provide a description of macro and microscopic aspects of the ectoderm-derived structures in chicken embryos / fetuses poultry (Gallus gallus domesticus) from 1st to 19th day of incubation. 40 fertilized eggs, from a strain of domestic chickens, with an incubation period of 2-19 days were subjected to macroscopic description, biometrics, light, and scanning microscopy. All changes observed during the development were described. The nervous system, skin and appendages and organs related to vision and hearing began to be identified, both macro and microscopically, from the second day of incubation. The vesicles from the primitive central nervous system-forebrain, midbrain, and hindbrain-were identified on the third day of incubation. On the sixth day of incubation, there was a clear vascularization of the skin. The optic vesicle was first observed fourth day of development and on the fifth day there was the beginning of the lens formation. Although embryonic development is influenced by animal line as well as external factors such as incubation temperature, this paper provides a chronological description for chicken (Gallus gallus domesticus) during its embryonic development. Microsc. Res. Tech. 76:1217-1225, 2013. © 2013 Wiley Periodicals, Inc. Copyright © 2013 Wiley Periodicals, Inc.
Embryonic development of endoderm in chicken (Gallus gallus domestics)
Microsc Res Tech. 2013 Aug;76(8):803-10. doi: 10.1002/jemt.22232. Epub 2013 Jun 3.
Alcântara D, Rodrigues MN, Franciolli AL, Da Fonseca ET, Silva FM, Carvalho RC, Fratini P, Sarmento CA, Ferreira AJ, Miglino MA. Source Department of Surgery, Faculty of Veterinary Medicine and Animal Science, University of São Paulo, São Paulo, Brazil. email@example.com
The poultry industry is a sector of agribusiness which represents an important role in the country's agricultural exports. Therefore, the study about embryogenesis of the domestic chicken (Gallus gallus domesticus) has a great economic importance. The aim of this study was to evaluate embryonic development of the endoderm in chicken (Gallus gallus domesticus). Forty fertilized eggs of domestic chickens, starting from the 1st day of gestation and so on until the 19 days of the incubation were collected from the Granja São José (Amparo, SP, Brazil). Embryos and fetus were fixed in 10% formaldehyde solution, identified, weighed, measured, and subjected to light and scanning electron microscopy. The endoderm originates the internal lining epithelium of the digestive, immune, respiratory systems, and the organs can be visualized from the second day (48 h) when the liver is formed. The formation of the digestive system was complete in the 12th day. Respiratory system organs begin at the fourth day as a disorganized tissue and undifferentiated. Their complete differentiation was observed at the 10 days of incubation, however, until the 19 days the syrinx was not observed. The formation of immune system at 10th day was observed with observation of the spleen, thymus, and cloacal bursa. The study of the organogenesis of the chicken based on germ layers is very complex and underexplored, and the study of chicken embryology is very important due the economic importance and growth of the use of this animal model studies such as genetic studies. Copyright © 2013 Wiley Periodicals, Inc. KEYWORDS: avian, chicken embryology, embryogenesis
Chicken primordial germ cells use the anterior vitelline veins to enter the embryonic circulation
Biol Open. 2012 Nov 15;1(11):1146-52. doi: 10.1242/bio.20122592. Epub 2012 Sep 18.
De Melo Bernardo A, Sprenkels K, Rodrigues G, Noce T, Chuva De Sousa Lopes SM. Source Department of Anatomy and Embryology, Leiden University Medical Center , Einthovenweg 20, 2333 ZC Leiden , The Netherlands.
During gastrulation, chicken primordial germ cells (PGCs) are present in an extraembryonic region of the embryo from where they migrate towards the genital ridges. This is also observed in mammals, but in chicken the vehicle used by the migratory PGCs is the vascular system. We have analysed the migratory pathway of chicken PGCs, focusing on the period of transition from the extraembryonic region to the intraembryonic vascular system.Our findings show that at Hamburger and Hamilton developmental stage HH12-HH14 the majority of PGCs concentrate axially in the sinus terminalis and favour transport axially via the anterior vitelline veins into the embryonic circulation. Moreover, directly blocking the blood flow through the anterior vitelline veins resulted in an accumulation of PGCs in the anterior region and a decreased number of PGCs in the genital ridges. We further confirmed the key role for the anterior vitelline veins in the correct migration of PGCs using an ex ovo culture method that resulted in defective morphogenetic development of the anterior vitelline veins.We propose a novel model for the migratory pathway of chicken PGCs whereby the anterior vitelline veins play a central role at the extraembryonic and embryonic interface. The chicken model of PGC migration through the vasculature may be a powerful tool to study the process of homing (inflammation and metastasis) due to the striking similarities in regulatory signaling pathways (SDF1-CXCR4) and the transient role of the vasculature.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial Share Alike License (http://creativecommons.org/licenses/by-nc-sa/3.0/).
Lhx1 in the proximal region of the optic vesicle permits neural retina development in the chicken
Biol Open. 2012 Nov 15;1(11):1083-93. doi: 10.1242/bio.20121396. Epub 2012 Aug 28.
Kawaue T, Okamoto M, Matsuyo A, Inoue J, Ueda Y, Tomonari S, Noji S, Ohuchi H. Source Department of Life Systems, Institute of Technology and Science, The University of Tokushima Graduate School , 2-1 Minami-Josanjima-cho, Tokushima 770-8506 , Japan.
How the eye forms has been one of the fundamental issues in developmental biology. The retinal anlage first appears as the optic vesicle (OV) evaginating from the forebrain. Subsequently, its distal portion invaginates to form the two-walled optic cup, which develops into the outer pigmented and inner neurosensory layers of the retina. Recent work has shown that this optic-cup morphogenesis proceeds as a self-organizing activity without any extrinsic molecules. However, intrinsic factors that regulate this process have not been elucidated. Here we show that a LIM-homeobox gene, Lhx1, normally expressed in the proximal region of the nascent OV, induces a second neurosensory retina formation from the outer pigmented retina when overexpressed in the chicken OV. Lhx2, another LIM-homeobox gene supposed to be involved in early OV formation, could not substitute this function of Lhx1, while Lhx5, closely related to Lhx1, could replace it. Conversely, knockdown of Lhx1 expression by RNA interference resulted in the formation of a small or pigmented vesicle. These results suggest that the proximal region demarcated by Lhx1 expression permits OV development, eventually dividing the two retinal domains.
Dual origins of the prechordal cranium in the chicken embryo
Dev Biol. 2011 Aug 15;356(2):529-40. doi: 10.1016/j.ydbio.2011.06.008. Epub 2011 Jun 15.
Wada N1, Nohno T, Kuratani S.
The prechordal cranium, or the anterior half of the neurocranial base, is a key structure for understanding the development and evolution of the vertebrate cranium, but its embryonic configuration is not well understood. It arises initially as a pair of cartilaginous rods, the trabeculae, which have been thought to fuse later into a single central stem called the trabecula communis (TC). Involvement of another element, the intertrabecula, has also been suggested to occur rostral to the trabecular rods and form the medial region of the prechordal cranium. Here, we examined the origin of the avian prechordal cranium, especially the TC, by observing the craniogenic and precraniogenic stages of chicken embryos using molecular markers, and by focal labeling of the ectomesenchyme forming the prechordal cranium. Subsequent to formation of the paired trabeculae, a cartilaginous mass appeared at the midline to connect their anterior ends. During this midline cartilage formation, we did not observe any progressive medial expansion of the trabeculae. The cartilages consisted of premandibular ectomesenchyme derived from the cranial neural crest. This was further divided anteroposteriorly into two portions, derived from two neural crest cell streams rostral and caudal to the optic vesicle, called preoptic and postoptic neural crest cells, respectively. Fate-mapping analysis elucidated that the postoptic neural crest cells were distributed exclusively in the lateroposterior part of the prechordal cranium corresponding to the trabeculae, whereas the preoptic stream of cells occupied the middle anterior part, differentiating into a cartilage mass corresponding to the intertrabecula. These results suggest that the central stem of the prechordal cranium of gnathostomes is composed of two kinds of distinct cartilaginous modules: a pair of trabeculae and a median intertrabecula, each derived from neural crest cells populating distinct places of the craniofacial primordia through specific migratory pathways. Copyright © 2011 Elsevier Inc. All rights reserved. PMID 21693114
4D fluorescent imaging of embryonic quail development
Cold Spring Harb Protoc. 2011 Nov 1;2011(11):1291-4. doi: 10.1101/pdb.top066613.
Canaria CA, Lansford R.
Traditionally, our understanding of developmental biology has been based on the fixation and study of embryonic samples. Detailed microscopic scrutiny of static specimens at varying ages allowed for anatomical assessment of tissue development. The advent of confocal and two-photon excitation (2PE) microscopy enables researchers to acquire volumetric images in three dimensions (x, y, and z) plus time (t). Here, we present techniques for acquisition and analysis of three-dimensional (3D) time-lapse data. Both confocal microscopy and 2PE microscopy techniques are used. Data processing for tiled image stitching and time-lapse analysis is also discussed. The development of a transgenic Japanese quail system, as discussed here, has provided an embryonic model that is more easily accessible than mammalian models and more efficient to breed than the classic avian model, the chicken.
- Analysis of retinal cell development in chick embryo by immunohistochemistry and in ovo electroporation techniques http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2822752&tool=pmcentrez&rendertype=abstract
Reference guide to the stages of chick heart embryology
Dev Dyn. 2005 Aug;233(4):1217-37.
Cardiac progenitors of the splanchnic mesoderm (primary and secondary heart field), cardiac neural crest, and the proepicardium are the major embryonic contributors to chick heart development. Their contribution to cardiac development occurs with precise timing and regulation during such processes as primary heart tube fusion, cardiac looping and accretion, cardiac septation, and the development of the coronary vasculature. Heart development is even more complex if one follows the development of the cardiac innervation, cardiac pacemaking and conduction system, endocardial cushions, valves, and even the importance of apoptosis for proper cardiac formation. This review is meant to provide a reference guide (Table 1) on the developmental timing according to the staging of Hamburger and Hamilton (1951) (HH) of these important topics in heart development for those individuals new to a chick heart research laboratory. Even individuals outside of the heart field, who are working on a gene that is also expressed in the heart, will gain information on what to look for during chick heart development. This reference guide provides complete and easy reference to the stages involved in heart development, as well as a global perspective of how these cardiac developmental events overlap temporally and spatially, making it a good bench top companion to the many recently written in-depth cardiac reviews of the molecular aspects of cardiac development. (c) 2005 Wiley-Liss, Inc.
Stages a HH, stages according to Hamburger and Hamilton (1951). Cell sources that contribute to cardiac development
Cardiac Progenitors (CP) HH 1, 2, 3, 4–5, 6–7, 10–11, 21–23 Primary heart field and primary heart tube formation HH 4–5, 6–7, 8, 10–11 Secondary heart field and outflow tract accretion HH 10–11, 21–23 Cardiac neural crest cells (CNC) HH 8, 9, 10–11, 12–13−, 13+, 14–15, 21–23 Outflow tract development HH 25–26, 27, 28, 31, 34 Cardiac innervation HH 27, 35, 36, 40 Proepicardial organ (PEO) HH 14–15 Epicardium HH 17–18, 19–20, 21–23, 27, 28, 32–33 Coronary vasculature (CV) HH 25–26, 27, 30, 31, 32–33, 34, 35, 36, 38, 40
Apoptosis HH 19–20, 21–23, 24, 25–26, 27, 30, 31, 32–33, 35 Epithelial-mesenchymal transformation See CP, CNC, PEO, Epicardium, CV, EC Migration See CP, CNC, PEO, Epicardium, CV, EC Proliferation of the myocardium HH 16, 17–18, 19–20, 27, 29, 34, 46
Cardiac remodeling and development
Cardiac looping HH 10–11, 12–13−, 13+, 16, 17–18, 24 Endocardial cushion (EC) development HH 12–13−, 16, 19–20, 21–23, 25–26, 29, 34 Valve development HH 21–23, 28, 30, 31, 34, 36 Cardiac septation See below Outflow tract septation HH 25–26, 27, 28, 31, 34 Ventricular septation HH 17–18, 19–20, 21–23, 29, 32–33, 34 Atrial septation HH 16, 21–23, 24, 28, 30, 31, 34, 36, 46 Pacemaker and conduction system HH 8, 10–11, 14–15, 28, 29, 34, 36, 46 Pacemaker (SA-node) HH 28 Central (AV-node, AV bundles) HH 14–15, 28, 34 Peripheral (Purkinje fiber network) HH 36, 46