Cardiac conduction: fish vs. mammals
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Many avian species develop an AV-ring which plays a role similar to that of the mammalian AV-node (Szabo et al. Anat Rec. 1986 May;215(1):1-9. PubMed PMID:3706789.) []
Steps in CCS develoment in zebrafish:
From Genetic and physiologic dissection of the vertebrate cardiac conduction system. PLoS Biol. 2008 May 13;6(5):e109. PubMed PMID: 18479184; PubMed Central PMCID: PMC2430899. []
- Initially, a linear conduction travels across the heart tube from the sinus venosus to the OFT (20–24 hours postfertilization (hpf)).
- next, a significant AV conduction delay develops during cardiac chamber formation (36–48 hpf)
- as the heart loops and develops ventricular trabeculations (72–96 hpf), an immature fast conduction network develops within the ventricle
- finally, this fast conduction network fully matures to an apex-to-base activation pattern when the ventricular apex has formed.
Notable bits from PMC:2323757 
Great coverage of different heart conduction systems across wide range of organisms
- Automaticity of heart rhythm can be neurogenic or myogenic depending on the organism
- Invertebrates have been proven to be outside the general concept of the myogenic origin of the heartbeat; pacemaking of a primitive heart originating from a neural ganglion placed behind the dorsal heart wall was documented in some tiny crustaceans ranked among stomatopods – Stomatopoda – for example, Squilla mantis or Squilla oratoria (9). Immediate heart arrest was demonstrated after the neural connection between the ganglion cordis and the heart muscle was cut (10). On the other hand, there was no heart innervation found in other primitive crustaceans – Branchiopoda (eg, Daphnia) – and their heart automaticity is considered to be myogenic.
- Myoendothelial cells of tunicates (primitive chordates) are electromecahnically coupled and endowed with automaticity. There is no dominant pacemaker. The locus of the temporary dominant automaticity is wandering – the pacemaking centre switches from one heart tube pole to another. After every few beats (which are variable), a new centre from the other end of the tube takes over the pacemaking activity. Electrical impulses and consequently peristaltic waves spread from the place of origin along the tube. They run in turns toward one pole and then the other pole, and thus, the bloodstream direction varies over time. There is no organized conducting system in tunicates (12), but myocytes specialized for conduction have been detected in the heart tube of Ciona species (13).
- Lancelet or amphioxus (Cephalochordata). These chordates differ essentially from tunicates, because they possess a head and their circulation system is closed, as in vertebrates. On the other hand, there is no developed heart at all. Instead, some vessels capable of contracting act as a pump. The main pump function relies on the aorta ventralis, but some other vessels produce autonomous contractions as well, such as the vena subintestinalis, vena portae and venae hepaticae. The walls of contracting vessels are coated by a single layer of myoepithelial cells and, therefore, differ from the structure of vessels in vertebrates. Vessels contract in sequence and thus actuate the circulation. Their co-ordination, however, is poor, sometimes even chaotic. Countercurrent contractions and flow are frequent. No specialized pacemaker cells or organized conducting system have been found in lancelets (14).
- Fish As in all higher vertebrates, the heart of the fish is segmented into clearly defined chambers. The four serially grouped chambers are the sinus venosus, atrium, ventricle and truncus arteriosus or bulbus arteriosus in teleosts. There has been no septation of the atrium or ventricle, thus, they have a single atrium and a single ventricle (5,16). a uniform (isotropic) impulse of velocity spreading through the atrium in fish, as well as a delay in conducting through the AV junction and the following rapid ventricular activation from the apex toward the base, have been described cogently. If stimulated ectopically, the excitation spreads through the ventricle more slowly. This anisotropy of activation suggests the existence of some pathways of preferential spread of excitation (18,19). Nevertheless, no histologically defined conducting tissue bundle has been discovered in the ventricle of fish so far. On the other hand, there are some subendocardial trabecular bands abridging the AV channel and the apex that conduct impulses anisotropically and can be stained by histochemical methods. These trabeculae may be a functional equivalent of the bundle branches and Purkynê (Purkinje) fibres (18).
Pubs discussing fish cardiac conduction system
- Notch1b and neuregulin are required for specification of central cardiac conduction tissue Milan et al.(2006) Dev 133(6):1125-1132
We demonstrate that a ring of atrioventricular conduction tissue develops at 40 hours post-fertilization in the zebrafish heart. Analysis of the mutant cloche reveals a requirement for endocardial signals in the formation of this tissue. The differentiation of these specialized cells, unlike that of adjacent endocardial cushions and valves, is not dependent on blood flow or cardiac contraction. Finally, both neuregulin and notch1b are necessary for the development of atrioventricular conduction tissue.