Lymph heart in chick - somitic origin, development and embryonic oedema

Abstract

The lymph heart is a sac-like structure on either side of avian tail. In some adult birds, it empties the lymph from the copulatory organ; however,during embryonic development, it is thought to circulate extra-embryonic lymph. Very little is known about the origin, innervation and the cellular changes it undergoes during development. Using immunohistochemistry and gene expression profiling we show that the musculature of the lymph heart is initially composed solely of striated skeletal muscle but later develops an additional layer composed of smooth myofibroblasts. Chick-quail fate-mapping demonstrates that the lymph heart originates from the hypaxial compartments of somites 34-41. The embryonic lymph heart is transiently innervated by somatic motoneurons with no autonomic input. In comparison to body muscles, the lymph heart has different sensitivity to neuromuscular junction blockers (sensitive only to decamethonium). Furthermore, its abundant bungarotoxin-positive acetylcholinesterase receptors are unique as they completely lack specific acetylcholinesterase activity. Several lines of evidence suggest that the lymph heart may possess an intrinsic pacing mechanism. Finally, we assessed the function of the lymph heart during embryogenesis and demonstrate that it is responsible for preventing embryonic oedema in birds, a role previously thought to be played by body skeletal muscle contractions.

Characterisation of Development and Electrophysiological Mechanisms Underlying Rhythmicity of the Avian Lymph Heart

“…Our in-situ hybridisation results showed that at HH30, En-1 specifically marked lymph heart myoblasts and not any other type of myoblasts. We have shown previously that lymph heart develops from the hypaxial regions [10] and our results show that En-1 is only expressed in the epaxial compartments during development suggesting that the expression of En-1 represents a de novo specification event and not simply a trace from its somitic origin. Secondly, the expression of En-1 is dependent on Hedgehog ( Hh ) signalling in Xenopus lymph heart and in the absence of Hh signalling and upon En-1 knockdown, lymph heart muscle fails to develop and embryos consequently develop oedema [11].…”

“…Particularly, we looked at E10, the stage at which avian lymph heart exhibits spontaneous contraction [12]. In skinned pelvis (HH36), lymph hearts contract at a rate of 5 beats per minute on heated stage (39–40°C) as reported previously [10]. ZD7288 is widely considered a selective blocker of HCN currents [24].…”

“…Avian lymph heart comprises three layers: an inner endothelial cell layer lining the organ’s cavity, an outer layer of fibrous connective tissue that facilitates attachment of the lymph heart to the vertebral column and a middle layer of muscle fibres which are highly branched and of smaller diameter reminiscent of cardiac muscle [9–11] Despite the cardiac similarity of myotubes, lymph hearts only express skeletal muscle markers [10] and not determinants of cardiac tissue, such as GATA-4 , -5 and -6 , and cardiac troponin [11]. Nevertheless, the organ exhibits spontaneous, rhythmic contractions [10–13].…”

“…Nevertheless, the organ exhibits spontaneous, rhythmic contractions [10–13]. Thus, the lymph heart is unique as a skeletal muscle that rhythmically contracts as a result of intrinsically generated action potentials [14] and displays structural characteristics similar to cardiac heart [10, 11]. Importantly, the avian lymph heart must be fully functional during in ovo development since its failure leads to oedema and embryonic death [10].…”

“…Furthermore, intracellular recordings from lymph heart muscle cells revealed summating depolarisation followed by a later, sustained hyperpolarisation [14]. In addition, Del Castillo and Sanchez [14] and Day et al [16]investigated pharmacological modulation of amphibian lymph heart electrophysiological properties to show that recorded activity was particularly sensitive to cholinergic modulators whilst subsequent studies revealed the presence of nicotinic acetylcholine receptors (nAChRs) in amphibian lymph heart [10, 17]. Thus, whilst the bioelectrical basis of the amphibian lymph heart contraction is well known and can be modulated via nAChRs in the same manner as skeletal muscle [10, 17], the underlying basis for avian lymph heart rhythmicity in birds remains unknown.…”