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Development of cardiac beat rate in early chick embryos is regulated by regional cues
1Department of Anatomy and Cell Biology, Emory University Health Science Center, Atlanta, Georgia 30322.
Insights
This study reveals that chick embryo heart development is influenced by location, not just origin. Transplanted heart cells adopt the beat rate of their new position, indicating environmental cues guide cardiac rhythm determination.
Area of Science:
- Developmental biology
- Cardiac embryology
- Cellular differentiation
Background:
- The developing chick embryo heart has distinct regions for conus, ventricle, and sinoatrial cells.
- These regions are initially arranged rostrocaudally within the heart-forming regions (HFRs).
- Intrinsic left-right and rostrocaudal beat rate gradients exist in early embryonic heart tissue.
Purpose of the Study:
- To investigate whether the intrinsic beat rate of embryonic heart cells is predetermined by their origin or influenced by their location.
- To understand the role of positional cues in cardiac rhythm development.
Main Methods:
- Microsurgical division of chick embryo HFRs into segments.
- Induction of cardia bifida (CB) by preventing HFR fusion.
- Exchanging mesodermal fragments between different HFR regions.
- Implantation of differentiated conus tissue into early HFRs.
Main Results:
- Segments of HFRs differentiate into beating cardiac vesicles with established beat rate gradients.
- Cardia bifida embryos show a consistent left-sided faster heart rate.
- Exchanged mesoderm develops a beat rate appropriate to its new location, overriding its origin.
- Implanted conus tissue slows the beat rate of adjacent developing heart tissue.
Conclusions:
- Cardiac cell beat rate determination is primarily influenced by the local environment within the developing heart.
- Positional cues within the HFR play a critical role in establishing cardiac rhythm.
- Early embryonic heart development is plastic and responsive to inductive signals from surrounding tissues.
Abstract:
The mesoderm of each of the paired lateral heart-forming regions (HFRs) in the stage 5-7 chick embryo includes prospective conus (pre-C), ventricle (pre-V), and sinoatrial (pre-SA) cells, arranged in a rostrocaudal sequence (C-V-SA). With microsurgery we divided each HFR into three rostrocaudally arranged segments. After 24 hr of further incubation, each segment differentiated into a spontaneously beating vesicle of heart tissue to form a multiheart embryo. The cardiac vesicles in these embryos expressed left-right and rostrocaudal beat rate gradients: the left caudal pre-SA mesoderm produced tissue with the fastest beat rate of the six while the rostral vesicle formed from right pre-C was the slowest. In another operation, we prevented the HFRs from fusing in the midline by cutting through the anterior intestinal portal at stage 8, to produce cardia bifida (CB) embryos with an independently beating half-heart on each side. In these cases, the left half-heart of 87.2% of CB embryos beat faster than the right, confirming the left-right difference in intrinsic beat rate. To assess whether the future beat rate of each region is already determined in the st 5-7 HFR, we exchanged rectangular fragments of left pre-SA mesoderm and attached endoderm with right pre-C fragments to yield a left HFR with the sequence C-V-C and a right HFR with the sequence SA-V-SA. A CB operation was subsequently performed on these exchange embryos to prevent fusion of the lateral HFRs. Preconus mesoderm, transplanted to the pre-SA region, differentiated into tissue with a rapid beat rate, while pre-SA mesoderm relocated to the preconus region formed heart tissue with a slow spontaneous rate typical of the conus. In 73% of the exchange CB embryos, the left half-heart beat faster than the right, despite the origins of its mesoderm. The exchanged mesoderm developed a rate that was appropriate for its new location rather than the site of origin of the mesodermal fragment. In a third set of operations, we implanted a fragment of st 15 differentiated conus tissue into a site lateral to the left caudal HFR in st 5, 6, and 7 embryos, and subsequently performed CB operations on them. The implant caused the adjacent half-heart to develop with a slower beat rate than in unoperated or sham-operated controls.(ABSTRACT TRUNCATED AT 400 WORDS)