Early Development
The mammalian heart is derived from embryonic mesoderm germ-layer cells that differentiate after gastrulation into mesothelium, endothelium, and myocardium. Mesothelial pericardium forms the outer lining of the heart. The inner lining of the heart, lymphatic and blood vessels, develop from endothelium. Heart muscle is termed myocardium.
From splanchnopleuric mesoderm tissue, the cardiogenic plates develops cranially and laterally to the neural plate. In the cardiogenic plates, two separate angiogenic cell clusters form on either side of the embryo. The cell clusters coalesce to form an endocardial tube continuous with a dorsal aorta and a vitteloumbilical vein. As embryonic tissue continues to fold, the two endocardial tubes are pushed into the thoracic cavity, begin to fuse together, and complete the fusing process at approximately 22 days.
The human embryonic heart begins beating at around 22 days after conception, or five weeks after the last normal menstrual period (LMP). The first day of the LMP is normally used to date the start of the gestation (pregnancy). The human heart begins beating at a rate near the mother’s, about 75–80 beats per minute (BPM).
The embryonic heart rate (EHR) then accelerates linearly by approximately 100 BPM during the first month to peak at 165–185 BPM during the early 7th week afer conception, (early 9th week after the LMP). This acceleration is approximately 3.3 BPM per day, or about 10 BPM every three days, which is an increase of 100 BPM in the first month.
After 9.1 weeks after the LMP, it decelerates to about 152 BPM (+/-25 BPM) during the 15th week post LMP. After the 15th week, the deceleration slows to an average rate of about 145 (+/-25 BPM) BPM, at term. The regression formula, which describes this linear acceleration before the embryo reaches 25 mm in crown-rump length, or 9.2 LMP weeks, is: the Age in days = EHR(0.3)+6. There is no difference in female and male heart rates before birth.
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