The primitive heart is truly a wonder of creation and develops quickly over a short period of time. During the initial stages of fetal development the ectoderm, mesoderm and endoderm of the trilaminar disc develop into all the organs of the embryo. We will focus is on the heart development and will thus exclude all other structures.
Two lateral endocardial tubes arrise from differentiated cardiac mesodermal cells in the cardiac crescent located under the head fold of the embryo (day 18 - 19 after fertilization). As the embryo folds along its long axis the tubes moves closer together and eventually fuse together to form one single heart tube (between days 20 - 25). This linear heart tube will now grow, fold and bulge to develop into the primitive heart.
Primitive heart development is complex and the structures can be difficult to visualise in 2d images. In this tutorial we aim to provide an overview of primitive heart development from day 20 to 28. Due to the amount of information and complexity of the structures the tutorial will grow over time and more content will be added as it becomes available.
Figure 1. This image shows a color legend of the primary and secondary heart fields at day 22 and day 28. These regions will continue to develop into the respective structures as observed in an adult heart.
Figure 2. This image shows the stages of the formation of the primitive heart from a single endocardial heart tube. The seperated lateral endocardial tubes form around the anterolateral aspect of the gut tube around day 20. The Theart tubes are joined into a single linear heart tube (day 21 - 24) that now starts to bulge, twist and grows to develop into the primitive heart. Labeled regions of the first and second heart fields are shown in colour.
Figure 3. This image shows the heart and some of the main vasculature of a 5 week old embryo. Note the blue left and right cardinal veins that return blood to the heart. The primitive heart is already folded and buldged into some of the structures later identified in an adult heart. At this stage the embryo measures around 1.5 - 2mm in length.