who feeds the heart bud
Answers
Answer:
Through the 20th century, knowledge of the events occurring during cardiac development was clouded by conflicting descriptions, coupled with use of notably different terminologies. Furthermore, not all accounts were based on direct study of embryonic material, instead being constructed on the basis of interpretations of previous reports, supported by inferences made from the structure of the congenitally malformed heart. Such processes, in themselves, are understandable, since it is axiomatic that proper appreciation of the events occurring during formation of the heart will aid in the analysis of the morphogenesis of cardiac malformations, this being a desirable prerequisite in the search for optimal treatment.
Over the past decade, this has all changed. There has been an explosion of work, both anatomical and molecular, devoted to cardiac development. Advances in technology, coupled with the use of suitable animal models, now enable us to provide a more accurate account of the steps involved in formation and septation of the cardiac chambers. Not all of this new information is concordant with the “classical” accounts. In these reviews, therefore, we will describe, first, the steps involved in formation of the primary heart tube, and its conversion to the four cardiac chambers and the paired arterial trunks. We will then look in greater detail at the events occurring during the separation of the initial solitary heart tube into discrete systemic and pulmonary circulations.
FORMATION OF THE HEART TUBE
The mesodermal tissues that give rise to the heart first become evident when the embryo is undergoing the process known as gastrulation. In the human, this occurs during the third week of development, while for the mouse, at a comparable stage of development, around seven days will have elapsed from fertilisation, and the embryo will be in the presomitic stage. The embryonic plate in humans, initially possessing two layers, is ovoid, and is formed at the union between the yolk sac and the amniotic cavity. In the midline of the long axis of the oval disc is found the primitive streak, with the node at its cranial end. Through this streak, cells migrate from the upper layer by the process called gastrulation to form the three germ layers of the embryo proper: the ectoderm, the endoderm, and the mesoderm. The mesoderm insinuates between the ectodermal and endodermal layers, which themselves are continuous with the amnion and yolk sac, respectively. Having insinuated, the mesoderm spreads laterally and cranially within the embryonic disc, ultimately giving rise to a variety of structures, such as the somites, which will produce the axial structures, and the lateral plate mesoderm, which will form the parietal body wall. The cells that are destined to form the heart are also derived from this mesodermal layer. They form a crescent virtually at the cranial border of the disc (fig 11).). As this heart forming region achieves its crescentic shape, the central region of the ectoderm transforms into the neural plate. This folds to become the neural tube, with the developing brain at its cranial end. In the human, the developing heart is initially cranial within the disc relative to the neural folds.