**2. The embryology of pulmonary veins**

The development of the cardiovascular system is complicated because it involves the process from before the folding of heart tube and extend to the later stage of vascular growth. In the vertebrate embryo, most discussion start from the Carnegie stage 12, which approximately equals to 28-30 days in human [1] and 2 days in chicken [2]. At this moment, the primitive pulmonary vein originates from the venous plexus of splanchnic

mesoderm. The staining characteristic of the pulmonary vein orifices in the developing heart can prove that the pulmonary vein is not part of the heart tube: it has no atrial natriuretic factor and has connexin 40 (a transmembrane protein that responsible for electrical coupling mostly found in the nodal tissue) [3]. In addition, an observation study of chicken embryo using image analysis and three-dimensional reconstruction technique also revealed that the pulmonary vein is developing from the splanchnic plexus [4]. The venous plexus of splanchnic mesoderm is a great capillary network that spread from the heart to the liver, connecting cardinal and umbilicovitelline veins. In other words, the pulmonary vein is communicating with systemic venous system in the beginning. In the subsequent developmental process, this communication will degenerate, therefore separating the systemic and pulmonary venous systems (**Figure 1**) [5].

#### **Figure 1.**

*The normal pulmonary venous development. A, the lung buds are surrounded the splanchnic plexus that communicates umbilical veins and cardinal veins. B, Common pulmonary vein is formed and connected with the sinoatrial part of the heart. C, the connection between pulmonary and splanchnic venous plexus is disappearing. D, the common pulmonary vein develops to four distinct pulmonary veins that incorporates separately with the left atrium. LA, left atrium; LCCV, left common cardinal vein; LLB, left lung bud; RA, right atrium; RCCV, right common cardinal vein; RLB, right lung bud; UV, umbilical vein.*

*Pulmonary Vein: Embryology, Anatomy, Function and Disease DOI: http://dx.doi.org/10.5772/intechopen.100051*

This common pulmonary vein connects the lung buds to the dorsal heart tube, where would develop to left atrium after the outgrowth of intertrial septum. At the level of left atrium, the common pulmonary vein would usually divide into four branches and incorporate with left atrium, forming the smooth part of the left atrium wall [6]. In a study using 26 normal human embryos, the initial process of formation of the human pulmonary vein is very similar to that seen in animal models; marked temporal and morphological difference between the development process of rightand left-side pulmonary veins was found: a much longer tributary being formed on the left than on the right [7].

Various congenital abnormalities of pulmonary veins can occur if anything is wrong during these developmental processes. The cor triatriatum sinister (CTS), a condition that left atrium is separated into two chambers by a membranous tissue, is thought to be the consequence of the inappropriate incorporation of pulmonary veins with the left atrium [7]. In addition, if the atrophy of connection between pulmonary veins and systemic venous system is fail, total or partial anomalous pulmonary venous connection (TAPVC or PAPVC) occurs, depending on the degree of remanent communication between systemic and pulmonary venous system [8].
