Chapter 20

### INTRODUCTION

Diseases of the pulmonary venous system have generally been poorly characterized and less frequently investigated compared with diseases of the bronchial tree and pulmonary arterial system. Generally speaking, disorders of the venous system follow a more indolent course. They are progressive in nature and generally do not have a dramatic presentation, with the exception of some congenital disorders that may present dramatically in infancy. Individually, these disorders have a low incidence, which impedes their study. Collectively, however, they affect a significant number of patients and are a challenge for physicians to diagnose and treat. The diagnosis of most of the diseases described in this section are only considered after more common causes of respiratory symptoms have been ruled out, specifically infection. Although delay in diagnosis is frustrating for physicians, there does not appear to be an effect on prognosis in most cases. This is most likely because of the slow progression of these diseases and the lack of effective therapies. This chapter reviews pulmonary vein embryology and describes diseases of pulmonary veins. It also discusses diseases of the pulmonary lymphatic system and briefly mentions disorders that are extremely rare and described only in case reports.

### EMBRYOLOGY

The tissue that gives rise to the pulmonary vasculature is hypothesized to have two distinct origins. The proximal vessels, including the pulmonary veins, are thought to develop through angiogenesis, a process by which existing vessels give rise to new ones by branching, beginning at the pulmonary truncus, an outgrowth of the primitive left atrium. The peripheral vessels, on the other hand, are thought to develop in situ from progenitor cells of lung mesenchyme. This process is a bit more complex, involving formation of a primitive primary plexus, which undergoes remodeling, resulting in a mature network of vessels varying in size. This process is termed vasculogenesis.1 The two separately formed groups of vessels subsequently fuse, communicating the peripheral lung to the heart.2 Morphogenesis and cell differentiation are controlled by numerous gene products, including transcription factors, peptide growth factors, intracellular adhesion molecules, and cell adhesion receptors. Ongoing research continues to provide insight into the genetic pathways that control pulmonary vasculature. Specific genes and pathways that have been identified as essential for normal lung vasculature development include vascular endothelial growth factor A (VEGF-A), endothelial monocyte activating polypeptide II (EMAP II), transforming growth factor β family (TGF-β), insulin like growth factors I and II (IGF-I, IGF-II), forkhead box transcription factors, and the Wnt signaling pathway. The absence of appropriate expression of these genes and pathways results in disrupted pulmonary vasculature development and early embryonic demise.3,5,6,7,8,9,10,11,12,13,14 Although the molecular and morphologic mechanisms that give rise to the pulmonary vasculature have not been clearly delineated, the chronologic events that take place have been well established for a considerable length of time.

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