The life of a human lung can be subdivided into five distinct phases: embryonic, pseudoglandular, canalicular, saccular, and alveolar. The embryonic period, during which the lung primordium is laid down as a diverticulum of the foregut, lasts for about seven weeks. From the 5th to the 17th week the lung looks much like a tubulo-acinar gland, with epithelial tubes sprouting and branching into the surrounding mesenchyme. In the last week of this pseudoglandular stage the prospective conductive airways have been formed, and the acinar limits can be recognized. The events of the subsequent canalicular phase (17th-26th week) can be summarized as the widening of the peripheral tubules, the differentiation of the cuboidal epithelium into type I and type II cells, the formation of the first thin air-blood barriers, and the start of surfactant production. During the saccular stage, which follows and lasts until birth, the growth of the pulmonary parenchyma, the thinning of the connective tissue between the airspaces, and the further maturation of the surfactant system are the most important steps towards life. At birth, although already functional, the lung is structurally still in an immature condition, because alveoli, the gas exchange units of the adult lung, are practically missing. The airspaces present are smooth-walled transitory ducts and saccules with primitive type septa that are thick and contain a double capillary network. During the first 1-3 years of postnatal life, alveoli are formed through a septation process that greatly increases the gas exchange surface area. The primitive septa with their capillaries undergo a complete remodeling, gaining the mature slender morphology found in the adult lung.
