59. Development of The Lower Respiratory System: The Bronchi and Surrounding Structures

  1. The bronchi: by the early part of week 5, the entodermal lung bud, surrounded by splanchnic mesenchyme, has developed 2 bronchial buds that will differentiate into the major bronchi as well as their ramifications in the lungs. Each bud enlarges to form a primitive primary bronchus with the right being slightly larger and more vertical than the left
    1. DURING WEEK 5, EACH PRIMARY BRONCHUS grows laterally into the medial walls of the pericardioperitoneal canals or primitive pleural cavities. Simultaneously, the right primary bud gives rise to 2 secondary buds (bronchi), whereas the left only gives rise to one. Thus, in the adult, there are 3 secondary bronchi and lobes (superior, middle, and inferior) on the right, but only 2 secondary bronchi and lobes on the left (superior and inferior)
      1. At the same time, the surrounding mesenchyme forms definitive masses which become the lung lobes, and all grow caudally into the coelomic cavity of the embryo
    2. EACH SECONDARY LUNG BUD then undergoes progressive dichotomous branching to form tertiary (segmental) bronchi: 10 in the right lung and 8 or 9 in the left, which begin to appear by week 7
      1. Each tertiary bronchus with its surrounding mass of mesenchyme will eventually form a bronchopulmonary segment
    3. BY WEEK 24, ABOUT 17 ORDERS OF AIRWAYS are formed, and the respiratory bronchioles are present. From then on to term, alveolar ducts and primitive alveoli develop
    4. AT BIRTH, THE LUNGS are not completely developed, and additional airways form until the age of 8 years when the full complement of about 24 orders of airways is established. Once complete, no further new airways replace those lost to trauma or disease
  2. Cartilage, smooth muscle, and connective tissue develop from the surrounding mesenchyme as the bronchial tree is developing
    1. CARTILAGE seen in primary bronchi by week 10, in segmental bronchi by week 12
    2. SMOOTH MUSCLE CELLS begin differentiating at the end of week 7 and by week 12 help form the posterior wall of the larger bronchi where cartilage is absent
      1. In the smaller bronchi and bronchioles, muscle forms an interlacing network in a double spiral pattern which extends into the alveolar ducts
    3. FIBROELASTIC STRUCTURES differentiate about week 7 with fibrils in the mesenchyme. By week 12, fibrils form bundles and look like mature, fibrous, collagenous tissue. In week 24, the elastic fibers begin to differentiate
  3. Epithelial structures: initially, the lining of the bronchi is nonciliated columnar cells, but by week 12?the cells become cuboidal and develop cilia
    1. BY WEEK 13 one sees cilia in the segmental bronchi, and at the end of week 20 the cells are cuboidal; at birth, cilia are present in the terminal bronchioles
    2. SUBMUCOSAL, MUCUS-SECRETING GLANDS of the bronchi and bronchioles arise from the epithelial cells which migrate into the submucosa
      1. By week 13, the cells produce mucous glands which secrete one week later
      2. At the end of week 28, over 85% of adult mucous glands are present in the system
  4. Pulmonary vessels
    1. EARLY IN WEEK 4, a primitive pulmonary artery arises from the ventral aspect of both right and left sixth (pulmonary) aortic arches and extends caudally toward the developing tubular lung bud, eventually being incorporated into the mesenchymal tissue around the primitive trachea and bronchial buds
      1. The major branch of each pulmonary arch is the pulmonary artery which accompanies each developing primary bronchus (see Circulatory system for details)
    2. THE VENOUS RETURN develops from the mesenchyme and the heart wall and, instead of following the bronchial tree, runs between bronchopulmonary segments
development of  the lower respiratory system:  the bronchi and surrounding structures: image #1