(BQ) Part 2 book "Larsen's human embryology" presentation of content: Development of the respiratory system and body cavities, development of the heart, development of the vasculature, development of the gastrointestinal tract, development of the urinary system, development of the reproductive system,. | Chapter 11 Development of the Respiratory System and Body Cavities SUMMARY As covered in Chapter 4, shortly after the three germ layers form during gastrulation, body folding forms the endodermal foregut at the cranial end of the embryo, thereby delineating the inner tube of the tube-within-a-tube body plan. On day twenty-two, the foregut produces a ventral evagination called the respiratory diverticulum or lung bud, which is the primordium of the lungs. As the lung bud grows, it remains ensheathed in a covering of splanchnopleuric mesoderm, which will give rise to the lung vasculature and to the connective tissue, cartilage, and muscle within the bronchi. On days twenty-six to twenty-eight, the lengthening lung bud bifurcates into left and right primary bronchial buds, which will give rise to the two lungs. In the fifth week, a second generation of branching produces three secondary bronchial buds on the right side and two on the left. These are the primordia of the future lung lobes. The bronchial buds and their splanchnopleuric sheath continue to grow and bifurcate, gradually filling the pleural cavities. By week twenty-eight, the sixteenth round of branching generates terminal bronchioles, which subsequently divide into two or more respiratory bronchioles. By week thirty-six, these respiratory bronchioles have become invested with capillaries and are called terminal sacs or primitive alveoli. Between thirty-six weeks and birth, the alveoli mature. Additional alveoli continue to be produced throughout early childhood. During the fourth week, partitions form to subdivide the intraembryonic coelom into pericardial, pleural, and peritoneal cavities. The first partition to develop is the septum transversum, a block-like wedge of mesoderm that forms a ventral structure partially dividing the coelom into a thoracic primitive pericardial cavity and an abdominal peritoneal cavity. Cranial body folding and differential growth of the developing head and neck regions .
