Respiratory Embryology — MBChB Year 1 Flashcards

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Gastrulation

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The process by which the bilaminar embryo forms the trilaminar germ disc with three germ layers: ectoderm, mesoderm, and endoderm. This sets up the basic embryonic tissue plan for organogenesis.

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Ectoderm

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The outer germ layer that forms the nervous system (central and peripheral), sensory organs (eye and ear), and the epidermis. It does not form internal visceral epithelia like the respiratory lining.

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Mesoderm

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The middle germ layer that gives rise to connective tissues, muscle, bone, blood, and the supportive stroma of many organs. In the respiratory system it forms lung parenchyma, pleura, and the muscular diaphragm.

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Endoderm

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The inner germ layer that forms the epithelial linings of the gastrointestinal and respiratory tracts. Respiratory epithelium, including the lining of bronchi and alveoli, derives from endodermal tissue.

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Lower respiratory tissues

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The lung's epithelium arises from **endoderm**, while the parenchymal connective tissues, pleura and diaphragm muscles derive from **mesoderm**. This distinction is important for understanding congenital defects.

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Embryonic period

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Weeks 4–8 of lung development when primary lung buds, trachea and main bronchi form. Early separation errors can produce tracheoesophageal malformations.

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Pseudoglandular period

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Weeks 5–17 during which extensive branching morphogenesis forms the conducting airways; by week 8 segmental bronchi are established. The epithelium has a gland-like appearance in this phase.

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Canalicular period

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Weeks 16–26 when respiratory bronchioles form and capillaries invade to form the future air–blood barrier. Differentiation of epithelium toward a gas-exchange phenotype occurs.

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Saccular period

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Weeks 24–38 when terminal saccules (primitive alveoli) form, type I cells flatten and capillaries elaborate, and type II pneumocytes begin producing surfactant. This increases surface area for gas exchange.

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Alveolar period

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Begins around week 36 and continues postnatally up to about 8 years, during which saccules mature into alveoli and alveolar number and complexity increase. Only a small proportion of adult alveoli are present at birth.

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Type I pneumocytes

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Flat, thin epithelial cells that form the surface across which gas exchange occurs. They provide a minimal barrier between airspace and capillary blood.

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Type II pneumocytes

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Cuboidal alveolar cells that produce pulmonary surfactant and act as progenitors for type I cells. Surfactant reduces alveolar surface tension and is essential for lung function at birth.

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Surfactant deficiency

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A condition in preterm infants leading to infant respiratory distress syndrome (RDS) due to inadequate surfactant production. It causes alveolar collapse, respiratory failure, and is treated with exogenous surfactant.

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Tracheoesophageal fistula

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A congenital connection between the trachea and oesophagus from faulty foregut separation during the embryonic period. It typically causes feeding difficulties and respiratory complications soon after birth.

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Congenital diaphragmatic hernia

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A defect where abdominal contents herniate into the thorax due to failure of pleuroperitoneal membrane closure, most commonly as a left posterolateral (Bochdalek) defect. This impairs lung development and can lead to pulmonary hypoplasia.

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Pleural cavities

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Cavities derived from lateral plate mesoderm after partitioning of the intraembryonic coelom. They become the potential spaces surrounding each lung lined by mesoderm-derived pleura.

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Pleuropericardial separation

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The embryologic process that partitions the thoracic cavity to form distinct pleural and pericardial spaces. Muscular and membranous folds drive this separation.

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Bronchial branching

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The iterative process of airway branching during the pseudoglandular period that forms segmental bronchi and many subsequent generations (about 14 more to reach terminal bronchioles). Proper branching is essential for normal lung architecture.