22.7 Embryonic Development of the Respiratory System
3 min read•june 18, 2024
The respiratory system's journey begins in the womb. From week 4 to birth, transform from a simple bud to a complex organ ready for life's first breath. This miraculous process involves precise timing and intricate cellular choreography.
play a crucial role in lung development. These rhythmic contractions stimulate growth, strengthen muscles, and maintain fluid balance. At birth, the dramatically shift from fluid-filled to air-filled, marking the start of independent breathing.
Embryonic Development of the Respiratory System
Stages of fetal respiratory development
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- Week 4-5: forms from the as an endoderm-derived structure that will develop into the , , , and lungs ()
- Week 5-6: form and fuse, separating the trachea from the esophagus to establish separate respiratory and digestive tracts
- Week 6-16: occurs with repeated branching of the to form the and , controlled by various signaling molecules () and transcription factors (, )
- During this stage, begins to differentiate, lining the developing airways
- Week 16-26: involves formation of the respiratory bronchioles, , and , along with epithelial differentiation and vascularization of the lungs to prepare for gas exchange
- Week 26-36: involves expansion of the peripheral airways and the formation of (primitive alveoli), with production beginning in to reduce surface tension
- Week 36-term: involves mature alveoli forming by the division of saccules, with increased surfactant production for lung stability after birth to facilitate efficient gas exchange
- continues postnatally, increasing the surface area for gas exchange
Purpose of fetal breathing movements
- Fetal breathing movements (FBMs) are rhythmic contractions of the and that occur intermittently starting around week 10 of gestation and become more regular and frequent as the fetus develops
- FBMs stimulate lung growth and maturation by promoting the development of respiratory muscles () and facilitating the clearance of
- FBMs help maintain lung liquid volume for proper lung development, as the lungs are filled with fluid in utero
- Absence or reduction of FBMs may indicate fetal distress or abnormalities, making monitoring FBMs a part of fetal well-being assessment during pregnancy ()
Lung inflation in newborns
- In utero, fetal lungs are filled with lung fluid secreted by the epithelial cells to maintain the lungs in an expanded state, promoting growth and development
- During labor and delivery, thoracic compression during vaginal delivery helps expel some lung fluid, while increased catecholamine levels () stimulate sodium absorption, reducing lung fluid volume
- At birth, the first breath is triggered by sudden changes in environment (temperature, light, and sound), increased carbon dioxide levels in the blood, and activation of the respiratory center in the brainstem ()
- As the newborn takes the first breath, the diaphragm and contract, expanding the thoracic cavity, allowing air to enter the lungs, and the remaining lung fluid is absorbed into the circulation or lymphatics
- Surfactant, produced by type II alveolar cells, reduces surface tension, preventing alveolar collapse during exhalation and facilitating lung inflation
- Subsequent breaths establish a regular breathing pattern and help maintain adequate lung inflation, with crying in newborns further helping to clear any remaining lung fluid and establish (volume of air remaining in lungs after passive exhalation)
Embryonic germ layers and respiratory system development
- The respiratory system primarily develops from the endoderm, which forms the lining of the airways and alveoli
- Mesoderm contributes to the development of connective tissue, smooth muscle, and cartilage in the respiratory tract
- The , including arteries, veins, and capillaries, develops from the mesoderm
- The , which surrounds and protects the lungs, also originates from the mesoderm
Key Terms to Review (51)
Alveolar Ducts: Alveolar ducts are short, narrow passages that connect the bronchioles to the alveolar sacs in the respiratory system. They serve as the final conduits through which air travels before reaching the alveoli, where gas exchange occurs.
Alveolar Stage: The alveolar stage is a critical phase in the embryonic development of the respiratory system, where the lungs undergo significant structural and functional changes to prepare for postnatal respiration.
Alveolarization: Alveolarization is the process by which the respiratory system develops the alveoli, the tiny air sacs within the lungs responsible for gas exchange. This critical stage of lung development occurs during the embryonic and fetal stages, laying the foundation for efficient respiration after birth.
Branching Morphogenesis: Branching morphogenesis is a developmental process in which an initially simple epithelial structure, such as a duct or tube, undergoes repeated branching to form a complex, tree-like network. This process is crucial for the formation and growth of many organs, including the lungs, kidneys, and mammary glands.
Bronchi: The bronchi are the primary airways that branch off from the trachea and carry air into and out of the lungs. They play a crucial role in the respiratory system by facilitating the movement of air during the process of breathing and enabling gas exchange within the lungs.
Bronchial artery: The bronchial artery is a blood vessel that supplies oxygenated blood to the lungs, supporting the tissues of the lungs outside of the alveoli where gas exchange occurs. Unlike the pulmonary arteries, which carry deoxygenated blood to the alveoli for oxygenation, bronchial arteries provide nutrients and oxygen to the lung tissue itself.
Bronchial bud: A bronchial bud is an early embryonic structure that develops into the major airways of the lungs, including the bronchi and bronchioles. It forms during the fifth week of embryonic development as an outgrowth from the foregut.
Bronchial Buds: Bronchial buds are the initial outgrowths from the embryonic foregut that eventually develop into the branching network of bronchi and bronchioles that make up the conducting portion of the respiratory system. They are a crucial early step in the embryonic development of the respiratory system.
Canalicular Stage: The canalicular stage is a critical phase in the embryonic development of the respiratory system, marked by the formation of branching air passages known as bronchioles and the establishment of the circulatory system to support gas exchange.
Conducting Airways: Conducting airways refer to the passages in the respiratory system that transport air to and from the lungs without being involved in gas exchange. These structures include the nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles, which play crucial roles during embryonic development by establishing the framework for the respiratory system's architecture and function.
Diaphragm: The diaphragm is a dome-shaped sheet of muscle and tendon that serves as the primary muscle for respiration, separating the thoracic (chest) cavity from the abdominal cavity. It contracts and flattens when you inhale, allowing the lungs to expand and fill with air, and relaxes and domes upwards during exhalation, pushing air out of the lungs.
Diaphragm: The diaphragm is a dome-shaped muscle that separates the thoracic and abdominal cavities. It is the primary muscle of respiration, playing a crucial role in the process of breathing and gas exchange within the body.
Embryonic germ layers: Embryonic germ layers are the three primary layers of cells formed during the early stages of embryonic development, specifically the ectoderm, mesoderm, and endoderm. These layers give rise to all the tissues and organs in the body, playing a crucial role in shaping the structure and function of the developing respiratory system and other systems.
Epinephrine: Epinephrine, also known as adrenaline, is a hormone and neurotransmitter produced by the adrenal glands that plays a crucial role in the body's fight-or-flight response by increasing heart rate, muscle strength, blood pressure, and sugar metabolism. It prepares the body for rapid action in situations perceived as stressful or dangerous.
Epinephrine: Epinephrine, also known as adrenaline, is a hormone and neurotransmitter produced by the adrenal glands. It plays a crucial role in the body's stress response, preparing the body to handle emergency situations by increasing heart rate, blood pressure, and blood sugar levels.
Fetal Breathing Movements: Fetal breathing movements are the rhythmic contractions and relaxations of the fetal diaphragm and intercostal muscles that occur during the prenatal period. These movements are an important indicator of fetal health and development, particularly in the context of the embryonic development of the respiratory system.
FGF10: FGF10, or Fibroblast Growth Factor 10, is a signaling protein that plays a crucial role in the embryonic development of the respiratory system. It is a member of the fibroblast growth factor family, which are involved in various developmental processes, including cell proliferation, differentiation, and tissue patterning.
Foregut: The foregut is an embryonic division of the gastrointestinal tract that gives rise to the upper parts of the digestive system, including the esophagus, stomach, and initial portion of the small intestine (duodenum), as well as respiratory structures like the trachea and lungs. It is crucial in the early development of the respiratory system, setting the foundation for breathing and digestion mechanisms.
Foregut: The foregut is the anterior portion of the embryonic digestive tract, which develops into the esophagus, stomach, and the first part of the small intestine (duodenum) in the mature organism. It plays a crucial role in the embryonic development of the respiratory system.
Functional Residual Capacity: Functional residual capacity (FRC) is the volume of air that remains in the lungs at the end of normal, relaxed exhalation. It represents the balance between the inward elastic recoil of the lungs and the outward elastic recoil of the chest wall, and is an important measure of lung volume and respiratory mechanics.
Functional residual capacity (FRC): Functional Residual Capacity is the volume of air remaining in the lungs after a normal, passive exhalation. It represents the balance point between the forces of the chest wall pushing outwards and the lungs recoiling inwards.
Intercostal muscles: Intercostal muscles are groups of muscles found between the ribs that help form and move the chest wall. They are primarily involved in the mechanical aspect of breathing, assisting with the expansion and contraction of the thoracic cavity to facilitate respiration.
Intercostal Muscles: The intercostal muscles are a group of skeletal muscles located between the ribs that play a crucial role in the mechanics of breathing and the structural integrity of the thoracic cavity. These muscles are closely associated with the respiratory system, the thoracic cage, and the abdominal wall.
Laryngotracheal bud: The laryngotracheal bud is an embryonic structure that gives rise to the trachea and the lungs. It forms from the ventral wall of the foregut during early fetal development.
Larynx: The larynx, commonly known as the voice box, is an organ situated in the neck of mammals that plays a critical role in breathing, producing sound, and protecting the trachea against food aspiration. It houses the vocal cords and manipulates pitch and volume, which is essential for phonation.
Larynx: The larynx, also known as the voice box, is a complex structure located in the neck that plays a crucial role in the respiratory and phonatory systems. It is responsible for producing sound, protecting the airway, and regulating airflow during various physiological processes.
Lung bud: The lung bud is a structure that forms early in embryonic development, which eventually gives rise to the major components of the respiratory system such as the trachea, bronchi, and lungs. It appears as a protrusion from the ventral wall of the foregut during the fourth week of human development.
Lung Fluid: Lung fluid refers to the thin layer of fluid that coats the alveoli, the tiny air sacs in the lungs where gas exchange occurs. This fluid is essential for maintaining the structural integrity of the alveoli and facilitating the efficient diffusion of oxygen and carbon dioxide across the alveolar-capillary membrane.
Lungs: The lungs are the primary respiratory organs responsible for gas exchange in the human body. They are located in the thoracic cavity and play a vital role in the respiratory system, facilitating the intake of oxygen and the removal of carbon dioxide from the bloodstream.
Medulla Oblongata: The medulla oblongata is the lower part of the brainstem, connecting the brain to the spinal cord. It is responsible for regulating vital autonomic functions, including respiration, heart rate, blood pressure, and other homeostatic processes.
Non-Stress Test: A non-stress test is a prenatal diagnostic procedure used to assess the well-being of the fetus by monitoring its heart rate in response to its own movements. It is a crucial tool in evaluating the health and development of the fetus during pregnancy, particularly in the context of embryonic development of the respiratory system.
Olfactory pit: The olfactory pit is a structure in the developing embryo that gives rise to the nasal cavity and is crucial for the formation of the sense of smell. It forms early in embryonic development as a depression on the surface of the head region.
Pleural cavity: The pleural cavity is a thin fluid-filled space between the two layers of the pleura, which are membranes surrounding each lung. This cavity provides lubrication and space for the lungs to expand and contract during breathing.
Pleural Cavity: The pleural cavity is the space between the two layers of the pleural membrane that surrounds the lungs. It is a fluid-filled space that allows the lungs to slide smoothly against the chest wall during breathing. This cavity is an essential component of the respiratory system, as it facilitates the expansion and contraction of the lungs during inhalation and exhalation.
Primitive Alveoli: Primitive alveoli are the earliest form of alveoli, the tiny air sacs in the lungs where gas exchange occurs. They develop during the embryonic stage of respiratory system formation and are the precursors to the mature alveoli that will be responsible for efficient oxygen and carbon dioxide exchange in the adult respiratory system.
Pulmonary surfactant: Pulmonary surfactant is a complex mixture of lipids and proteins produced by cells in the lungs, which reduces surface tension within the alveoli to prevent their collapse during exhalation. This substance is crucial for efficient gas exchange and lung stability.
Pulmonary Vasculature: The pulmonary vasculature refers to the network of blood vessels that carry deoxygenated blood from the heart to the lungs and oxygenated blood from the lungs back to the heart. This intricate system plays a crucial role in the respiratory system by facilitating gas exchange and maintaining the body's oxygen and carbon dioxide balance.
Respiratory bronchioles: Respiratory bronchioles are the smallest airways in the lungs that have alveoli attached to their walls, marking the transition from conducting airways to respiratory zones where gas exchange occurs. They play a crucial role in the respiratory system as they allow for both the passage of air and the exchange of oxygen and carbon dioxide. Their unique structure, featuring alveoli, is essential for efficient respiration and connects them closely to the overall function of the lungs.
Respiratory Diverticulum: The respiratory diverticulum is an outpouching or protrusion that forms the initial structure of the developing respiratory system during embryonic development. It arises from the ventral aspect of the foregut and serves as the precursor to the larynx, trachea, bronchi, and lungs.
Respiratory Epithelium: Respiratory epithelium is the specialized lining that covers the inner surfaces of the respiratory system, including the nasal cavity, trachea, bronchi, and alveoli. It plays a crucial role in the embryonic development and function of the respiratory system.
Respiratory Tree: The respiratory tree refers to the branching network of airways that carry air to and from the lungs. It is the part of the respiratory system responsible for conducting air during the breathing process.
Saccular Stage: The saccular stage is a critical phase in the embryonic development of the respiratory system, marked by the formation of saccular structures that will eventually mature into the alveoli - the functional units of the lungs responsible for gas exchange.
Saccules: Saccules are small, pouch-like structures found in the developing respiratory system, specifically as part of the alveolar stage of lung development. They play a crucial role in the formation of alveoli, the tiny air sacs where gas exchange occurs, and are essential for creating a functional respiratory surface in the fetus. Understanding saccules is important for comprehending how the lungs mature and prepare for breathing air at birth.
SOX2: SOX2 (Sex-determining region Y-box 2) is a transcription factor that plays a crucial role in the embryonic development of the respiratory system. It is a key regulator of stem cell pluripotency and is essential for the maintenance and differentiation of various cell types, including those involved in the formation of the lungs and airways.
SOX9: SOX9 is a transcription factor that plays a crucial role in the embryonic development of the respiratory system. It is a member of the SRY-related HMG-box (SOX) family of proteins and is essential for the specification and differentiation of various cell types during lung and airway formation.
Surfactant: Surfactant is a complex mixture of lipids and proteins produced by type II alveolar cells in the lungs. It plays a critical role in the respiratory system by reducing surface tension within the alveoli, facilitating gas exchange, and preventing alveolar collapse during exhalation.
The Lungs: The lungs are a pair of spongy, air-filled organs located on either side of the chest (thorax), primarily responsible for gas exchange between the air and the bloodstream. They remove carbon dioxide from the blood and replace it with oxygen during the breathing process.
Trachea: The trachea is a cartilaginous tube that connects the larynx to the bronchi, allowing for the passage of air between the lungs and the outside environment. It plays a vital role in the respiratory system by facilitating the movement of air during inhalation and exhalation.
Trachealis muscle: The trachealis muscle is a band of smooth muscle that connects the ends of the C-shaped cartilages at the back of the trachea, allowing for flexibility and constriction during breathing. It plays a key role in adjusting airflow resistance and maintaining an open airway.
Tracheoesophageal ridges: Tracheoesophageal ridges are embryonic structures that form during the early development of the respiratory and digestive systems, specifically separating the trachea from the esophagus. These ridges develop from lateral outgrowths of the foregut and play a critical role in ensuring that the trachea and esophagus become distinct pathways, preventing potential anomalies such as tracheoesophageal fistulae. Their proper formation is essential for normal respiratory function and swallowing.
Type II Alveolar Cells: Type II alveolar cells, also known as type II pneumocytes, are a specialized epithelial cell type found in the alveoli of the lungs. These cells play a crucial role in the embryonic development and maintenance of the respiratory system.