Respiratory System Embryology Review

Respiratory System Embryology Review

I. Structure

The respiratory system is divided into two main parts:

• Lungs: The main organs of the respiratory system, where gas exchange takes place.

• Airways: Structures that conduct air from the nose to the lungs, including the nasal cavity, nasopharynx, larynx, trachea, and bronchi.

II. Function

The respiratory system performs two main functions:

• Air Conduction: Conducts air into and out of the lungs, and also cleans, warms, and humidifies the air.

• Gas Exchange: Exchanges gases between the lungs and the blood.

III. Formation and Development

• The respiratory system begins to form from the third to fourth week of gestation as a pouch from the primitive foregut of the digestive tube.

• Alveoli and alveolar ducts form during the sixth month of gestation.

• At first, the alveolar epithelium is simple columnar or cuboidal, and the lumen is very narrow.

IV. Airways

• Extra-pulmonary Airways: Includes the nasal cavity, nasopharynx, larynx, trachea, and main bronchi.

• Vestibule: Lined by stratified squamous non-keratinized epithelium, with hair and some sebaceous and sweat glands.

• Respiratory Part of Nasal Cavity: Lined by ciliated pseudostratified columnar epithelium.

• Sensory Nerve for Nasal Cavity: Trigeminal nerve (V).

• Nasal Cavity includes: Vestibule, respiratory part, and olfactory part.

• Olfactory Epithelium: A type of pseudostratified epithelium, consisting of 3 types of cells:

• Supporting Cells: Columnar, with short microvilli.

• Basal Cells: Small, less differentiated, divide to regenerate.

• Olfactory Cells: Bipolar neurons.

• Under the Basement Membrane of the Nasal Cavity: Connective tissue, with many vessels and Bowman’s glands.

• Pharynx: Divided into 3 parts: nasopharynx, oropharynx, and laryngopharynx. Of these, the nasopharynx and laryngopharynx are in contact with air, while the oropharynx is in contact with food.

• Ciliated pseudostratified columnar epithelium: Located in the part of the pharynx that is in contact with air.

• Laryngeal Wall: Composed of 2 layers: mucosa and fibrocartilage.

• Mucosa: Consists of ciliated pseudostratified columnar epithelium, a lamina propria with many elastic fibers and mixed glands.

• Fibrocartilage: Consists of hyaline and elastic cartilage with surrounding fibrous connective tissue.

• Vocal Cords: Covered by stratified squamous non-keratinized epithelium.

• Trachea and Main Bronchi: Have a similar structure, consisting of 3 layers: mucosa, submucosa, and fibrocartilage.

• Trachea-Bronchial Mucosa: Covered by ciliated pseudostratified columnar epithelium.

• Dynein: A protein located on the cilia of ciliated columnar cells, plays a role in cilia movement.

• Kartagener Syndrome: Due to dynein deficiency, cilia are almost immobile.

V. Lungs

• Right Lung: Has more lobes than the left lung.

• Clara Cells: Lack cilia but have microvilli, are shaped similar to goblet cells, play a role in forming surfactant on the respiratory surface.

• Endocrine Cells of the Trachea-Bronchial Mucosa: Secrete serotonin, gastrin, calcitonin.

• Submucosal Layer: Does not have a role in killing bacteria and cooling the respiratory tract.

• Fibrocartilaginous Layer in the Trachea: Has 16-20 C-shaped rings of hyaline cartilage, due to the esophagus passing behind it.

• Correct order of bronchial branching:

1. Main Bronchus

2. Lobar Bronchus

3. Segmental Bronchus

4. Subsegmental Bronchus

5. Bronchiole

6. Alveolar Duct

7. Alveolus

• Parasympathetic Nervous System: Causes contraction of the Reissessen muscle, leading to bronchoconstriction.

• Reissessen Muscle: Located between the mucosa and hyaline cartilage.

• Bronchioles: Lack cartilage, glands, and have a simple columnar epithelium.

• Clara Cells in Bronchioles: More abundant than in bronchi and contain more cytochrome P450.

• Terminal Bronchioles: Diameter of 0.2-0.5 mm, mucosa is lined by simple cuboidal (or low columnar) epithelium with cilia.

• Alveolar Wall: Lined by a special epithelium composed of 2 types of cells: type I pneumocytes and type II pneumocytes.

• Type I Pneumocytes: Occupy 97% of the alveolar surface area, have a weak ability to divide when damaged.

• Type II Pneumocytes: Are spherical or polyhedral cells that protrude into the alveolar lumen, can differentiate into type I pneumocytes, secrete lamellar bodies (membrane-bound granules rich in phospholipids) containing surfactant.

• Surfactant: Reduces alveolar collapse and prevents the entry of bacteria from the air into the blood.

• Surfactant has 2 phases: Liquid phase and lipid layer, mainly dipalmitoyl phosphatidylcholine, the surfactant layer also contains many glycosaminoglycans.

• Alveolar Macrophages: Also called dust cells, have many vacuoles, many dust particles, and cigarette smoke in the cytoplasm.

• Correct order of structures in the blood-gas barrier:

1. Surfactant

2. Type I Pneumocytes

3. Basement membrane of the respiratory epithelium

4. Basement membrane of the capillaries

5. Endothelial cells

• Pleura: Consists of 2 layers: parietal pleura and visceral pleura.

• Space between the 2 layers of pleura: Called the pleural cavity.

• Correct order of the 3 layers of pleura:

1. Epithelial layer: a single layer of mesothelial squamous cells, similar to endothelial cells.

2. Sub-epithelial layer: loose connective tissue, very few cells and no blood vessels.

3. Fibro-elastic layer: many connective tissue cells and vessels.

• Endocrine Function: Endocrine cells (K cells) secrete serotonin, bradykinin, norepinephrine.

• Correct order of airways from the subsegmental bronchus:

1. Subsegmental Bronchioles

2. Bronchioles

3. Respiratory Bronchioles

4. Alveolar Ducts

5. Alveoli

VI. Comparison Table of Airway Segments

| Feature | Trachea | Subsegmental Bronchus | Bronchioles | Terminal Bronchioles | Respiratory Bronchioles | Alveoli |

|—|—|—|—|—|—|—|

| Epithelium | Ciliated pseudostratified columnar | Ciliated pseudostratified columnar | Simple columnar with cilia | Simple cuboidal with or without cilia | Simple cuboidal with or without cilia | Alveolar epithelium |

| Reissessen Muscle | Present | Present | Present | Absent | Absent | Absent |

| Cartilage | Present | Present | Absent | Absent | Absent | Absent |

| Glands | Present | Present | Absent | Absent | Absent | Absent |

| Lumen | Round, large | Round, large | Wrinkled | Not wrinkled | Not wrinkled | Not wrinkled |

VII. Notes:

• Bronchioles are also called proper bronchioles.

• The smooth muscle around the airways forms a ring called the Reissessen muscle, terminal bronchioles have very little smooth muscle, so it can be said that they do not have a ring but only smooth muscle.

• From the terminal bronchioles onwards, the Reissessen muscle is no longer present.

• From the bronchioles onwards, cartilage and glands are no longer present.

• Subsegmental bronchi have a round, large lumen, while bronchioles have a wrinkled lumen.

VIII. Ratio:

• Ratio of type I pneumocytes to type II pneumocytes in alveoli: 97%: 3%.

IX. Note:

• Surfactant is secreted by Clara cells and type II pneumocytes.

• The 3 criteria used to classify subsegmental bronchi and proper bronchioles are epithelium, cartilage, and glands.

• Among the criteria used to identify subsegmental bronchi and proper bronchioles, cartilage is given priority.