Respiratory System Histology

• Module: Histology/Embryology
• Topic: Respiratory System
• Professor: TABYAOUI
• Academic Year: 2024_25 for 2nd year Medicine students

Educational Objectives

• Understand the general organization of respiratory tract conduction.
• Identify the respiratory mucosa and its components.
• Identify the olfactory mucosa and its components.
• Identify the mucosa of the nasopharynx.
• Identify the tracheal wall.
• Compare the tracheal wall with the walls of the large, medium, and small bronchi.
• Recognize a bronchiolar wall and identify its components.
• Recognize the segments of the pulmonary acinus and describe the components of each segment.
• Identify an alveolar wall.
• Understand the components of the alveolar-capillary membrane.
• Know the histological components of the pleura.

Plan of Study

• Outline of the general organization of the respiratory system.
• Study the conduction system:
  • Nasal fossae
  • Pharynx
  • Larynx
  • Trachea
  • Bronchial tree, including bronchi and bronchioles
• Zone of exchange (pulmonary parenchyma):
  • Respiratory bronchiole
  • Alveolar ducts
  • Alveolar sacs / Alveoli
  • Interalveolar septum
• Understand the pulmonary vascularization.
• Study the pleura.

Anatomical Organization

• The respiratory system is divided into upper and lower airways.
• The upper respiratory tract includes the sphenoidal and frontal sinuses, nasal cavity, and pharynx.
• The lower respiratory tract includes the larynx, trachea, bronchi, lungs, bronchioles, respiratory bronchioles, alveolar ducts, alveoli, pleura, and diaphragm.

Functional and Histological Organization

• The respiratory system is divided into conducting and respiratory pathways.
• The airways condition and transfer inspired air to the exchange environment.
• The respiratory tract facilitates gas exchange via the pulmonary parenchyma.

Conducting Airways

Extrapulmonary Structures

• Composed of nasal fossae, the pharynx, larynx, trachea, and primary bronchi.

Intrapulmonary Structures

• Bronchial tree and terminal bronchioles

Respiratory Airways

• Respiratory bronchioles, alveolar ducts, alveolar sacs, and pulmonary alveoli

Nasal Fossae

• The nasal fossae consist of two cavities separated by a cartilaginous nasal septum

Boundaries

• Anteriorly by the vestibule, a space just inside the nostrils
• Laterally by the turbinates

Classification of Mucosa

• Vestibular mucosa houses stratified squamous epithelium
• Respiratory mucosa lines the nasal cavity
• Olfactory mucosa lines the apical section of the nasal fossa

Respiratory Mucosa Structure

• The respiratory mucosa features pseudostratified ciliated columnar epithelium with goblet cells laying on a basale membrane
• The lamina propria includes seromucous glands, elastic fibers, a large venous plexus, and isolated lymphocytes or in nodules

Respiratory Epithelium

• Includes ciliated columnar cells, goblet cells, basal cells, brush cells, and endocrine cells.
• Endocrine cells aren't visible with standard HE staining.

Olfactory Mucosa features neurosensory pseudostratified columnar epithelium

• Includes Olfactory receptor cells, which are bipolar neurons and have:
  • An apical dendrite ending in an olfactory knob and immobile olfactory cilia
  • A basal axon that passes through the basement membrane and forms nerve fascicles
• Supporting columnar cells with apical microvilli, and basal cells.
• The lamina propria houses Bowman's serous olfactory glands.

Pharynx

• A structure that follows the nasal fossae
• Lined with:
  • Respiratory-type mucosa and a lamina propria rich in lymphoid elements in the nasopharynx
  • Non-keratinized stratified squamous epithelium in the oropharynx and hypopharynx.

Nasopharynx

• Features respiratory-type epithelium and a lamina propria rich in lymphoid tissue and mixed glands

Oropharynx/Laryngopharynx

• Features non-keratinized stratified squamous epithelium
• Includes mucous glands and lymphoid elements in the lamina propria

Larynx

• The organ of phonation, which contain the vocal cords
• Supported by a fibromusculocartilaginous framework of ligaments, membranes, elastic (epiglottis) and hyaline cartilage, and skeletal muscle
• Consist of two sets of folds projecting inwards which are the vestibular folds and true vocal cords

Ventricle Laryngis

• Located between the true and false vocal cords

Composition of the Mucosa of the Larynx

• Respiratory-type epithelium, containing a lamina propria containing mixed glands in the vestibular folds
• Non-keratinized stratified squamous epithelium, containing a lamina propria devoid of glands but including vocal ligaments and striated muscle in the vocal cords.

Epiglottis

• A mobile sheet of elastic cartilage
• Forms a lingual surface, a laryngeal surface, and a free upper border
• The lingual surface, free border, and upper part of the laryngeal face is non-keratinized stratified squamous epithelium
• The remainder of the laryngeal surface is respiratory-type epithelium

Trachea

• A flexible airway
• Composed of these main histological layers:
  • Mucosa
  • Submucosa
  • Cartilaginous tunic
  • Adventitia
• The cartilaginous layer consists of 16-20 C-shaped hyaline cartilage rings
• The muscle is the band of smooth muscle that connects the posterior parts of the cartilage rings

Trachea: Mucosa

• The trachea features respiratory-type epithelium
• Includes a thick basal membrane
• Features a lamina propria with numerous lymphocytes and abundant elastic fibers

Trachea: Submucosa

• Contains mixed tubuloacinar glands and visible excretory ducts

Trachea: Cartilaginous Tunic

• Cartilaginous Tunic features 16 to 20 C-shaped cartilage rings linked by dense fibroelastic connective tissue

Trachea: Adventitia

• Composed of loose connective tissue containing blood vessels, lymphatics, nerves, and adipocytes

Air Conduction

• Conducted via the bronchiole tree consisting of a series of conducting airways that facilitate the movement of air from the trachea to the pulmonary alveoli

Bronchial Tree

• Consists of primary, lobar, and segmental branches within each lung
• Terminal branches end in alveolar sacs specialized for gas exchange

Bronchi

• Structurally similar to the trachea but smaller and with incomplete cartilage rings

Bronchi: Primary Extrapulmonary

• Histologically similar to the trachea but with smaller diameter and incomplete cartilage rings

Bronchi: Intrapulmonary

• Includes cartilage plates and smooth muscle
• Become integrated among cartilage fragments
• Characterized by:
  • Cartilaginous plates and smooth muscle, as well as less mucus cells and mixed glands
  • Smooth muscle lies between cartilage pieces
  • Decreased mucous cells and mixed glands

Bronchi: Histological Layers

• The layers consist of the mucosa, muscularis, submucosa, cartilage, & peribronchium
• Mucosa: respiratory epithelium on an elastic fiber rich lamina propria
• Muscularis: smooth muscle fibers arranged spirally as the Reissessen's muscle, which controls bronchial diameter.
• Submucosa: containing seromucous bronchial glands.
• Cartilaginous tunic: cartilage plates in larger bronchi, replaced by cartilage nodules in smaller ones.
• Peribronchium: fusion of the bronchus and pulmonary artery's adventitia

Transition from Bronchi to Bronchioles

• Includes the disappearance of cartilage and glands
• Results into a thinner, more folded wall with lower epithelium
• Dissociation of Reissessen's muscle into dispersed smooth muscle fibers.
• Decreased, then disappeared goblet cells within terminal bronchioles
• Club cells replace goblet Cell
• Abundant elastic fibers

Smaller Diameter

• Bronchioles are characterized as a diameter, simple histological structure: bronchiolar mucosa, smooth muscle layer, and peribronchiolar sheath, lacking both cartilage and glands

Components of Bronchioles

• Bronchioles is made of bronchiolar mucosa, smooth muscle layer, and peribronchiolar sheath

Types of Bronchioles

• The larger bronchioles are lined by pseudostratified ciliated columnar epithelium with goblet cells.
• Intermediate bronchioles have ciliated simple columnar epithelium.
• Terminal Bronchioles have Cuboidal Simple Epithelium
• Respiratory bronchioles are simple cuboidal epithelium.

Terminal Bronchiloes

• Contain club cells and does lack of goblet cell

Club Cells (Clara cells)

• Non-ciliated cells whose exocrine functions include:
  • Producing a surfactant agent
  • Antimicrobial peptides
  • Cytokines

Respiratory parenchyma is alveolar

Pulmonary Acinus

• Defined as the functional unit of the pulmonary lobule
• It consists of a respiratory bronchiole, alveolar ducts, alveolar sacs, and alveoli.

Zone Description

Respiratory Bronchioles

• Composed of cuboidal simple epithelium
• Contain a few ciliated cells and club cells
• They has a thin wall interrupted by alveoli
• Characterized by a lamina propria containing a few dispersed bundles of smooth muscle

Alveolar Ducts

• Featuring reduced walls with cuboidal to squamous alveolar cells
• They contain an annulus of smooth muscle and elastic fibers.

Alveolar Sac/Alveoli

• Air-filled space that is surrounded by groups of alveoli lacking traditional walls

Septum (interalveolar wall)

• Consists of a thin layer of connective tissue containing fibroblasts, elastic fibers, collagen, reticular fibers, and alveolar capillaries
• Capillaries are in direct contact with the alveoli.

Consituents of the Simple Alveolar Epithelium

• Type I pneumocytes
• Type II pneumocytes
• Macrophages

Type I Pneumocytes

• Thin squamous cells that comprise 95% of the alveolar surface
• They facilitate gas exchange between alveoli and capillaries during the air/blood barrier

Type II Pneumocytes

• Are granular and globular cells
• They secrete pulmonary surfactant, which reduces surface tension and keeps the alveoli open.
• Located at the Alveoli corner
• Comprise 5% of the alveolar surface

Corps Multilamellaire

• Synthesizes the surfactant that covers the alveolar surface

Alveolar Macrophages

• They eliminate particules by the alveolo-capillaires and by the air and blood layer.

Alveolar Septum

• Location for Macrophages
• This Macrophages located at the alveolar space where they are ready to engulf particulate matter

Air/Blood Barrier

• The barrier facilitates gas exchange
• Consists of:
  • Type I pneumocytes
  • Endothelial cells of capillaries
  • Fused basement membranes

Double Pulmonary Vascularization

• The dual-supply includes both pulmonary and bronchial arteries
• The pulmonary artery carries deoxygenated blood from the heart to the alveoli while the Bronchioalveolar carries oxygenated blood from the aorta
• Pulmonary Arty does not ramifies after the alveoli
• Bronchial artery distributes to the various respiratory structures.

Pleura

• A serous membrane composed of visceral and parietal layers and is divided by a pleural cavity. The visceral pleura covers the lungs, and the parietal pleura lines the thoracic cavity.
• The both layers is lined with mesothelium and connective tissue with collagen and elastic fibers alongside lymphatic vessels.