respiratory histology

• Respiration


  The process of obtaining oxygen (O2) from the external environment and eliminating (CO2)
• Components of respiration


  • External respiration - oxygen and carbon dioxide exchanged between the external environment and the body cells
  • Internal respiration - cells use oxygen for ATP production (and produce carbon dioxide and water in the process)
• C6H12O6 + 6O2 6CO2 + 6H2O + 36 ATP
• Mammalian Respiratory System


  • Has a conducting portion and a gas exchange portion
• Structures of the conducting portion

  • Nostrils
  • Nasal cavity
  • Nasopharynx
  • Laryngopharynx
  • Larynx
  • Trachea
  • Primary bronchi
  • Lobar bronchi
  • Segmental Bronchi
  • Bronchioles
  • Terminal bronchioles
• Structures of the gas exchange portion

  • Respiratory bronchioles
  • Alveolar ducts
  • Alveolar sacs
  • Alveoli
• Functions of the conducting portion

  • Voice production (vocal cords in the larynx)
  • Air modulation (warming, humidification)
  • Removal of foreign particles
  • Regulation of airflow
  • Olfaction
  • Cooling of the entire body
• Nostrils


  The two openings in the anterior part of the nose
• Nasal cavity


  A large air filled space above and behind the nose in the middle of the face
• The nasal cavity is divided into right and left cavities by a cartilaginous nasal septum
• The nasal cavity is lined by a typical respiratory epithelium, i.e. pseudostratified columnar ciliated epithelium containing goblet cells
• The olfactory epithelium lines the roof of the nasal cavity and the medial aspect of the vomeronasal organ
• Typical Respiratory Epithelium


  • Composed of columnar ciliated cells, goblet cells, and basal cells
  • Responsible for mucociliary clearance
• Olfactory Epithelium


  Thicker than respiratory epithelium, no goblet cells, responsible for detection of smell
• Olfactory cells


  • Bipolar nerve cells with a bulbous projection (knob) where the cilia extend from
  • Cilia are non motile and act as receptors
• Fila olfactoria is a collective term denoting the numerous olfactory filaments: slender fascicles each composed of the thin, unmyelinated axons of 8-12 of the bipolar olfactory receptor cells
• Bowman's glands


  Purely serous glands in the olfactory mucosa, beneath the olfactory epithelium
• Vomeronasal Organ (VNO)


  An auxiliary olfactory sense organ that is found in many animals, mainly used to detect pheromones
• Mammals have paired arytenoids, cricoid , thyroid and several other small cartilages including the epiglottis in the larynx
• The true vocal cords contain underlying skeletal muscle called the vocalis muscles
• The false vocal cord is covered by respiratory epithelium
• The epiglottis has stratified squamous epithelium with modified salivary glands that secrete thick mucous
• The glottis is the space between the two vocal cords
• The true vocal cords are covered by stratified squamous epithelium with no / rare submucosal glands
• Trachea


  A flexible tube whose wall is reinforced by C-shaped hyaline cartilages
• The trachea is lined by a typical respiratory epithelium
• The primary bronchi have a structure similar to that of the trachea up to the point of entry into lungs
• Segmental bronchi


  • The cartilaginous rings are replaced by small irregular plates of cartilage
  • The epithelium is lined with typical respiratory epithelium changing to tall columnar with little pseudostratification at the final branches
• Bronchioles


  • Absence of cartilage
  • Abundance of smooth muscle within the wall
  • Epithelium goes through a transition from the pseudostratified ciliated columnar respiratory epithelium to cuboidal ciliated epithelium
  • Goblet cells are replaced by Club cells
• Segmental bronchi


  • Bronchi are lined with the pseudostratified ciliated columnar epithelium (respiratory epithelium) with glands within the submucosa
• Segmental bronchus has typical respiratory epithelium
• Bronchioles


  Bronchi further separate into bronchioles, which are defined by the absence of cartilage and abundance of smooth muscle within the wall
• Transition from bronchi to bronchioles

  Epithelium changes to simple columnar containing ciliated cells and non-ciliated (club) cells
• Club cells


  • Dome-shaped cells with short microvilli, found in the bronchioles
  • Secrete a more watery substance than mucus to moisten, warm, and clean the air
  • Secrete glycosaminoglycans to protect the bronchiole lining
  • Secrete club cell secretory protein, uteroglobin and a solution similar to the component of the lung surfactant
  • Have a detoxifying effect due to cytochrome P450 enzymes
  • Act as a stem cell, multiplying and differentiating into ciliated cells to regenerate the bronchiolar epithelium
• Transition from bronchioles to respiratory bronchioles

  Terminal bronchioles continue as respiratory bronchioles, which then open into alveolar ducts and individual alveoli
• Respiratory bronchioles


  • Walls are interrupted at intervals by alveoli
  • Initially lined by a ciliated simple cuboidal epithelium which gradually becomes non-ciliated low cuboidal
  • Terminate as alveolar ducts, in which the alveoli are so numerous and so closely spaced than the limits of the duct proper
• Alveoli


  • Wall is formed by a thin sheet (~2μm) of tissue separating two neighboring alveoli
  • Wall consists of the surface epithelium, connective tissue and blood capillaries
  • Neighboring alveoli may be connected to each other by small interalveolar pores or pores of Kohn
• Alveolar type I cells

  • Extremely flattened and form the bulk (~95%) of the surface of the alveolar walls
  • Nuclei are flattened and slightly protrude into the interior of the alveoli
  • Involved in the process of gas exchange between the alveoli and blood
  • Extremely thin (sometimes only 25 nm) to be readily permeable for enabling an easy gas exchange
• Alveolar type II cells

  • Irregularly (sometimes cuboidal) shaped found interspersed between the type I cells
  • Occur singly or in small groups and occupy ~5% of the area but ~90% of the total numbers of alveolar pneumocytes
  • Cytoplasm contains large number of osmiophilic granules called cytosomes (or lamellar bodies)
  • Secrete pulmonary surfactant consisting mainly of Dipalmitoylphosphatidylcholine
  • Surfactant spreads over the thin film of fluid that coats the surface of alveolar epithelium, reducing surface tension and preventing small alveoli from emptying into large ones
• Alveolar macrophages


  • Also called dust cells
  • Found in the inter-alveolar septae as interstitial macrophages and within the lumen of the alveoli adhering to the epithelial cells
  • Phagocytize dust and other particles that have escaped entrapment by the mucus layer in the upper respiratory tract