Histology

Question 1

Structure and function of upper airways (conduction zone)

Answer 1

• Every tissues in upper portion consists of: respiratory mucosa, submucosa, cartilage/or smooth muscle, and adventitia, majority cells are pseudostratified columnar epithelial
• goblet cells are mucin secreting, goblet cells + ciliated columnar cells + serous and mucous glands = mucociliary escalator, mucus traps particulates, cilia beats to move to pharynx for swallowing (smokers have more goblet and ciliated cells due to respiratory tract irritated by smoke, decreased movement of large production mucus → bronchitis and damage to underlying epithelial cells)
  1. Ciliated pseudostratified columnar epithelium: bulk of typical respiratory epithelium = columnar epithelial, goblet, and basal cells (stem cells), other cells include: brush cells (chemosensory), small granule (neuroendocrine), and intraepithelial immune cells
• Respiratory epithelium found through length of conducting portion, some slight changes going down tract: cartilage plates rather than rings, fewer glands in submucosa, epithelium changes from pseudostratified to simple, lamina propria and submucosa are separated with layer of smooth muscle, and fewer goblet cells
  2. Nasal cavity: submucosa has lot of serous glands (help produce moist environment, condition air, and produce mucus)
• Multiple vessels responsible for warming air to provide optimum conditions for gas exchange
  3. Olfactory epithelium: 3 main cells = bipolar olfactory neurons, supporting columnar (sustentacular cells), and basal cells replace bipolar olfactory neurons when damaged (olfactory serous glands known as glands of Bowan)
  4. Nasopharynx: respiratory epithelium involved in mucociliary action to trap and remove pathogen/particulates, has lymphoid tissue = first line defence (MALT – mucosal associated lymphoid tissue), makes up tonsils to protect airways and digestive tract
  5. Larynx: mostly lined with respiratory epithelium, apart from over vocal cord and upper epiglottis which has stratified epithelium, extra layers to provide protection to underlying structures

Question 2

Structure and function of lower airways (respiratory zone)

Answer 2

1. Trachea: ‘C’ shaped hyaline cartilage helps keep airways patent, posteriorly are connected by trachealis muscle and help to clear airway
   - Many glands: mucus, serous, has respiratory epithelia, lamina propria, and submucosa merges with either hyaline cartilage or elastic tissue of cartilaginous rings
2. Main bronchus: intercalated plates then cartilage becomes more sparse down bronchiole (< 5mm, have neither cartilage or glands, epithelial changes to simple cuboidal in smaller terminal bronchioles, few goblet cells, contains more elastin that allows for structural support during expansion, smooth muscle on outside for constriction)
   - Respiratory bronchioles have bronchiolar exocrine (club) cells instead of goblet cells: non-ciliated, produce component of surfactant, stem cells, contains enzymes that ‘detoxify’ noxious signals, and secrete snit-microbial factors and cytokines
3. Alveoli: formed from surface epithelial, supportive tissue, and capillaries
   - alveoli are pockets lined with flattened epithelial cells called pneumocytes
   - Types of pneumocytes: type I (squamous cells, large cytoplasm, cover most of lining, nuclei rarely seen), and type II (more numerous, cuboidal so cover less area)
   - Type II = pneumocytes produce surfactant (with help of club cells ), can differentiate into type I if needed
   - Connected tissue at alveolar septum: reticular + collagen + elastic fibres, and occasional fibroblasts
   - Air-blood interface at alveolus = thin for gas exchange
   - Macrophages + other leukocytes migrate from capillaries in alveoli to ingest dust