recognise cartilage, ciliated epithelium, goblet cells, squamous epithelium of alveoli, smooth muscle and capillaries in microscope slides, photomicrographs and electron micrographs

The Gas Exchange System

The respiratory system of mammals is specialised for efficient exchange of oxygen (\$O2\$) and carbon dioxide (\$CO2\$) between the external environment and the blood. Understanding the microscopic structures involved is essential for A‑Level examinations.

Key Structures to Recognise

• Cartilage (supporting trachea and bronchi)
• Ciliated pseudostratified columnar epithelium
• Goblet cells (mucus‑secreting)
• Simple squamous epithelium of alveoli (type I pneumocytes)
• Smooth muscle in bronchi and bronchioles
• Capillary networks surrounding alveoli

Microscopic Identification

1. Cartilage

In routine light microscopy (H&E stained), cartilage appears as dense, basophilic extracellular matrix with scattered chondrocytes in lacunae. In electron micrographs the matrix shows abundant collagen fibrils and proteoglycan granules.

2. Ciliated Epithelium

Rows of columnar cells with apical motile cilia. Cilia are visible as regular, hair‑like projections in photomicrographs; in electron micrographs each cilium shows a 9+2 arrangement of microtubules.

3. Goblet Cells

Scattered among the ciliated cells, goblet cells contain large, pale‑staining mucin droplets that push the nucleus to the basal side. Electron microscopy reveals secretory granules with a characteristic “halo” of mucin.

4. Alveolar Squamous Epithelium

Extremely thin, flat cells (type I pneumocytes) forming the alveolar wall. In light microscopy they appear as a delicate, continuous lining; electron microscopy shows a thin cytoplasmic rim with abundant surface microvilli to increase surface area.

5. Smooth Muscle

Located in the walls of bronchi and bronchioles. Light microscopy shows spindle‑shaped cells with centrally placed nuclei. Electron micrographs reveal dense bodies and abundant smooth endoplasmic reticulum.

6. Capillaries

Thin‑walled vessels closely apposed to alveolar epithelium. In photomicrographs they appear as red‑staining lumina; electron microscopy shows a single layer of flattened endothelial cells with fenestrations.

Photomicrograph Features

1. Identify the orientation of the tissue (transverse vs longitudinal).
2. Look for characteristic staining patterns:

   • Basophilic cartilage matrix (deep blue/purple).
   • Pink mucin droplets in goblet cells.
   • Clear spaces indicating alveolar air sacs.

3. Note the arrangement of cilia – they should be parallel and uniform.
4. Observe the relationship between alveolar epithelium and capillaries (thin barrier).

Electron Micrograph Features

Electron microscopy provides ultrastructural detail that confirms the identity of each component.

• Cartilage: Collagen fibrils (diameter \overline{50} nm) embedded in a ground substance rich in proteoglycans.
• Cilia: 9+2 microtubule arrangement with dynein arms.
• Goblet cells: Secretory granules with electron‑lucent mucin cores.
• Alveolar type I cells: Thin cytoplasm, extensive surface microvilli, tight junctions.
• Smooth muscle: Dense bodies (analogous to Z‑lines) and abundant smooth ER.
• Capillary endothelium: Fenestrations (\overline{50} nm) and a basal lamina.

Summary Table

Exam Tips

• Always label the orientation of the slide (e.g., “transverse section of trachea”).
• Use the presence of cilia and goblet cells to differentiate respiratory epithelium from other columnar epithelia.
• Remember that type I pneumocytes are the only cells thin enough for efficient diffusion; type II cells are not a focus for this objective.
• When interpreting electron micrographs, look for characteristic ultrastructural markers (e.g., 9+2 cilia, dense bodies in smooth muscle).