Microstructure of the lungs and pleura

Question 1

Describe the embryological development of the lungs (origin, timing)

Answer 1

• Trachea, major bronchi and lungs are epitheliam mesenchymal outgrowths of the anterior foregut
• Tubes form which branch as they grow. Larger proximal tubes become the conducting airways which do not take part in gas exchange (trachea, bronchi, bronchioles, terminal bronchioles). More distal parts form respiratory bronchioles, alveolar ducts and sacs, alveoli
• Respiratory development occurs late - alveoli and surfactant develop in third trimester

Question 2

Structure of the trachea (layers, anteiror and posterior wall)

Answer 2

Trachea consists of mucosa, submucosa and outer adventitia,

Supported by cartillagenous rings which occupy 2/3 diameter anteriorly
Posterior wall consists of smooth muscle bundles and collagen

Mucosa has 3 layers:
1. Respiratory epithelim: **ciliated pseudostratified columnar epithelium **
2. Lamina propria: thin layer of connective tissue
3. Elastic lamina: longitudinally arranged elastic fibres with some blood vessels

Submucosa consists of connective tissue embedded with smooth muscle, glands, blood vessels, lymphoid tissue, nerves
* Glands contain serous and mucous cells which produce the sheets of mucous within the trachea

Between the cartillages, forming part of the outer adventitia, is a layer of connective tissue containing blood vessels, nerves and fat

Question 3

What type of cells are present in tracheal respiratory epithelium? What are their functions

Answer 3

Main cell types:
* Ciliated columnar cells: secretion clearance. Cilial beating rate varies with mechanical stimulation and inflammatory mediator presence
* ** Goblet cells: **contain secretory vacuoles filled with mucinogen. Number increases with epithelium is irritated
* Basal cells: mitotic stem cells for other cell types
* Brush cells: slender, non-ciliated cells with stiff microvilli. Sensory function: in contact with afferent nerve fibres

Lymphocytes and mast cells migrate into the epithelium from underlying connective tissue. Mast cells in the basal areas of epithelim release histamine in response to inhaled irritants or allergens

Question 4

What are the layers of tracheal mucosa?

Answer 4

Mucosa has 3 layers
1. Respiratory epithelim: ciliated pseudostratified columnar epithelium
2. Lamina propria: thin layer of connective tissue
3. Elastic lamina: longitudinally arranged elastic fibres with some blood vessels

Question 5

Answer 5

Question 6

What are two main differences between the structure of the bronchi compared to the trachea

Answer 6

1. Smooth muscle is present in a spiral arrangement (constriction causes bronchospasm)
2. Cartilagenous rings are replaced by plates which reduce in size as the bronchial diameter reduces

Picture shows structure of a normal bronchus

Question 7

Which conducting airways are extrapulmonary, which are intrapulmonary

Answer 7

Extrapulmonary: trachea, main bronchi, lobar bronchi
Intrapulmonary (within the lung tissue): segmental bronchi down to terminal bronchioles

Question 8

What are 4 structural features specific to bronchioles (compared to other conducting airways)

Answer 8

• Epithelium is pseudostratified columnar, +/- cilia through simple columnar to cuboidal. Goblet cells are only present in larger bronchioles
• Smooth muscle is present in bundles
• No cartillage
• No glands

Question 9

What is the overall alveolar area of the lung. How many alveoli

Answer 9

Over 140m2
>3 million alveoli

Question 10

Describe the structure of a respiratory bronchiole

Answer 10

• Respiratory bronchioles (unlike terminal bronchioles) have alveolar out-pockets along their walls
• Tube like alveolar ducts which consist almost entirely of alveoli and alveolar sacs (terminal clusters of alveoli)

Question 11

Structure of the wall of an alveolus (thickness, cell types, functions)

Answer 11

• Single layer of squamous epithelial cells (type I cells) surrounded by a basal lamina
• Type II cells are smaller cuboidal epithelial cells scattered amount the Type I cells. Contain secretory granules full of surfactant. Almost as abundant by type I but overall much smaler surface area
• External surfaces are covered by mesh of capillaries. Gas exchange occurs across the alveolar and capillary walls and their fused basement membrane
• Total alveolar wall thickness is 0.5 micrometres
• Adjacent alveoli interconnect via pores which permit pressure equalization

Question 12

Pleura (structure, cell type, blood and nerve supply)

Answer 12

• ** Parietal pleura**: covers the internal surface of the chest wall, superior and lateral surfaces of the diaphragm. Extends to the lung root where becomes continuous with visceral pleura
• Visceral pleura covers external lung surface and cannot be separated from it

Layers:
* Single layer of mesothelial cells
* Lamina propria of loose connective tissue
* Basal lamina

Blood supply: bronchial arteries, which drain into bronchial veins

Nerve supply: anterior and posterior pulmonary plexi

Question 13

How does the structure of the airways change from trachea –> broncioles (cartillate, epithelium, muscle)

Answer 13

Cartilage
* incomplete rings in the trachea
* plates in the bronchi
* absent at the level of the bronchioles.

Epithelium
* changes from pseudostratified columnar to simple columnar then simple cuboidal epithelium in the terminal and respiratory bronchioles.
* Cilia and goblet cells are absent in these small bronchioles.

Muscle
* Muscle in the posterior tracheal wall (trachealis muscle) extends into the bronchi and forms helical bands around the smaller bronchi and bronchioles.
* Smooth muscle thins towards the end of the bronchial tree and is absent in the alveoli.