Type II pneumocytes and surfactant in the foetal lung
Type II pneumocytes, the cube-shaped alveolar cells, make surfactant, which lowers surface tension (the inward pull that would otherwise collapse the alveoli) from about 26 weeks.
Remember
Surfactant comes from type II pneumocytes and is made from about 26 weeks. It lowers alveolar surface tension, so the lungs inflate more easily and resist collapse, but a baby born very early often has too little.
Memory aid
type I is thin for gas, and type II makes the surfactant
The thin type I cell is for gas, and the type II cell makes surfactant.
The alveoli (the air sacs of the lung) begin to form from about 26 weeks. This is the terminal sac stage, which lasts from about 26 weeks until birth. Two main epithelial (lining) cell types appear in the alveolar wall.
Type I pneumocytes (type I alveolar cells) are thin, flat (squamous) cells that cover most of the alveolar surface, so gas has a short distance to cross. Type II pneumocytes are smaller, cube-shaped (cuboidal) cells that make surfactant, a mixture of phospholipids (fat-based molecules) and proteins.
Surfactant spreads across the thin film of fluid lining each alveolus and lowers its surface tension. Surface tension is the inward pull of water molecules at the air-liquid surface, which would otherwise collapse a small air sac. With less of it, smaller pressures are enough to open the alveoli, and they are less likely to collapse at the end of a breath. That is why the first breaths at birth are much easier.
Production starts before there is necessarily enough for stable breathing. Type II cells release small amounts early, but the amount is generally not adequate to give effective lung compliance (how easily the lungs stretch) until about the eighth month, and the baby's own production is often not enough before 34 to 36 weeks. A baby born early can therefore have the right cells but too little surfactant. The alveoli collapse and the lung becomes stiff, which is respiratory distress syndrome.
Key facts
- Cell that makes surfactant
- type II pneumocytes, the cube-shaped (cuboidal) alveolar cells.
- What surfactant is
- a mixture of phospholipids and proteins.
- What it does
- it lowers alveolar surface tension, so the alveoli can be opened by smaller pressures and are less likely to collapse.
- When it starts
- the alveoli begin to form from about 26 weeks, and type II pneumocytes make surfactant from about 26 weeks.
- Type I pneumocytes
- thin, flat cells built for gas exchange, which do not make surfactant.
- Born early
- the baby's own production is often not enough before 34 to 36 weeks, so respiratory distress syndrome can develop.
Exam traps
- Type I pneumocytes are thin, flat cells for gas exchange, so they do not make mucus; surfactant is secreted by type II pneumocytes.
- Muscle tone in airway walls can alter airway diameter, but it does not keep the alveoli open, because that depends on surfactant.
- Fibroblasts lay down collagen, and extra collagen would stiffen the alveolar wall, so it would make the lungs harder to expand, not easier.
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