Essay 8

Do not memorise these answers! These are guidelines. Be creative, devise your own expression and organisation! You can elaborate on the points in your own language.
Remember that there is no 'perfect' answer - a little shortfall in content is easily compensated by presentation.
You should aim at writing an essay in this examination in approximately 20 minutes.

Development of the lung parallels that of a gland : Comment.

A gland is a specialisation of an epithelium. A portion of the epithelium grows down into the supporting connective tissue. In a large gland the duct branches repeatedly. Each of the finer branches has a specialised secretory portion at the tip. The supporting connective tissue separates portions of the gland as lobes and lobules.

The respiratory system begins its development as an endodermal diverticulum (outgrowth) from the foregut. In effect, it is an epithelial derivative. The "lung bud" has two main branches. Each of these branches divides repeatedly forming a system of tubes. The blind ends of these tubes have epithelium which gradually flatten out for gas exchange.

This pattern is very similar to the development of a gland. The first branches of each side in fact cater to the lobes of the lungs. Subsequent divisions cater to segments and then smaller lobules. Before the epithelium at the tips flattens out, it is columnar. In fact there is a stage of development in the foetal lung that is called the pseudoglandular stage - it appears like a gland, but it is not.

The similarity, though striking, stops here. The lung is going to cater to the function of gas exchange. The functional epithelium at the tips is therefore simple squamous. The 'ducts', trachea and bronchi are designed to carry air and must remain open at all times. Their walls therefore are strengthened by hyaline cartilage.