Histological Differentiation – Lung Development

by John McLachlan, PhD

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    00:01 segment if that’s where a tumour should happen to lie. Now, let’s look at the development and histological differentiation of the lungs. This is one instance where the histology is particularly important for the function. In fact, it can make the difference between life and death. This maturation, this histological maturation of lungs is divided into four periods, which I’ll name briefly here, and then we’ll look at each of them in more detail subsequently.

    00:27 So these are first, the pseudoglandular period, then the canalicular period, next the terminal sac period, and finally, the alveolar period. We’ll look at the times that are associated with each of these. I’ve marked out as 5 to 17 weeks of pregnancy for pseudoglandular, and for canalicular, 16 to 25 weeks. So you can see that for each of these stages, there’s a slight amount of overlap between them. And this is because the lungs are not evenly developed all the way through. They tend to be a little more advanced towards the cranial, that’s towards the head end of the embryo. So at a certain point where some parts of the lung have reached the canalicular stage, others are still in the pseudoglandular period.

    01:13 And this is, in fact, clinically extremely significant. You can also see that much lung development does not begin until after birth. So they will progress during childhood. So let’s look at each of these stages in turn, first of all, the pseudoglandular period.

    01:29 As you can see in the image, the lining, the endodermal lining of the lung buds are still relatively thick and the blood vessels are some distance away and at this stage, respiration is not possible. It’s not possible for oxygen to pass from the lungs into the blood system because the distance is too great and the cells are too well differentiated to allow this to happen. So the process that’s going to go on is one of blood vessels becoming closer and the walls becoming thinner. If we look at the canalicular period, this process is already underway. So we can see that the walls of the lung buds are thinning down and the blood vessels are coming into closer and closer association with the lung buds themselves.

    02:17 These terminal bronchioles are giving rise to respiratory bronchioles, and in turn as we move into the alveolar period, this will become the alveolar ducts. The mesoderm which surrounds the developing lung buds is highly vascularised, rich in blood vessels.

    02:35 At the very end of this canalicular period, it is just possible that a baby might be able to breathe unassisted because some of the terminal sacs will have differentiated sufficiently for respiration to take place. Well, there’s a differentiation, as I said, from head to tail, craniocaudal differentiation. And it’s the cranial parts that are most advanced and most highly vascularised. But in general, survival before 25 weeks is very difficult. After 25 weeks and say six days, then we begin to see a better chance of a baby born at this stage surviving without major trauma resulting from the early birth process. Next, we have the terminal sac period from 24 weeks up till birth. Further terminal sacs are developing and the epithelium has become even thinner. The capillaries are in direct contact with the epithelium. Now we can see that the blood-air barrier has formed. Surfactant, which is effectively a detergent that helps keep the sacs inflated is also being secreted from the cells.

    03:46 In that terminal sac period, the epithelium is giving rise to both - type I pneumocytes, and these are the ones involved in gas exchange, and the type II pneumocytes which are secreting surfactant. Now let’s look at the development of the lungs as seen in a transverse section

    About the Lecture

    The lecture Histological Differentiation – Lung Development by John McLachlan, PhD is from the course System-Specific Embryology with John McLachlan.

    Author of lecture Histological Differentiation – Lung Development

     John McLachlan, PhD

    John McLachlan, PhD

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