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.
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.
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.
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.
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.
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.
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