Histology Of The Lower Respiratory Tract
First: Bronchioles
-The bronchioles are divided into three parts: large, terminal, and respiratory.
General characteristic of bronchioles:
1) The diameter: as we said before when we move distally in the respiratory tract the
diameter decreases.
-The diameter in the bronchioles is less than 5mm, because the last part in the
bronchi has a diameter of 5mm.
-The diameter of the large bronchioles is 1mm, 0.5mm for terminal bronchioles
and less than 0.5 mm for respiratory bronchioles.
2) The bronchioles don't have glands, cartilage or lymphocytes.
- Despite the fact that we have a numerous lymphocytes and lymphatic
nodules in the distal part of the bronchus, in the bronchioles the lymphocytes
were replaced by macrophages which were named, in the alveoli, dust cells.
- There are only scattered goblet cells within the epithelium of the large
bronchioles while the terminal and respiratory bronchioles are devoid from
goblet cells. So saying that bronchioles in general don't contain goblet cells in
their epithelium is a correct statement, but when we apply this statement to
the large bronchioles only it will be incorrect, because it contains scattered
goblet cell.
3) The epithelium: remember that we don't have abrupt changes in the respiratory
tract. So from pseudostratified ciliated columnar to simple columnar ciliated then
to simple cuboidal ciliated finally in the terminal part, esp. in the distal part of the
terminal bronchioles and the respiratory bronchioles, the epithelium is formed
mainly from Clara cells.
- These Clara cells are simple cuboidal epithelium devoid from cilia and they
contain secretory granules that contain substances which participate in the
formation of the pulmonary surfactant which lines the apical surface of the
alveolar epithelium to reduce the alveolar surface tension, so the formation of
the surfactant is very important and must occur adequately before delivery to
make the first inflation of the lungs passive (without any muscular effort from
the baby).
4) The lamina propria: it contains a large amount of smooth muscle cells and
elastic fibers. So in the case of Asthma there is a contraction of the smooth
muscles in the bronchioles that can be relieved by the administration of
sympathomimetic drugs like adrenaline causing bronchodilation.
5) The bronchioles also exhibit specialized cells called neuroepithelial cells, their
number is between 80-100 cells, they receive cholinergic nerve endings, they
contain secretory granules, and they also contain chemoreceptors.
Second: The respiratory bronchioles
- Around those bronchioles we will always find alveoli.
The characteristics of the respiratory bronchioles:
1) The epithelium: is simple columnar or simple cuboidal and it may be ciliated or
non-ciliated (Clara cells).
2) The smooth muscle cells: the amount of the smooth muscle cells will decrease
gradually until they form knobs surrounding the openings of the alveoli that act as
sphincters. Also we will have an increase in the elastic fibers and reticular fibers in
the area between the alveoli to help in the inflation and deflation of the alveoli.
The terminal part of the respiratory bronchioles is the alveolar ducts which will
bifurcate distally to give the atria that communicate with the alveolar sac, so the
end of the alveolar duct opens into the alveolar sac.
Third: Alveoli
- They are saclike evaginations; they have a diameter of 200μm. There are 300
million alveoli in each lung so the surface area is about 140m2. These alveoli are
responsible for the spongy structure of the lungs because of the presence of
elastic fibers and reticular fibers between the alveoli.
-The space between two alveoli is called the inter-alveolar septum, within this
space we have the interstitium which consist of capillaries and connective
tissue, also within this septum there are alveolar pores which have a diameter
between 10-15μm these pores equalize the alveolar air pressure and promote
the collateral circulation between the alveoli esp. if there is a distal
obstruction.
1) The wall of the alveoli contains: type 1 alveolar cells (type 1 pneumocytes)
which are simple squamous cells and they form 97% of the wall, and type2 alveolar
cells which form only 3% of the alveolar wall and they are present particularly at
the corners of the alveolar wall .while in the interalveolar septum type 2 cells
form16% of the septum (because of this they are also called septal cells) and type 1
form 8%.
- Type 2 alveolar cells (septal cells) contain lamellar bodies which contain
surfactant , the production of this substance will be increased in the last month
before delivery, and as we mentioned this substance will line the inner aspect
of the alveolar wall to form a surfactant film that reduces the surface tension .
2) The interalveolar septum: it's the septum between two alveolar walls starting
from the epithelium of the first to the epithelium of the second (the epithelium of
the alveolar walls are part of the septum) and within this septum we have the
interstitium (capillary and connective tissue) also we have mast cells, fibroblasts
(they form36%), elastic fibers, reticular fibers and macrophages (which form 10%). so
the interstitium consists of : 1)capillaries 2)connective tissue which contains elastic
fibers, reticular fibers, fibroblasts, and leukocytes.
3) The macrophages (dust cells):
-They are derived from monocytes.
- They are found in the interalveolar septum, in the alveolar wall and in the
pleura.
- in the case of congestive heart failure the lungs become congested with blood
and some RBCs will enter the alveoli where they are phagocytosed by
macrophages , as a result we will find those hemosiderin-laden macrophages
(heart failure cells) in the sputum of the patients with heart failure .
4) The respiratory membrane (blood air barrier): consists of the epithelium of the
alveolar wall and the endothelial cells of the capillary and between them there are
fused basal laminae to decrease the thickness of the barrier and facilitate gas
exchange, so the total thickness is around 0.1-1.5μm. This membrane permits only
O2 to pass into blood capillaries and CO2 to diffuse in the opposite direction so this
membrane is highly selective. The CO2 is released from the H2CO3 by the action of
the carbonic anhydrase enzyme which is present within the erythrocytes.
- The cells that form the respiratory membrane:
- Type 1 cells (pneumocytes type 1): they form 97% of the alveolar wall, they
are simple squamous cells, they are extremely attenuated cells, and their
thickness is about 25nm. The organelles of those cells are grouped around the
nucleus leaving a large portion of the cytoplasm free from organelles to
facilitate gas exchange; also the cytoplasm contains abundant pinocytic vesicles
which play a role in the turnover of the surfactant and the removal of small
particulate contaminants. The lateral wall of those cells contains desmosomes
and occluding junctions and these junctions are numerous between them and
type 2 cells to prevent the leakage of tissue fluid into the alveolar air space.
- Capillary endothelial cells: they are extremely thin, continuous, not
fenestrated, and their organelles are clustered together to clear a space for gas
exchange.
- Type two cells (septal cells): they are interspersed among type 1 alveolar cells,
with which they have occluding and desmosomal junctions. They are cuboidal
cells resting on the basement membrane of the alveolar epithelium and they
have the same origin as the type 1 cells that line the alveolar wall. They are
found mainly at the angles of the alveolar wall. They divide by mitosis to
replace their own population and the type1 population. In histological sections
they exhibit a characteristic foamy or vesicular cytoplasm, and they have
lamellar bodies which contain phospholipids, proteins and glycosaminoglycans,
all those contents will form the pulmonary surfactant which lowers the alveolar
surface tension, the deficiency of this substance in the newborn will cause
“Respiratory Distress syndrome (RDS) of the newborn”. Those cells and type 1
cells are destroyed by the inhalation of NO2 gas.
-The surfactant layer consists of an aqueous hypophase, lipid monolayer and several
proteins. The functions of the surfactant are:
- Decrease the surface tension of the alveolar cells, so less inspiratory force is
needed to inflate the alveoli.
- Stabilizing the alveoli; without the surfactant the alveoli would tend to
collapse during expiration.
- It also acts as bactericidal substance.
- In fetal development, surfactant appears in the last weeks (last month) of gestation
and the production of it is mainly by type 2 cells (which contain lamellar bodies).
- The surfactant is not static but is constantly being turned over; the lipoproteins are
gradually removed from the surface by the pinocytic vesicles of the squamous
epithelium, by macrophages, and by type 2 alveolar cells.
- During intrauterine life the fetus receives oxygen from his mother through the
placenta, but after delivery the lungs must inflate and start working so the doctor
will carry the baby from his legs and stimulate the respiratory center in the brain by
patting his back, then the lungs will inflate and the baby will cry marking the
beginning of respiration. This information is important because it can help in
differentiate between stillborn babies and babies who have died after delivery;
because if the baby wasn’t stillborn he would have breathed and his lungs would be
inflated and their density would be lower (they contain air) so they would float if
placed in water.
After delivery the delicate body of the baby is covered with amniotic fluid and is
therefore extremely slippery and handling the baby requires extra caution to avoid
accidental injuries or deaths.
- The synthesis of surfactant can be induced by the administration of glucocorticoids.
Fourth: The pulmonary blood vessels
-Within the lungs we have bronchopulmonary segments; each consists of alveoli and
connective tissue that contains the segment's artery, vein, and lymphatic vessel.
Within this connective tissue the pulmonary vein is in the lateral side while the
bronchiole, the lymphatic vessel and the pulmonary artery are in the middle of the
connective tissue (the branches of the artery are surrounded by the adventia of the
bronchioles).
- The circulation within the lungs includes both nutrient (systemic) and functional
(pulmonary) vessels. The pulmonary arteries have thin walls as a result of the low
pressure (25mm Hg during systole and 5mm Hg during diastole). Within the lung the
pulmonary arteries accompany the bronchial tree, the arteries then give many
branches that end into a network of capillaries that surround the alveoli. On the
other end of this network the small venules will originate and those venules are
found singly in the parenchyma then in the interlobular septum then after they leave
the lobules they follow the bronchial tree toward the hilum and finally they end in
the left atrium as the 4 pulmonary veins.
Done by: Ibraheem Suleiman Obaidat