Dental Histology M. Hall
2003
(G & H Chap. 15)
The
respiratory system includes the lungs and a system of tubes that link the sites
of gas exchange with the external environment.
It is customary to divide the respiratory system into two principal
regions:
(1) Conducting Portion
nasal cavity
nasopharynx
larynx
trachea
bronchi
bronchioles
terminal bronchioles
The conducting portion serves
two main functions:
1). to provide a conduit
through which air can travel to and from the lungs,
2). to condition the
inspired air.
(2) Respiratory Portion
respiratory bronchioles
alveolar ducts
alveoli - it is here that O2 and
CO2 are exchanged between inspired air and blood—which is the principal function of the lungs.
1) In order to
ensure that the airways remain open, a combination of cartilage, elastic
fibers and smooth muscle provide the conducting portion with a rigid
structural support as well as flexibility and extensibility.
2) Before it
enters the lungs, inspired air is cleansed,
moistened and warmed. To carry out
these functions, the mucosa of the conducting portions is lined by a
specialized respiratory epithelium, and there are numerous mucous and serous glands, as well as a rich superficial vascular network in
the lamina propria. As the air enters
the nose, hairs serve to remove coarse particles of dust, etc. Once the air reaches the nasal
fossae, particulate and gaseous impurities are trapped in a layer of
mucous. This mucous, in conjunction
with serous secretions, also serves to moisten the incoming air, which protects
the delicate alveolar lining from dessication.
In addition, the incoming air is warmed by a rich, superficial vascular
network.
Respiratory Epithelium
Most of the conducting portion is lined by respiratory epithelium—a
pseudostratified, columnar ciliated epithelium containing a rich population of
goblet cells. This respiratory
epithelium continues from the nasal fossae, through the nasopharynx, larynx,
trachea, bronchi and larger bronchioles, unchanged except for a gradual
decrease in the number of goblet cells.
However, the ciliated cells
continue all the way into the respiratory bronchioles--these cilia serve to
prevent mucous from accumulating in the respiratory portion of the system. The beating of the cilia in an anterior
direction moves the mucous toward the oral cavity, where it is either swallowed
or expectorated. Were this not to
occur, we would drown in our saliva.
Respiratory epithelium consists of 5 cell types, although only 3 are
easily visible under the light microscope:
viz: the ciliated columnar cells, mucous
goblet cells, and basal cells (which give rise to the
other cell types by division and differentiation). Underlying the respiratory epithelium is a lamina propria which contains
serous and mucous glands and a rich vascular bed. The former two components decrease in frequency as one passes
down the conducting passages. There is
a change in the epithelium at the level of the terminal and respiratory
bronchioles--there is a loss of goblet cells, and the pseudostratified
epithelium changes to a simple columnar/cuboidal epithelium. The submucosa
contains hyaline cartilage as well as some serous and mucous glands.
Let’s now look at some of the individual portions of this
respiratory system starting at the nose and traveling down to the smallest
units, the alveolar sacs.
Nasal Cavity:
The two nasal cavities are separated by a
septum. Each cavity is divided
into 2 parts: a vestibule, and the internal nasal fossa. The vestibule is the short chamber just
internal to the external nasal orifice.
It is continuous with the skin of the face and is lined by stratified
squamous epithelium which undergoes a transition to typical respiratory
epithelium upon entering the fossa. The
medial wall of the fossa is smooth, but the lateral wall is uneven due to the
presence of 3 bony, shelflike projections known as conchae. The middle and lower conchae are covered by
typical respiratory epithelium, while the upper concha is covered by a
specialized olfactory epithelium.
The narrowed passages formed by the conchae improve the conditioning of
the inspired air by increasing the surface area of respiratory epithelium, and
by creating turbulence in the air flow, resulting in increased contact with the
mucous layer. The rich venous networks (swell bodies) in the lamina propria of
the respiratory epithelium are capable of becoming greatly engorged with blood.
One side swells every 20-30 min, thus directing air through the other nasal
fossa and allowing the delicate respiratory epithelium to recover from
dehydration The rich and complexly organized vascular system of the fossae thus
results in the efficient warming of the inspired air.. Allergic reactions also cause engorgement of
swell bodies, thus restricting air flow.
The paranasal sinuses are large airspaces within the frontal, ethmoid,
and sphenoid bones and in the maxilla.
They open into the nasal cavity.
They are covered by typical respiratory epithelium with underlying
glands. Coordinated ciliary activity
moves the secretions of the sinuses to the nasal cavities. Thus, in its passage through the nasal
cavity, air is cleaned, warmed and moistened in preparation for its passage down
the trachea to the lungs.
Olfactory Epithelium is found only lining a small area of the roof of the
nasal cavity, the upper portions of the nasal septum and the superior
concha. This pseudostratified columnar epithelium with microvilli is
modified to serve as a receptor for smell and contains the olfactory cells, which are actually bipolar neurons. The apical
surface of the olfactory cell possesses long cilia, which lie parallel to the
surface of the epithelium in the fluid secretions of the mucous membranes. These cilia contain receptors for odiferous
substances. Excitation of the receptors
stimulates cAMP production in the cell, which eventually results in the generation
of a receptor potential. The base of
the bipolar neuron gives rise to a long axonal process which enters the
underlying connective tissue and joins axons of other bipolar olfactory
neurons, to form the olfactory nerves (largely unmyelinated).
Two other cell types found in the olfactory epithelium are columnar sustentacular cells with
microvilli (probably a metabolically active supportive cell), and basal cells. The underlying lamina propria contains numerous glands called Bowmans’ glands.
These branched, tubuloaveolar glands empty their secretions via
ducts onto the olfactory surface. The
olfactory epithelium ends abruptly at a boundary with respiratory epithelium.
Pharynx
The pharynx
serves both the respiratory and digestive systems as a passageway for air and
food. The pharynx communicates
anteriorly with the nasal and oral cavities, and is called the nasopharynx or the oropharynx. The nasopharynx
is lined with respiratory epithelium, while the oropharynx is surfaced by
non-keratinized, stratified squamous epithelium as it is continually abraded by
food.
Larynx
The larynx is an irregular tube that connects the pharynx
to the trachea. It has a cartilaginous
framework which serves to maintain an open airway. It serves as the organ of phonation. The epiglottis is a leaflike cartilage
which projects from the rim of the larynx and extends into the pharynx. It prevents food from entering the
trachea. The epiglottis has a lingual
and a laryngeal surface. The entire
lingual (upper) surface is covered with a stratified squamous epithelium, as it
is constantly abraded by the passage of food.
Toward the base of the epiglottis, on the laryngeal (lower) side, the
epithelium undergoes a transition to an atypical respiratory epithelium. As you already know, the epiglottis is
comprised of elastic cartilage which
allows it to bend easily, thus closing off the larynx during swallowing.
Below the epiglottis, the mucosa forms 2 pairs of folds
that extend into the lumen of the larynx.
These are the vocal cords. The
upper pair are the false vocal cords, while the lower pair are the true
vocal cords. These are quite
easy to distinguish, as the false vocal
cords are covered by atypical respiratory epithelium with numerous serous
glands in the lamina propria, while the
true cords are covered by stratified squamous epithelium. Beneath the
epithelium is the vocal ligament and
bundles of skeletal muscle, the vocalis muscle. This muscle regulates the tension of the
fold and its ligaments. When air is
made to pass through the glottis, the vocal cords are made to vibrate by the
passing air. The vibrations are altered
by altering the tension placed on the cords by the vocalis muscle. This change in the vibrations produces
sounds of different pitch.
Trachea
Connecting the larynx to the bronchi is a thin walled
tube, the trachea. It extends from the larynx to about the
middle of the thorax. This is still
part of the conducting system, and like most of the airways, is lined by
typical respiratory epithelium. The trachea
is lined with 16-20 C-shaped rings of hyaline cartilage, which serve to keep
the trachea open. At the posterior,
open end of the C, a fibroelastic cartilage and a bundle of smooth muscle (trachealis
muscle) bind to the periochondrium.
This arrangement allows for contraction of the lumen and is used in the
cough reflex.
Bronchial Tree
The bronchial tree stretches from the primary bronchi to
the smallest respiratory bronchioles.
This bifurcating airway is largely lined with respiratory epithelium,
although the complexity of this epithelium decreases as we reach down to the
smaller bronchioles.
The trachea bifurcates into 2 primary bronchi, which
enter the lungs at the hilum. Shortly thereafter, the primary bronchi divide to give rise to 3 secondary
bronchi in the right lung, and 2 in the left lung. The secondary bronchi divide repeatedly,
giving rise to numerous smaller bronchi.
The histological features of these bronchi are similar to those of the
trachea except that the C-shaped rings of cartilage are replaced by irregular
rings, and then by plates of cartilage.
Thus, as long as cartilage is
present, the airway is classified as a bronchus
(large or small).
After the bronchi lose their cartilage and are reduced to
less than 1 mm in diameter, they are classified as bronchioles. Each bronchiole enters a pulmonary
lobule where it branches to form 5-7 terminal bronchioles. It is here that we start to see major
changes in the respiratory epithelium.
Bronchioles and terminal bronchioles contain no glands or cartilage and
few or no goblet cells. Additionally,
the epithelium is reduced to a simple
columnar or even cuboidal epithelium,
with ciliated cells. An additional
feature at this level of the airways is an increased amount of smooth muscle which is wrapped around
the bronchiole in criss-crossing bundles.
Additionally, the lamina propria contains large numbers of elastic fibers. Thus, the diameter of these airways can
change due to contraction of the smooth muscle bundles.
Each terminal bronchiole subdivides into 2 or more respiratory
bronchioles that serve as regions
of transition between the conducting and respiratory portions of the respiratory
system. The mucosa is identical to
that of the terminal bronchioles with a simple, ciliated cuboidal epithelium. A
distinguishing feature of respiratory bronchioles is the presence of scattered alveoli budding from the walls of the
tubules. The number of alveoli
increases distally, until eventually the wall consists only of alveoli and the
tube is now called an alveolar duct. At the termination of the alveolar duct,
clusters (2 or 3) of alveoli share a common opening to the alveolar duct. Such a cluster of alveoli is referred to as
an alveolar
sac. Each alveolar sac contains
many alveoli. The openings of the
alveolar sacs and alveoli are surrounded by elastic and reticular fibers. The elastic fibers allow the alveoli to
expand upon inspiration and to passively contract upon expiration, while the
reticular fibers prevent overdistension of the alveoli, which would cause
damage to the delicate capillaries and alveolar septa.
Alveoli
The alveoli are
the small air sacs with thin walls which allow for the ready diffusion of gases
between the air space and the blood.
They are the terminal portions of the bronchial tree and are responsible
for the spongy structure of the lungs.
The structure of the alveolar walls is specialized for enhancing
diffusion between the internal and external environments. Each wall is a partition, or septum, between
2 adjacent alveoli, and is called an interalveolar septum. An interalveolar septum consists of 2 thin
squamous epithelial layers between which lie capillaries, elastic and collagen
fibers, macrophages and fibroblasts.
The capillary network in the septum is the richest in the body, and the
extreme thinness of the septum between the capillary lumen and the alveolus
allows for the ready exchange of 02 and C02. In order to allow for the free flow of air
between the alveoli, the septa are perforated by alveolar pores.
The interalveolar septum is
comprised of 4 main cell types, and can be thought of as a
"sandwich"
1. Type I cells which covers 97% of the alveolar surface and
comprise the “bread” of the septal sandwich.
Tight junctions between these cells contribute towards the blood-air
barrier. The main role of this cell is
to provide a very thin barrier that is readily permeable to gases.
2. Endothelial cells of the capillaries are extremely thin. The capillaries comprise the “filling” of
the septal sandwich. These are
continuous capillaries without fenestrations.
3. Type II Cells
(Great Alveolar Cells) are found located at septal junctions, and bulge
slightly into the airspace of the alveolus.
These cells are responsible for the synthesis and secretion of pulmonary
surfactant, a phospholipid,
protein and glycosaminoglycan-rich secretion that spreads over the surface of
the entire alveolus, providing an extracellular coating that lowers the surface
tension of the alveolus. This
surfactant facilitates the expansion and contraction of the alveoli. Surfactant is continuously removed by
macrophages and replaced by type II cells.
(Hyaline membrane disease in
newborns is due to insufficient surfactant production, so that the alveoli have
difficulty in expanding.
4. Alveolar macrophages: Chronic
inflammation is rare in the lungs due to the presence of macrophages (dust
cells) found both within the alveolar septum and on the surface of the alveolar
wall. These macrophages scavenge dust
and other particles which find their way into the alveoli. They may then be passed up the bronchial
tree by the cilia and swallowed; others remain in the connective tissue of the
septum and scavenge along the capillary wall.
The alveolar membrane, across which O2 and CO2
must diffuse, is less than half the thickness of the interalveolar septum, since it consists only of i) the thin
cytoplasm of the type I cell, ii) the fused basal laminae of the type I cell
and the capillary endothelial cell, and iii) the thin cytoplasm of the
capillary endothelial cell.
5. Other cells found within the alveolar septum include fibroblasts, mast cells and brush cells.
Emphysema
results from destruction of the alveolar wall, with subsequent reduction in
area of the respiratory portion of the lungs.
It is clearly associated with smoking and air pollution).
Pulmonary Blood Supply
Circulation to the lungs includes both functional (pulmonary) and nutrient (systemic) vessels.
The functional
circulation (i.e. blood to be “treated”) is represented by pulmonary
arteries and veins. (Remember,
pulmonary arteries carry C02-rich blood, while pulmonary veins carry
02-rich blood). The
pulmonary circulation branches and accompanies the bronchial tree. At the level of the alveolar duct, the
branches of the pulmonary artery form a capillary network which courses in the
interalveolar septa between all alveoli.
The lung has the best developed capillary network in the body. Venules which arise from this capillary
network enter the interlobular septa (connective tissue septa), and after
leaving the lobule, follow the bronchial tree towards the hilum.
The nutrient
supply to the lungs is provided by the bronchial
arteries which are smaller than the pulmonary vessels. These convey nutrient and oxygen rich blood
to the larger masses of connective tissue of the lung. The arteries extend as far as the
respiratory bronchioles, at which point they anastomose with the pulmonary
artery and capillaries. Thus, most of
the blood entering the lungs via the bronchial arteries leaves by the pulmonary
veins (bronchial veins drain only the upper region of the lungs).
Pleura
The surface of the lung is covered by a serous membrane. It consists of two layers - parietal and visceral - which are continuous at the region of the hilum. Both layers of the pleura are covered by mesothelial cells. In the healthy state, this pleural cavity contains only a film of
liquid that allows the smooth sliding of one surface over the other during
respiratory movements. In certain
pathological states (pleurisy), the
pleural cavity can become filled with fluid which is derived from blood plasma
by exudation.
Exam Questions, 1993.
Written Questions
1A. (Q) What are the 3 main
functions of the conducting portion of the respiratory system--besides
conducting air?
(A) To clean,
moisten and warm inspired air.
1B (Q) What structural
component, found throughout the conducting portion of the respiratory system,
is mainly responsible for conditioning inspired air
(A) Respiratory
epithelium
2A (Q) What structure
prevents food from entering the trachea?
(A) Epiglottis
2B (Q) In what portion
of the airway is this structure situated?
(A) Larynx
3A (Q) What structural
feature distinguishes a bronchus from a bronchiole?
(A) Presence of
cartilage in bronchus.
3B. (Q) What structural feature
distinguishes a terminal bronchiole from a respiratory bronchiole?
(A) Presence of
alveoli (in the respiratory bronchiole)
4 (Q) Distinguish
between an interalveolar septum and an alveolar membrane.
(A) Interalveolar
septum is the structure which separates two alveoli
(B) Alveolar
membrane is the structure across which oxygen and CO2 diffuse.
5 (Q) What cellular
components comprise the alveolar membrane?
(A) Type 1 Septal
cell, fused basement membrane, capillary endothelial cell.
Lab identifications.
1. Identify this
cell. (Macrophage in alveolus)
2. Classify this
tissue. Be very specific. (Respiratory epithelium)
3. Name this
structure Be very specific. (Bowmans gland)
4. Name this
structure Be very specific. (Olfactory nerve)
5. Identify this
structure. Be very specific. (Vocalis muscle)
6. Identify this
structure. Be specific. (False vocal cord)
7. Classify this
tissue. Be very specific (Olfactory epithelium)
8. Name this
structure Be very specific. (Bronchiole)
9. Name this
tissue. Be specific (Perichondrium).
10. Identify this structure. (Capillary in interalveolar septum).