A presentation on the histology of the respiratory system

3. OVERVIEW OF THE RESPIRATORY SYSTEM
• The respiratory system consists of
the paired lungs and a series of air
passages that lead to and from the
lungs.
• functions :
- air conduction
- air filtration
- gas exchange (respiration).
- to produce speech
- stimuli for the sense of smell.
- Participates to a lesser degree in endocrine functions
- as well as regulation of immune responses to inhaled
antigens.

4. RESPIRATORY SYSTEM
• Conducting portion
- consisting of airways that deliver air to
the lungs
- Nasal cavities
- Nasopharynx
- Larynx
- Trachea
- Paired main (primary) bronchi
- bronchioles.
• Respiratory portion
- is that part of the respiratory tract in
which gas exchange occurs.
- Respiratory bronchioles
- Alveolar ducts
- Alveolar sacs
- Alveoli

5. • Blood vessels enter the lung with the bronchi.
• The arteries branch into smaller vessels as they follow them
bronchial tree into the lung.
• Capillaries come into intimate contact with the terminal
respiratory units, the alveoli.

6. • Conditioning of the air occurs in the conducting portion of the respiratory
system and includes warming, moistening, and removal of particulate materials.
• Mucous and serous secretions play a major role in the conditioning process.

7. 1- Conducting portion

8. • Nasal cavities
- are paired chambers separated by a bony and cartilaginous septum
- The chambers are divided into three regions:
- Nasal vestibule
- Respiratory region
- Olfactory region
Histology of the Nasal cavities

10. Nasal cavity
1- Vestibule of the Nasal Cavity
- lined with stratified squamous epithelium,
- vibrissae
- Sebaceous glands
- Posteriorly: the lining changes to respiratory epithelium

12. Nasal cavity
2- Respiratory Region
• The medial wall of the respiratory region, the
nasal septum
• the lateral walls are thrown into folds bythe
presence of three shelf-like, bony projections
called conchae or turbinates.
• It is lined by the respiratory mucosa that
contains a ciliated, pseudostratified columnar
epithelium on its surface.
• lamina propria
- It has a richly vascularized (many venous
plexuses) and contains seromucous glands.
• The mucosa of the respiratory region
warms, moistens, and filters inspired air.

14. • The ciliated, pseudostratified columnar epithelium of the respiratory mucosa is
composed of five cell types:
1. • Ciliated cells,
2. • Goblet cells
3. • Brush cells,
4. • Small granule cells (Kulchitsky cells)
5. • Basal cells, stem cells from which the other cell types arise

18. • 3- Olfactory epithelium
- on the superior nasal conchae.
- It is lined with a specialized olfactory mucosa.
- total surface area of the olfactory mucosa is only about 10 cm2
- It has a lamina propria that contains numerous blood and lymphatic vessels, unmyelinated
olfactory nerves, myelinated nerves, and olfactory glands (bowman ).

19. • The olfactory epithelium is composed of the following cell types:
1. Olfactory receptor cells
2. Supporting or sustentacular cells= They synthesize and secrete odorant-binding
proteins.
3. Basal cells
4. Brush cells

21. olfactory receptor cell
• has a single dendritic process that
projects above the epithelial
surface as a knoblike structure
called the olfactory vesicle.
• A number of long, thin cilia (10 to
23) arise from the olfactory
vesicle
• The cilia are usually up to 200 m
long
• The basal domain of the cell gives
rise to an unmyelinated axonal
process
• The collections of axons from
olfactory receptor cells form the
olfactory nerve (cranial nerve I).

23. 1. Olfactory receptor cells

24. • olfactory transduction pathways

25. Olfactory epithelium
• Supporting or sustentacular
cells
• provide mechanical and metabolic support
for the olfactory receptor cells.
• are the most numerous cells in the olfactory
epithelium and occupy a more apical position
• They have numerous microvilli and abundant
mitochondria, sER, rER, lipofuscin granules
• secretion of OBP molecules
• Adhering junctions are present with the
olfactory receptor cells, but gap and tight
junctions are absent.
• comparable to that of neuroglial cells

26. Olfactory epithelium Brush cells
• With the electron microscope (EM), brush cells
exhibit large, blunt microvilli
• The basal surface:
- in synaptic contact with nerve fibers (trigeminal
nerve) that penetrate the basal lamina.
• functions:
- involved in transduction of general sensory
stimulation of the mucosa.
- involved in an absorptive as well as a secretory
functions.
 Basal cells
• are progenitors of the other mature cell
types.
• are small, rounded cells located close to the
basal lamina.
• Their nuclei are frequently invaginated and lie
at a level below those of the olfactory receptor
cell nuclei.

29. Olfactory epithelium
• olfactory gland (Bowman’s glands)
• are a characteristic feature of the olfactory mucosa.
• Lipofuscin granules are prevalent in the gland cells
• Short ducts composed of cuboidal cells

30. • identifying feature of the olfactory
region
• is the presence of the olfactory nerves in
combination with olfactory glands in the
lamina propria.

33. Paranasal Sinuses
• are air-filled spaces in the bones of
the walls of the nasal cavity.
• are extensions of the respiratory region
of the nasal cavity
• The mucosal surface of the sinuses is a
thin, ciliated, pseudostratified columnar
epithelium with numerous goblet cells.
• The sinuses are named for the bone in
which they are found
• sinuses communicate with the nasal
cavities via narrow openings onto the
respiratory mucosa.

35. Pharynx
 Oral cavity at posterior part is
contineous with pharynx and
called oropharynx.
 Epithelium of oropharynx is
SSE
 epithelium of nasopharynx is
pseudostratified epithelium

37. pharynx
• The auditory (Eustachian) tubes connect the nasopharynx to each middle ear.
• Diffuse lymphatic tissue and lymphatic nodules are present in the wall of the nasopharynx.
• The concentration of lymphatic nodules at the junction between the superior and posterior walls of
the pharynx is called the pharyngeal tonsil.

38. Larynx
• The passageway for air between the oropharynx and
trachea is the larynx
• This complex tubular region of the respiratory system
is formed by irregularly shaped plates of hyaline
(thyroid, cricoid, and lower part of arytenoids) and elastic
cartilage (the epiglottis and the vocal processes of the
arytenoid cartilages)
 Vestibular folds (false vocal cords)
 vocal cords
- stratified squamous nonkeratinized epithelium

40. Larynx
• Vestibular folds
- located above the vocal folds are the
“false vocal cords.”
- is an elongated recess in the larynx called
the ventricle.
- mucous glands are prominent
- Numerous lymph nodules
- ciliated, pseudostratified columnar epithelium
• vocal cords
- stratified squamous epithelium
- The epithelium serves to protect the
mucosa from abrasion caused by the
rapidly moving air stream. The rest of the
larynx
• Inferior to the vocal cords through the trachea and intrapulmonary bronchi, epithelium change
to respiratory epithelium

41. lymph nodules

42. Trachea
• is a short, flexible, air tube about 2.5 cm in diameter and about 10 cm long.
• extends from the larynx to about the middle of the thorax, where it divides into the two main
(primary) bronchi.
• The wall of the trachea consists of four definable layers:
1. Mucosa
2. Submucosa
3. Cartilaginous or muscle layer
4. Adventitia

43. In this specimen, the tracheal ring has been transformed, in part, to bone, a process that occurs in aging.

46. Trachea
• Tracheal Epithelium
- similar to respiratory epithelium in other parts of
the conducting airway.
1- Ciliated cells
- the most numerous of the tracheal cell types
- Each cell has approximately 250 cilia
- dark line formed by the aggregated ciliary basal
bodies
- In effect, the ciliated cells function as a “mucociliary
escalator”
• 2- Mucous cells = Goblet cells
- They are interspersed among the ciliated cells
- They are readily seen in the light microscope
- the number of mucous cells increases during chronic
irritation of the air passages.

48. • basal body associated
structures:
Transitional zone
- extends from the upper end of the
basal body into the axomene
• alar sheet (transitional
fiber)
• basal foot
• striated rootlet

49. Trachea
• Tracheal Epithelium
3- Brush cells
- They are columnar cells that bear blunt microvilli.
- The basal surface of the cells is in synaptic contact with an afferent nerve ending
(epitheliodendritic synapse).
- Thus, the brush cell is regarded as a receptor cell.

50. Trachea
4- Small granule cells (Kulchitsky cells)
- Enteroendocrine cells of the gut and gut derivatives
- usually occur singly in the trachea
- The nucleus is located near the basement membrane;
- cytoplasm exhibits numerous, membrane-bounded, dense-core granules.
- In one type of small granule cell, the secretion is a catecholamine.
- A second cell type produces polypeptide hormones such as serotonin, calcitonin, and
gastrin-releasing peptide (bombesin).
- Some are present in groups in association with nerve fibers, forming neuroepithelial bodies
Tracheal Epithelium

51. Trachea
5- Basal cells
- Basalcells tend to be prominent because their nuclei form a row in
close proximity to the basal lamina.
Tracheal Epithelium

53. Trachea
Tracheal Epithelium
- A thick “basement membrane” is characteristic of tracheal epithelium.
- approximately 25 to 40 µm thick.
- it consists of densely packed collagenous fibers an unusually thick and dense reticular lamina and, as
such, is part of the lamina propria.
• Basement Membrane

54. Trachea
Mucosa
- loose connective tissue. It is very cellular,
containing numerous lymphocytes, many of
which infiltrate the epithelium.
- Lymphatic tissue, in both diffuse and nodular
forms (BALT).
- Elastic membrane marks the boundary between
the lamina propria and submucosa.
• Lamina Propria

56. Trachea
submucosa
- is a relatively loose connective tissue similar
in appearance to the lamina propria
- Diffuse lymphatic tissue and lymphatic
nodules characteristically extend into this
layer from the lamina propria.
- The submucosa contains the larger distributing
vessels and lymphatics of the tracheal wall.
- Submucosal glands composed of mucus-
secreting acini with serous demilunes are also
present in the submucosa.
- The glands are especially numerous in the
posterior portion of the trachea.

59. Trachea
• The tracheal cartilages and trachealis muscle separate submucosa from adventitia.
• The tracheal cartilages, which number about 16 to 20 in humans, represent the next layer of the
tracheal wall.
• With age, the hyaline cartilage may be partially replaced by bone tissue, causing it to lose much of its
flexibility
• Adventitia:
- lies peripheral to the cartilage rings and trachealis muscle.
- contains the largest blood vessels and nerves that supply the tracheal wall,

62. Trachea and oesophagus

65. BRONCHOPULMONARY SEGMENTS

68. Main (primary) bronchi= bronchi
• can be identified by their cartilage
plates and a circular layer of smooth
muscle(circumferential layer).
• The smooth muscle becomes an
increasingly conspicuous layer as the
amount of cartilage diminishes.

69. Main (primary) bronchi
bronchi
• Mucosa
- pseudostratified epithelium with the same
cellular composition as the trachea.
• Muscularis
- a continuous layer of smooth muscle in the
larger bronchi.
- Regulates the appropriate diameter of the
airway.
• Submucosa
- loose connective tissue
- Glands are present as well as adipose tissue in the
larger bronchi.
• Cartilage layer
• Adventitia
- dense connective tissue
- that is continuous with that of adjacent structures,
such as pulmonary artery and lung parenchyma.

73. Bronchioles
• The bronchopulmonary segments are
further subdivided into pulmonary lobules;
each lobule is supplied by a bronchiole.
• Pulmonary acini
- are smaller units of structure that make up the
lobules.
- Each acinus consists of:
- terminal bronchiole
- respiratory bronchioles
- Alveoli
• Respiratory bronchiolar unit
- smallest functional unit of pulmonary
- It consists of :
- single respiratory bronchiole
- the alveoli

76. Bronchiolar Structure
• Bronchioles are air-conducting ducts
that measure 1 mm or less in diameter.
1. Larger-diameter bronchioles=
segmental bronchioles
2. Terminal bronchioles
3. Respiratory bronchioles
• Cartilage plates and glands are not
present in bronchioles.
• A relatively thick layer of smooth muscle
is present in the wall of all bronchioles.

78. 1. Segmental bronchioles
Primary bronchioles (Larger-diameter bronchioles)
- diameter of 1 mm or less.
- ciliated, pseudostratified columnar
epithelium that gradually transforms
into a simple ciliated columnar
epithelium
- Goblet cells
- form several terminal bronchioles
after entering the pulmonary
lobules.

80. 2. Terminal bronchioles
• diameter of less than 0.5 mm.
• ciliated simple cuboidal epithelium in
which Clara cells are interspersed
among the ciliated cells
• Occasional brush cells and small
granule cells are also present.
• A small amount of connective tissue
underlies the epithelium, and a
circumferential layer of smooth muscle

83. Terminal bronchioles
• Clara cells
- nonciliated cells
- rounded or dome-shaped apical surface projection.
- protein-secreting cells
- They have rER, a lateral or supra nuclear Golgi,
secretory granules, and numerous cisternae of Ser
• Function:
- secrete a surface-active agent, a lipoprotein
- secretory protein (CC16), which is an abundant
component of the airway secretion.
- differentiate to form ciliated cells.
- They secrete glycosaminoglycans.
- They metabolize airborne toxins, a process that is
carried out by cytochrome P450 enzymes in their
abundant smooth endoplasmic reticulum (SER).

86. surrounding alveoli

87. • are the first part of the bronchial tree that allows gas exchange.
• They are lined by a simple cuboidal epithelium that contains mostly Clara cells
and some ciliated cells
• where their walls are interrupted by alveoli, the sites where gas exchange
3. Respiratory bronchioles

92. Alveolar ducts
alveolar ducts

94. Alveolar ducts
• Alveolar ducts are linear passageways
continuous with the respiratory
bronchioles.
• Their walls consist of adjacent alveoli,
• which rim the openings of adjacent
alveoli and which often appear as knobs
in histological sections.
• Alveolar ducts are lined by type II
pneumocytes and the type I
pneumocytes.
alveolar ducts

95. Alveoli

96. Alveoli
• Alveoli are the site of gas exchange.
• Each alveolus is surrounded by a network of capillaries
• About 150 to 250 million alveoli are found in each adult lung;
• their combined internal surface area is approximately 75 m2, roughly the size of a
tennis court.
• alveolar septum or septal wall
• Alveolar epithelium
- is composed of type I and II alveolar cells and occasional brush cells.

97. Alveoli
• In section, they resemble a honeycomb
• Alveoli are separated by interalveolar septum
- lying between thin epithelial lining of two neighbouring alveoli
- contains anetwork of capillaries supported by reticular and elastic fibres, occassionally fibroblasts,
macrophages and mast cells.
- contain spores (ALVEOLAR PORES OF KOHN) help in passage of air from one alveolus to another,
thus equalizing Pressure in the alveoli
• Elastic fibres-enable the alveoli to expand during inspiration and passively contract during
expiration.
• Reticular fibres support and prevent overdistention of the alveoli

98. Respiratory bronchiole
Alveolar duct
Alveolar sac
Alveolar
pore
Dust cell
(macrophage)
Lamellar bodies
Type II pneumocyte

100. Alveoli

101. Cells in the Alveoli
• Type I Pneumocytes
• Type II Pneumocytes
• Macrophages or Dust cells

102. Alveolar cells
• cover about 95% of the alveolar surface
• form tight junctions with adjacent cells and form part of the blood-gas barrier
• where exchange of oxygen and carbon dioxide occurs.
• have an extremely thin cytoplasm that may be less than 80 nm thick
• may have phagocytic capabilities.
• are not able to divide.
• preventing leakage of blood from capillaries to the alveolar lumen
Type I pneumocytes (type I alveolar cells)

103. Alveolar cells
• Type II pneumocytes
- septal cells
- are cuboidal and are most often found near
septal wall.
- bulge into the alveolus and have a free surface
that contains short microvilli
- are able to divide and regenerate both types of
alveolar pneumocytes.
- form tight junctions with adjacent cells.
- synthesize pulmonary surfactant, which is
stored in cytoplasmic lamellar bodies.

106. Alveolar cells
• Alveolar macrophages (alveolar
phagocytes; dust cells)
- Derived from Monocytes and are part
mononuclear phagocytic system
- remove inhaled dust, bacteria, and other
particulate matter
- Cytoplasm contains phagocytosed inhaled
carbon and dust particles
- Inhaled carbon and dust particles are passed
on to them from pneumocyte I through
pinocytic vesicles

107. Alveolar cells
• Alveolar macrophages (alveolar
phagocytes; dust cells)
- migrate to the bronchioles after filling with
debris. From there, they are carried via ciliary
action to the upper airways,
- They phagocytize red blood cells that may enter
the alveoli in heart failure
- Main function is to clean the alveoli of invading
microorganisms and inhaled particulate
matter by phagocytosis

109. Surfactant
• decreases the alveolar surface tension and actively participates in the clearance of
foreign materials.
• dipalmitoylphosphatidylcholine (DPPC)
• synthesis in the fetus occurs after the 35th week of gestation and is modulated by a
variety of hormones, including cortisol, insulin, prolactin, and thyroxine.

110. • contains phospholipids, proteins and glycosaminoglycans
• Is constantly renewed.
• Removed from the surface by Type I pneumocytes and macrophages
• The reduced surface tension in the alveoli decreases the force that is
needed to inflate alveoli during inspiration.
• Therefore surfactant stabilizesthe alveolar diameters, facilitates
their expansion and prevents their collapse by minimizing the
collapsing forces

111. Surfactant proteins
• help organize the surfactant layer and modulate
alveolar immune responses.
• Surfactant protein A (SP-A)
- the most abundant surfactant protein.
- is responsible for surfactant homeostasis and it also
modulates immune responses to viruses, bacteria, and
fungi.
• Surfactant protein B (SP-B)
- For the transformation of the lamellar body into the
thin surface film of surfactant.
• Surfactant protein C (SP-C)
- represents only 1%
- Along with SP-B, SP-C aids in orientation of
DPPC
• Surfactant protein D (SP-D)
- a primary protein involved in host defense.

112. Alveolar septum
• Air–blood barrier
• The thinnest air–blood barrier consists
Of:
- (a) layer of surfactant lining the alveolar airspace
- (b) Type I pneumocytes and Fused basal laminae of
type I pneumocytes and capillary endothelial cells
- (c) Endothelium of the continuous capillaries within
the interalveolar septum
- Is responsible for most of the gas exchange
• Thick portion:
- Connective tissue cells and fibers that may be present
between the two basal laminae
- macrophage in the thick portion that extends its
processes into the lumen of the alveolus.
- plays an important role in fluid distribution

119. pulmonary circulation

121. Blood supply of lung
• Bronchial arteries (branches of descending thoracic
aorta)….. supply oxygenated blood to bronchi , lung
tissue & visceral pleura.
• Bronchial veins : drain into azygos & hemiazygos
veins.
• Pulmonary artery which carries non-
oxygenated blood from right ventricle to the lung
alveoli.
• 2 pulmonary veins : carry oxygenated blood from
lung alveoli to the left atrium.

122. Lymph drainage of the lungs
• There are 2 lymphatic plexuses…. superficial & deep plexuses.
• Superficial plexus (subpleural): lies under the visceral pleura and drain to
bronchopulmonary nodes in the hilum of lung.
• Deep plexus:
• Lies along the bronchial tree & pulmonary blood vessels and drain into the pulmonary nodes
within the lung substance.
• Then into bronchopulmonary nodes in the hilum of lung.
• Then into the tracheo-bronchial nodes at the bifurcation of trachea , and finally into broncho-
mediastinal lymph trunks to end in thoracic duct (left) or in right lymphatic duct (right).

124. Nerve Supply of the lung
• Pulmonary plexus :
- at the root of lung
- is formed of autonomic N.S. from
sympathetic & parasympathetic
fibres.
1- Sympathetic F.
- from sympathetic trunk
- broncho-dilatation
- vasoconstriction.
2- Parasympathetic F.
-fromVagus nerve
- Broncho-constriction
- secretomotor to bronchial glands
- vasodilatation.