Histology of the Respiratory System, Alveoli and Cells, UAG School of Medicine

Histologic Characteristics and Functions

Conducting Portion

• Conducting portion, nose, paranasal sinuses, pharynx, larynx, trachea, bronchi, bronchioles, terminal bronchioles.

Respiratory Portion

• Respiratory bronchioles, alveolar ducts, pulmonary alveoli, identify the components of the air-blood barrier.

Respiratory System Components

Functionally the respiratory system has 2 components:

Conducting Portion

-cleans and humidifies inspired air
-provides conduits for air movement to
and from alveoli
-support, flexibility and extensivility:
cartillage, collagen, elastic fibers,
smooth muscle

Nasal cavity, pharynx, larynx, trachea,
bronchi, bronchioles and terminal
bronchioles.

Respiratory Portion

Alveoli
-sites of exchange of 02 and C02
between inspired air and blood
-small, air-filled, saclike structures

Respiratory
bronchioles,
alveolar
ducts and alveoli.

Respiratory Epithelium

Nasal Cavity and Conducting Portion

Ciliated pseudostratified columnar epithelium

5 major cell types:

• Ciliated Columnar
  Apical surface
  Move particles and mucus
• Goblet
  Filled with granules of mucin
  glycoproteins
  Secrete mucus
• Brush
  Chemosensory receptors
• Small granule (or
  Kulchitsky)
  Part of the neuroendocrine
  system
  Monitor oxygen and carbon dioxide in
  the lumen of the airway
• Basal
  Mitotically active stem and progenitor cells that give rise to other cells

LM of respiratory mucosa lining
the nasal cavity.
basal cells (B)
goblet cells (G)
columnar cells
bearing apical cilia (short arrow)
Particulate matter (long arrow) on
the ciliated surface.
Capillaries (C)
Basement membrane (BM) 450x.
H&E.
all of which
contact an
unusually thick
basement
membrane

The respiratory epithelium has:
epithelium.
Mucus secreting:
.

The respiratory epithelium has 5 cell types and they
all rest on a thick
.

Lumen
Mucus
Goblet
cell
Ciliated Basal Small granule Brush
cell
cell
cell
cell
Nerve
ending

1. Brush cells have
   receptors.
2. Small granule cells are also called
3. cells give rise to other epithelial cells.

Olfactory Epithelium

Pseudostratified columnar epithelium
olfatory
chemoreceptors
for the sense of
smell

3 major cell types:

• Olfactory neurons
  Apical surface
  Transmembrane chemoreceptors: respond to
  odoriferous substances and send signals to the brain.
• Supporting cells
  Help maintain microenvironment.
• Basal cells
  Stem cells for the other 2 types - replace the olfactory neurons every 2-3
  months and support cells less frequently.

Lamina propria: olfactory glands (of Bowman) - produce a constant flow of fluid
surrounding the olfactory cilia and facilitating the access of new odoriferous
substances.

Olfactory epithelium
Bowman's gland
olfactory nerve fibre
lamina
propria
basal cell
olfactory
epithelium
olfactory
receptor cell
cilia
supporting cell
lumen of nasal cavity
odour molecules

Paranasal Sinuses

Frontal sinus
Ethmoid sinus
Maxillary sinus
Sphenoid sinus

Respiratory epithelium
-Goblet cells
-Communicate with the nasal cavities
through small openings
-Mucus produced is moved into the nasal
passages via ciliated epithelial cells.

Pharynx

-Nasal cavities open posteriorly into the nasopharynx
-Nasopharynx is continuous caudally with the oropharynx
-Posterior part of the oral cavity leads to the larynx and
esophagus
Nasopharynx:
Respiratory epithelium
Oropharynx: stratified squamous epithelium

Larynx

Epiglottis
Anterior commissure
False cords
Cricoid ring
True cords
Posterior cartilages
Interaretynoid notch
Tongue
Epiglottis
Supraglottis
Vocal cord
Glottis
Subglottis
Esophagus
Trachea

Epiglottis

Upper, or lingual, surface: stratified squamous epithelium
Ciliated pseudostratified columnar (respiratory) epithelium
Mixed mucous and serous glands are found in the lamina
propria beneath the epithelium.

Vestibular Folds

respiratory epithelium
overlying numerous seromucous glands and occasional
lymphoid nodules.

Vocal Folds

nonkeratinized stratified squamous epithelium that protects the mucosa from
abrasion and desiccation from rapid air movement.

Larynx
Hyoid bone
Epiglottis
Vestibular fold
(false vocal cord)
Thyroid cartilage
Vocal fold
(true vocal cord)
Cricoid cartilage
Tracheal cartilages
(b) Sagittal view; anterior surface to the right
Squamous
Respiratory
Vocal
Cord

Trachea

-Lamina propria (LP): has numerous seromucous glands (G) producing
watery mucus

4 layers:
Mucous membrane
(respiratory mucosa)
Hyaline
cartilage ring
Adventitia

-Adjacent to the submucosa: C-shaped rings of hyaline cartilage (C) that
reinforce the wall and keep the tracheal lumen open.
-Lined with
respiratory epithelium (RE)

Histologic Features of the Upper Respiratory Tract, Larynx, and Trachea

Vestibules of Nasal Cavities

Stratified squamous,
keratinized to
nonkeratinized
Sebaceous and sweat
glands
Hyaline cartilage
Vibrissae (stiff hairs) and
moisture both filter and
humidify air

Most Areas of Nasal Cavities

Respiratory
Seromucous glands
Bone and hyaline cartilage
Rich vasculature and glands
warm, humidify, and clean air

Superior Areas of Nasal Cavities

Olfactory, with bipolar
neurons
Serous (Bowman) glands
Bone (ethmoid)
Solubilize and detect odorant
molecules in air

Nasopharynx and Posterior Oropharynx

Respiratory and
stratified squamous
Seromucous glands
Bone and skeletal muscle
Conduct air to larynx;
pharyngeal and palatine tonsils

Larynx

Respiratory and
stratified squamous
Mucous glands, smaller
seromucous glands
Elastic and hyaline
cartilage, ligaments,
skeletal muscle
Site for phonation; epiglottis
closes while swallowing

Trachea

Respiratory
Mainly mucous glands,
some serous or mixed
glands
C-shaped rings of hyaline
cartilage, with smooth
(trachealis) muscle in
posterior opening of each
Conduct air to primary
bronchi entering lungs; some
MALT

Bronchial Tree and Lung

The Airways

Trachea
Conducting zone
(anatomical dead space)
Primary bronchi
(left and right)
Secondary (lobar) bronchi
(2 left and 3 right)
Tertiary (segmental)
bronchi
(10 right, 8 left)
Terminal bronchioles
(<1 mm)
Respiratory bronchioles
(<0.5 mm diameter)
Respiratory zone
(ascini)
Alveolar ducts
(n.b. lined with alveoli)
Alveolar sacs
Alveoli

Bronchi

Respiratory epithelium (E)
Large
-Respiratory epithelium and mucosa fold due to
contraction of its smooth muscle (SM)
-Surrounded by hyaline cartilage (C)
- Contains seromucous glands (G) which drain
into the lumen.

Bronchioles

They lack
suppporting
cartilage and
glands!

Largest Bronchioles

-Folded ciliated pseudostratified columnar (E)
-Smooth muscle
-Supported by connective tissue (CT)

Terminal Bronchioles

-Simple cuboidal epithelium (E)
Consists of: club cells (C) or bronchiolar exocrine cells (previously called
Clara cells) with nonciliated, dome-shaped apical ends containing secretory
granules
They secrete components
of surfactant which
reduces surface tension and
helps prevent collapse of
the bronchioles.

Alveoli

-Give spongy structure to the lungs
-Gas exchange
-Alveolar septa: connective tissue
-Alveolar pores (of Kohn): penetrate the interalveolar septa and
connect neighboring alveoli that open to different bronchioles.

Alveolar
connective
tissue
Nucleus of capillary
endothelial cell
Pulmonary
capillaries
Nucleus
of alveolar
type I cell
Erythrocyte
Capillary
Alveolar
type I cell
Alveolar
type II cell
Diffusion of CO2 Diffusion of O2
Alveolar
macrophages
Alveolus
Alveolar pores
Respiratory
membrane
Alveolar epithelium
Fused basement membranes
of the alveolar epithelium and
the capillary endothelium
Capillary endothelium
Interalveolar
septum
Erythrocyte

• Type I alveolar cells (or type I pneumocytes): thin and flat squamous cells (I).
  Gas exchange.
• Type II alveolar cells (or type II pneumocytes or septal cells): cuboidal cells (II). They produce surfactant.

Lung H&E
capillaries
alveolus
alveolar type II
cell
nucleus of
alveolar type I
cell

• Alveolar macrophages or dust cells (M): phagocytosis.

1. Capillaries
2. Great alveolar cell
   (type Il pneumocyte)
   type
   alveolar cell
3. Alveoli
   type II
   alveolar cell
4. Interalveolar septa
5. Great alveolar cell
   (type Il pneumocyte)
6. Alveolar macrophages
   (dust cells)
7. Alveolar cells
   (type | pneumocytes)
8. Smooth muscle fibers

Features of Airways within the Lungs

Bronchi

Respiratory
Prominent spiral bands of smooth Repeated branching; conduct air
muscle; irregular hyaline cartilage
deeper into lungs
plates

Bronchioles

Simple ciliated cuboidal to
columnar, with exocrine club cells
muscle; no cartilage
Prominent circular layer of smooth Conduct air; important in
bronchoconstriction and
bronchodilation

Terminal Bronchioles

Simple cuboidal, ciliated cells and
club cells
Thin, incomplete circular layer of
smooth muscle; no cartilage
Conduct air to respiratory portions of
lungs; exocrine club cells with several
protective and surfactant functions

Respiratory Bronchioles

Simple cuboidal, ciliated cells and
club cells, with scattered alveoli
Fewer smooth muscle fibers,
mostly around alveolar openings
Conduct air deeper, with some
gas exchange, and protective and
surfactant functions of club cells

Alveolar Ducts and Sacs

Simple cuboidal between many
alveoli
Bands of smooth muscle around
alveolar openings
Conduct air, with much gas exchange

Alveoli

Types I and II alveolar cells
(pneumocytes)
None (but with network of elastic
and reticular fibers)
Sites of all gas exchange; surfactant
from type II pneumocytes; dust cells