Hsitology of Alveoli

							                  Objectives
Define the lower respiratory tract, Respiratory
 Portion & different parts of respiratory portion
Mention the detail histological features of
 respiratory bronchioles
Describe the histological features of alveolar duct,
 atrium and alveolar sac
Discuss the histology and type cells of alveoli
Mention the inter-alveolar septum and blood air
 barrier
Mention the clinical correlation related to
 respiratory system
Lower Respiratory System
Respiratory Portion of Respiratory System
The terminal
   bronchiole is 1 mm
   in diameter and last
   part of conducting
   portion of
   respiratory system.
It divided into three
   respiratory
   bronchioles these
   are actually
   beginning of
   respiratory portion
   of respiratory system
             Respiratory Portion
The respirator
  system
  consists of
Respiratory
  Bronchioles
Alveolar Ducts
Atria
Alveolar Sacs
Alveoli
             Respiratory Bronchioles
The epithelium of the
   respiratory bronchioles is
   devoid of goblet cells and
   largely consists of ciliated
   cuboidal cells and smaller
   numbers of non-ciliated
   cells called Clara cells.
   Respiratory bronchioles
   also contain a small
   number of single alveoli
   in their walls
In the most distal part of the
   respiratory bronchioles.
   Clara cells become the
   predominant cell type.
                     Clara Cells
Clara cells have three
  functions:
They produce one of the
  components of
  surfactant.
They act as stem cells,
  i.e. they are able to
  divide, differentiate
  and replace other
  damaged cell types.
They contain enzyme
  systems which can
  detoxify noxious
  substances.
            Respiratory Bronchioles
The walls respiratory
  bronchiole in distal portion
  contain increasing number
  of alveoli and the distance
  between them is markedly
  reduced. Between alveoli,
  the bronchiolar epithelium
  consists of ciliated cuboidal
  epithelium while the cilia
  may be absent in more
  distal portions. Smooth
  muscle and elastic
  connective tissue lie
  beneath the epithelium of
  respiratory bronchioles.
                  Alveolar Ducts
Distal to respiratory
  bronchioles the number of
  alveolar openings into the
  bronchiolar wall becomes
  more and the entire the
  wall consists of alveoli,
  now this tube is as called
  an alveolar duct. Both the
  alveolar ducts and the
  alveoli are lined with
  extremely attenuated
  squamous alveolar cells.
                    Alveolar Ducts
In the lamina propria
   surrounding the rim of
   the alveoli is a network
   of smooth muscle cells.
   These sphincter like
   smooth muscle bundles
   appear as knobs
   between adjacent
   alveoli. Smooth muscle
   disappears at the distal
   ends of alveolar ducts. A
   rich matrix of elastic and
   reticular fibers provides
   the only support of the
   duct and its alveoli.
             Atria & Alveolar Sacs
Alveolar ducts open
  into atria that
  communicate with
  alveolar sacs, two
  or more of which
  arise from each
  atrium. Elastic and
  reticular fibers form
  a complex network
  encircling the
  openings of atria,
  alveolar sacs, and
  alveoli.
               Role of elastic Fibers
The elastic fibers
  enable the alveoli to
  expand with
  inspiration and to
  contract passively
  with expiration. The
  reticular fibers serve
  as a support that
  prevents over
  distention and
  damage to the
  delicate capillaries
  and thin alveolar
  septa.
Alveoli are saclike
   evaginations (about 200
   mm in diameter) and            Alveoli
   present in respiratory
   bronchioles, alveolar
   ducts, and alveolar sacs.
Alveoli are responsible for
   the spongy structure of the
   lungs. Structurally, alveoli
   resemble small pockets
   that are open on one side.
Within these cuplike
   structures, O2 and CO2 are
   exchanged between the air
   and the blood. The
   structure of the alveolar
   walls is specialized for
   enhancing diffusion
   between the external and
   internal environments.
       Interalveolar Septum, Or Wall.
Generally, each wall lies
  between two neighboring
  alveoli and is therefore called
  an interalveolar septum, or
  wall.
An interalveolar septum
  consists of two thin
  squamous epithelial layers
  between which lie capillaries,
  elastic and reticular fibers,
  and connective tissue matrix
  and cells. The capillaries and
  connective tissue constitute
  the interstitium. Within the
  interstitium of the
  interalveolar septum is found
  the richest capillary network
  in the body.
                  Blood Air Barrier
Air in the alveoli is separated
   from capillary blood by three
   components referred to
   collectively as the blood air
   barrier:
Surface lining and cytoplasm of
   the alveolar cells
Fused basal laminae of the
   closely apposed alveolar and
   endothelial cells
Cytoplasm of the endothelial
   cells
The total thickness of these
   layers varies from 0.1 to 1.5
   mm.
Components of Inter-alveolar Septum
Within the interalveolar
  septum, anastomosing
  pulmonary capillaries are
  supported by a meshwork
  of reticular and elastic
  fibers.
These fibers are helpful for
  the expasion and
  contraction of the
  interalveolar septum they
  are the primary means of
  structural support of the
  alveoli.
Components of Inter-alveolar Septum
The basement membrane,
  leukocytes,
  macrophages, and
  fibroblasts can also be
  found within the
  interstitium of the
  septum. The fusion of
  two basal laminae
  produced by the
  endothelial cells and the
  epithelial (alveolar) cells
  of the interalveolar
  septum forms the
  basement membrane.
           Capillary Endothelial Cells
Capillary endothelial cells are
  extremely thin. The
  endothelial lining of the
  capillaries is continuous and
  not fenestrated. Clustering of
  the nuclei and other
  organelles allows the
  remaining areas of the cell to
  become extremely thin,
  increasing the efficiency of gas
  exchange. The most
  prominent feature of the
  cytoplasm in the flattened
  portions of the cell is
  numerous pinocytotic vesicles.
                    Alveolar Cell Types
Each alveolus lines by two types of
   cells
Type I cells, or squamous alveolar
   cells most abundant 97 %
Type II cells scanty 3%
Type I cells, or squamous alveolar
   cells These cells are so thin
   (sometimes only 25 nm) that the
   electron microscope was needed
   to prove that all alveoli are
   covered with an epithelial lining.
Organelles such as the Golgi complex,
   endoplasmic reticulum, and
   mitochondria are grouped around
   the nucleus, reducing the
   thickness of the blood air barrier
   and leaving large areas of
   cytoplasm virtually free of
   organelles.
Type I cells, or squamous alveolar cells
The cytoplasm in the thin
  portion contains abundant
  pinocytotic vesicles, which
  may play a role in the
  turnover of surfactant and
  the removal of small
  particulate contaminants
  from the outer surface.
Types of junctions present
  between the type I cells are
  desmosomes and occluding
  junctions that prevent the
  leakage of tissue fluid into the
  alveolar air space. The main
  role of these cells is to
  provide a barrier of minimal
  thickness that is readily
  permeable to gases.
                       Type II cells
Type II cells present among
  the type I alveolar cells
  with which they have
  occluding and
  desmosomal junctions.
Type II cells are rounded cells
  that are usually found in
  groups of two or three
  along the alveolar surface
  at points at which the
  alveolar walls unite and
  form angles.
                     Type II cells
These cells, which rest
  on the basement
  membrane, are part
  of the epithelium,
  with the same origin
  as the type I cells
  that line the alveolar
  walls.
They divide by mitosis
  to replace their own
  population and also
  the type I population.
                  Lamellar Bodies
Histochemical studies show
  that these bodies, which
  contain phospholipids,
  glycosaminoglycans, and
  proteins, are continuously
  synthesized and released
  at the apical surface of the
  cells. The lamellar bodies
  give rise to a material that
  spreads over the alveolar
  surfaces, providing an
  extracellular alveolar
  coating, pulmonary
  surfactant, that lowers
  alveolar surface tension.
                  Lamellar Bodies
In histological sections, type
   II cells exhibit a
   characteristic vesicular or
   foamy cytoplasm. These
   vesicles are caused by the
   presence of lamellar
   bodies that are preserved
   and evident in tissue
   prepared for electron
   microscopy. Lamellar
   bodies, which average 1to
   2 nm in diameter, contain
   concentric or parallel
   lamellae limited by a unit
   membrane.
       Respiratory Distress Syndrome
The respiratory distress
  syndrome of the newborn is
  a life-threatening disorder of
  the lungs caused by a
  deficiency of surfactant. It is
  principally associated with
  prematurity and is the
  leading cause of mortality
  among premature infants.
  The incidence of respiratory
  distress syndrome varies
  inversely with gestation age.
  The immature lung is
  deficient in both the amount
  and composition of
  surfactant.
  Expansion of Lungs in Normal Baby
In the normal newborn, the
   onset of breathing is
   associated with a
   massive release of stored
   surfactant, which
   reduces the surface
   tension of the alveolar
   cells. This means that
   less inspiratory force is
   needed to inflate the
   alveoli, and thus the
   work of breathing is
   reduced.
     Expansion of Lungs in RDS Baby
In the respiratory distress
   syndrome the alveoli are
   collapsed, and the
   respiratory bronchioles and
   alveolar ducts are dilated
   and contain edema fluid. A
   fibrin-rich eosinophilic
   material called hyaline
   membrane lines the
   alveolar ducts. This
   explains why respiratory
   distress syndrome was
   initially named hyaline
   membrane disease.
   Management of Respiratory Distress Syndrome
Synthesis of surfactant can be
  induced by administration
  of glucocorticoids, a
  medication used in cases
  of respiratory distress
  syndrome. Recently,
  surfactant has also been
  suggested to have a
  bactericidal effect, aiding
  in the removal of
  potentially dangerous
  bacteria that reach the
  alveoli.
                     Emphysema
Emphysema is a chronic
  lung disease
  characterized by
  enlargement of the
  air space distal to the
  bronchioles, with
  destruction of the
  interalveolar wall.
  Emphysema usually
  develops gradually
  and results in
  respiratory
  insufficiency.
Features    Trachea    Bronchus Tertiary      Bronchiole Respiratory
                                   bronchus             bronchiole
Epithelium Pseudostrati Pseudostrat Pseudostra Columnar Cuboidal
           fied         ified       tified
Goblet      +++        ++          ++         +         Absent
cells
Clara     Absent       Absent      Absent     +         +
cells
Musculari Absent       +           ++         +++       +++
s mucosae
Elastic     +          +           ++         ++        +++
Fibers
Mucous      +++        ++          +          Absent    Absent
glands
Cartilage   +++        ++          +          Absent    Absent
Alveoli     Absent     Absent      Absent     Absent    Squamous