Download Developmental Anatomy of the Respiratory system

Developmental Anatomy of the Respiratory system
<< Mechanics of respiration
Since the embryo develops in a craniocaudal (head- to- tail) direction, the upper respiratory
structures (nose and pharynx) appear before the lower ones. By week 4, a thickened plate of
ectoderm called the Olfactory placode (= plate) has appeared on each side of the future face.
These placodes quickly invaginate to form
olfactory
pits
that
form the future nasal cavity. The nasal cavity then connects with the future Pharynx of the
developing foregut.
The lower respiratory organs develop from a tubular outpocketing off the foregut called the the
Respiratory diverticulum
or the
laryngeotracheal bud
. As the respiratory diverticulum elongates, its distal end enlarges to form a globular tracheal
bud, which gives rise to the trachea. Soon after, the tracheal bud divides into bronchial buds,
which branch repeatedly and develop with the bronchi.
By 24 weeks, 17 orders of branches have formed and respiratory brochioles have developed.
During weeks 6 - 16, all major elements of the lungs have been formed, except for those
involved in gaseous exchange (i.e. respiratory bronchioles, alveolar ducts, and alveoli).
During weeks 16 - 26, the lung tissue becomes highly vascular, and respiratory bronchioles,
alveolar ducts, and some of the primitive alveoli develop.
From 26 weeks to birth, many more primitive alveoli develop; they consist of type I alveolar
cells (main site of gaseous exchange) and
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Developmental Anatomy of the Respiratory system
type II
surfactant-producing cells (
surfactant is necessary to lower surface tension of alveolar fluid and thus reduce the tendency
of alveoli to collapse on exhalation
) . Blood capillaries also establish close contact with the primitive alveoli.
Although surfactant production begins by 20 weeks, it is present only in small quantities.
Amounts sufficient to permit survival of a premature infant are not produced until 26 - 28 weeks
of gestation. It is not until 26 - 30 weeks that a prematurely born baby can survive and breathe
on its own. Infants born before 26 - 28 weeks are at high risk of Infant Respiratory Distress
Syndrome
(IRDS), in which the
alveoli collapse ( due to lack of surfactant) during exhalation and must be reinflated during
inhalation.
At about 30 weeks, mature alveoli develop. However, it is estimated that only about one-sixth of
the full complement of alveoli develop before birth; the remainder develop after birth during the
first eight years of life.
All the respiratory tubes are lined by endoderm and covered by splanchnic mesoderm. The
endoderm becomes the lining epithelium and glands of the trachea, brochial tree, and the
alveoli. The splanchnic mesoderm, by contrast, gives rise to all other layers of the tracheal and
bronchial walls ( including cartilage and smooth muscle) and to the
stroma
(substance) of the lungs.
As the lungs develop, they acquire their pleural sacs. The visceral pleura and the parietal pleura
develop from
Mesoderm. The space
between the pleural layers is the
pleural cavity.
During fetal life, the lungs are filled with fluid, and all respiratory exchanges occur across the
placenta.At birth, the first breaths bring air to the deflated lungs, and the alveoli inflate and begin
to function. However, it takes nearly two weeks for the lungs to become fully inflated. At birth,
only one-sixth of the final number of lung alveoli are present. The lungs continue to mature
throughout childhood, and more alveoli are formed until young adulthood.
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Developmental Anatomy of the Respiratory system
CONGENITAL DEFECTS OF THE RESPIRATORY SYSTEM
Other than the Infant respiratory distress syndrome (IRDS), these defects also include
Agenesis of lungs, lung hypoplasia, and cystic fibrosis.
Agenesis of lungs: It results from failure of the bronchial buds to develop. Agenesis of one
lung is more common than bilateral agenesis, but mboth conditions are rare. Unilateral
pulmonary agenesis is compatible with life. The heart and other mediastinal structures are
shifted to the affected side and the existing lung is hyperexpanded.
Lung hypoplasia: In infants with congenital diaphragmatic hernia, the lung is unable to develop
normally because it is compressed by the abnormally positioned abdominal viscera. Lung
hypoplasia is characterised by a markedly reduced lung volume. Most infants with congenital
diaphragmatic hernia die of pulmonary insufficiency, despite optimal postnatal care, because
their lungs are too hypoplastic to support extrauterine life.
Cystic fibrosis is an inherited disease in which the function of exocrine glands are disrupted
throughout the body. Cystic fibrosis affects the respiratory system by causing an oversecretion
of a viscous mucus by the bronchial glands. This mucus clogs the respiratory passageways and
predisposes the child to early death through repeated respiratory infections. Cystic fibrosis
accounts for about 5% of childhood deaths.
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