Each person is born to one possession which
outvalues all his others — his last breath.
Mark Twain
Pharynx and Respiratory
Development
This chapter will introduce you to the key structures and systems that arise
from the endodermal linings of the embryonic pharynx. The pharyngeal
pouches and the floor of the pharynx give rise to numerous glands, tonsils,
and the respiratory system. These structures will arise as endodermal
outgrowths of the pharynx that associate with surrounding mesenchyme and
neural crest.
• You should be familiar with the anatomy of the respiratory system
and the glands and tonsils of the phryngeal and oral cavities.
• Understand which pharyngeal pouches contribute to the formation
of the various glands and tonsils.
• Understand the process that accounts for the development of the
respiratory passageways and their relationship to the gut tube.
• Understand which glands have ectodermal origins and which glands
have endodermal origins.
91
Anatomy Review
Review the basics of the digestive system
Topography of the pharynx and associated glands
Note location of glands
Relation of esophagus and pharynx
Associated structures
Tonsils
Thyroid gland
Parathyroid gland
Thymus
Salivary glands
Basic structure of the respiratory tubes
Understand the different tunics and their tissues
Organs of the respiratory system
Larynx
Trachea
Bronchial tree
Alveolar tubes
92
R e s p i r a t o r y
D e v e l o p m e n t
Derivatives of the Pharynx
Germ layers of origin
Ectoderm
Mesenchyme
Endoderm
Review of the Pharyngeal Pouches
Endodermally lined infoldings of the pharynx
Pouch 1
Pouch 2
Pouch 3
Pouch 4
93
Derivatives of the Pharyngeal Pouches
First pharyngeal pouch
Pharyngotympanic tube and tympanic cavity
Second pharyngeal pouch
Intratonsillar cleft
Tonsillar buds – tonsillar crypts
Palatine tonsils
Third pharyngeal pouch
Dorsal bulbar portion
Parathyroid III (inferior)
Placodal origin?
Ventral elongation
Thymus gland
Fourth pharyngeal pouch
Dorsal bulbar portion
Parathyroid IV (superior)
Placodal origin?
Ventral elongtion
Ultimobranchial body
Calcitonin producing C-cells of the thyroid gland
Neural crest origin
Fifth pharyngeal pouch
Possibly the ultimobranchial body is annexed from this arch
E m b r y o l o g y
94
L e c t u r e
M a n u a l
b y
M a r k
N i e l s e n
R e s p i r a t o r y
D e v e l o p m e n t
Review of pouch development
Other Stomodeal and Pharyngeal Derivatives
Thyroid gland
Thyroid diverticulum
Thyroglossal duct
Foramen caecum
Thyroid gland
Salivary glands
Parotid gland
Ectomermal invagination of stomodeum
Submandibular gland
Endodermal invagination
Sublingual gland
Endodermal invagination
95
Respiratory System Development
Early Respiratory Passageways
Laryngotracheal groove in endodermal floor of the pharynx
Laryngotracheal diverticulum
Lung bud
Tracheoesophageal septum
Endoderm contribution
Mesenchyme contibution
E m b r y o l o g y
96
L e c t u r e
M a n u a l
b y
M a r k
N i e l s e n
R e s p i r a t o r y
D e v e l o p m e n t
Larynx and Trachea Formation
Larynx
Arytenoid swellings
Epiglottal swelling
Other cartilages
Trachea
Endoderm
Mesenchyme
Tracheal rings
Fibromuscular membrane
97
Bronchial Tree and Lungs
Bronchial buds
Lobes
Right superior lobe
Right middle lobe
Right inferior lobe
Left superior lobe
Left inferior lobe
Bronchopulmonary segments within lobes
Histological changes in lung development
Pseudoglandular period
Weeks 5 to 17
Canalicular period
Weeks 16 to 25
Terminal sac period
Weeks 24 to birth
Alveolar period
Late fetal period to 8 years of age
E m b r y o l o g y
98
L e c t u r e
M a n u a l
b y
M a r k
N i e l s e n