Note: The respiratory system consists of: nasal cavity, pharynx, larynx, trachea and lungs. The lining
of the nasal cavity is derived from ectoderm; the lining of the rest of the respiratory system is
derived from endoderm.

Nasal Cavity Development (Summary) — details are presented under Face Development
• initially, bilateral nasal placodes (olfactory placodes) appear; the placodes are ectoderm
thickenings at the ventral tip of the frontonasal prominence
• placodes become nasal pits, by outward growth of the surrounding medial and lateral nasal processes
of the frontonasal prominence
• continued outgrowth of medial & lateral nasal processes elongates the nasal pits and transforms
them into a nasal cavity
• right and left medial nasal processes fuse to form a primary palate (incisive bone & rostral upper lip)
and a nasal septum; lateral nasal processes become nose cartilage and nasal & lacrimal bones
• formation of the secondary palate divides a common naso-oral space into three separated cavities
(right & left nasal cavities and the oral cavity); also, the secondary palate ultimately divides
the pharynx into three compartments (nasopharynx, oropharynx, & laryngopharynx)
• conchae (scrolls of thin bone covered by mucosa) arise as cartilaginous ridges from bones of the
nasal cavity wall
• paranasal sinuses (diverticula of the nasal cavity) develop postnatally

Laryngotrachealgroove — larynx, trachea & lungs all originate from the laryngotracheal groove
• the laryngotracheal groove originates from the floor of the pharynx as an evagination of endoderm
• ridges grow medially from lateral walls of the laryngotracheal groove; the ridges fuse, establishing
a tracheoesophageal septum
• the septum separates a laryngotracheal tube (future trachea & lung buds) from the esophagus
• rostrally, the lumen of the groove retains communication with the pharynx (the future larynx)

Larynx — develops where the laryngotracheal groove communicates with the pharynx
• bilateral laryngeal swellings (future cricoid, thyroid, & arytenoid cartilages) and a rostral epiglottal
swelling form the wall of the larynx
• a bilateral diverticulum of the lateral laryngeal wall produces a lateral ventricle & vocal fold
(except in cats & cattle)
• epithelium lining the larynx is derived from endoderm; cartilages of the laryngeal wall come from
splanchnic mesoderm of pharyngeal arches IV & VI
• laryngeal muscles develop from somite myotomes that migrate into pharyngeal arches:
IV (cricothyroideus m., innervated by the cranial branch of the vagus n.) &
VI (other intrinsic laryngeal muscles, innervated by the vagal recurrent laryngeal n.)

Trachea and Bronchi — lined by respiratory epithelium derived from endoderm
• the laryngotracheal tube grows caudally, ventral to the pharynx, into splanchnic mesoderm that
contributes cartilage and connective tissue to the developing trachea
• tracheal elongation shifts the future bronchi caudally into the thorax; the blind, caudal end of the
growing trachea develops bi-lobed bronchial buds that become the right & left principal bronchi
• outgrowths from each principal bronchus become future lobar bronchi, each lobar bronchus gives rise
to outgrowths that become future segmental bronchi, each segmental bronchus gives rise
to more than a dozen additional levels of bronchial branches
• final branches of the bronchial tree are terminal bronchioles (which branch into respiratory
bronchioles in carnivores); alveolar sacs and alveoli arise from terminal (or respiratory)
bronchioles
Note: Lung lobation is defined by lobar bronchi. In domestic mammals generally, the right principal
bronchus divides into four lobar bronchi (to cranial, middle, caudal & accessory lobes),
while the left gives rise to two lobar bronchi (to cranial & caudal lobes). However, there are
right side exceptions: the horse lacks a middle lobe, humans lack an accessory lobe, and
in ruminants and swine the right cranial lobe is supplied by a tracheal bronchus.

Lungs — formation of new alveoli occurs post-natally to a considerable extent in all mammals
• the endoderm-lined bronchial tree grows into splanchnic mesoderm; the mesoderm forms the
cartilage, fascia, smooth muscle, vessels, and the visceral pleura coating of the lung
• initially, bronchiole-lung branches are solid cores of cells growing into mesoderm similar to the
way exocrine glands grow, eventually the branches must become hollow
• continued branching of the bronchial tree results in lung tissue occupying more and more of the
pleural cavity, resulting in pleural cavity excavation and expansion into somatic mesoderm of
the body wall
• terminal bronchiole branches give rise to alveolar sacs, featuring endoderm as a thin epithelium;
finally, alveoli are created by the formation of septae that partition the alveolar sacs
• some endodermal alveolar cells become cuboidal rather than flat and produce a phospholipid
surfactant that reduces surface tension and thus facilitates alveolar expansion
(as opposed to alveolar collapse)
• bronchial branching continues as lungs enlarge throughout the fetal period and even into the
postnatal period; species differ in degree of lung maturity at birth; also, within a single lung,
distal regions are less mature than proximal ones
• subsequent to post-natal production of new alveoli, lung growth is attained by hypertrophy
(increased size) of alveoli and air passageways
• during fetal development lungs are filled with fluid and non-functional, although breathing
movements that take place in utero to prepare for postnatal respiration; at birth,  
lung fluid drains or is absorbed as air is breathed