Thymus Embryology
The pharyngeal glandular organs have complex developmental origins.
Thymus,
thyroid,
parathyroids,
and ultimobranchial bodies,
develop and migrate concurrently from their sites of origin to their final positions along the ventral midline of the pharyngeal and upper thoracic region.
The thymus originated from a ventral sacculation of the 3rd (and probably 4th) pharyngeal pouch on each side,
during the 6th week of gestation.
Endodermal cells proliferation within the sacculation gives rise to a thymic primordium (ectodermal and mesenchymal elements are further incorporated).
(Fig 1)
The thymic primordia elongates into tubular structures,
by caudal growth,
forming the thymopharyngeal duct.
(Fig 1)
Fig. 1: Fig 1.: Embryological development of the thymus
(Images modified from Manley, NR et al. Developmental Biology (1998) 195:1-15)
References: Sandra Monica Tochetto
The descent path of the thymopharyngeal duct would pass the piriform sinus,
through the thyroid membrane,
downward lateral to the thyroid gland.
By the 7th week of gestation,
the thymic primordia lose their connections with the pharyngeal wall,
join in the midline and give rise to the thymus.
Together with the parietal pericardium,
the thymus descend to its final anatomic position in the anterior superior mediastinum by the 9th week.
Although they arise independently,
the pharyngeal organs primordia interact with one another during the migration.
(Fig 2)
Fig. 2: Fig 2.: Embryological development of the thymus
(Images modified from Manley, NR et al. Developmental Biology (1998) 195:1-15)
References: Sandra Monica Tochetto
Initially,
the 3rd parathyroid migrates in association with thymic lobes and then stops and becomes closely associated with the thyroid gland.
The ultimobranchial bodies separate from the pharynx and migrate close to the cranial aspect of the thyroid lobes,
subsequently merging with them.
(Fig 2)