The pituitary gland, tongue and thryoid gland are midline structures located within the head and neck.
In this article, we will look at the embryology of the pituitary, tongue and thyroid gland – their origin, development and clinical correlations.
The pituitary gland is an endocrine gland located within the middle cranial fossa of the skull base.
Anatomically, the pituitary is a ‘’two-in-one’’ structure consisting of the anterior pituitary gland and the posterior pituitary gland – which develop from two different sources:
- Rathke’s pouch – outpocketing of the ectoderm of the stomatodeum (future oral cavity).
- Becomes the anterior pituitary gland.
- Infundibulum – downward growth of the forebrain.
- Becomes the pituitary stalk and posterior pituitary gland.
Development begins at week 3 of gestation. Rathke’s pouch appears, and extends upwards towards the infundibulum. By week 8, it has detached from the oral cavity and lies tightly apposed to the infundibulum – forming the pituitary gland.
The tongue begins development in the 4th week of gestation. It is derived from pharyngeal arches 1-4 (forms the mucosa of the tongue) and the occipital somites (forms the musculature of the tongue).
Pharyngeal Arches (Mucosa)
In the first stage of development, lingual and medial swellings appear:
- Lateral lingual swellings (x2) – derived from the 1st pharyngeal arch. Contributes to the mucosa of the anterior 2/3 of the tongue.
- Medial swellings (x3):
- Tuberculum impar – derived from the 1st pharyngeal arch. Contributes to the mucosa of the anterior 2/3 of the tongue.
- Cupola (hypobranchial eminence) – derived from the 2nd, 3rd and 4th pharyngeal arches. Forms the mucosa of the posterior 1/3 of the tongue.
- Epiglottal swelling – derived from the 4th pharyngeal arch. Forms the epiglottis.
During the 4th week, the lateral lingual swellings overgrow the tuberculum impar and merge together – forming the mucosa of the anterior 2/3 of the tongue. Their line of fusion is marked by the median sulcus of the tongue.
Within the cupola, the 3rd pharyngeal arch component overgrows the 2nd arch, and forms the mucosa of the posterior 1/3 of the tongue. The anterior 2/3 and posterior 1/3 fuse – forming a V-shaped groove known as the terminal sulcus. At the centre of this groove is the foramen cecum, a pit which represents the place of origin of the thyroid gland.
As the tongue forms, it is initially is tethered to the floor of the oral cavity. A process of carefully programmed cell death known as sculpting apoptosis releases the tongue, leaving in place the lingual frenulum to anchor the tongue in the mouth.
Occipital Somites (Musculature)
The intrinsic and extrinsic muscles of the tongue are derived from occipital somites, which are segments of mesoderm in the region of the upper neck.
The somites migrate from the neck anteriorly to give rise to the muscles of the tongue.
The thyroid gland begins development as a proliferation of endodermal cells between the cupola and tuberculum impar of the primitive tongue (the site of the future foramen cecum).
This proliferation of cells bifurcates, and descends into the neck as a two-lobed diverticulum. By week 7, it has reached its destination in the anterior neck, and is formed of two lateral lobes connected by a central isthmus.
The descent of the developing thyroid gland forms the thyroglossal duct – an epithelialised tract that connects the gland to its origin at the foramen cecum. It usually regresses by the 10th week of gestation, but can persist in some individuals.
Clinical Relevance Thyroglossal Cyst
The thyroglossal duct is an epithelialised tract which connects the thyroid gland to the foramen cecum of the tongue. If it fails to regress, the duct can give rise to cysts or fistulae.
A thyroglossal cyst results from a build-up of secretions within the duct. It typically presents as a midline lump in the anterior neck, and characteristically rises on tongue protrusion. If left untreated, this cyst can become infected, and form a cutaneous fistula – discharging out onto the skin of the anterior neck.
Thyroglossal cysts and fistulae are usually treated with complete exicsion. Recurrence is quoted at approximately 2.5%.