11. Head and Neck - Development of the Midline Structures (Pituitary G, Tongue, Thyroid G) Flashcards
1
Q
What is the pituitary gland and where is it located?
A
- A ‘two in one’ gland
- Tissues in both lobes are completely different to each other
- Posterior lobe = neuroendocrine secretions
- Anterior lobe = endocrine secretions
- Size of a pea, sits in the sphenoid bone surrounded by the sella tursica
2
Q
How does the pituitary gland develop?
A
- Ecotderm differentiates into neuroectoderm (which forms the neural tube –> CNS tissue)
- Neuroectoderm gives rise to infundibulum (downward out-growth of the forebrain which grows toward the roof of the pharynx), which gives rise to the posterior pituitary
- Outpocketing of ectoderm (or invagination of roof of oropharynx) grows up and thickens as it differentiates
- Gives rise to ‘Rathke’s pouch’, which gives rise to anterior pituitary
- i.e. infundibulum growing down to oropharynx, rathke’s pouch up from oropharynx
- Grow and form around each other forming two discrete lobes
- Rathke’s pouch pinches off and sphenoid bone continues to ossify beneath it, walling it off from cavity of the oropharynx
3
Q
Summarise what parts of the pituitary glands are derived from what parts of the embryology
A
- Infundibulum = pos pit and stalk
- Rathke’s pouch = ant pit
4
Q
Describe the anatomy of the tongue
A
- Lies partly in oral cavity and partly in the pharynx
- Needs to be highly mobile for its function of mastication and swallowing
- Tethered to floor of oral cavity by short cord called the lingual frenulum
- Boundary of anterior 2/3 and posterior 1/3 separated by sulcus terminalis (V shaped) – foramen cecum means a blind ending pouch – when we look at thyroid, we see significance of this anatomical landmark
- Comprised of intrinsic muscles (work and contract to change shape of tongue) and extrinsic muscles (move tongue around mouth)
5
Q
How does the tongue develop?
A
- Gets contribution from all pharyngeal arches
- Pharyngeal arches:
2 lateral lingual swellings - derived from 1st arch
Tuberculum impar - derived from 1st arch
Cupola - derived from 2nd, 3rd, little bit of 4th arch
Epiglottal swelling - derived from 4th arch
- Lateral lingual swelling overgrow tuberculum impar (lose blue shading)
- 3rd arch over grows 2nd arch component
- Apoptosis - frees up tongue from floor of cavity - what remains = lingual frenulum allowing tongue to stay attached to the oral cavity
6
Q
What happens if the degeneration that frees up the tongue from the oral cavity (normally leaving the lingual frenulum) is not complete?
A
- lingual frenulum = tight
- ‘tongue tied’ so movement of tongue not optimal
- easy procedure to loosen
4.
7
Q
What is the sensory innervation of the tongue?
A
- Mucosa of anterior 2/3 derived from first and third pharyngeal arches (as we lose the contribution from the 2nd). The nerves that innervate these arches = 5th trigeminal and 9th glossopharyngeal CN
- Posterior 1/3 derived from third and 4th therefore = 9th glossopharyngeal and 10th vagus CN
- Taste - develop in discrete structures called papillae - derived from second arch therefore = facial CN = branch is chorda tympani (pass from 2nd arch into first arch structure which derives middle ear and therefore chorda tympani passes through middle ear)
8
Q
What is the motor innervation of the tongue?
A
Both intrinsic and extrinsic muscles of the tongue develop from myogenic precursors (small occipital somites) that migrate into the developing tongue - innervated by 12th hypoglossal CN
9
Q
What and where is the thyroid gland?
A
- The thyroid gland is an endocrine gland
- In the neck
- Consists of two lobes connected by an isthmus
- TSH (from ant. pit) stimulates TRH (from hypo) stimulates T3 and T4 from it –> inc metabolic rate, inc rate and strength of heart beat, inc growth rate in young people
10
Q
How does the thyroid gland develop?
A
- Begins development in the floor of the oropharynx between the tuberculum impar and cupola
- Descends to its final position in the anterior neck
11
Q
What is the path of descent of the thyroid gland?
A
- Foramen caecum marks point of origin for descent (boundary of pos1/3 and ant2/3 of tongue)
- bifurcates and descends as a bilobed diverticulum connected by the isthmus
- during descent, developing thyroid gland remains connected to the tongue by thyroglossal duct (atrophys in adult life)
- migration is ant to pharyngeal gut, hyoid bone and laryngeal cartilages
12
Q
What is a thyroglossal cyst?
A
- a fibrous cyst that forms from a persistent thyroglossal duct
- anywhere along path from foramen caecum to position of thyroid gland in neck
- maintained part of duct - can collect fluid
13
Q
What is ectopic thyroid tissue?
A
- anywhere along path from foramen caecum to position of thyroid gland in neck
- presence of thyroid tissue in locations other than the normal anterior neck region between the second and fourth tracheal cartilages
- can lead to congenital hypothyroidism - is a condition of thyroid hormone deficiency present at birth.
14
Q
What is cleft lip and palate?
A
- Palate formed by palatal shelves (which grow medially into oral cavity from maxillary prominence)
- Once mandible sufficiently enlarged, tongue drop on palatal shelves, meets in midline and fuses
- Abnormality = failure of FNP to fuse with maxillary prominence and failure of palatal shelves to fuse
15
Q
What is first arch syndrome?
A
- Problems related to development of eyes, ears, mandible and palate
- Example: Treacher- Collins syndrome – mandible is hypoplastic, hypoplasia of facial bones, low set ears, abnormal appearance of external ear, haploinsufficiency of treacle protein (doesn’t work) = neural crest cells cannot adequately arrive in first arch and begin normal program of development. Doesn’t stop completely but ability to function properly is. Impacts on neural crest migration.
