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Chapter 18: Development of the Structures of Head and Neck- Part 2
1. True/ False- The 1st pharyngeal pouch forms the middle ear cavity and the auditory tube in mammals.
2. True/ False- The dentary part of the reptilian mandible evolved into the mandible of mammals.
3. True/ False- The columella is phylogenetically newer addition in vertebrates. (The columella is
only in primitive animals (birds and reptiles) and is the oldest bone of the middle ear cavity)
4. True/ False- Meckel’s cartilage (of the 1st pharyngeal arch) forms part of the mandible in the dog.
(In mammals, this cartilage form the incus and malleus, bones of the middle ear. In submammalian vertebrates, the arch I cartilage forms the mandible)
5. True/ False- The auricular hillocks are 8 in number and are derived from lateral plate mesoderm.
(They are 6 in number and are derived from pharyngeal pouch endoderm)
6. True/ False- Due to the migration of the external ear to the dorsal aspect of the head, part of the
external ear canal is also supplied by the CN IX. (Migration of the external ear dorsally catches
branches of the vagus n. (CN X))
7. True/ False: The intermediate lobe of the pituitary gland is derived from the pharyngeal endoderm.
(The pituitary gland is derived entirely of ectoderm)
8. True/ False- Some cranial nerves carry only GSE fibers that supply muscles derived from
somitomeres 3 and 4. (Somitomere 4 contributes to pharyngeal arch I, and all skeletal muscles
derived from pharyngeal arches are innervated by SVE. Somitomeres 1, 2, 3 and 5 develop into
extra-ocular muscles that are innervated by GSE fibers)
9. True/ False- The palatopterygoquadrate is a cartilage in the mandibular process of Arch I. (It is the
cartilage in the maxillary process of branchial arch I)
10. True/ False- Oligodendrocytes are derived from cranial neural crest cells. (Schwann cells are)
11. True/ False- 1° neurulation results in filum terminale. (Results from 2° neurolation)
12. True/ False- The thyroid gland forms later in life after the formation of pituitary gland, which
controls it. (The thyroid gland is the first endocrine gland to develop in the embryo)
13. True/ False- Primary neurulation process results in conus medullaris. (Forms the neural tube)
14. True/False- The remnant of the 5th branchial arch forms part of external ear canal. (In mammals,
the ultimobranchial body forms the parafollicular “C” cells of the thyroid gland, which secrete
thyrocalcitonin. In birds/reptiles, it forms a third parathyroid gland)
15. True/False- Cardiac neural crest cells migrate to the level of the left colic flexure.
16. True/ False- In birds, part of the mandible is derived from the mandibular arch cartilage.
17. Describe the development of the thyroid gland.
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The thyroid gland is NOT formed from pharyngeal pouches but from the floor of the pharynx. It is the
first endocrine gland to form in the embryo, indicating that the thyroid hormones it secretes (thyroxin)
are vital for normal development of the embryo.
The pharyngeal endoderm, between pouches I and II, forms a mid-ventral out-pouching called the
thyroid primordium, which elongates to form the thyroid diverticulum. Originally, the diverticulum
is connected to the floor of the pharynx via the thyroglossal duct. As the diverticulum elongates and
the duct disintegrates, it detaches from the duct. Now, the diverticulum migrates caudally to its final
location. During this migration, the parafollicular “C” cells of the ultimobranchial body (remnant of
pharyngeal pouch V) and the internal parathyroid glands become associated with it. In some species
(carnivores) the gland divides into a right and left lobe. In others (horses, ruminants, humans) the lobes
are partially connected by a glandular isthmus. In pigs, the thyroid gland is a single mass.
The thyroid glandular tissue proliferates to form epithelial cords that are highly vascularized and form
cellular clusters with a central lumen. The clusters are known as thyroid follicles, which are populated
by the C cells.
18. Describe the fate of the 1st pharyngeal pouch.
The first gill cleft forms the external ear and external auditory meatus. In early development, the ear is
formed on the ventrolateral aspect of the head. This auricular structure eventually migrates to the
dorsolateral aspect of the head. The 1st pharyngeal pounch, which is in contact with the 1st gill cleft,
also migrates dorsolaterally with the external ear, and its contact with the pharynx is retained as a long,
narrow tube. This 1st pouch eventually forms the auditory tube and the middle ear cavity, in which
resides the 3 middle ear ossicles (malleus, incus, stapes) and 2 middle ear muscles (tensor tympani m.
and stapedius m.).
19. Describe the development of parathyroid glands in mammals
Pharyngeal pouches III and IV (on both sides of the body) develop distinct dorsal and ventral
diverticulae. The dorsal diverticulae (composed of pharyngeal endodermal cells) of pouch III detaches
and proliferates, invading the surrounding mesenchyme. These cells eventually from the external
parathyroid gland. The surrounding mesenchyme condenses to form the capsule of the parathyroid
gland. The dorsal diverticulum of pouch IV also detaches, proliferates, and forms the internal
parathyroid gland, which eventually become associated with the thyroid gland.
20. What is a spiracle? Among which vertebrates do you find it as a functional adult structure?
The spiracle is the cleft between the 1st and 2nd pharyngeal arches. It is found only in cartilaginous
fish. In mammals, the auditory tube and middle ear cavity are modifications of the spiracle.
21. Describe the development of the thymus in mammals.
The thymus is one of the primary lymphatic organs in all mammals. Recall the ventral diverticulae of
pharyngeal pouches III and IV. The epithelial cells of these pouches proliferate and form a solid
structure. The epithelial lining sprouts numerous branches of epithelial cords, some clustering to form
thymic (Hassal’s) cords. The developing thymic diverticulum differentiates into a cranial and caudal
component, which eventually form the cranial (cervical) and thoracic thymus. The thymic primordium
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is soon invaded by prothymocytes (from hemopoietic cells, CFU-L). These cells develop into
thymocytes (“T” lymphocytes), which mature and are protected by a “blood-thymic barrier”
(extremely tight endothelial cells that are important for preserving the separation of antigenic and
immunogenic T-cells from systemic circulation).
It’s important to understand that the thymus epithelium is mostly derived from the ventral diverticulum
of pharyngeal pouch III, though pouch IV contributes to minor epithelium. Actual thymus tissue is
composed of invading lymph (T lymphocytes). The thymus, in addition to providing the main source
of T-lymphocytes, also secretes thyroid hormones (thyroxin).
22. Describe the development of the pituitary gland and its 3 major components
The development of the pituitary gland is entirely from ectoderm and influenced by inductive factors.
It develops from 2 distinct primordia, one from the roof of the stomodeum and one, called the
infundibular primordium, from diencephalon of the forebrain. The infundibular stalk from
diencephalon secretes paracrine factors (BMP-4 followed by FGF-8) that induce stomodeal ectoderm
to form the adenohypophyseal (Rathke’s) pouch. The pouch elongates and gradually detaches to
form the adenohypophyseal vesicle.
This vesicle migrates towards the infundibulum and fuses with it. During migration, its rostral and
caudal walls become hyperplastic and form a cavity. The rostral wall gives rise to the anterior lobe
(pars distalis and pars tuberalis), while the caudal wall forms the intermediate lobe (pars intermedia).
The cavity remains (not in horses) as pituitary cleft located between anterior and intermediate lobes.
The infundibulum forms the posterior lobe (pars nervosa).
23. The only muscle derived from the somitomere 3 is the:
a. Dorsal rectus m.
b. Ventral rectus m.
c. Lateral rectus m.
d. Dorsal oblique m.
e. Ventral oblique m.
24. All of the following are derived from somitomeres 1 and 2 except:
a. Ventral rectus m.
b. Medial rectus m.
c. Lateral rectus m.
d. Ventral oblique m.
e. Dorsal rectus m.
25. Select the wrong item:
a. The maxillary process forms part of upper lip
b. Nasolateral process is involved in the formation of nasolacrimal duct
c. Nasal placode is derived from ectoderm
d. The nasal pit olfactory epithelium is derived from neural crest cells (Derived from nasal
mucosa, ectodermally-derived epithelium, at the caudal end of the nasal cavity)
e. Union of nasomedial processes form nasal philtrum
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