Development Of The Head

Question 1

How many pairs of cranial nerves are there and how are they numbered

Answer 1

12 pairs number in Roman numerals in rostro-caudal direction
Each has a name according to function, anatomical characteristics and distribution

Question 2

What are the cranial nerves

Answer 2

I = Olfactory
II = Optic
III = Occulomotor
IV = Trochlear
V = Trigeminal
VI = Abducent
VII = Facial
VIII = Vestibulo-cochlear
IX = Glosso-pharyngeal
X = Vagus
XI = Accessory
XII = Hypo-glossal

Question 3

In the trunk of cavitation what does mesoderm result in

Answer 3

Splanchic and somatic lateral mesoderm enclosing a coelomic cavity

Question 4

What doesn’t occur in the head

Answer 4

Cavitation does not occur
Lateral mesoderm remains as solid belt of tissue surrounding gut tube
Intermediate mesoderm fails to develop

Question 5

What do the seven segments of the head consist of

Answer 5

In adult the trunk is clearly segmental even though head appears not to be
Seven segments consist of - ectoderm, somites (S1-7), endoderm (gut tube), lateral plate mesoderm (A1-6), segmental arteries, neuraxis, nerves

Question 6

What is the function of the ectoderm

Answer 6

Forms outer epithelial covering (skin)
Function of somatic sensation (touch, pressure, pain ect)
Innervated by somatic afferents

Question 7

What are segments marked externally by

Answer 7

Lateral indentations (pharyngeal clefts)
1st segment doesn’t have a pharyngeal cleft
Equivalent to gill slits in fish

Question 8

What is the ectoderm deeply invaginated into and what is the oral plate

Answer 8

Deeply invaginated into segments 1 + 2
Forming stomodeum (oral opening)
Consequently segments 1 + 2 don’t have endoderm
Oral plate is thin plate of tissue where stomodeal ectoderm meets pharyngeal endoderm - lies at boundary between segments 2 + 3

Question 9

What does the endoderm (gut tube) form

Answer 9

Forms inner epithelial covering - sensation supplied via autonomic afferents. Motor (sympathetic and parasympathetic) supplied via autonomic efferents
Cephalic part of fore gut becomes pharynx (flattened doso-ventrally)
Marked off into 6 divisions by 6 internal gill slits
In mammals = pharyngeal pouches. In fish = these perforate into external gills slits
In mammals no perforation occurs

Question 10

What is the lateral plate mesoderm how is it separated and innervated

Answer 10

Belt of striated skeletal muscle that surrounds pharynx of all segments except segment 1
Each belt forms a donut (each selected by pharyngeal cleft externally and pharyngeal pouches internally)
Each belt referred to as pharyngeal or brachial arch
Forms special visceral muscle of gut tube
Innervated by special visceral efferents
Also forms skeletal elements

Question 11

What are somites

Answer 11

Each segment has a pair of somites
Form striated muscle of eyeball and tongue
Innervated by somatic efferents

Question 12

What are segmental arteries

Answer 12

All but 1st segment have a pair of segmental arteries
Forms 6 pairs of aortic arches (embedded in lateral plate mesoderm)
Innervated by autonomic afferents/efferents

Question 13

What are neuraxis

Answer 13

Differentiates into brain
Each segment of neuraxis referred to as a neuromere

Question 14

What does obliteration include and what fails to develop further

Answer 14

Somites 4 + 5 (and associated nerves)
Pharyngeal arch 5 (associated endoderm, ectoderm, lateral plate mesoderm, nerves and arterial arches)
Arterial arches (1 + 2)
Pharyngeal clefts/pouches 5 + 6
Ectoderm or segments 6 + 7
Fails to develop - endoderm of arch 2, ectoderm of segments 3-5

Question 15

What does migration include

Answer 15

Somites 6 + 7

Question 16

When the nasal pits form what separates them

Answer 16

Right and left nasal cavity separated by nasal septum

Question 17

What does the nasal cavity break through into and what then grows

Answer 17

Nasal cavity breaks through into primitive oral cavity
Palatine process grows (soft palate endodermal, if lateral palatine folds fail to develop = cleft palate

Question 18

What is the tongue made up of

Answer 18

Epithelium - front of tongue = ectoderm of arch 1, back of tongue = endoderm of arch 3 (endoderm of arch 2 fails to develop further)
Muscle - migration of paired somites 6 + 7

Question 19

How does the pituitary gland develop

Answer 19

Anterior and intermediate lobes arise as diverticulum (= Rathke’s pouch) of ectoderm from arch 1 (posterior pituitary develops from brain stem)

Question 20

How does the thyroid gland develop

Answer 20

Arises as diverticulum of endoderm from arch 2
Thyroglossal duct is remnant of this

Question 21

How does the larynx, trachea and lungs develop

Answer 21

Depression in floor of arches 4 + 6 (5 has disappeared) forms laryngitis-tracheal groove
Trachea and lungs develop from this
Epithelial lining of these = endodermal

Question 22

What do the pharyngeal pouches and clefts equal

Answer 22

Cleft 1 = external ear (boundary = ear drum)
Pouch 1 = middle ear cavity/Eustachian tube
Clefts 2 and 4 disappear
Pouch 2 = palatine tonsil
Pouch 3 = parathyroid + thymus
Pouch 4 = parathyroid, thymus ultimobrachial body

Question 23

What does the ectoderm develop into

Answer 23

Ectoderm 1 = skin of top of head
Ectoderm 2 = the rest of the skin of the head
Ectoderm 3, 4 + 5 = small are of skin in external ear
Ectoderm 6 + 7 fail to develop

Question 24

What do the somites develop into

Answer 24

S1-S3 = extrinsic muscle of the eye
S1 = all rectum muscles (except lateral rectus) and ventral oblique
S2 = dorsal oblique
S3 = lateral rectus and retractor oculi
S4 + S5 disappear
S6-S7 = muscles of the tongue
Intrinsic = tongue itself
Extrinsic attach tongue to other structures (all end in term glossus)

Question 25

What does the lateral plate mesoderm develop into

Answer 25

Arch 1 (seg 2) = chewing muscles. Malleus and incus
Arch 2 (seg 3) = facial muscles. Stapes and hyoid
Arch 3 (seg 4) = stylopharyngeus muscle/hyoid
Arch 4 (seg 5) = pharyngeal muscles and cricothyroid muscle/laryngeal cartilages
Arch 5 (seg 6) disappears
Arch 6 (seg7) = laryngeal muscles/laryngeal cartilages

Question 26

What do the aortic arches develop into

Answer 26

Aortic arch 1 and 2 disappear
Aortic arch 3 = internal carotid, carotid sinus and body
Aortic arch 4 right = right subclavian. Aortic arch 4 left = aortic arch, aortic baroreceptor site and aortic bodies
Aortic arch 5 disappears
Aortic arch 6 = pulmonary trunk and ductus arteriosus on left side