Embryological development of the primary and secondary palates

Question 1

What is the ‘stomodeum’?

Answer 1

Primitive oral cavity

Question 2

What is the stomodeum delimited by?

Answer 2

When the stomatodeum first forms, it is delimited rostrally by the frontal
prominence and caudally by the developing
cardiac bulge

Question 3

What seperates the stomodeum from the foregut?

Answer 3

The buccopharyngeal membrane, a
bilaminar structure consisting of apposed
ectoderm and endoderm,

Question 4

What is the stomoderum limited by laterally?

Answer 4

The first pair of pharyngeal arches

Question 5

Where do the pharyngeal arches form?

Answer 5

The pharyngeal arches form in the
pharyngeal wall as a proliferation of
mesoderm infiltrated by migrating NCCs.

They expand from the lateral wall of the
pharynx and approach their anatomic
counterparts expanding from the
opposite side.

Question 6

What seperates the stomodeum from the developing heart?

Answer 6

The pharyngeal arches

Question 7

What are branchial grooves?

Answer 7

Bulges that seperate the pharyngeal arches

Question 8

What are pharyngeal pouches?

Answer 8

On the inner aspect of the pharyngeal wall are corresponding small
depressions called pharyngeal pouches that separate each of the
branchial arches internally

Question 9

What is teh basic anatomy of a pharyngeal arch?

What is an exception to this?

Answer 9

On the inner aspect of the pharyngeal wall are corresponding small
depressions called pharyngeal pouches that separate each of the
branchial arches internally

except for the first arch because it forms in front of the buccopharyngeal membrane and therefore derives completely from ectodermally covered surfaces

The central core consists of mesenchyme.

Question 10

What is the arch cartilage?

Answer 10

This “neural-derived” mesenchyme condenses to form a bar of
cartilage, the arch cartilage

Question 11

What are the names of the cartilage belonging to the first and second arches?

Answer 11

The cartilage of the first arch is called Meckel’s cartilage, and that of the
second Reichert’s

Question 12

What does reichart’s cartilage give rise to?

Answer 12

Reichert’s cartilage gives rise to a bony process, the stylohyoid
ligament and the upper part of the body and lesser horns of the hyoid
bone.

Question 13

What does the cartilage of the 3rd arch give rise to?

Answer 13

The cartilage of the third arch gives rise to the lower part of the body
and greater horns of the hyoid bone

Question 14

What does the cartilage of the 4th carch give rise to?

Answer 14

Cartilages of the larynx

Question 15

What is the floor of the mouth formed by?

Answer 15

epithelium covering the mesenchyme of the first, second, and third
branchial arches.

Question 16

What happens at about day 24 of gestation?

Answer 16

At about day 24 of gestation, the first
branchial arch establishes another process,
the maxillary process, so that the
stomatodeum is limited cranially by the frontal
prominence covering the rapidly expanding
forebrain, laterally by the newly formed
maxillary process, and ventrally by the first
arch (now called the mandibular process

Question 17

Label this

Answer 17

Question 18

What is early development of the face dominated by?

Answer 18

proliferation and migration of ectomesenchyme involved in the formation of the primitive nasal cavities

Question 19

What happens at about day 28 of gestation?

Answer 19

localised thickenings develop within the
ectoderm of the frontal prominence, just above the opening of the
stomodeum

These thickenings are the olfactory placodes.

Question 20

How is the nasal pit formed?

Answer 20

Rapid proliferation of the underlying mesenchyme around the placodes
bulges the frontal eminence forward and also produces a horseshoeshaped
ridge

Question 21

What is the lateral arm of the horseshoe called?

What is the medial arm called?

Answer 21

lateral nasal process

Medial nasal process

Question 22

Explain how the maxillary process grows

Answer 22

The maxillary process grows
medially and approaches the lateral
and medial nasal processes but
remains separated from them by
distinct grooves, the nasolacrimal
groove and the bucconasal groove.

As the process continues to grow,
the medial nasal process is
displaced toward the midline, where it merges with its anatomic
counterpart from the opposite side. In this way the middle portion of the
upper lip or philtrum is formed.

• The merging of the two medial nasal processes also results in the
formation of that part of the maxilla carrying the incisor teeth and the
primary palate.

Question 23

What does fusion between the forward extent of the maxillary process
and the lateral aspect of the medial nasal process result in?

Answer 23

obliterate the bucconasal groove and result in the formation of the lateral aspects of the upper lip.

• The lower lip is formed by merging of the two streams of
ectomesenchyme of the mandibular processes.

Question 24

When can you distinguish between the oral and nasal cavities?

Answer 24

Only after the development of the secondary palate is distinction
between the oral and nasal cavities possible.

Question 25

When does formation of the secondary palate begin and end?

Answer 25

commences between 7 and 8 weeks of gestation and completes
around the third month of gestation

Question 26

How does development of the palate begin?

Answer 26

Three outgrowths appear in the oral cavity; the
nasal septum grows downward from the frontonasal
process along the midline, and two palatine shelves
or processes, one from each side, extend from the
maxillary processes toward the midline.
• The shelves are directed first downward on each
side of the tongue. After 7 weeks of development,
the tongue is withdrawn from between the shelves,
which now elevate and fuse with each other above
the tongue and with the primary palate.
• The septum and the two shelves converge and fuse
along the midline, thus separating the primitive oral
cavity into nasal and oral cavities.

Question 27

Describe the relation of the of the head and thoracic region of the developing embryo at 7-8 weeks of gestation?

What happens at 9 weeks of gestation?

Answer 27

• Between 7 and 8 weeks of gestation the tongue and mandible in the
embryo are small relative to the upper facial complex,
and the lower lip is positioned behind the upper one.
The head is folded onto the developing thoracic
region, and the tongue occupies an elevated position
between the palatine shelves.

• By 9 weeks of gestation, the upper facial complex has
lifted away from the thorax and thus permits the
tongue and lower jaw to grow forward so that the
lower lip now is positioned in advance of the upper lip
and the tongue is situated below the palatine shelves.

Question 28

What must occur for the fusion of the palatine shelves and other processes to occur?

Answer 28

elimination of the epithelial covering of the shelves is necessary

Question 29

How does elimination of the epithleium occur?

Answer 29

As the two palatine shelves meet, adhesion of the epithelia occurs
so that the epithelium of one shelf becomes indistinguishable from
that of the other, and a midline epithelial seam that consists of two
layers of basal epithelial cells forms.
‣ This midline seam must be removed to permit ectomesenchymal
continuity between the fused processes.
‣ As palatal growth proceeds, the seam first thins down and then
breaks up into discrete islands of epithelial cells.
‣ The basal lamina surrounding these cells then is lost, and the
epithelial cells lose their epithelial characteristics and assume
fibroblast-like features. In other words, epithelial cells transform
into mesenchymal cells; that is, they undergo an
epitheliomesenchymal transformation (transition