Development of maxillary prominences and secondary palate: - PowerPoint Presentation

Slide1

Development of maxillary prominences and secondary palate:

Slide2

New outgrowths from the medial edges of the maxillary prominences form the shelves of the secondary palate. These palatal shelves grow downward beside the tongue, at which time the tongue partially fills the nasal cavities. At about the

ninth gestational week

; the shelves elevate, make contact, and fuse with each other above the tongue. In the anterior region, the shelves are brought to the horizontal position by a rotational (hinge like) movement. In the more posterior regions, the shelves appear to alter their position by changing shape (remodeling) as well as by rotation.

Slide3

Available evidence indicates that the shelves are incapable of elevation until the tongue is first withdrawn from between them. Fusion of palatal shelves requires alterations in the epithelium of the medial edges that begin prior to elevation. These alterations consist of cessation of cell division, which appears to be mediated through distinct underlying biochemical pathways, including a rise in

cyclic AMP levels.

There is also loss of some surface epithelial

(

peridermal

)

cells and production of extracellular surface substances, particularly

glycoproteins

, that appear to enhance adhesion between the shelf edges as well as between the shelves and inferior margin of the

nasal septum

.

Slide4

The ultimate fate of these remaining epithelial cells is controversial. Some of them appear to undergo cell death and eventually are

phagocytized

, but many undergo direct transformation in mesenchymal cells. Some of the epithelial cells remain indefinitely in clusters (cell rests) along the fusion line. Eventually, most of the hard palate and all of the soft palate form from the secondary palate.

Slide5

Development of visceral arches and tongue

The pituitary gland develops as a result of inductive interactions between the ventral forebrain and oral ectoderm and is derived in part from both tissues Following initial neural crest cell migration , these cells invade the area of the developing pituitary gland and are continuous with cells that will later form the maxillary prominence. Eventually, neural crest cells form the connective tissue components of the gland.

Slide6

In humans there is a total of six visceral arches, of which the fifth is rudimentary. These arches are also known as

pharyngeal or

branchial

arches

. The gills (

branchii

) of the fish are modified to give rise to these arches. The proximal portion of the first (mandibular) arch becomes the maxillary

prominence.As

the heart recedes caudally; the mandibular and hyoid arches develop further at their distal portions to become consolidated in the ventral midline.

Slide7

Slide8

As noted previously, the

mesodermal

core of each visceral arch is concerned primarily with the formation of

vascular endothelial cells

. As noted below, these cells appear to be later replaced by cells that eventually form visceral arch

myoblasts

. The first

(mandibular) and second (hyoid)

visceral arches undergo further developmental changes. As the heart recedes caudally, both arches send out bilateral processes that merge with their opposite members in the ventral midline. Nerve fibers from the

fifth, seventh, ninth, and tenth cranial

nerves extend into the mesoderm of the first four visceral arches.

Slide9

Slide10

The mesoderm of the definitive mandibular and hyoid arches gives rise to the

fifth and seventh nerve musculature

, while mesoderm associated with the less well developed

third and fourth arches forms the ninth and tenth nerve

musculature.

Myoblast

cells in the visceral arches actually originate from mesoderm more closely associated with the neural tube (cells that form the hypoglossal and extrinsic eye musculature).

Slide11

They would then migrate into the visceral arches and replace the

mesodermal

cells that initiated blood vessel formation earlier. It therefore appears that

myoblasts

forming voluntary striated muscle fibers of the facial region would then originate from mesoderm adjacent to the neural tube. Groups of visceral arch

myoblasts

that are destined to form individual muscles each take a branch of the appropriate visceral arch nerve.

Slide12

Myoblasts

from the

second visceral arch

, for example, take branches of the seventh cranial nerve and migrate very extensively throughout the head and neck to form the contractile components of the

“muscles of facial expression

.”

Myoblasts

from the

first arch contribute

mostly to the

muscles of mastication,

while those from the

third and fourth

arches contribute to the pharyngeal and soft palate musculature.

Slide13

Connective tissue components of each muscle in the facial region are provided by mesenchymal cells of crest origin. The crest mesenchymal cells of the visceral arches give rise to skeletal components such as the temporary visceral arch cartilages (e.g.,

Meckel’s cartilage

,

middle ear cartilages, and mandibular bones

).

Slide14

Also visceral arch crest cells form connective tissues such as dermis and the connective tissue components of the tongue.

The tongue forms in the ventral floor of the pharynx after arrival of the hypoglossal muscle cells. The lateral lingual tubercles or swellings form the tongue has not been carefully documented. It is known that the anterior two thirds of the tongue is covered by ectoderm whereas endoderm covers the posterior one third.

Slide15

Slide16

The thyroid gland forms by invagination of the most anterior endoderm (

thyroglossal duct)

. A residual pit (

the foramen

cecum

) left in the epithelium at the site of invagination marks the junction between the anterior two thirds and posterior one third of the tongue, which are, respectively, covered by epithelia of ectodermal and

endodermal

origin. It is also known that the connective tissue components of the

anterior two thirds

of the tongue are derived from

first-arch

mesenchyme

,

Slide17

Slide18

Where as those of the

posterior

one third

appear to be primarily derived from the

third-arch

mesenchyme

.

The epithelial components of a number of glands are derived from the

endodermal

lining of the pharynx.

In addition to the

thyroid,

these include the

parathyroid and thymus

. The epithelial components of the

salivary and anterior pituitary

glands are derived from oral ectoderm.

Slide19

Finally, a lateral extension from the inner groove between the

first and second

arch gives rise to

the

eustachian

tube

, which connects the pharynx with the ear. The

external ear, or

pinna

, is formed at least partially from tissues of the

first and second arches .

Slide20

FINAL DIFFERENTIATION OF FACIAL TISSUES

The extensive cell migrations referred to above bring cell populations into new relationships and lead to further inductive interactions, which, in turn, lead to progressively more differentiated cell types. For example, some of the crest cells coming into contact with pharyngeal endoderm are induced by the endoderm to form visceral arch cartilages. Recent studies indicate the early epithelial interactions are also involved in bone formation. The exact interactions involved in tooth formation are somewhat controversial.

Slide21

Mesenchymal cells of crest origin must be involved, and these cells form the dental papilla and the

mesenchyme

surrounding the epithelial enamel organ. Whether the epithelium or

mesenchyme

is initially responsible for determining which tooth (e.g., incisor or molar) forms from a tooth germ is controversial.

Slide22

only crest mesenchymal cells and not

mesodermal

mesenchymal cells will respond to inducing tissues such as pharyngeal endoderm. In other cases, as in the differentiation of dermis and

meninges

, it appears that the origin of the

mesenchyme

is of no consequence.

Slide23

The formation of skeletal and connective tissues, ordinarily performed by

mesodermal

cells in other regions, has been occupied by neural crest cells in the facial region. The crest cells therefore play a very dominant role in facial development, since they form all

nonepithelial

components except endothelial cells and the contractile elements of

skeletal (voluntary

) muscle.

The onset of bone formation or the establishment of all the organ systems (about

the eighth week of development

) is considered the termination of the embryonic period.