embryology l Oral histology and biology MCQs for dental students

 

 

devlopment of pharngyeal (branchial) arches l Oral histology and biology MCQs for dental students

Pharyngeal arches development

During the pre –Embryonic period, the cells differentiate into three germ layers:

Ectoderm (outside layer)

• gives rise to the CNS, peripheral nervous system; sensory epithelia of the eye, ear, and nose; epidermis and its appendages (hair and nails).

Mesoderm (middle layer)

• gives rise to connective tissue; cartilage; bone; striated and smooth muscles; blood, and lymphatic vessels; heart, kidneys; ovaries; testes; genital ducts; serous membranes lining the body cavities (pericardial, pleural, and peritoneal); spleen; and cortex of suprarenal glands.

Endoderm (inside layer)

• gives rise to the epithelial lining of the gastrointestinal and respiratory tracts, urinary bladder , tympanic cavity, tympanic antrum, and auditory tube parenchyma of the tonsils, thyroid and parathyroid glands, thymus, liver, and pancreas.

NEURAL CREST CELL:

• A multipotential population of migratory cells arises from epithelial cells at border between prospective epidermis and neural plate (Neuroectoderm), sometimes called forth germ layer

• give rise to the cells of the spinal, cranial (cranial nerves V, VII, IX, and X), and autonomic ganglia; unsheathing cells of the peripheral nervous system; pigment cells of the dermis; muscle, connective tissues, and bone of pharyngeal arch origin and meninges (coverings) of the brain and spinal cord.

 

The branchial apparatus =The branchial (pharyngeal) arches

• The development of the pharyngeal arches is complex involving a number of disparate embryonic cell types: ectoderm, endoderm, neural crest and mesoderm, During the 4th week of embryonic development, each of the three germ layers give rise to a number of specific tissues and organs, the major features of the external body form are recognizable by the end of second month.

• Pharyngeal arches are paired structures associated with the pharynx that contribute greatly to the formation of the face, jaw, ear, and neck.

• The pharyngeal arches are a series of externally visible anterior tissue bands lying under the early brain. Each arch though initially formed from similar components will differentiate to form different head and neck structures.

• The pharyngeal arches begin to develop early in the fourth week as neural crest cells migrate into the future head and neck regions.

• During human and all vertebrate development, pharyngeal arch pairs project forward from the back of the embryo toward the front of the face and neck. Each arch develops its own artery, nerve that controls a distinct muscle group, and skeletal tissue. These grow and join in the ventral midline. The first arch, as the first to form, separates the mouth pit or stomodeum from the pericardium.

• The 1st pharyngeal arch appears at about the beginning of the 4th week and others are added more caudally later such that there are ultimately 5 arches by the end of the 4th week; the 5th arch fails to form, so the arches are numbered 1, 2, 3, 4, and 6. The entire apparatus consists of paired pharyngeal arches, pharyngeal pouches, pharyngeal clefts (or grooves), and pharyngeal membranes .

• The arches are covered by ectoderm, The ectoderm between the arches form clefts (grooves) called pharyngeal (branchial) clefts (grooves). The arches are bordered medially by the pharynx which is lined by endoderm. Medially each of the pharyngeal arches is separated by a pharyngeal pouch. These pouches approach the corresponding branchial cleft. The approximation of the ectoderm of the pharyngeal cleft with the endoderm of the pharyngeal pouch forms the pharyngeal membrane. The grooves and pouches are named (numbered) the same as the preceding arch.

 

Derivatives of pharyngeal archs:

1-The first pharyngeal arch (mandibular arch)

• Skeletal elements

1. Malleus & Incus of the middle ear
2. maxilla & mandible

• Muscles

1. Muscles of mastication (chewing)
2. Mylohyoid muscle
3. Digastric muscle, anterior belly
4. Tensor palati muscle
5. Tensor tympani muscle

• Nerve;

1. Trigeminal nerve

2-The second pharyngeal arch

• The second pharyngeal arch or hyoid arch (or second branchial arch) assists in forming the side and front of the neck.
• Skeletal elements;

1. Stapes
2. Styloid process (Temporal bone)
3. Stylohyoid ligament
4. Lesser horn and superior body of the hyoid bone.

• Muscles;

1. Muscles of facial expression
2. Occipitofrontalis muscle
3. Platysma
4. Stylohyoid muscle
5. Posterior belly of Digastric
6. Stapedius muscle
7. Auricular muscles

• Nerve supply;

1. Facial nerve

3-The third pharyngeal arch:

• Skeletal elements

1. Hyoid (greater horn),and lower part of body,thymus ,inferior parathyroid.

• Muscles;

1. Stylopharyngeus

• Nerve;

1. Glossopharyngeal nerve(IX)

4-The fourth pharyngeal arch:

• Skeletal elements;

1. Thyroid cartilage  
2. Epiglottic cartilage.

• Muscles;

1. Cricothyroid muscle
2. Pharyngeal muscle
3. All intrinsic muscles of the soft palate including levator veli palatine

• Nerve;

1. Vagus nerve
2. Superior laryngeal nerve.

Pharyngeal pouches

• Pharyngeal or branchial pouches form on the endodermal side between the branchial arches, and pharyngeal grooves (or clefts) form the lateral ectodermal surface of the neck region to separate the arches.

First pouch

• The endoderm lines the future auditory tube (Pharyngotympanic Eustachian tube), middle ear, mastoid antrum, and inner layer of the tympanic membrane.

Second pouch

• Contributes to the middle ear, palatine tonsils, supplied by the facial nerve.

Third pouch

• Third pouch possesses dorsal and ventral wings. Derivatives of the dorsal wings include the inferior parathyroid glands, while the ventral wings fuse to form the cytoreticular cells of the thymus. The main nerve supply to the derivatives of this pouch is Cranial Nerve IX, glossopharyngeal nerve.

Fourth pouch: Derivatives include:

• Superior parathyroid glands and ultimobranchial body which forms the parafollicular C-Cells (calcitonin producing cells) of the thyroid gland.
• Musculature and cartilage of larynx (along with the sixth pharyngeal pouch).

Fifth pouch

• Rudimentary structure, becomes part of the fourth pouch contributing to thyroid C-cells.

Sixth pouch

• Along with the fourth pouch, the contributes to the formation of the musculature and cartilage of the larynx.

 

Pharyngeal clefts

• Ectodermal lined recesses that appear on the outside of the pharynx between the arches .

Pharyngeal cleft1

• Develops into the external auditory meatus (the corresponding 1st pharyngeal pouch develops into auditory , and the intervening membrane develops into the tympanic membrane).
• Defects in the development of pharyngeal cleft1 can result in preauricular cysts and/or fistulas(in front of the pinna of the ear).

Pharyngeal cleft 2 , 3 and 4 :

• are overgrown by expansion of the 2nd pharyngeal arch and usually obliterated.
• The clefts between the branchial arches of the embryo, formed by rupture of the membrane separating corresponding endodermal pouch and ectodermal groove. 

Fate of Pharyngeal Arches

1. The pharyngeal arches contribute exclusively to the formation of the face, nasal cavities, mouth, larynx, pharynx and neck.
2. During the fifth week, the second pharyngeal arch enlarges and overgrows the third and fourth arches, forming the ectodermal depression called cervical sinus.
3. By the end of seventh week ,the second to fourth pharyngeal grooves and the cervical sinus have disappeared, giving the neck a smooth contour

Branchial cyst

• Branchial cyst is a congenital epithelial cyst that arises on the lateral part of the neck usually due to failure from an incompletely closed branchial cleft, usually located between the 2nd and 3rd branchial arches.

• Remnants of pharyngeal clefts 2-4 can appear in the form of cervical cysts or fistulas found along the anterior border of the sternocleidomastoid muscle.

• Most branchial cleft fistulae are asymptomatic, but they may become infected.

• Treatment: Conservative (i.e. no treatment), or surgical excision. With surgical excision, recurrence is common, usually due to incomplete excision. Often, the tracts of the cyst will pass near important structures, such as the internal jugular vein, carotid artery, or facial nerve, making complete excision impractical.

Key Points

1. The formation of blastocyst from the zygote and implantation of the blastocyst in the endometrium of the uterus take place by the end of the first week.
2. The bilaminar germ disc comprises a superior epiblast layer composed of tall columnar cells and an inferior hypoblast layer composed of cuboidal cells.
3. The characteristic event in the third week of development is gastrulation—the formation of the three germ layers.
4. Gastrulation begins with the formation of the primitive streak and the primitive node.
5. At the end of the third week, two embryonic membranes—the buccopharyngeal membrane or oropharyngeal membrane at the cephalic end and a cloacal membrane at the caudal end—appear in the germ disc.
6. The buccopharyngeal membrane and the cloacal membrane consist only of ectoderm and endoderm without an intermediate mesoderm and denote the locations of the future oral cavity and the anus.
7. Invading prenotochordal cells from the epiblast, through the primitive streak, interpose with the hypoblast to form the notochordal plate, which gives rise to notochord.
8. The cells of the notochord are formed from the mesoderm. The notochord serves as the basis for the axial skeleton.
9. Neuroectoderm cells differentiate from the ectoderm during the later part of the third week of intrauterine life (IUL).
10. Neuroectoderm cells proliferate to form the neural plate. The neural groove appears in the neural plate and is surrounded by the neural folds. During the fourth week, the neural folds fuse to form the neural tube.
11. The anterior portion of the neural tube bulges to form the forebrain, midbrain and hind brain, and the spinal cord develops caudal to the hind brain.
12. Neural crest cells are often referred to as the fourth germ layer.
13. The paraxial mesoderm differentiates into 38 paired cuboidal aggregates called somites which give rise to the sclerotome (bone and cartilage), myotome (muscles) and dermatome (dermis of the skin), forming the structures of the head, neck and trunk.
14. At the fourth week of IUL, the buccopharyngeal membrane ruptures and a communication is established between the primitive oral cavity and the primitive foregut.
15. The branchial arches are separated by the branchial grooves or clefts on the lateral aspect and by the pharyngeal pouches on the inner aspect. In between the consecutive arches, the ectodermal lining of the pharyngeal cleft and the endodermal lining of the pharyngeal pouch meet without the intervening mesoderm, forming the branchial membrane.
16. Each branchial arch consists of a cartilage, a branchiomere, a vascular component and a neural component.
17. The derivatives of each branchial arch are supplied by a pre-trematic nerve and a post-trematic nerve.
18. The cartilage of the first arch is Meckel’s cartilage and that of the second arch is Reichert’s cartilage.
19. The special sensory component of the facial nerve (nerve of the second arch), the chorda tympani, supplies the anterior two-third of the tongue as a pre-trematic nerve to the first arch.
20. The endodermal lining of the tympanic cavity, along with the ectoderm of the first branchial groove (branchial membrane), aids in the formation of the tympanic membrane or eardrum.
21. The period of sensitivity to teratogens that affect development is between 3 and 8 weeks, the period of organogenesis.