Pharyngeal apparatus. - At the third week, it is a 3 layered structure: ectoderm, mesoderm and endoderm. This is called trilaminar disc

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Denis Newman
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1 Pharyngeal apparatus Remember from the first year embryology - The embryo was disc shaped in the second week of development (this is called embryonic disc) and it is a 2 layered disc (composed of two layers)---bilaminar disc - At the third week, it is a 3 layered structure: ectoderm, mesoderm and endoderm. This is called trilaminar disc - Then folding starts at the beginning of the 4 th week. The embryonic disc starts to fold from sides and cranio-caudally ( take a cylindrical shape instead of a flat disc) - Note that when the disc is folded, the ectoderm forms the outer layer of the cylinder, lining the inside is the endoderm, and the filling material in between is the mesoderm - The cranial part will form the head - The mesoderm after folding is called mesenchyme - Mesenchyme is the connective tissue of the embryo, and it differs from the connective tissue that we have now (postnatal), it has the ability to form many structures according to its location in the embryo. It can form muscles, blood vessels, cartilage, bone (pluripotent) - Remember that ectoderm before folding forms the neural groove, then neural tube and neural crest ( the neural tube and neural crest have derivatives that you will learn in the nervous system) - The rest of ectoderm will form the epidermis of the skin (remember that ectoderm is the outermost layer in the embryo) - The endoderm will form the mucous membranes of the gut and what arise from the gut (like respiratory system, part of the urinary bladder and the glands associated with the gut like the liver and pancreas). remember that endoderm is the innermost lining layer of the embryo - The mesoderm derivatives depend on the location of mesenchyme

2 The mesenchyme in the neck region: Look at the embryonic disc before folding, look at the superior surface of the disc, you will see the ectoderm Remove the ectoderm layer from the embryonic disc, u will see the mesoderm This layer is called intra-embryonic mesoderm Structures seen from this view are: Notochord in the middle Paraaxial mesoderm: divides into somites (squares) Intermediate mesoderm Lateral plate mesoderm: forms U-shaped curve (horse-shoe in shape) directed cranially. Inside the lateral plate a cavity will form called intraembryonic coelom. The intra-embryonic coelom is divided into one pericardial and two pleural and peritoneal areas. In front the pericardial area is the septum transversum which gives rise to diaphragm. Behind the pericardial area is the cardiogenic area which gives rise to heart. Note the cardiogenic area is located between the buccopharyngeal membrane and the pericardial area (heart starts to form in the beginning of day 18). End of revision Now after folding, we will take a longitudinal section (from head to tail) The blue line is the ectoderm The green line is the endoderm In-between is the mesenchyme Note that the cylindrical tube lined by the endoderm (green) will form the gut The gut is divided into: foregut, midgut and hindgut The head fold enlarges forward (because of the growing brain) and forms the head The cranial part of foregut is called pharynx The pharynx is lined by endoderm, and surrounded by mesenchyme then ectoderm

3 After folding, the buccopharyngeal membrane lies between the pharynx and the stomodeum (the future oro-nasal cavity) So the pharynx is separated from the future nasal/oral cavity by the buccopharyngeal membrane Note that postnatally the pharynx is located behind the nasal cavity, oral cavity and the larynx, and then continues as esophagus The pharynx in the neck area of the embryo is endoderm, like any other endoderm in the embryo; the pharynx is surrounded by mesenchyme then ectoderm. The neck is a cylindrical structure, inside there is a tube of endoderm, surrounded by mesenchyme, then ectoderm Now during folding (not after folding) at the 4 th week of development what will happen? At the sides of the pharynx, the mesoderm at the sides of pharynx thickens The mesoderm outgrows, proliferates to form long cylindar There are 6 mesodermal thickenings at the side of the pharynx due to proliferation of the mesoderm in this area along six lines Each thickening is cylindrical in shape and arched (curved) why? Because they are inside a cylinder (the neck is rounded and cylindrical) These 6 structures are called pharyngeal (branchial) arches Why pharyngeal? Because they are formed at the side of the pharynx Why arches? Because they are curved (not straight) Why branchial? Because when you look from outside at the side of the neck of the embryo, you see ectodermal elevations (because of the outgrowth of the underlying mesoderm).. These elevations look like the gills of the fish (branchia: gills) So branchial according to shape Pharyngeal according to site So what are pharyngeal arches?

4 Pharyngeal arches are six ventrodorsal, cylindrical and curved thickenings in the mesoderm on both sides of pharynx appear at the 4 th and 5 th weeks of development Note that the pharyngeal arches are covered by ectoderm from outside There are depressions or grooves between the arches seen from outside, these grooves are called clefts If you are inside the pharynx, and looking inside, you will also see grooves between the arches lined by endoderm, these grooves are called pouches So the pharyngeal arches when formed, cause the formation of clefts from outside and pouches from inside Pharyngeal arches+ clefts+ pouches= are called pharyngeal apparatus Now we will study the derivatives of pharyngeal arches+ clefts+ pouches Note that not all the six pharyngeal arches will form at the same time, first 3 arches will form, the first arch converts to its derivatives, then the 2 nd and third. In the 5 th week, the 4 th arch appears then 5 th then 6 th and convert to their derivatives (5 th week),,,,, so its impossible to see the 6 arches at the same time..so the pharyngeal arches appear gradually and we can see maximum four arches at the same time Stomodeum: the depressed area between the forebrain bulge (head bulge) and the pericardial bulge (heart bulge) which will form the nasal and oral cavities Each arch has a ventral end and a dorsal end

5 First arch The first arch has ventral end and dorsal end, but because it s a special arch (forms the lower part of the face), the dorsal end grows ventrally also along with the ventral end The ventral end of the first arch is called mandibular process The dorsal end is called maxillary process (to fill the face from the sides) The first arch is the only arch which has two processes Note that this happens on both sides of the pharynx Now. Each arch is composed of mesenchyme, this mesenchyme gives rise to bones, cartilages and muscles Each arch has blood supply and nerve to supply the muscles in each arch If you know the nerve of each arch, you will know the muscles originate from each arch For example, the nerve of the first arch is the trigeminal (5 th ) The second arch is supplied by facial nerve (7 th ) The third arch is supplied by glossopharyngeal nerve (9 th ) The fourth and sixth arches are supplied by vagus nerve (10 th ) Branches of vagus are: superior laryngeal (external) and recurrent laryngeal nerves 4 th arch is supplied by superior laryngeal branch 6 th arch is supplied by recurrent laryngeal branch Note the fifth arch degenerates and disappears and does not form any thing The blood supply for each arch is formed by the 6 aortic arches (right and left), each aortic arch run in each arch (the formation of aortic arches will be explained later in cardiovascular system) First arch is supplied by trigeminal nerve. The trigeminal nerve divides into 3 branches The ophthalmic nerve: goes to orbit

6 The maxillary nerve supplies the maxillary process of first arch The mandibular supplies the mandibular process of first arch now the mucous membrane of the anterior two thirds of the tongue originates from the first arch and supplied by the mandibular nerve (general sensations) but there is no taste sensations in the mandibular nerve, so the first arch borrows some nerve fibers (taste fibers) from the second arch (facial nerve). This nerve is called chorda tympani. Chorda tympani is a branch of facial nerve, it carries taste sensations from the anterior two thirds of the tongue and it is called pre-trematic nerve What is pre-trematic nerve? Pre-trematic nerve crosses from one arch to other example: chorda tympani Note Post trematic nerve: is the original nerve of the arch, like facial nerve of the second arch Now in a coronal section, you see the pharyngeal arches on both sides, lined internally by endoderm and covered externally by ectoderm. In each arch, there are muscles, nerve, artery and cartilage The cartilage either stays as cartilage or is replaced by a bone (endochondral ossification) From inside you see pouches From outside, you see clefts Derivatives of pharyngeal arches First arch: Nerve of first arch is trigeminal

7 Maxillary process forms lower part of temporal bone, zygomatic bone and maxilla Mandibular process will form a cylindrical shaped cartilage called Meckel's cartilage The dorsal end of Meckel s cartilage forms two bones in the middle ear (malleus and incus) by endochondral ossification The Ventral end of Meckel's cartilage form part of the mandible (ramus) by endochondral ossification The middle part of Meckel s cartilage disappears but its perichondrium persists and forms two ligaments 1- Anterior ligament of malleus 2- sphenomandibular ligament (from sphenoid to lingula of mandible) These two ligaments are not required So not all Meckel's cartilage forms bone The rest of the mandible forms a membrane which then forms bone by intramembranous ossification Muscles of first pharyngeal arch: the muscles supplied by the mandibular nerve (not the maxillary as it has no motor) 1- muscles of mastication 2- tensor tympani 3- anterior belly of digastric 4- mylohyoid Second arch: Forms a cartilage called Reichert's cartilage The dorsal end of Reichert's cartilage forms 2 bones: stapes and styloid process The ventral end of Reichert's cartilage forms the upper part of the body of hyoid bone and the lesser horn

8 The middle part forms the ligament between them called Stylohyoid ligament (The middle part of Reichert's cartilage disappears and its perichondrium persists to form Stylohyoid ligament) Muscles of second pharyngeal arch: the muscles supplied by the facial nerve 1- muscles of facial expression 2- posterior belly of digastrics 3- stylohyoid 4- stapedius Third arch The nerve of the 3 rd arch is the glossopharyngeal nerve (9 th ) The third arch forms the rest of hyoid bone (lower part of the body and greater horn) No ligaments Only one muscle: stylopharyngeus (supplied by 9 th ) Fourth arch The nerve of the 4 th arch is the superior laryngeal nerve (branch of vagus 10 th ) It forms: 1- laryngeal cartilages 2- One muscle : cricothyroid (the only laryngeal muscle that is supplied by external laryngeal nerve (branch of superior laryngeal n) Fifth arch: disappears Sixth arch: The nerve of the 6 th arch is the recurrent laryngeal nerve It continues the derivatives of the fourth arch 1- Laryngeal cartilages 2- Laryngeal muscles (except cricothyroid)

9 Now we have finished the derivatives of the mesenchyme core inside each pharyngeal arch now we will talk about the derivatives of the clefts (covered by ectoderm) and pouches (lined by endoderm) Clefts and pouches - Clefts are 4 in number (not 5, as the 5 th arch disappears from outside then from inside) - Pouches are 5 in number, as the inside of 5 th arch disappear slowly (slower that the outer aspect) Pharyngeal Clefts - Dorsal end of first cleft forms external auditory meatus and the outer layer of ear drum Remember that eardrum has three layers - Outer layer: skin (from ectoderm) - Middle : fibrous tissue (from mesoderm in between) - Inner layer: mucous membrane (from endoderm) Extra information: the auricle is formed by tubercles above and below the dorsal end of the first cleft; these tubercles arise in a very regular manner and merge to form the auricle with its characteristic shape. Note that your auricle is located between the first and second arches Now the 2 nd,3 rd and 4 th clefts will disappear and will not give any derivatives, How? The second arch will externally grow, the growth is external and downward, it descends as a curtain to cover the 2 nd,3 rd and 4 th clefts This curtain closes first dorsally and forms a ventral sinus called cervical sinus. Then it closes ventrally, so the sinus becomes a cyst (called cervical cyst). Then it obliterates and the nice contour of our neck is formed. So again, the cervical sinus is formed on either side of the neck. Cervical cyst (branchial cleft cyst) is a cyst in the skin of the lateral part of the neck.

10 Sometimes it can remain anterior to the sternocleidomastoid muscle. It can communicate with the skin (external fistula) or with the pharynx (internal fistula). Fistula is an abnormal communication or connection between two spaces. It is prone to infection. If it opens to both the interior and exterior, it forms a pharyngocutaneous fistula, which allows saliva to run out during mastication Again As we said before, the cylindrical embryo is covered externally by ectoderm, the gut tube inside is lined by endoderm, the filling material between the endoderm and ectoderm is the mesenchyme. Note that the ectoderm and endoderm are in direct contact with each other (no mesoderm in between) in only two places 1- The buccopharyngeal membrane : cranially 2- The cloacal membrane: caudally The buccopharyngeal membrane lies between stomodeum and the pharynx (upper part of the foregut) stomodeum will form the nasal and oral cavities. So the buccopharyngeal membrane should rupture later on to allow communication between the oro-nasal cavity in front and the pharynx behind At the 4 th week of development (folding time), the mesoderm at the sides of the pharynx proliferate to form pharyngeal arches with clefts outside and pouches inside. They are six in number, appear in the 4 th and 5 th weeks of development. All pharyngeal arches will form derivatives except the 5 th arch which will disappear We talked about the derivatives for each pharyngeal arch. Pharyngeal clefts: only the first cleft gives rise to important derivatives The 2 nd,3 rd and 4 th clefts will be covered by the second arch The second arch proliferates downward and fuses with epicardial ridge

11 Now from inside the pharynx, you will see also grooves, the endodermal grooves are called pouches. Pharyngeal Pouches - 5 in number - Lined by endoderm - Remember endoderm forms mucous membranes or extra derivatives - Each pouch has ventral end and dorsal end First pouch - The dorsal end of first pouch is opposite to dorsal end of first cleft - As we said that first cleft (ectoderm) forms the external auditory meatus and the skin of eardrum (outer surface of ear drum) - So the first pouch (endoderm) forms the inner mucous membrane of the tympanic membrane (ear drum) and middle ear and auditory tube (remember auditory tube opens into nasopharynx).. all these structures are lined with mucous membrane - Mesoderm in between forms the fibrous middle layer of ear drum - Ventral end of first pouch will be occupied by the developing tongue The second pouch: The dorsal part of the second pouch forms the palatine tonsils The ventral parts of the first and second pouches Note that the ventral parts of the first and second pouches form the floor of the pharynx The ventral parts of the first and second pouches are occupied by the developing tongue this area is where the tongue develops and when the

12 buccopharyngeal membrane ruptures, the tongue grows toward the oral cavity The third pouch: Ventral part: thymus Dorsal part: inferior parathyroid gland Note that post-natally, thymus gland is located behind the sternum in thorax Inferior and superior parathyroid glands are on the posterior wall of thyroid gland in the neck During development, thymus gland will develop from the ventral part of third pouch, and the inferior thyroid gland will develop from the dorsal part of third pouch. Both glands will separate from the pouch, and the thymus then descends (in a caudal and a medial direction), pulling the inferior parathyroid with it. The thymus continues to reach the thorax, while the inferior parathyroid attaches to posterior surface of thyroid gland Growth and development of the thymus continue until puberty In the young child, the thymus occupies considerable space in the thorax and lies behind the sternum and anterior to the pericardium and great vessels In older it is atrophied and replaced by fatty tissue The fourth pouch: Ventral part: unknown Dorsal part: superior parathyroid gland

13 The fifth pouch Forms ultimobranchial body which forms the parafollicular cells of thyroid gland Note: thyroid gland contains two types of cells Follicular and parafollicular cells, these two types have different embryonic origins The follicular cells originate in the thyroid gland itself (between the anterior 2/3 and post 1/3 of the tongue, then thyroid gland descends in the neck) The parafollicular cells originate from the 5 th pouch and incorporate inside the thyroid gland Follicular cells secrete the thyroid hormone while parafollicular cells secrete calcitonin (controls the Ca++ blood levels)

04 Development of the Face and Neck. Development of the Face Development of the neck

04 Development of the Face and Neck Development of the Face Development of the neck Development of the face Overview of facial development The fourth week ~ the twelfth week of prenatal development Between