Embryology 4 PDF

EMBRYOLOGY 4 Dr.Ban A.Ghani THE INTRAEMBRYONIC MESODERM The intraembryonic mesoderm is formed by proliferation of cells in the primitive streak and it separates the ectoderm and the endoderm. As the notochord and neural tube developed, the intraem...

EMBRYOLOGY 4 Dr.Ban A.Ghani THE INTRAEMBRYONIC MESODERM The intraembryonic mesoderm is formed by proliferation of cells in the primitive streak and it separates the ectoderm and the endoderm. As the notochord and neural tube developed, the intraembryonic mesoderm on each side of notochord forms three longitudinal columns which are: 1-Paraxial mesoderm Paraxial mesoderm gives final count of approximately 42-44 pairs of somites. The somites form distinct surface elevations and are triangular in shape when seen in a transverse section These somites further differentiate into 3 components: a- Sclerotome (cartilage and bone) b- Myotome (muscles) c- Dermatome (dermis of skin) 2- Intermediate mesoderm 3-Lateral mesoderm Intermediate mesoderm (gives kidneys& genital system) Lateral mesoderm differentiates into: a-Somatopleure (gives the voluntary muscles of chest and abdomen and the parietal layer of pleura and peritoneum ). b- Splanchnopleure (gives the involuntary muscles of heart, bronchial tree, and gut and the visceral layer of pleura and peritoneum). The cavity between these two layers is the intraembryonic coelom. EMBRYOLOGY 4 Dr.Ban A.Ghani NEURAL CREST CELLS As the neural folds elevate and fuse, cells at the lateral border or crest of the neuroectoderm begin to dissociate from their neighbours. This cell population, the neural crest, will undergo an epithelial-to- mesenchymal transition as it leaves the neuroectoderm by active migration and displacement to enter the underlying mesoderm. Neural crest cells are so fundamentally important and contribute to so many organs and tissues that they are sometimes referred to as THE FOURTH GERM LAYER. Derivatives of NCC  EMBRYOLOGY 4 Dr.Ban A.Ghani DEVELOPMENT OF HEAD AND NECK(PHARYNGEAL ARCH,POUCH & CLEFT) The development of the head and neck begins in the 4th and 5th week. Growth of mesenchymal tissue (connective tissue) in the cranial region of the embryo results in the formation of arches, separated by clefts. These are the pharyngeal arches and pharyngeal clefts, a number of outpocketings appear on the lateral wall of the pharynx – the pharyngeal pouches. The pouches separate the arches on the internal (endodermal) surface whilst the clefts separate the arches on the external (ectodermal) surface.  As the embryo develops, each pharyngeal arch includes cartilaginous, arterial, muscular and nervous components. These will subsequently give rise to structures of the developing fetus  There are six pharyngeal arches, however, the 5th arch regresses before development is complete. Therefore, the numbering of first, second, third, fourth, sixth pharyngeal arches remains. The first two arches are named (as the mandibular and hyoid, respectively) while the rest of the arches remain unnamed.  EMBRYOLOGY 4 Dr.Ban A.Ghani  The entire pharyngeal apparatus consists of arches, pouches, grooves, and membranes, all of which are early embryonic structures with roles in the formation of the face and neck structures. The arches are formed bilaterally as adjacent c-shaped swellings or rolls. Each arch is separated from the next on the embryo’s external surface by pharyngeal grooves (clefts), and on the internal surface by depressions called pharyngeal pouches which are developed from the endoderm and they open towards the pharyngeal clefts. The first pair of pharyngeal arches are the primordium of the jaws. These are seen as two elevations lateral to the future pharynx. These elevations are further divided into maxillary (dorsal and upper positioned) and mandibular (ventral and lower) prominences. After the first pair of pharyngeal arches, the second one successively appears as well as the others. By the end of the fourth week, four visible pairs of pharyngeal arches are revealed while the fifth and sixth arches are invisible on the surface of the embryo.  EMBRYOLOGY 4 Dr.Ban A.Ghani DERIVATIVES OF PHARYNGEAL ARCHES First arch  It is comprised of two parts: Maxillary prominence ,mandibular prominence associated with Meckel’s cartilage  The first arch gives rise to the muscles of mastication and the mylohyoid, anterior belly of digastric, tensor veli palatini and tensor tympani  The artery the terminal portion of the maxillary artery  Nerve is the trigeminal nerve branches Second arch  Reichart’s cartilage is the cartilage component of the 2nd arch.It is the precursor to the stapes, the styloid process, the stylohyoid ligament and the upper body and lesser horn of the hyoid bone  muscular derivatives are the muscles of facial expression, stapedius, stylohyoid, platysma and the posterior belly of digastric.  There are two arteries: Stapedial artery & hyoid artery  Innervated by the facial nerve Third arch  The artery is common carotid artery and internal carotid artery.  It’s cartilaginous component is the lower body and greater horn of the hyoid.  It’s nerve is the glossopharyngeal nerve (CN IX).  Muscle is stylopharyngeus Fourth arch  The 4th arch gives rise to laryngeal cartilages  Muscular derivatives : the constrictors of the pharynx, levator palatini and cricothyroid.  The vascular derivatives :subclavian artery, aortic arch  The associated nerve is the superior laryngeal branch of the vagus nerve (CN X)  EMBRYOLOGY 4 Dr.Ban A.Ghani Sixth Arch  The muscles derived from the sixth arch include the intrinsic muscles of the larynx (except the cricothyroid)  The vascular derivatives : proximal portion of the pulmonary arteries ,ductus arteriosus  The nerve is recurrent laryngeal branch of the vagus nerve (CN X). PHARYNGEAL CLEFTS They are 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 the auditory or eustacian tube, and the intervening membrane develops into the typanic membrane. Pharyngeal cleft 2,3 and 4 are overgrown by expansion of the 2nd pharyngeal arch and usually obliterated. PHARYNGEAL POUCHES The pharyngeal pouches are endodermal-lined pockets that form on the inside of the pharynx between the arches. Pharyngeal pouches derivatives produce tissues necessary for hearing, calcium homeostasis, and adequate immune response  First pouch Develops into the middle ear cavity and the eustachian tube, which joins the tympanic cavity to the nasopharynx.  Second pouch Develops into the palatine tonsils, a secondary lymphoid organ playing a role in protecting the body from pathogens passing through the pharynx.  EMBRYOLOGY 4 Dr.Ban A.Ghani  Third pouch Develops into the thymus and inferior portion of the parathyroid  Fourth pouch Development of the superior region of the parathyroid and the ultimobranchial bodies.  Fifth pouch Rudimentary structure, becomes part of the 4th pouch contributing to thyroid C- cells.  Sixth pouch Contributes to the formation of the musculature and cartilage of the larynx along with the 4th pouch. FATE OF PHARYNGEAL ARCHES During the 5th week, the second pharyngeal arch enlarges and overgrows the third and fourth arches, forming the ectodermal depression called cervical sinus. By the end of 7th week the 2nd to 4th pharyngeal grooves and the cervical sinus have disappeared, giving the neck a smooth contour. 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. EMBRYOLOGY 4 Dr.Ban A.Ghani