BMS 150 Embryology 4 Win2023 PDF

Embryology The Oropharyngeal Apparatus References: The Developing Human – Clinically-Oriented Embryology BMS 150 Editors: Moore, Persaud, Torchia Week 10 Chapter 9 The pharyngeal arches Pharyngeal arch = a core of mesenchyme (embryonic connective tissu...

Embryology The Oropharyngeal Apparatus References: The Developing Human – Clinically-Oriented Embryology BMS 150 Editors: Moore, Persaud, Torchia Week 10 Chapter 9 The pharyngeal arches Pharyngeal arch = a core of mesenchyme (embryonic connective tissue) covered externally by ectoderm and internally by endoderm ▪ Originally, this mesenchyme is derived from mesoderm in the third week ▪ During the fourth week, most of the mesenchyme is derived from neural crest cells that migrate into the pharyngeal arches migration of neural crest cells into the arches and their differentiation into mesenchyme produces the maxillary and mandibular prominences The pharyngeal arches A pharyngeal arch contains: ▪ A pharyngeal arch artery that arises from the truncus arteriosus of the primordial heart and passes around the primordial pharynx to enter the dorsal aorta ▪ A cartilaginous rod that forms the skeleton of the arch ▪ A muscular component that differentiates into muscles in the head and neck ▪ Sensory and motor nerves that supply the mucosa and muscles derived from the arch derived from neuroectoderm of the primordial brain. The pharyngeal arches The pharyngeal arches are the main formative elements of the face, nasal cavities, mouth, larynx, pharynx, and neck ▪ During the fifth week, the second pharyngeal arch enlarges and overgrows the third and fourth arches, forming an ectodermal depression-the cervical sinus ▪ By the end of the seventh week, the second to fourth pharyngeal grooves and the cervical sinus have disappeared, giving the neck a smooth contour. Pharyngeal grooves and pouches The primordial pharynx, derived from the foregut, widens cranially where it joins the stomodeum, and narrows caudally where it joins the esophagus The pharyngeal endoderm lines the internal aspects of the pharyngeal arches and passes into diverticula (outpouchings) = pharyngeal pouches ▪ There are four pairs of pharyngeal pouches; the fifth pair is rudimentary or absent ▪ The endoderm of the pouches contacts the ectoderm of the pharyngeal grooves and together they form s double-layered pharyngeal membrane separates the pharyngeal pouches from the pharyngeal grooves 28 days Days 33- 41 The pharyngeal apparatus The pharyngeal arches begin to develop early in the fourth week ▪ neural crest cells migrate into the ventral parts of the future head and neck regions ▪ The first pair of pharyngeal arches, the primordium of the jaws, appears as surface elevations lateral to the developing pharynx ▪ Soon other arches appear as rounded ridges on each side of the future head and neck By the end of the fourth week, four pairs of pharyngeal arches are visible externally The fifth and sixth arches are rudimentary and are not visible on the surface of the embryo The pharyngeal arches are separated from each other by the pharyngeal grooves ▪ pharyngeal grooves are also numbered in a cranial – caudal sequence Bony and cartilaginous derivatives of the pharyngeal arches The first pharyngeal arch (mandibular arch) separates into two prominences ▪ The maxillary prominence gives rise to the maxilla, zygomatic bone, and a portion of the vomer. ▪ The mandibular prominence forms the mandible The proximal mandibular prominence also forms the squamous temporal bone ▪ The first arch plays a major role in the overall formation of the face The second and third pharyngeal arches form the hyoid bone Bony and cartilaginous derivatives of the pharyngeal arches The first and second pharyngeal cartilages give rise to the ossicles of the middle ear and the styloid process of the temporal bone The fourth and sixth pharyngeal arches give rise to the laryngeal cartilage The third and fourth pharyngeal arches also eventually give rise to the epiglottis The fifth pharyngeal arch is rudimentary – nothing is derived from it, and it disappears 4 weeks fetus Muscular derivatives of the pharyngeal arches first pharyngeal arch: muscles of mastication, muscles of middle ear second pharyngeal arch: stapedius, stylohyoid, posterior belly of digastric, auricular, and muscles of facial expression third pharyngeal arch: stylopharyngeus fourth pharyngeal arch: cricothyroid, levator veli palatini, and constrictors of the pharynx sixth pharyngeal arch: intrinsic muscles of the larynx. 4 weeks fetus  Arch Nerve Muscles Bones Muscles of mastication: temporalis, Maxilla, mandible masseter, medial and lateral palatine bone, vomer Trigeminal pterygoids zygomatic bone, First (but not V1) mylohyoid, anterior belly of the squamous part of the digastric, tensor veli palatini temporal bone tensor tympani malleus, incus Muscles of facial expression Upper part of hyoid stylohyoid, posterior belly of the bone Second facial digastric Stapes stapedius Styloid process glosso- Lower part of the hyoid Third stylopharyngeus pharyngeal bone 4th – superior 4th – mostly swallowing muscles laryngeal Fourth (pharyngeal – cricothyroid, levator veli (vagus) Both contribute to the + palatini, pharyngeal constrictors) 6th – cartilages of the larynx Sixth recurrent 6th – mostly muscles of the larynx laryngeal (vagus) The pharyngeal pouches The first pharyngeal pouch expands into a tubotympanic recess ▪ The expanded distal part of this recess contacts the first pharyngeal groove → contributes to the formation of the tympanic membrane (eardrum) ▪ The cavity of the tubotympanic recess becomes the tympanic cavity (where the bones of the middle ear are found) and mastoid antrum ▪ The connection of the tubotympanic recess with the pharynx elongates and forms the pharyngotympanic tube (auditory tube, Eustachian tube) The pharyngeal pouches The second pharyngeal pouch eventually gives rise to parts of the palatine tonsils By the sixth week, the third pouch develops: ▪ Into parathyroid glands – dorsal part (inferior pair) ▪ Into a thymus – ventral part The fourth pouch also develops into parathyroids – the superior pair There are no fifth or sixth pouches 5 weeks 6 weeks 7 weeks Number Pouch Membrane Groove External Middle ear, mastoid Tympanic First auditory antrum, auditory tube membrane meatus Second Palatine tonsils - - Inferior parathyroids, Third - - thymus Fourth Superior parathyroids - - Development of the thyroid thyroid gland is the first endocrine gland to develop in the embryo It begins to form 24 days after fertilization from a median endodermal thickening in the floor of the primordial pharynx This thickening forms a small outpouching-the thyroid primordium As the embryo and tongue grow, the developing thyroid gland descends in the neck, passing ventral to the developing hyoid bone and laryngeal cartilages ▪ For a short time, the thyroid gland is connected to the tongue by a narrow tube, the thyroglossal duct ▪ By 7 weeks, the thyroid gland is usually located in its final site in the neck By this time, the thyroglossal duct usually degenerates  4, 5, & 6 weeks Last image – mature thyroid