Neural Crest, Eye, and Ear Development PDF

NEURAL CREST, EYE AND EAR Class Year 2, Semester 1 Lecturer DR. VIJAYALAKSHMI S B Department of Anatomy Email id: [email protected] Date 26-11-2024 1 LEARNING OUTCOMES Describe the origin of neural crest c...

NEURAL CREST, EYE AND EAR Class Year 2, Semester 1 Lecturer DR. VIJAYALAKSHMI S B Department of Anatomy Email id: [email protected] Date 26-11-2024 1 LEARNING OUTCOMES Describe the origin of neural crest cells and their migration Explain how failure of normal neural crest migration can give rise to pathology (Hirschsprung's disease) Describe the development of the eye and explain how embryological abnormalities such as coloboma or cataract formation occur Describe the development of the ear and explain how problems such as congenital deafness occur Describe the pharyngeal arches and their derivatives NEURAL CREST CELLS  Neural crest cells Special cell population at the lateral border or crest of the neuroectoderm Begins to dissociate from their neighbors while neural folds elevate and fuse each other to form neural tube These special cells undergoes an epithelial-to-mesenchymal transition Leaves the neuroectoderm by active migration to many specific location in the body giving rise to variety of structures Neural crest Derivatives Connective tissue and bones of the face and skull Cranial nerve ganglia C cells of the thyroid gland Conotruncal septum in the heart Odontoblasts Dermis in face and neck Spinal (dorsal root) ganglia Sympathetic chain and pre-aortic ganglia Parasympathetic ganglia of the gastrointestinal tract Adrenal medulla Schwann cells Glial cells Meninges (forebrain) Melanocytes Smooth muscle cells to blood vessels of the face and forebrain  Hirschsprung’s disease Congenital megacolon Failure of migration of neural crest cells into the bowel wall Defective parasympathetic (Auerbach’s and Meissner’s) plexus courtesy of surgical-tutor.org.uk Aganglionic segment Especially sigmoid and rectum Delayed passage of meconium Constipation Vomiting Abdominal distension Hirschsprung disease. A contrast radiograph shows marked dilation of the rectosigmoid colon proximal to the narrowed rectum Development of Eye  OPTIC CUP & LENS VESICLE Derived from four sources: Neuroectoderm of the forebrain Surface ectoderm of the head Mesoderm between the previous two layers Neural crest cells THE EARLIEST SIGN OF EYE DEVELOPMENT is seen in the 22-day embryo as a pair of shallow grooves on each side of the forebrain Closure of neural tube – grooves form outpocketings – Optic vesicles  DEVELOPMENT OF EYE Embryonic structure Adult derivative Neuroectoderm Retina, posterior layers of iris, and optic nerve Surface ectoderm Lens of the eye and the corneal epithelium Mesoderm between Fibrous and vascular coats of neuroectoderm and surface the eye ectoderm Neural crest cells Migrate into the mesenchyme and differentiate into the choroid, sclera, and corneal endothelium Courtesy: Dr Aamir Hameed  OPTIC CUP & LENS VESICLE Lens vesicle – optic vesicles comes in contact with the surface ectoderm and induce changes Optic vesicles begin to invaginate and form the double-walled optic cup. The inner and outer walls of the cup are initially separated by the intraretinal space, but with development, the lumen disappears, and the walls oppose each other.  Presence of choroid fissure Fissure allows the hyaloid vessels Lips of choroid fissure fuse – 7th week The hyaloid artery and vein later become the central artery and vein of the retina  Cells of the surface ectoderm in contact with the optic vesicle – lens placode Lens placode invaginates – lens vesicle During 5th week – lens vesicle loses contact with the surface ectoderm and lies in the mouth of the optic cup  RETINA, IRIS AND CILIARY BODY Outer wall of the optic cup – Pigment layer of retina Posterior four-fifths Inner wall of the optic cup – Neural layer of retina  RETINA, IRIS AND CILIARY BODY Anterior fifth of the inner wall of optic cup divides into: Pars iridica retinae – inner layer of the iris Pars ciliaris retinae – ciliary body Sphincter & dilator pupillae, ciliary muscles – mesoderm present between optic cup & the overlying surface epithelium  CHOROID, SCLERA AND CORNEA Undifferentiated mesenchyme around the eye primordium Mesenchyme differentiates into two layers: Inner layer – develops into highly vascularized pigmented called choroid Outer layer – develops into sclera  Anterior chamber is formed through vacuolisation – splits the mesenchyme into two layers: Inner layer (in front of lens & iris) – forms iridopupillary membrane, which later disappears completely Outer layer (continuous with sclera) – forms substantia propria of the cornea Posterior chamber – is a space between the iris anteriorly and the lens and ciliary body posteriorly With further development, the optic cup forms the CILIARY BODY and IRIS, while the overlying ectoderm forms the CORNEA and EYELIDS. The optic stalk has become the OPTIC NERVE. The optic stalk contains axons from the ganglion cell layer of the retina. The choroid fissure closes during week 7, so that the optic stalk, together with the axons of the ganglion cells, forms the optic nerve (CN II), optic chiasm, and optic tract. The optic nerve (CN II) is a tract of the diencephalon and has the following characteristics: The optic nerve is not completely myelinated until 3 months after birth; Myelinated by oligodendrocytes The optic nerve is not capable of regeneration after transection  CLINICAL CORRELATES Coloboma iridis Aniridia due to failure of closure of Persistence of (Absence of iris) choroid fissure iridopupillary membrane  Congenital Aphakia Absence of lens Synophthalmia / Cyclopia Congenital Cataracts fused or single eye Lens become opaque Development of Ear  Mature components of the ears The ear is composed of inner, middle and external ear portions External ear -auricle - external auditory meatus - tympanic membrane Middle ear - malleus, incus & stapes - auditory tube Internal ear - cochlear duct - semicircular canal  Formation of the ear Dual origin thickened ectodermal placode at rhombencephalon (hindbrain) first and second pharyngeal arches Rhombencephalon region:formation of otic vesicles Derivatives of the 1st & 2nd pharyngeal arches  Development of internal ear Induction – rhombencephalon ~ surface ectoderm Otic placode Otic vesicles Dorsal vestibular region Ventral cochlear region  Membranous labyrinth Ventral cochlear region Saccule Cochlear duct Utricle Semicircular Endolymphatic canals duct Development of the otic vesicle showing a dorsal utricular portion with endolymphatic duct, and a ventral saccular portion Development of the semicircular canals  Neural crest forms surrounding connective tissue (mesenchyme) Cartilaginous, then ossifies to form a bony labyrinth  1st arch = malleus & incus Mesenchyme 2nd arch = stapes Otic vesicle 1st pharyngeal cleft = EAM (ectoderm) 1st pharyngeal pouch = tubotympanic recess (endoderm) JC Holland  Tympanic membrane (Eardrum) The definitive eardrum is derived from the first pharyngeal membrane and is thus composed of pharyngeal cleft ectoderm and pharyngeal pouch endoderm, plus neural crest cells (ectoderm) that infiltrate the space between the cleft ectoderm and the pouch endoderm. Therefore, the definitive eardrum is a three-layered structure derived from two germ layers.  Development of Auricle Develops from six mesenchymal proliferations at the dorsal ends of the first and second pharyngeal arches, surrounding the first pharyngeal cleft (auricular hillocks) Congenital malformation of the ear Congenital deafness - inner ear deafness(rubella), middle ear deafness (1st & 2nd arches), agenesis of external ear Auricular anomalies  External ear deformities Congenital Accessory tragus Microtia Anotia Preauricular pits or sinuses  Pharyngeal arches and their derivatives Pharyngeal apparatus Pharyngeal arches not only contribute to the formation of the neck but also play an important role in the formation of the face Pharyngeal arches Each arch consists of a core of mesenchymal tissue covered on outside by surface ectoderm and inside by epithelium of endodermal origin Neural crest cells migrate into the arches to contribute to skeletal components of the face Mesenchymal cells of mesoderm of the arches gives rise to the muscles of face and neck Each arch have their own cranial nerve and arterial component  Fate of pharyngeal arch First pharyngeal consists of a dorsal portion, the maxillary process and a ventral portion, the mandibular process, which contains Meckel’s cartilage Derivatives of the pharyngeal arches and their innervation Pharyngeal pouches Pharyngeal pouches  Pharyngeal clefts Among the four pharyngeal clefts, only the first cleft contributes to the formation of external auditory meatus The epithelial lining – pharyngeal membrane – forms tympanic membrane (ear drum)  Pharyngeal clefts Active and proliferation of mesenchymal tissue in the second arch causes it to overlap the third and fourth arches. Merges with the epicardial ridge in the lower part of the neck, and the 2nd and 3rd clefts lose contact with the outside. Clefts form a cavity lined with ectodermal epithelium – cervical sinus- later disappears Brachial fistulas – when 2nd arch fails to grow caudally over the 3rd & 4th arches Cervical cyst – remnants of cervical sinus  1st Arch Defect Mandibulofacial dysostosis Autosomal dominant  1st Arch Defect Micrognathia Poor growth of the mandible Cleft palate Glossoptosis Posteriorly placed tongue fails to drop between palatal shelves Genetic and/or environmental causes  3rd & 4th Pouch Syndrome Absence of thymus Absence of parathyroid glands Persistent truncus arteriosus Abnormal external ear Micrognathia  Practice References questions