Head and Neck Structure Notes


Osmosis High-Yield Notes

This Osmosis High-Yield Note provides an overview of Head and Neck Structure essentials. All Osmosis Notes are clearly laid-out and contain striking images, tables, and diagrams to help visual learners understand complex topics quickly and efficiently. Find more information about Head and Neck Structure:

Pharyngeal arches, pouches, and clefts

Development of the ear

Development of the eye

Development of the teeth

Development of the nervous system

NOTES NOTES HEAD & NECK STRUCTURE PHARYNGEAL ARCHES, POUCHES, & CLEFTS osms.it/pharyngeal-arches-pouches-clefts ▪ Week 4: pharyngeal apparatus begins to form, develop into various head, neck structures ▪ Bars of mesoderm form six pharyngeal arches in craniocaudal fashion ▫ Numbered from one to six ▫ 5th quickly regresses, does not form any structures ▪ Between pharyngeal arches, four pharyngeal clefts cover each arch’s external part with ectoderm ▪ Four pharyngeal pouches line each arch’s internal part with endoderm ▪ Each pharyngeal arch carries its own cranial nerve First pharyngeal arch ▪ Innervated by mandibular branch of trigeminal nerve (CN V3) ▪ Bones ▫ Forms maxilla, mandible temporal, zygomatic bones ▫ Two small portions of mandible form incus, malleus bones of middle ear ▪ Muscles ▫ Muscles that help with chewing: temporalis, masseter, pterygoid muscles, tensor tympani muscles ▫ Muscles that help with swallowing: tensor veli palatini, mylohyoid muscles, anterior belly of digastric muscle Figure 30.1 Locations of the pharyngeal arches, clefts, and pouches. OSMOSIS.ORG 249
Figure 30.2 Bones and muscles originating from the first pharyngeal arch. Second pharyngeal arch ▪ Innervated by facial nerve (CN VII) ▪ Bones ▫ Lesser horns, upper portion of hyoid bone ▫ Styloid process of temporal bone ▫ Stapes bone of middle ear ▪ Muscles ▫ Stylohyoid muscle, posterior belly of digastric muscle ▫ Stapedius muscle of middle ear Third pharyngeal arch ▪ Innervated by glossopharyngeal nerve (IX) ▪ Bones ▫ Rest of hyoid bone ▪ Muscles ▫ Stylopharyngeus muscle in throat 250 OSMOSIS.ORG Fourth pharyngeal arch ▪ Innervated by superior laryngeal branch vagus nerve (CN X) ▪ Muscles ▫ Levator palatini, pharyngeal constrictors, cricothyroid muscle Sixth pharyngeal arch ▪ Innervated by recurrent laryngeal branch of CN X ▪ Muscles ▫ Rest of intrinsic muscles of larynx PHARYNGEAL CLEFTS AND POUCHES First pharyngeal cleft, pouch ▪ Form ear ▪ Cleft gives rise to external auditory meatus, ear drums
Chapter 30 Embryology: Head & Neck Structure Figure 30.3 Bones and muscles originating from the second pharyngeal arch. Figure 30.4 Structures originating from the third, fourth, and sixth pharyngeal arches. Muscles from fourth and sixth not shown. ▪ Pouch gives rise to internal auditory meatus, AKA middle ear, eustachian tube Second-fourth clefts ▪ Fade as embryo grows ▫ Cells lining second pharyngeal pouch multiply, migrate to form primitive tonsils Third, fourth pouches ▪ Both divide into dorsal, ventral portions ▪ Dorsal portion of third pouch becomes inferior parathyroid gland ▪ Ventral portion becomes primitive thymus ▫ Later descends down to chest ▪ Dorsal portion of fourth pouch becomes superior parathyroid gland ▪ Ventral portion becomes ultimo-pharyngeal body ▫ Contains cells which differentiate into parafollicular/C-cells, migrate into thyroid Thyroid and parathyroid glands ▪ Thyroid develops from endoderm at base of tongue independent of pharyngeal apparatus, descends down neck ▪ Parathyroid glands latch onto thyroid OSMOSIS.ORG 251
Figure 30.5 Thyroid develops from endoderm at base of tongue independent of pharyngeal apparatus, descends down neck. Parathyroid glands latch on as it passes by them. DEVELOPMENT OF TEETH osms.it/development-of-teeth ▪ Tooth development, AKA odontogenesis, involves epithelial, neural crest-derived mesenchymal interaction ▪ Week 6: basal layer of oral epithelium has formed C-shaped dental lamina ▫ Gives rise to 10 dental buds in each jaw Cap stage ▪ Invagination of deep surface of buds → dental cap ▪ Each dental cap consists of: ▫ Outer dental epithelium ▫ Inner dental epithelium ▫ Central core of stellate reticulum ▪ Mesenchyme forms dental papilla, which form odontoblasts ▫ Produce dentin ▪ Remainder of dental papilla forms pulp Bell stage ▪ Dental cap grows, indentation deepens, forming bell-shaped configuration ▪ Inner dental epithelium cells transform into ameloblasts ▫ Produce enamel deposited over dentin 252 OSMOSIS.ORG ▪ As enamel thickens, ameloblasts retreat into stellate reticulum, regress ▪ Also form enamel knot, which regulates early tooth development Root formation ▪ Inner and outer dental epithelial layers invade underlying mesenchyme, form epithelial root sheath ▪ Pulp begins to narrow as more dentin laid down ▫ Forms canal containing nerves, blood vessels ▪ Mesenchymal cell differentiation ▫ Cementoblasts produce cementum (AKA type of specialized bone) ▫ Periodontal ligament gives structural integrity to tooth ▪ As root lengthens, it pushes crown into oral cavity ▫ Deciduous teeth (AKA milk teeth) arise 6–24 months of age ▪ Permanent teeth buds form during third month of development, remain dormant until sixth year of life
Chapter 30 Embryology: Head & Neck Structure DEVELOPMENT OF THE BRAIN osms.it/development-of-the-brain DEVELOPMENT OF BRAIN VESICLES ▪ ▪ ▪ ▪ Neural plate folds, forming neural tube Rostral region develops into brain Week 4: primary brain vesicles develop Week 6: vesicles develop Primary vesicle: forebrain/prosencephalon ▪ Secondary vesicles ▫ Telencephalon: cerebral hemispheres, caudate, putamen, amygdaloid, claustrum, laminal terminalis, olfactory bulbs, hippocampus ▫ Diencephalon: epithalamus, subthalamus, thalamus, hypothalamus, mammillary bodies, neurohypophysis, pineal gland, globus pallidus, renina, iris, ciliary body, optic nerve (CN II), optic chiasm, optic tract Primary vesicle: midbrain/mesencephalon ▪ Secondary vesicle ▫ Mesencephalon Primary vesicle: hindbrain/ rhombencephalon ▪ Secondary vesicles ▫ Metencephalon: pons, cerebellum ▫ Myelencephalon: medulla DEVELOPMENT OF HINDBRAIN/ RHOMBENCEPHALON ▪ Alar, basal plates separated by sulcus limitans Basal plate ▪ Contains three groups of motor nuclei ▪ General somatic efferent ▫ Cranial nerves III, IV, VI (metencephalon); XII (myelencephalon) ▫ Innervation: somatic striated muscle (extrinsic eye muscles, tongue) ▪ Special visceral efferent ▫ Cranial nerves V, VII (metencephalon); IX, X (myelencephalon) ▫ Innervation: striated muscle of pharyngeal arches, AKA pharynx ▪ General visceral efferent ▫ Cranial nerve III (metencephalon); IX, X (myelencephalon) ▫ Innervation: parasympathetic pathway to sphincter pupillae; smooth muscles of airways, heart, salivary glands, viscera Alar plate neuroblasts ▪ Contain three groups of sensory relay nuclei ▪ General visceral afferent ▫ Cranial nerve X (myelencephalon) ▫ Innervation: viscera, AKA gastrointestinal tract ▪ Special afferent ▫ Cranial nerves VII, IX (metencephalon, myelencephalon); VIII (metencephalon) ▫ Innervates: tongue, palate, epiglottis, AKA taste; cochlea, semicircular canals, AKA balance, hearing ▪ General somatic afferent ▫ Cranial nerves V, VII (metencephalon); IX (myelencephalon) ▫ Innervation: touch, temperature, pain in head, neck MYELENCEPHALON ▪ Gives rise to medulla oblongata ▫ Transitional zone between brain, spinal cord ▪ Alar plate sensory neuroblasts give rise to ▫ Cochlear nuclei, vestibular nuclei, spinal trigeminal nucleus, solitary nucleus, dorsal column nuclei, inferior olivary nuclei ▪ Basal plate motor neuroblasts give rise to ▫ Nuclei of CN X, IX, XI ▪ Roof plate lined by ependymal cells covered by vascular mesenchyme, AKA pia mater ▫ Collectively known as tela choroidea ▫ Projects into ventral cavity, invaginations form choroid plexus OSMOSIS.ORG 253
▫ Choroid plexus produces cerebrospinal fluid METENCEPHALON ▪ Develops from rostral rhombencephalon, gives rise to cerebellum, pons Cerebellum ▪ Functions as center for coordination, posture ▪ Neuroectoderm cells proliferate ▫ In ventricular zone, form cerebellar nuclei, Purkinje cells, golgi cells ▫ In external germinal layer, form basket, granule, stellate cells ▫ External, internal germinal layers form astrocytes, oligodendrocytes, Bergmann cells Pons ▪ Serves as pathway for nerve fibers between spinal cord, cerebrum, cerebellum ▪ Base of pons contains ▫ Pontine nuclei from alar plate ▫ Corticobulbar, corticospinal, corticopontine fibers from cell bodies in cerebral cortex; pontocerebellar fibers ▫ Alar plate sensory neuroblasts (CN V, CN II, CN III) ▫ Basal plate motor neuroblasts (CN V, CN VI, CN VII) DEVELOPMENT OF MESENCEPHALON ▪ Gives rise to midbrain ▪ Basal plate neuroblasts give rise to motor nuclei ▫ Oculomotor (III) nucleus → general somatic efferent column ▫ Edinger–Westphal nucleus of oculomotor nerve (III) → general visceral efferent ▫ Substantia nigra ▫ Red nucleus ▫ Trochlear (IV) nucleus, part of CN V migrate to metencephalon ▪ Alar plate sensory neuroblasts gives rise to superior, inferior colliculi ▪ Crus cerebri contains corticobulbar, corticospinal, corticopontine fibers 254 OSMOSIS.ORG DEVELOPMENT OF THE PROSENCEPHALON Diencephalon ▪ Develops from median portion of prosencephalon ▪ Consists of one roof plate, two alar plates; basal plate regresses ▪ Alar plates give rise to ▫ Epithalamus: also develops from roof plate; gives rise to pineal body, habenular nuclei, commissure, posterior commissure, tela choroidea, third ventricle choroid plexus ▫ Thalamus: gives rise to thalamic nuclei, lateral geniculate body, medial geniculate body ▫ Subthalamus: gives rise to subthalamic nucleus; zona incerta; lenticular, thalamic fasciculi (AKA fields of Fortel) ▫ Hypothalamus: also develops from floor plate; gives rise to hypothalamic nuclei, mammillary bodies, neurohypophysis ▪ Optic vesicles, cups, stalks derivatives of diencephalon ▫ Give rise to retina, iris, ciliary body, CN II, optic tract ▪ Hypophysis (AKA pituitary) develops from two different structures ▪ Anterior lobe/adenohypophysis ▫ Develops from Rathke’s pouch ▫ Ectodermal diverticulum of primitive oral cavity/stomodeum ▪ Posterior lobe/neurohypophysis ▫ Develops from the infundibulum ▫ Neuroectodermal evagination of hypothalamus Telencephalon ▪ Gives rise to cerebral hemispheres, caudate, putamen, amygdaloid, claustrum, lamina terminalis, olfactory bulbs, hippocampus ▪ Week 5: cerebral hemispheres begin emerging as two outpocketings of prosencephalon ▫ Contain cerebral cortex, white matter, lateral ventricles Basal ganglia ▪ Basal part of hemispheres grow, bulge into the lateral ventricles, giving rise to part of hemisphere wall (AKA corpus striatum)
Chapter 30 Embryology: Head & Neck Structure ▪ Expands: gives rise to caudate nucleus, putamen, amygdaloid nucleus, claustrum ▪ Divided by fibers of internal capsule ▫ Single layer of ependymal cells form choroid plexus ▫ Thickened wall of hemisphere forms hippocampus ▫ Only globus pallidum arises from neuroblasts of subthalamus that migrated into the hemispheres Hemispheres ▪ Rapid, extensive growth of hemispheres creates many convolutions (AKA gyri) ▫ Separated by grooves (AKA sulci) fissures ▪ Hemispheres develop frontal, parietal, occipital, temporal lobes, which overlie insula Cerebral cortex ▪ Develops from paleopallium/archipallium, neopallium ▪ Initially has neuroepithelial, mantle, marginal layers ▪ Neuroblasts proliferate, migrate to subpial regions to differentiate into mature neurons ▫ Continues until all layers are formed ▪ Early formed neuroblasts have deep position in cortex, whereas later formed neuroblasts more superficially positioned ▪ Classified into neocortex, allocortex ▪ Neocortex: AKA isocortex ▪ Allocortex: subdivided into 2 parts ▫ Archicortex: includes hippocampal formation ▫ Paleocortex: includes olfactory cortex ▪ Telencephalon also gives rise to olfactory bulbs, tracts DEVELOPMENT OF COMMISSURES ▪ Bundles of nerve fibers connecting corresponding areas in right, left hemispheres ▪ Cross in midline of brain via lamina terminalis (AKA commissural plate) ▪ Anterior commissure ▫ Appears first ▫ Connects olfactory bulbs, middle, inferior temporal gyri ▪ Hippocampal commissure/fornix commissure ▫ Appears second ▫ Fibers arise in hippocampus → connect to lamina terminalis → mammillary body, hypothalamus ▪ Corpus callosum ▫ Appears third ▫ Largest commissure ▫ Forms bundle in lamina terminalis, connects two homologous neocortical areas of cerebral hemispheres OSMOSIS.ORG 255
DEVELOPMENT OF CRANIAL NERVES & AUTONOMIC NERVOUS SYSTEM osms.it/development-cranial-nerves-ANS DEVELOPMENT OF CRANIAL NERVES ▪ By week 4: nuclei for all cranial nerves present ▪ Except olfactory (I), optic (II) nerves, all cranial nerves arise from hindbrain ▪ Motor nuclei derived from rhombomeres produced by neuroepithelium ▫ Gives rise to motor nuclei of cranial nerves IV, V, VI, VII, IX, X, XI, XII ▫ Motor neurons for these nuclei reside within brain ▪ Cranial nerve sensory ganglia originate from neural crest cells, ectodermal placodes DEVELOPMENT OF AUTONOMIC NERVOUS SYSTEM ▪ Comprised of efferent motor fibers ▫ Innervate smooth muscle, cardiac muscle, secretory glands ▫ Divided into sympathetic, parasympathetic systems Sympathetic nervous system ▪ Ganglia arise from basal plate of neural tube, neural crest cells ▫ Basal plate gives rise to preganglionic sympathetic neurons in intermediolateral horns of spinal cord 256 OSMOSIS.ORG ▫ Neural crest cells give rise to postganglionic sympathetic neurons of sympathetic chain ganglia, prevertebral sympathetic ganglia, adrenal chromaffin cells ▪ Cell bodies of preganglionic neurons reside at T1–L2 of spinal cord ▪ Preaortic ganglia located at major vessel branches Parasympathetic nervous system ▪ Ganglia arise from basal plate of neural tube, neural crest cells ▫ Basal plate gives rise to preganglionic parasympathetic neurons of cranial nerve nuclei—CN III (midbrain), CN VIII (pons), CN IX, X (medulla), spinal cord at S2–S4 ▫ Neural crest cells give rise to postganglionic parasympathetic neurons of ciliary ganglion (CN III), pterygopalatine ganglion (CN VII), submandibular ganglion (CN VII), enteric ganglion (Meissner, Auerbach, CN X), ganglia of abdominal, pelvic cavities ▪ Neuron cell bodies reside in brainstem, S2– S4 of spinal cord
Chapter 30 Embryology: Head & Neck Structure DEVELOPMENT OF THE SPINAL CORD osms.it/development-spinal-cord NEURAL TUBE ▪ Neural plate folds in cephalocaudal manner, forming neural tube ▫ Open at each end, forming cranial, caudal neuropores ▪ Three layers: neuroepithelial cells/ ventricular zone, mantle layer/intermediate zone, marginal layer/outermost layer Neuroepithelial cells ▪ Form thick layer of pseudostratified epithelium ▫ Rapid division forms more neuroepithelial cells, produces neuroepithelium ▫ Neuroepithelium gives rise to neuroblasts (AKA primitive nerve cells) Mantle layer ▪ Forms around neuroepithelial layer ▪ Composed of neuroblasts that migrated from neuroepithelial layer ▪ Gives rise to gray matter of spinal cord Marginal layer ▪ Contains neuroblast nerve fibers ▪ Gives rise to white matter ▪ Myelination → color Thickening of mantle layer ▪ Ventral, dorsal thickening occurs as more neuroblasts form ▪ Ventral thickening produces basal plates ▫ Basal plates form ventral motor horn of spinal cord ▪ Dorsal thickening produces alar plates ▫ Alar plates form dorsal sensory horn of spinal cord ▪ Sulcus limitans divides basal, alar plates ▪ Intermediate horn develops between motor, sensory horns ▫ Located at T1–T12, L2/L3 ▫ Contain sympathetic portion of autonomic nervous system ▪ Dorsal midline portion (AKA roof plate) ventral midline portion (AKA floor plate) of neural tube do not contain neuroblasts ▫ Serve as crossover pathways CELL DIFFERENTIATION Development of nerve cells ▪ Start out as round, apolar cells ▪ Differentiate as primitive axons, dendrites develop ▫ Bipolar neuroblast differentiates into multipolar neuroblast ▫ Eventually develops into neuron Development of glial cells ▪ Glioblasts formed by neuroepithelial cells that migrate to the mantle and marginal layers ▪ Differentiate into glial cells ▫ Protoplasmic astrocytes, fibrillar astrocytes: provide support, metabolic functions ▫ Oligodendroglial cells: myelination in CNS ▫ Microglia cells: phagocytic activity ▪ Neuroepithelial cells cease to produce neuroblasts, glioblasts ▫ Differentiate into ependymal cells, which line central canal of spinal cord DEVELOPMENT OF SPINAL NERVES AND GANGLIA ▪ Week 4: development of spinal nerves begins ▪ Motor nerve fibers arise from cell bodies in basal plates (AKA ventral horns) ▫ Form bundles (AKA ventral motor roots) ▪ Processes from nerve cell bodies in spinal cord ganglia OSMOSIS.ORG 257
▫ Form bundles (AKA dorsal sensory roots) ▪ Spinal nerves split into rami containing both motor, sensory fibers ▪ Dorsal primary rami ▫ Innervate dorsal axial musculature, vertebral joints, skin of back ▪ Ventral primary rami ▫ Innervate limbs, ventral body wall ▫ Form brachial, lumbosacral plexus MYELINATION OF THE NERVOUS SYSTEM ▫ Originate from neural crest cells ▫ Each Schwann cell myelinates just one axon of peripheral nerve, wrapping around axon to form neurilemma (AKA myelin, sheath) Myelination in CNS ▪ Carried out by oligodendrocytes ▫ One oligodendrocyte can myelinate ≤ 50 axons ▫ Myelination of corticospinal tracts incomplete until first one-two years of postnatal life Myelination in PNS ▪ Carried out by Schwann cells DEVELOPMENT OF THE EAR osms.it/development-of-the-ear ▪ Comprised of internal, middle, outer ear DEVELOPMENT OF THE INTERNAL EAR ▪ Around day 22, otic placodes formed ▪ Ectoderm thickens each side of rhombencephalon ▪ Sides invaginate, form otic/auditory vesicles (AKA otocysts) ▪ Otocystic cells of vesicles differentiate into ganglion cells for vestibulocochlear/ statoacoustic ganglia ▪ Each vesicle divides, forming two components that will become membranous labyrinth ▫ Ventral component: forms saccule, cochlear duct ▫ Dorsal component: forms utricle, semicircular canals, endolymphatic duct DEVELOPMENT OF THE COCHLEA ▪ Week 6: cochlear duct forms as saccule forms tubular outgrowth ▫ Cochlear duct spirally penetrates mesenchyme ▫ Completes 2.5 turns by week 8 258 OSMOSIS.ORG ▪ Week 7: cochlear duct cells give rise to spiral organ of Corti ▫ Cochlear duct remains connected to saccule via ductus reuniens ▫ Mesenchyme surrounding cochlear duct differentiates into cartilaginous shell ▪ Week 10: large vacuoles appear in cartilage ▫ Form two perilymphatic spaces: scala vestibuli, scala tympani ▫ Cochlear duct now separated from scala vestibuli by vestibular membrane, from scala tympani by basilar membrane ▫ Lateral wall of cochlear duct remains attached to cartilage by spiral ligament ▫ Median angle of cochlear duct connected to cartilaginous process called modiolus DEVELOPMENT OF ORGAN OF CORTI ▪ Epithelial cells of cochlear duct form two ridges ▫ Inner ridge gives rise to spiral limbus ▫ Outer ridge gives rise to sensory hair cells of auditory system ▪ Tectorial membrane covers sensory cells
Chapter 30 Embryology: Head & Neck Structure while attached to spiral limbus ▫ Sensory cells, tectorial membrane: organ of Corti DEVELOPMENT OF SEMICIRCULAR CANALS ▪ Week 6: flattened outpouchings appear on dorsal component/utricle of otic vesicle ▫ Central portion of their walls eventually disappear, semicircular canals develop ▪ Each canal has two ends ▫ Crus ampullare: dilated end ▫ Crus nonampullare: does not dilate ▫ Cells in ampullae form crista ampullaris ▪ Maculae acusticae develop in walls of utricle, saccule ▫ Maintenance of equilibrium: change in position of head, body generates impulses in sensory cells of cristae, maculae; carried by cranial nerve VIII/ vestibular fibers DEVELOPMENT OF THE MIDDLE EAR ▪ Composed of tympanic cavity, Eustachian tube/auditory tube ▪ Tympanic cavity develops from first pharyngeal pouch/endoderm ▪ Pouch expands, reaches floor of first pharyngeal cleft ▫ Distal part of pouch widens, becomes primitive tympanic cavity ▫ Proximal part remains narrow, becomes auditory tube DEVELOPMENT OF THE OSSICLES Malleus and incus ▪ Derived from cartilage of first pharyngeal arch ▫ Tensor tympani muscle innervated by mandibular branch of trigeminal nerve Ossicles ▪ Appear during the first half of fetal life ▪ Remain embedded in mesenchyme until it dissolves in eighth month ▫ Space around ossicles forms ▪ Endodermal epithelium of primitive tympanic cavity covers space’s wall ▫ Connects ossicles to cavity wall like mesentery ▪ During late fetal life, tympanic cavity expands dorsally to form tympanic antrum ▪ After birth, epithelium of tympanic cavity extends to the mastoid process ▫ Forms air sacs (AKA pneumatization) ▫ Mastoid air sacs communicate with tympanic antrum, tympanic cavity DEVELOPMENT OF THE EXTERNAL EAR ▪ External auditory meatus derived from dorsal portion of first pharyngeal cleft ▪ During third month, epithelial cells of meatus’ floor proliferate, form solid epithelial plate (AKA meatal plug) ▫ During seventh month meatal plug dissolves, creating definitive eardrum ▫ Meatal plug persists until birth → congenital deafness ▪ Composition of eardrum ▫ Ectodermal epithelial lining of auditory meatus ▫ Endodermal epithelial lining of tympanic cavity ▫ Intermediate mesoderm layer of connective tissue ▪ Auricle ▫ Auricle develops from six mesenchymal proliferations/auricular hillocks at dorsal ends of first, second pharyngeal arches surrounding first pharyngeal cleft ▫ These proliferations later fuse, form definitive auricle Stapes ▪ Derived from cartilage of second arc ▫ Stapedius muscle innervated by facial nerve OSMOSIS.ORG 259
DEVELOPMENT OF THE EYE osms.it/development-of-the-eye KEY POINTS ▪ Day 22: begins with formation of optic grooves on both sides of forebrain ▪ As neural tube closes, optic grooves form outpouchings (AKA optic vesicles) ▪ Optic vesicles reach surface ectoderm, induce lens formation ▫ Optic vesicles invaginate, form double layered optic cups ▫ Inferior surface of optic cup forms choroid fissure pathway for hyaloid artery ▪ Week 7: choroid fissure closes, gives rise to pupil ▪ Ectoderm cells elongate, form lens placode ▪ Lens placode invaginates, forms lens vesicle DEVELOPMENT OF THE RETINA ▪ Optic cup has two layers ▫ Inner, outer layer initially separated by intraretinal space; obliterated in adult ▫ Outer/pigmented layer: gives rise to pigmented layer of retina ▫ Inner/neural layer: gives rise to neural layer of retina ▪ Posterior 4/5: pars optica retinae ▪ Cells bordering the intraretinal space differentiate into rods and cones ▪ Adjacent mantle layer: gives rise to neurons and supporting cells ▫ Outer, inner nuclear layers, ganglion cell layer ▪ Surface fibrous layer contains nerve cell axons of deeper layers ▫ Nerve fibers converge towards optic stalk ▫ Optic stalk develops into optic nerve ▪ Anterior 1/5: pars ceca retinae ▫ Pars iridica retinae: forms inner layer of iris ▫ Pars ciliaris retinae: forms ciliary body 260 OSMOSIS.ORG Iris ▪ Three layers ▪ Outer, pigmented layer of optic cup ▪ Inner, neural layer of optic cup ▪ Richly vascularized connective tissue layer containing pupillary muscles ▫ Sphincter, dilator pupillae develop from ectoderm of optic cup ▪ Pars ciliaris retinae ▫ Externally covered by mesenchyme layer, forms ciliary muscle ▫ Internally connected to lens by suspensory ligament/zonula DEVELOPMENT OF THE LENS ▪ Cells of optic vesicles elongate, fill lumen of vesicle with primary lens fibers ▫ End of week 7: fibers reach anterior vesicle wall ▫ Secondary fibers area added to central core DEVELOPMENT OF CHOROID, SCLERA & CORNEA ▪ End of week 5: loose mesenchyme surrounds eye primordium, differentiates into 2 layers ▫ Inner layer: similar to pia mater, forms highly vascularized pigmented layer, AKA choroid ▫ Outer layer: continuous with dura mater, forms sclera ▪ Anterior chamber forms on anterior aspect of the eye ▫ Splits loose mesenchyme via vacuolization ▫ Inner layer: iridopupillary membrane, sits in front of lens, iris ▫ Outer layer: substantia propria of cornea, continuous with sclera ▪ Cornea now contains 3 layers ▫ Epithelial layer derived from surface ectoderm
Chapter 30 Embryology: Head & Neck Structure ▫ Substantia propria ▫ Epithelial layer bordering anterior chamber ▪ Posterior chamber: space between iris, lens ▪ Anterior, posterior chambers filled with aqueous humor produced by ciliary process of ciliary body ▫ Aqueous humor circulates from posterior into anterior chamber through pupil ▫ In anterior chamber, fluid flows through canal of Schlemm (AKA scleral venous sinus) at iridocorneal angle, resorbs into bloodstream DEVELOPMENT OF THE VITREOUS BODY ▪ Mesenchyme invades inside of optic cup through choroid fissure ▫ Forms hyaloid vessels, which supply lens during intrauterine life ▪ Invading mesenchyme also forms fibrous network between lens, retina ▫ Interstitial spaces of network fill with vitreous body ▪ During fetal life, hyaloid vessels eventually disappear, replaced by hyaloid canal DEVELOPMENT OF THE OPTIC NERVE ▪ Develops from optic stalk, which connects optic cup to brain ▪ Initially, optic cup has ventral groove (AKA choroid fissure) ▫ Fissure contains hyaloid vessels ▫ Nerve fibers of retina line stalk’s inner wall ▪ Week 7: choroid fissure closes ▫ Narrow tunnel forms inside optic stalk ▫ Nerve fibers fill tunnel, forming optic nerve ▪ Contents of optic nerve ▫ Inner layer provides neuroglia supports optic nerve fibers ▫ Hyaloid artery later transforms into central artery of retina ▪ Choroid: continuation of pia arachnoid, sclera continuation of dura layer of nerve OSMOSIS.ORG 261

Osmosis High-Yield Notes

This Osmosis High-Yield Note provides an overview of Head and Neck Structure essentials. All Osmosis Notes are clearly laid-out and contain striking images, tables, and diagrams to help visual learners understand complex topics quickly and efficiently. Find more information about Head and Neck Structure by visiting the associated Learn Page.