Histology                                                                                                                     Stanley Shostak
BioSc 1450                                                                                                                  Spring 03

Lectures 21 and 22: Special sense organs

Specific neural receptors are incorporated in a non-neural structure which enhances and refines reception of stimuli

taste buds:

gustation; in epithelium of circumvallate papillae of tongue; serous glands (von Ebner) drain into troughs; opens at surface via taste pore; contains 20 to 30 cells; gustatory cells, supporting (sustentacular) & basal (stem) cells; innervated by non-myelinated nerve fibers; cells renewed continuously olfactory receptors
olfactory epithelium: very tall, pseudostratified columnar, containing three cell types: olfactory receptor cells: bipolar neurons; nuclei in middle of epi; single dendritic process extends to free surface, terminating in small swelling producing about 12 extremely long, modified (nonmotile) cilia which lie flat on surface; single fine nonmyelinated axon at base penetrates basement membrane; bundles of axons pass via about 20 small holes of cribriform plate of ethmoid bone to reach olfactory bulbs of forebrane; supporting (sustentacular) cells: elongated with tapered bases; nuclei uppermost; long microilli extend from luminal surface; form tangled mat with cilia; bound to sensory cell by typical junctional complexes forming terminal bar; basal (stem) cells: small; conical cells;

underlying CT: afferent nerve fibers & numerous serous olfactory glands (Bowmans glands)

eye anterior and posterior chambers: anterior compartment: anterior to lens, suspensory ligament and ciliary body; posterior compartment: small posterior to lens, suspensory ligament and ciliary body contain aqueous humor (clear, watery fluid similar to CSF and hypotonic to plasma) secreted into posterior chamber by ciliary body and circulated through pupil to drain into scleral venous sinus (= canal of Schlemm) vitreous body: gelatinous mass consisting of vitreous humor 

wall of eye (3 layers)

corneoscleral layer: tough, fibro-elastic capsule; scleral layer = posterior 5/6 opaque; insertion extra-ocular muscle; dense, fibro-elastic tissue; fibers arranged in bundles parallel to surface; little ground substance and few fibroblasts; cornea = anterior 1/6; transparent, smaller radius of curvature; principal refractive medium, rough focus on retina; limbus = corneo-scleral junction cornea: avascular structure of 5 layers: (1) outer stratified squamous epithelium; not normally keratinized; (2) lamina limitans anterior = Bowmans membrane; (3) substantia propria highly regular, dense collagenous tissue in thin lamellae; (4) lamina limitans posterior = Descemets membrane; thick, elastic basement membrane; (5) corneal endothelium; pumps fluid from substantia propria (discuss endothelium = lining convave surface) uveal layer or tract = choroid + ciliary body + iris choroid (btwn sclera and retina): heavily pigmented by melanocytes; merges anteriorly with ciliary body) layer loose vascular supporting tissue; separated from retina by - basement membrane of pigmented epithelium = Bruchs membrane

ciliary body (circumferential thickening beneath limbus; surrounds coronal equator of lens; attached to lens by suspensory ligament or zonule; smooth muscle controls shape of lens); bulges into eye at coronal equator of lens, between ora serrata and limbus = forward continuation of choroid layer of uveal tract; highly vascular and pigmented; continuous anteriorly with iris; lined by double layer of cuboidal epithelium; highly pigmented deep layer - forward continuation of pigmented epithelial layer of retin; outer, non-photosensitive extension of neural retina; smooth muscle derived from neural crest (?); contraction reduces tension upon suspensory ligament, increasing curvature of lens = accommodation; innervated by parasympathetic nerve fibers; ciliary processes extend into posterior chamber; rich in fenestrated capillaries; produce aqueous humor;

iris: diaphragm in front of lens from ciliary body; highly pigmented; aperture called pupil; divides anterior compartment into anterior & posterior compartments; loose, highly vascular, pigmented stroma (determines color of eye); anterior surface, irregular, discontinuous layer fibroblasts and melanocytes; lined by endothelial cells in fetus; posterior surface, relatively smooth, lined by epithelium derived from two layers of retina; superficial layer heavily pigmented; deep epi. layer transformed into myoepithelial layer = dilator pupillae radially oriented muscle; innervated by sympathetic nerve fibers; constrictor pupillae = constrictor muscle of the pupil; circumferentially oriented smooth muscle situated in stroma near free edge of iris; innervated by parasympathetic nervous system

 retina: photosensitive layer; terminates along ora serrata, a scalloped line; anteriorly, non-photosensitive epithelial layer lines ciliary body and posterior surface of iris; inner layer of neurone suport cells, epithelium and neurons; fovea (depression): depression in retina; area of greatest visual acuity; surrounded by yellow pigmented macula lutea (1) outermost layer = pigmented epithelium; single layer resting on Bruchs membrane; cuboidal cells with nuclei basal; apically cramed with melanin; long microvilli extend btwn photoreceprors; plane of detachment retinal photoreceptors neurons = photoreceptor cells

(2) rods & cones processes = outer segment

rods: membraneous disks continuously shed; incorporate rhodopsin; rod proper also has inner segment connected by thin eccentric strand of cytoplasm containing nine microtubule doublets but no inner pair);

cones: outer segment long conical structure about 2/3 length of rod; containing flattened membraneous disks; disk membrane continuous with plasma membrane; on one side spaces btwn disks continuous with extracellular environment; disks not shed; pigment receptive to blue, green and red; bodies generally continuous with inner segment; nuclei form lower row; deep to outer limiting membrane

 (3) outer limiting membrane: thin, eosinophilic structure; line of intercellular junctions between Muller cells and photoreceptor cells

(4) outer nuclear layer; densely packed nuclei of rod and cone cells

(5) plexiform layer: contains synaptic connections btwn short axons of photoreceptor cells and integrating neurons

neuronal interconnections in retina interposed btwn receptors and ganglion cells; also Mullers cells = neuroglia; long cytoplasmic processes embrace and may encircle retinal neurons; fill all intervening spaces;

(6) inner nuclear layer: cell bodies of integrating neurons interdigitating (bipolar) neurons (subdivided into three types: bipolar cells, horizontal cells & amacrine cells)

(7) inner plexiform layer: integrating neurons make synaptic connections with dendrites of ganglion cells

(8) optic tract neurons or ganglion cells

(9) layer of afferent fibers passing toward optic tract ganglion cells of afferent fibers passing into optic nerve;

(10) inner limiting membrane: innermost aspect of retina; basement membrane of Mullers cells; separates it from vitreous body

 optic nerve: formed by convergence of afferent (i.e., toward brain) nerve fibers; leave eye through lamina cribrosa medial to visual axis; overlaid by optic papilla (disk), devoide of photoreceptors = blind spot; myelination commences at the optic disk; invested by meninges: dura mater = sclera; pia-arachnoid = uveal tract; central artery of the retina moves up optic nerve; divides at optic disc into four branches that course through the optic nerve fiber layer, supplying the quadrants of retina

scleral venous sinus = canal of Schlemm: circumferential; at angle of anterior chamber; reabsorbs aqueous humor; lined by endothelium;

lens: biconvex transparent; shape varied for fine focus;

conjunctiva: = epithelium + loose vascular supporting tissue; covering exposed part of sclera and inner surface of eyelids; stratified columnar; containing goblet cells in surface layers;

lacrimal gland: secrete lysozyme and electrolytes ~ isomorphic with plasma; resemble salivary glands; lobular structure; compound tubulo-acinar form; serous glands; drain via into superior fornix to inner aspect of eye and nasal cavity via nasolacrimal duct.

eyelid: dense fibro-elastic plate = tarsus; skeletal muscle of orbicularis oculi & levator palpebrae (in upper lid) lie above tarsal plate; separated from it by layer of supporting tissue; = forward continuation of sub-aponeurotic layer (tendon-like) of scalp (in upper lid)

glands producing oily layer covering tear layer

tarsal glands = Meibomian gland: 12 to 30 within tarsal plate; oriented vertically and opening at free margin of eyelid via minute foramina; modified sebaceous glands;

glands associated with eyelashes

glandulae ciliares = glands of Moll; modified apocrine glands

sebaceous ciliary gland = gland of Zeis

 Ear = vestibulo-cochlear apparatus = stato-acoustic system external pinna = auricle: elastic cartilage coverd by skin
external auditory meatus: wall of elastic cartilage
canal: elastic cartilage in petrous part of temporal bone; lined by hairy skin containing sebaceous glands and ceruminous glands modified apocrine sweat glands; ecrete cerumen. middle: air-filled cavity; in petrous temporal bone; separated from external ear by tympanic membrane;

tympanic membrane: ear drum; pars flaccida superior; small triangular area; pars tensa; thin fibrous membrane: cuticular layer (thin, hairless skin; epidermis devoid of ridges; dermis of plump fibroblasts and fine vascular network); fibrous layer (outer layer of fibers radiating from center of membrane toward circumference; inner inner layer of fibers disposed circumferentially at periphery; fibers of unique composition); mucous layer (inner; continuation of modified respiratory-type epithelium; single layer cuboidal cells devoid of cilia and goblet cells; lamin propria thinwith own blood supply; also invests ossciles, small muscles and nerves of middle ear cavity);

ossicles (malleus, incus & stapes); formed by endochondral ossification; articulate via synovial joints; communicate with vestibule (perilymph) of internal ear through oval window; malleus and incus pivot on tiny ligaments attached to wall; tensor tympani & stapedius dampen excessive vibrations;

communicates with nasopharynx via auditory (Eustacian) tube (canal); lined by typical pseudostratified respiratory epithelium with numerous glblet cells particularly toward pharyngeal end; salpingo-pharyngeus, tensor palati and levator veli palati muscles connected to fibrocartilaginous part (lower) of tube cause it to dialge during swallowing.

communicates with air spaces within mastoid part of temporal bone; lined by simple squamous or cupoidal epithelium;

communicates with scala tympani (perilymph) via round window

internal

osseous labyrinth lying within temporal bone; separated from membraneous labyrinth (mainly) by fluid-filled space (continuous with subarachnoid space lined by squamous epithelium) containing perilymph = CSF; vestibulo-cochlear apparatus divided into three main areas:

vestibule: central space; gives rise to other parts; contains two components of membraneous labyrinth: utricle (posterior) & saccule (anterior) ; separated from middle ear by thin bony plate contining oval (surrounded by annular ligament) and round windows = secondary tympanic membrane

semicircular canals: from posterior aspect of vestibule; two vertical at right angles; one nearly horizontal

cochlear: extends from anterior aspect of vestibule; modiolus forms central bony axis; osseous spiral lamina spicule spiralling outwards from modiolus; supports spiral limbus: thickened mass tissue above free edge; attached to apex of scala media; scala vestibuli & scala tympani lined byunspecialized squamous epithelium of mesodermal origin

 membraneous labyrinth: filled with endolymph: closed; interconnected fluid-filled; lined by simple epithelium except in endolymphatic sac (in subdural space) lined by columnar epithelium [possibly site ofendolymph absorption] utricle & saccule (contained in vestibule of osseous labyrinth): connected by Y-shaped duct arising from endolymphatic sac; walls contain macula consisting of sensory (hair: goblet- & columar-shaped) receptor cells (central nuclei; single, eccentric cilium & many stereocilia embedded in gelatinous plaque of glycoprotein containing crystals mainly of calcium carbonate known as otoliths) & support cells (tall, columnar, basally-located nuclei and apical microvilli; secrete glycoprotein); dendrites synapse with sensory cells; also modulatory [inhibitory] neurons from CNS; axons pass into vestibular nerve;

semicircular membraneous duct: filled with endolymph; continuous at both ends with utricle; an ampulla lies at one end of each duct; crista ampullaris (elongated epithelial structure on ridge arising from membraneous wall at right angles to direction of flow; receptor cells [both goblet- and columnar-shaped] with numerous sterocilia and single cilium situated at margin nearest utricle; embedded in gelatinous glycoprotein = cupula) supported by single layer of columnar cells continuous with cuboidal epi lining membraneous labyrinth) containing sensory receptors with axons converging on vestibular nerve

scala vestibule & scala tympani on either side of scala media; continuous through helicotrma at tip of cochlea

membraneous component of cochlear: arises from saccule; triangular canal divides osseous space into 3 spiral components

scala vestibule: contains perilymph

scala media: contains endolymph; organ of Corti = spiral organ; contains receptors for sound;

separated from scala tympani by basilar membrane supporting spiral organ; composed of fibrous tissue; attached axially to spiral lamina and laterally to spiral ligament (marked thickiening of endosteum on lateral wall of cochlear canal); stria vascularis: thickened outer wall; highly vascular; maintains ionic composition of endolymph

vestibular (Reissners) membrane separates scala media from scala vestibuli; extremely delicate fibrous tissue lined by simple squamous epithelium on both sides;

 scala tympani: contains perilymph spiral organ: contains receptors for sound; epithelium derived from simple epithelium lining membraneous labyrinth of ectodermal origin; gives rise to spiral ganglion in modiolus at base of spiral lamina = cell bodies of bipolar sensory neurons and proximal axons from auditory component of VIII cranial nerve; tectorial membrane of GAGs extends as a flap-like mass from border cells over spiral limbus; overlie sensory cells; tips of stereocilia embedded; inner phalangeal & pillar cells support single row of inner sensory (hair) cells and pillar & outer phalangeal cells support triple row of outer sensory (hair) cells on either side of inner tunnel (= tunnel of Corti); each sensory cell may make contact with several primary sensory neurons; also inhibitory neurons arising from brain stem

spiral ganglion: contains bipolar primary sensory cells; axons (?) fibers ramify into dendrites and make synaptic contact with several sensory cells; secondary sensory cells and inhibitory neurons from brain stem; efferent fibers (leave spiral ganglia) pass to center of modiolus to form cochlear nerve. auditory component of VIII cranial nerve

 vestibulo-auditory nerve

last revised: 04-09-03