Anatomy & Development of the Eye, Ear & Olfactory Systems Flashcards
what are the three germ layers?
ectoderm (top)
mesoderm (middle)
endoderm (bottom)
what happens to the three germ layers?
the flat layers roll up into a tube:
ectoderm outside
mesoderm in the middle
endoderm inside lining
what do the three germ layers make?
they eventually make all the animal’s tissues and organs
development of the eye - meaning of development cascade?
development of one part triggers the next
what is the first stage of the development of the eye?
first, the optic vesicle forms from an outgrowth of the neural tube
how does the lens placode form?
the optic vesicle contacts the overlying ectoderm, inducing it to form the lens placode
optic cup?
the optic vesicle flattens and folds inwards to become a double-walled optic cup
this will form the retina
how does the lens vesicle form?
the lens placode also folds inwards and pinches off to become the lens vesicle
sits inside the optic cup
this will become the lens
what does the lens vesicle induce?
induces the overlying ectoderm to become the cornea
what form the retina?
two layers of the optic cup form the retina
Inner layer –> neural retina
Outer layer –> pigmented retina
mesenchyme surrounding the optic cup?
this forms two more layers on top of the retina - these will become the vascular and fibrous layers, later
what does the optic stalk become?
the optic nerve
how does a non-functional hyaloid canal form?
the hyaloid vessels eventually regress and leaves a non-functional hyaloid canal
what are the three main layers of the eyeball called?
tunicae
what are the three main layers of the eyeball?
the outer layer is the fibrous tunicae
the middle layer is the vascular tunic
the innermost layer of the eyeball is the retina
describe the outer layer of the eyeball?
fibrous tunic
dense, tough collagen tissue
protects and holds the eyeball’s shape
describe the middle layer of the eyeball?
vascular tunic
contains blood vessels
also structures that hold the lens, regulate pupil size, and also produce fluid that fills the eyeball
describe the innermost layer of the eyeball?
the retina
still connected to the brain
converts light information to nerve signal
what does the fibrous tunic of the eye consist of?
sclera (neural crest cells)
cornea (surface ectoderm + neural crest)
meet at corneoscleral junction (aka limbus)
sclera?
the white of the eye
extra-ocular muscles attach to it
cornea?
transparent part at front of eye
allows light to enter
has superficial epithelium, deep stroma and even deeper endothelium
what does the vascular tunic of the eye consist of?
choroid
ciliary body
iris
what are the choroid, ciliary body and iris all from?
all from mesenchymal cells
(mixture of mesoderm and neural crest cells) around optic cup
vascular tunic AKA?
uvea
where does the choroid occupy?
the choroid occupies most of the back of the eyeball
describe the choroid?
it is highly vascular
blood vessels nourish the outer layers of retina
what supplies the choroid w/ blood?
choroid supplied by branches of the opthalmic artery
there are also retinal vessels entering via the optic nerve, supplying the inner layer of the retina
choroid may contain?
may contain tapetum lucidum
ciliary body?
thickened part of vascular tunic between choroid and iris
what is the ciliary muscle attached to?
attached to the lens by suspensory ligaments (aka zonules)
what does the ciliary muscles do of the vascular tunic of the eye?
controls the shape of the lens to keep objects in focus (‘accommodation’)
what covers the ciliary body?
the ciliary epithelium (covering the body) produces aqueous humour
this fluid fills the front chambers of the eye (anterior and posterior)
where does the aqueous humour go?
it circulates through the pupil and is re-absorbed by the trabecular meshwork at the iridocorneal angle
if the drainage is reduced, eye pressure can rise dangerously (glaucoma)
iris contains?
blood vessels
collagen fibres
smooth muscle
pigment cells
the iris is?
a continuation of the ciliary body shaped into a thin ring with a circular opening (the pupil)
what does the iris do?
the iris functions to let in more or less light as needed via the two smooth muscles
what are the two smooth muscles that control the iris?
sphincter of pupil
Dilator of pupil
different species, smooth muscles of retina?
these are different shapes and additional fibres reinforce the muscles causing different pupil shapes in different species
what are the two layers of the retina?
neural layer from inner part of optic cup
and
pigmented layer develops from the outer part of optic cup
the neural layer of the nervous tunic?
incoming light stimulates the receptor cells (rods and cones)
pigmented layer of nervous tunic - retina?
makes the inside of the eye dark (like a camera)
doesn’t reflect scattered light back into retina again
Another network of blood vessels lies on top of the retina:
light passes through this fine mesh of vessels before hitting rods and cones (light sensitive cells)
problematic - chronic problems e.g. cats w/ hypertension high bp they can get bleeding in retinal blood vessels and lots of bleeding over extended periods of time = blindness
inside eyeball, space is divided into how many chambers and what are these chambers?
3 chambers
Anterior chamber - in front of lens
Posterior chamber - in front of lens
Vitreous chamber - behind lens
how do the anterior and posterior chambers communicate?
communicate through the pupil - both are filled with watery aqueous humour
what separates the posterior and vitreous chambers?
the lens and the suspensory ligaments holding it - vitreous chamber is filled with gel-like vitreous humour
Where does the eye sit?
within the bony orbit of the skull
bony orbit?
houses and protects eyeball
complete in humans
incomplete in most domestic animals
extra-ocular muscles?
these muscles move the eye in orbit - act together to produce coordinate eye movement
four rectus muscles?
rotate left/right and up/down
two oblique muscles?
rotate around visual axis
one retractor bulbi?
retracts eyeball into head
eyelids aka?
palpebrae
eyelids?
skin folds that cover the eye
block light
protect the cornea
keep the surface moist
nictitating membrane?
some species also have a third eyelid
covered in conjunctiva on both sides
contains a T-shaped cartilage
plus an extra tear gland (accessory lacrimal gland)
what keeps the third eyelid retracted?
kept retracted by a smooth muscle under sympathetic influence
might be visible when animal is sleepy or unwell
nictitating membrane - what does it do?
slides over the eye when it is pulled into the orbit
retractor bulbi and third eyelid together protect the eye
can be a source of eye problems in some breeds e.g. cherry eye
tear glands?
the eye is covered in tear film
washes eye of debris
different glands contribute layers and function:
A. lipid top-layer (tarsal glands) - prevents evaporation
B. Watery middle layer (lacrimal & gland of 3rd eyelid) - moistens and nourishes cornea
C. Mucus bottom layer (goblet cells in eyelid conjunctiva) - sticks to tear film in eye
blinking distributes?
distributes tear film over eye
tear glands - tear film drains?
drains through nasolacrimal duct into nose
overflow onto face with excessive production or impaired drainage
what part of the ear forms first? how?
inner ear forms first - the ectoderm thickens into otic placodes
roll inwards to form otic pit
pinch off to form otic vesicle
what does the otic vesicle form?
will form the membranous labyrinth
middle ear space forms?
forms from an outgrowth of the 1st pharyngeal pouch
the inside of the embryo’s ‘throat’
endoderm
external ear space forms?
forms from the 1st pharyngeal groove
the outside of the embryo’s ‘neck’
ecotderm
the ear drum?
aka tympanic membrane
is formed where the pouch and groove touch
ear bones develop how?
they develop endochondrally from neyral crest (NC) cells in between
adult ear is a result of?
result of complex development from different embryological components
congential problems may only affect one part of ear, and may have other distant effects
external ear - mostly outside of the skull - parts?
the pinna (aka auricle) - skin and cartilage
auricular muscles
ear canal (external auditory meatus)
ends at ear drum (tympanic membrane)
what does the external ear do?
it directs and transmits sound to the middle ear
trauma to great auricula artery?
can result in an aural haematoma - a collection of blood within the cartilage plate of the pinna
shape of external ear - why?
funnel shape to collect and direct sound
shape and size of external ear - different species?
varies greatly in size and shape between animals
shaped determined by auricular cartilage
‘domestication syndrome’ - weakened ear cartilage, floppy ears
how does the external ear help with thermoregulation?
surface area and blood supply
pinna?
highly mobile pinna moved by auricular muscles
located sounds
important communication function
external ear innervated by?
by CNVII (facial n.)
e.g. dropped ear with facial nerve paralysis
external ear - pinna, hairs?
few hairs on concave surface except some long hairs protecting entrance to meatus
on tragus = tragi hairs
external ear lined by skin?
common site of skin problems
contains sebaceous and modified apocrine (ceruminous) glands
cerumen (earwax) traps particles that might damage delicate tympanic membrane
in carnivores and pigs - external auditory meatus?
there is a bend
vertical canal
horizontal canal
when can you see tympanic membrane?
just possible to see with otoscope when canal is straightened
shape of stria malleris
some dogs have hairy canals
tympanic membrane - what does it do?
it separates external and middle ear
where is tympanic membrane found?
stretched like drumskin over tympanic ring
tympanic ring incomplete dorsally = flaccid tympanic membrane here
what does the first ossicle (malleus) attach to?
attaches to tympanic membrane
visible as stria malleris
effect of sound waves on tympanic membrane?
sound waves cause membrane to vibrate
vibration transmitted along the chain of ossicles
middle ear found where?
housed inside temporal bone of skull
includes a bulbous expansion of temporal bone (tympanic bulba)
what is the middle ear lined with?
lined with mucous membrane
middle ear communicated with pharynx via?
via auditory tube
pressure equalisation
drainage of gland secretions
middle ear - ossicles structures?
hammer (malleus)
anvil (incus)
stirrup (stapes)
what do the ossicles of the middle ear do?
transmit vibrations
connect tympanic membrane to oval window of inner ear
how do ossicles amplify vibrations?
tympanic membrane much larger than oval window
muscles attaching to ossicles alter sensitivity
inner ear consists of?
membranous labyrinth
osseous labyrinth
perilymph
membranous labyrinth?
a series of inter-connected ducts and chambers, filled with endolymph
osseous labyrinth?
hollow cavity in temporal bone of skull
perilymph?
fills the space between the osseous and membranous labyrinths
two functions of the inner ear?
hearing by the cochlea (and cochlear duct)
balance by the vestibular system (saccule, utricule and semicircular ducts)
cochlear is divided into how many parts, what are these?
3 parts
the cochlear duct
the scala vestubuli
the scala tympani
the two scala communicate at the tip of the cochlea’s spiral
the vestibular part of the inner ear is responsible for?
for sensing balance and movement
- three semicircular canals
- enlargements of the membranous labyrinth: saccule and utricle
what do the semicircular canals detect?
detects rotations of the head
what do the saccule and utricle detect?
accelerations
information is carried by - inner ear?
carried by the vestibular part of the vestibulocochlear nerve to the brain
olfactory organ?
olfactory mucosa in dorsocaudal nasal cavity
covers lateral wall and ethmoidal conchae here
distinguished histologically by presence of olfactory neuronal cells
why is sense of smell much better in domestic animals - give e.g.
dog = 300 million receptors
humans = 6 million receptors
olfactory neurone dendrites reach?
reach epithelium
present cilia (hair-like projections) into cavity
axons combine to form?
the fascicles of the olfactory nerve
pass through cribriform plate to connect directly to olfactory bulb on brain
vomeronasal organ?
used to detect hormones
‘sexual nose’
role in reproductive and social behaviours
Flehmen reaction e.g.?
detection of oestrus by males
following birth and interaction with newborn
interesting smells
vomeronasal organ aka?
Jacobson’s organ
where is vomeronasal organ found?
found in nasal cavity
two narrow parallel ducts
embedded in hard palate
end blindly (caudally)
open into incisive ducts (rostrally)
part-lined by olfactory mucosa
incisive ducts?
connect oral and nasal cavities through hard palate
oral opening absent in equids
choroid may also contain?
tapetum lucidum
what is the tapetum lucidum?
a half-moon shaped reflective area at back of eyeball
reflect light back through retina for better low-light vision
tapetum lucidum in different species?
different colour in different species
Tapetum lucidum present in all domestic mammals (except pig)
what is the lens composed of?
composed of clear, transparent cells
what is the lens contained within?
contained within an elastic capsule, under tension
when not pulled by suspensory ligaments, it would fall back into an almost-spherical shape
why does the lens change shape?
changes shape (accommodation) to change focal distance
allows objects at different distances to be focused on the retina
what is cataracts?
clouding of the lens reduces vision
due to age, trauma, disease
binocular vision?
front-facing eyes with overlap in visual fields
gives good depth perception, limited field of view e.g. predators
monocular vision?
laterally-facing eyes with little overlap
gives little depth perception but wide field of view e.g. prey
what are the upper and lower ‘true’ eyelids lined by?
by tactile hairs (eyelashes)
what glands do the upper and lower ‘true’ eyelids contain?
contain tarsal (aka meibomian) glands - lipid part of tear film
what is found on the inside of the upper and lower ‘true’ eyelids?
musculofibrous inside and
mucous membrane conjunctiva inner surface, continuous over sclera of eye and corneal epithelium
what is found on the outside of the upper and lower ‘true’ eyelids?
skin outside
what do equids have that may function in brain cooling?
equids have outpouchings of the auditory tubes - these are known as GUTTURAL pouchings
what are guttural pouches divided into and by what?
divided into two compartments by stylohyoid bone - form medial and lateral compartments
what blood vessels are near the guttural pouches in horses?
e.g. ext and int carotid arteries
what nerves are near the guttural pouches in horses?
CNIX, X, XI, XII
infections/disorders of guttural pouches in horses can lead to?
can leads to severe complications
what happens as the stapes vibrates in the oval window?
it compresses the perilymph in the scala vestibuli
what happens to the pressure waves after the perilymph is compressed?
the pressure waves are transmitted to the endolymph in the cochlear duct via Reissner’s membrane
movement of the endolymph causes?
causes pressure on the tectorial membrane inside the cochlear duct
the tectorial membrane puts pressure on hair cells - what do these do next?
they send an impulse along the (vestibulo) cochlear nerve to the brain
