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THE SENSES
OPTIC CUP

EYE

     Technically, the eye is a part of the brain since the embryonic optic vesicles form as part of the forebrain.  These optic vesicles subsequently form an optic cup which will form the retina.  The “optic nerve” is technically an optic tract since it does not connect a peripheral sensory structure to the brain but rather connects the retina which developed from the embryonic brain with the rest of the brain.

The optic cup induces the formation of the lens vesicle (Hall, p. 112).  The mechanisms of lens induction can apparently vary, even among closely related animals, since the lenses of congeneric amphibians differ in developing with or without induction by the optic cup (Hall, p. 186).

Frog

EYE IN FROG EMBRYO EYE IN FROG EMBRYO
EYE IN FROG EMBRYO EYE IN FROG EMBRYO
EYE IN CHICK EMBRYO
EYE IN FROG EMBRYO EYE IN FROG EMBRYO
EYE IN FROG EMBRYO EYE IN FROG EMBRYO
EYE IN FROG EMBRYO EYE IN FROG EMBRYO
     It has long been observed that the vertebrate eye is “built backwards”.  Light which enters the eye does not immediately strike the photoreceptor cells of rods and cones but rather is filtered through several layers of other cells, the bipolar, amacrine, and horizontal cells.  Once light does reach the rods and cones, the axons which are sent to communicate with the brain are sent back through these layers of cells rather than directly to the brain.  The axons which have traveled away from the brain then are collected to form the optic nerve which must then penetrate the retina again.  At the site where the optic nerve must pass through the retina, a blind spot in the visual field exists since no photoreceptors can be located here.
OPTIC NERVE OPTIC NERVE
CHICK EYE OPTIC NERVE
EYE IN A PIG EMBRYO
EYE IN A PIG EMBRYO EYE IN A PIG EMBRYO
EYE IN A PIG EMBRYO EYE IN A PIG EMBRYO OPTIC NERVE

Although vertebrate vision depends on the detection of light by opsin molecules expressed in the eye itself, there are a variety of proteins of the opsin gene family known to be expressed in the brain tissue of vertebrates (including humans).  It appears that various regions of the brain were once involved in the perception of light (although this perception of light would obviously have served purposes such as the regulation of circadian rhythms and rather than vision).  A fair amount of light still reaches brain tissue in many vertebrates (such as hatchling fish and those vertebrates which still retain the ancestral pineal foramen between parietal bones).  Even in animals which possess a thick skull, there may be minute quantities of some light wavelengths which reach the opsin-expressing tissue of the deep brain. 

     A number of congenital abnormalities can affect the formation of the eyes, including cyclopia.

CYCLOPIA BIRTH DEFECT

EAR

    Several ectodermal thickenings known as placodes initiate the development of important vertebrate head structures. Sensory placodes contribute to the development of the eye, ear, lateral line system, and olfactory system. Neurogenic placodes produce sensory neurons for several ganglia found in the head.The olfactory placode develops into the tissues which perceive smell, the hypophyseal placode develops into the anterior pituitary gland, lens placodes for the lenses, trigeminal placodes form trigeminal ganglia, otic placodes form the ear, and epibranchial placodes form taste and visceral ganglia.  Fish and some amphibians also possess placodes for lateral line organs.  Tunicate embryos also develop placode-like areas of ectoderm which seem to be homologous to the vertebrate hypophyseal placode and the otic/lateral line placode. Other placodes evolved in basal vertebrates (Mazet, 2005; Shimeld, 2000).

Frog

EAR IN FROG

  Mammalian embryos develop kinocilia in the organ of Corti but do not possess them as adults. (Webster, 1974, p. 230). 

 

     The mammalian middle and inner ear regions are enclosed by the temporal bone.  This occurred through a number of steps in mammalian evolution as the temporal bone formed from the fusion of several ancestral bones (primarily the squamosal and petrosal) and a bony auditory bulla formed over the auditory regions.  In reptiles and amphibians, the quadrate and articular bones form the jaw joint: this is where the lower jaw articulates with the upper jaw.  In mammals, the jaw joint occurs between the dentary (mandible) and squamosal.  During mammalian evolution, the quadrate and articular were freed from their original functions and formed the middle ear bones that only mammals have (Kemp, 1982; Carroll, 1988; Kardong, 2002).  .  In marsupial newborns, the quadrate and the articular form in the jaw joint and are incorporated into the middle ear after birth.
EAR BONES
 In human fetuses, the quadrate and articular form in the vicinity of the jaw joint and slowly become encased in a bony part of the temporal bone.  In human fetuses, the angular bone (formerly a bone in the ancestral lower jaw) is still visible as a separate bone before it fully fuses with the temporal.  The auditory ossicles form in the first half of fetal development but they are surrounded by mesenchyme until the eighth month at which point the mesenchyme dissolves (Sadler, p.334).  FETAL SKULL
COILING OF COCHLEA
The cochlea begins as a straight structure (as is the condition in more primitive vertebrates) and gradually becomes coiled. 
malformation of pinna
Birth defects include malformations of the pinna.

VOMERONASAL SYSTEM (VNS)

     In 1703 a Dutch military surgeon noticed the vomeronasal organ (VNO) in a soldier with a facial wound.  In 1813, the  Anatomical description of a new organ in the nose of domestic animals” by Ludwig Jacobson was published and the vomeronasal organ was known to exist in human fetus.  In 1891, a French doctor found it in 1/4 of 200 patients.  In the 1930s, neuroanatomist Elizabeth Crosby noted that accessory olfactory bulb (where VNO connections run in vertebrates with a functional VNS) is not present in human fetuses after the first trimester and it was assumed that any VNO would be vestigial.  

1) VOMERONASAL ORGAN (VNO)

    The vomeronasal organ is a pair of structures on either side of the nasal septum (vomer) near the base.  Vomeronasal ducts are openings which range in size from a tenth of an inch to small ducts invisible to the naked eye to being absent. A study on the percentage of the human population with pits opening into these ducts concluded that 16% of people retain them; a second study which used an endoscope concluded this number was 76%.  The lining of duct is pseudostratified columnar epithelia with kinocilia and microvilli with myelinated and unmyelinated neurons under the basement membrane.  This is anatomical arrangement is unique in the body.  In some animals, these ducts connect to oral cavity as well (carnivores, ungulates).  A capsule made of cartilage encloses these ducts.

    Most people seem to possess a vomeronasal organ as adults.  It consists of neurons which constantly regenerate, just as in the main olfactory system.   Neuron-specific markers do bind to cells in the VNO indicating that the VNO possesses neurons and these neurons send projections into the brain.  These neurons possess microvilli rather than the cilia observed in the main olfactory system.  Some putative pheromones blown onto the VNO (not MOS) caused measurable electrical signals from VNO. 

     Human embryos at stage 17 have no evidence of a VNO which is formed in all embryos during stage 18 or later.  It continues to develop and its full size not achieved by birth.  Some vertebrates lack a VNO: it is absent in birds and adult catarrhine mokeys.  Only 1 of 18 bat families have; in some bats it is even absent in the embryo.  The flehmen behavior (a kind of grimace displayed after some mammals sample urine conspecific urine) may make VNO ducts more accessible to pheromones.

 

2) Vomeronasal Nerve

  The axons of bipolar neurons from the VNO form a nerve that passes through the cribiform plate of the ethmoid bone where neurons synapse in the accessory olfactory bulb.  The VNO can also receive innervation from the trigeminal nerve.


3) Accessory Olfactory Bulb (AOB)

     The accessory olfactory bulb lies in dorsocaudal portion of olfactory bulb.  Neurons here produce LHRH and TRH (best known as hypothalamic releasing hormones which apparently serve another role here).  Lesions in the accessory olfactory bulb affect reproductive behavior in mice and male mice alter the LHRH levels when exposed to urine of other mice.    The accessory olfactory bulb is believed to be absent in fish.  It is present in most tetrapods, although it is absent in crocodiles, birds, aquatic mammals, and most bats.  Although humans do not have a recognizable accessory olfactory bulb, its tissue is still present in the brain although its existence as a discrete structure is altered by the embryonic growth of the neocortex.