Vision ECE6397, Lecture 8

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Title
ECE 6304 Visual System Physiology,
Computation, and Methods
Prof. Valery Kalatsky Dept. of Electrical
Computer Engineering University of Houston
Lecture 19 Development
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Embryology
Study of development of the embryo
5 major stages  

• Gametogenesis
• Fertilization
• Cleavage
• Gastrulation
• Organogenesis

http//fig.cox.miami.edu/cmallery/150/devel/anima
l_development.htm
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Human Embryo
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Human Embryo Animation
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Fertilization
Sperm Ovum (egg) ? Zygote The egg cell is
always asymmetric, having animal pole - future
ectoderm and mesoderm two of three primitive
tissue types vegetal pole - future endoderm
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Cleavage (Cell Division)
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Morula
Morula is an embryo at an early stage of
embryonic development, consisting of
approximately 12-32 cells (blastomeres). The
morula is produced by embryonic cleavage, the
rapid division of the zygote. After reaching
the 16-cell stage, the cells of the morula
differentiate. The inner blastomeres will become
the inner cell mass and the blastomeres on the
surface will later flatten to form the
trophoblast. In mammals the morula travels to
the uterus around 3-4 days after fertilization,
and at about 4 days after fertilization a
fluid-filled space called the blastocystic
cavity appears and the morula becomes a
blastocyst.
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Blastula
The blastula is an early stage of embryonic
development in animals. It is produced by
cleavage of a fertilized ovum and consists of a
spherical layer of around 128 cells surrounding
a central fluid-filled cavity. The blastula
follows the morula and precedes the gastrula in
the developmental sequence. In mammals,
blastulation leads to the formation of the
blastocyst. The blastocyst consists of two
primary cell lines the inner cell mass, also
known as the epiblast, and the trophoblast. The
former is the source of embryonic stem cells and
gives rise to all later structures of the adult
organism. The latter combines with the maternal
endometrium to form the placenta.
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Early stages
The fertilised egg divides rapidly, first to
form a morula and then a blastocyst. After about
six to seven days, the blastocyst reaches the
uterine cavity where it implants in the
endometrium. When the blastocyst attaches to
the endometrium on day 9, it has an inner mass
of cells which develops into the embryo and an
outer mass of cells that are called
trophoblasts. The trophoblast cells begin to
develop their own blood supply and this outer
cell mass becomes known as the chorion which
later develops into the placenta. The cells
produce a hormone called human chorionic
gonadotrophin (hCG). This is the hormone that is
detected in a pregnancy test. It is first
produced in very small amounts and can be
detected by sensitive tests at the time of the
missed period.
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Gastrulation
Gastrulation - period of cell migrations, which
converts embryo from hollow ball of cells into a
3 layered stage called gastrula embryo It forms
3 primary germ cell layers Ectoderm - outer
epidermal layers of organs, skin (epithelial
nerve) Endoderm - digestive tract tissue (gut
endocrine glands) Mesoderm - fills in space in
between (notochord, muscle connective
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Gastrulation Invertebrate
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Organogenesis
Organogenesis is the process by which the
ectoderm, endoderm, and mesoderm develop into the
internal organs of the organism. Organ Formation
via - organizing centers, groups of cells
that control fate of (organize) other
cells. Almost all organs form from flat plates
into tubes -gt 3D shape followed by segmented
development via somites.
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Neurulation
During late gastrulation and early neurulation,
the notochord forms by invagination of the
mesoderm in the region of the primitive streak.
The ectoderm overlying the notochord becomes
defined as the neural plate.
The neural plate begins to fold at the midline,
forming the neural groove and ultimately the
neural tube. The neural plate immediately above
the notochord differentiates into the floorplate,
whereas the neural crest emerges at the lateral
margins of the neural plate.
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Neurulation
Once the edges of the neural plate meet in the
midline, the neural tube is complete. The
mesoderm adjacent to the tube then thickens and
subdivides into structures called somites the
precursors of the axial musculature and skeleton.
The neural tube adjacent to the somites ecomes
the rudimentary spinal cord, and the neural crest
gives rise to sensory and autonomic ganglia (the
major elements of the peripheral nervous system).
Finally, the anterior ends of the neural plate
(anterior neural folds) grow together at the
midline and continue to expand, eventually giving
rise to the brain.
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Neural Crest, Migration

• Four distinct migratory paths lead
• to differentiation of neural crest
• cells into specific cell types and
• structures.
• and (2) give rise to sensory and autonomic
  ganglia, respectively.
• (3) The precursors of adrenal neurosecretory.
  They eventually aggregate around the dorsal
  portion of the kidney.
• (4) Non-neural tissues migrate
• along this pathway.

Each pathway permits the migrating cells to
interact with different kinds of cellular
environments, from which they receive inductive
signals
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Cell Differentiation
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Brain Development

• The elaborate architecture of the adult brain is
  the product of
• Genetic instructions
• Cell-to-cell signals
• Interactions between the developing child and
  the external world.
• The early development of the nervous system is
  dominated by events that occur prior to the
  formation of synapses and are therefore
  activity-independent.

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Brain Development Early Events

• The early events include
• Establishment of the primordial nervous system
  in the embryo
• Initial generation of neurons from
  undifferentiated precursor cells
• Migration of neurons from sites of generation to
  their final positions
• Formation of the major brain regions
• These processes set the stage for the subsequent
  formation of axon pathways and synaptic
  connections.

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Brain Development
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Brain Development
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Postnatal Brain Development
The weight of the brain of the newborn is
approximately 300 grams (or around 10 of body
weight) in contrast to the adult brain which
weighs approximately 1400 grams (only 2 of body
weight). Brain weight increases with age and
achieves 'adult' weight between six and fourteen
years of life. We are born with our full
compliment of neurons.
Postnatal growth of the brain is due to an
increase in the size of neurons, and subsequent
increase in number of supporting cells (glia),
development of neural processes and synapses and
the laying down of the insulation of nerve
processes (myelin sheaths). Synapses are formed
at a very rapid rate during the early months of
life, usually achieving maximum density between
six and twelve months after birth. There is a
decrease after this due to disuse or natural
attrition. The infant's brain forms and retains
only those synapses that it frequently uses.
Early sensory experiences are vital to the
formation and retention of synapses.
http//home.iprimus.com.au/rboon/StagesofBrainDeve
lopment.htm
http//www.uib.no/med/avd/miapr/arvid/UiB50/anatom
i/facts.htm
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Postnatal Brain Development
Anatomically and physiologically, infant sensory
systems are not fully mature at birth
Cone
Photoreceptor mosaic
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Cortical Development
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Nature vs Nurture

• Infants perceptual abilities are quite immature
  at birth
• Questions
• Are perceptual abilities pre-programmed, or are
  they moulded by experience?
• To what extent is development a result of
  maturation or a result of experience?

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Monocular Deprivation
What happens if one eye occluded for a period of
time after birth?
Hubel and Wiesel - Monocular deprivation studies
Brief periods of monocular deprivation have
profound effects on the visual system
Effects show up at different levels
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Effects of Monocular Deprivation
In the lateral geniculate nucleus deprived eye
cells shrink
Abnormal layers appear in the left andright LGNs
of a monkey whose right eye was closed at age
two weeks for eighteen months. On both sides,
the layers receivinginput from the eye that was
closed (theright eye) are paler layers 1, 4,
and 6 onthe left layers 2, 3, and 5 on the
right,numbered from below. The cells in the
af-fected layers are smaller.
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Effects of Monocular Deprivation
A kitten was visually deprived after having its
right eye closed at about ten days, the time at
which the eyes normally open. The duration of
closure was two-and-a-half months. In this
experiment we recorded from only twenty-five
cells. (In subsequent experiments we were able
to record more cells, and we found a small
percentage that were influenced from the eye
that had been closed.)
The results were very similar for a baby monkey.
It had its right eye closed at two weeks, and
the eye remained closed for eighteen months. We
subsequently found that the result is the same
if the eye is closed for only a few weeks.
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Effects of Monocular Deprivation
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Effects of Monocular Deprivation
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Critical Period
Critical period is a limited time in which an
event can occur, usually to result in some kind
of transformation. A "critical period" in
developmental psychology and developmental
biology is a time in the early stages of an
organism's life during which it displays a
heightened sensitivity to certain environmental
stimuli, and develops in particular ways due to
experiences at this time. If the organism does
not receive the appropriate stimulus during this
"critical period", it may be difficult, or even
impossible, to develop some functions later in
life. For example, the critical period for the
development of a human child's binocular vision
is thought to be between one and three years, and
further critical periods have been identified for
the development of hearing and the vestibular
system.
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