ORGANOGENESIS
Organs and Organogenesis - General
1.
Uniqueness
a. Origin - germ layer(s)
b. Position
c. Structure
d. Function
2.
Organ primordia form from a specific germ layer
(sometimes layers), usually in the region of the body
where the organ will be located.
3.
Differentiation ===> Histogenesis
===>
Function
Organogenesis - general
4.
Organogenesis involves
a. inductions
b. migration of cells in some instances
c. shape change (at both cellular and tissue
levels)
d. changes in cellular gene activity differentiation/histogenesis
e. growth - organ size, cell number
f. in some cases, cell death
5.
All these factors work in concert to “mold” the
primordium into a functional organ.
ORGANOGENESIS - ECTODERM
P. 234, Carlson
Neurulation (human embryo)
http://courses.temple.edu/neuroanatomy/lab/embryo/ntube.htm
Neural Crest Cells
1.
Multipotent cells that arise from the edges of the forming neural
plate
2.
Migrate throughout body to form many different tissues/structures.
3.
Two paths of migration
a. Dorsolateral (superficial pathway)
b. Ventral (deep pathway, between and through the somites)
http://www.erin.utoronto.ca/~w3bio380/lecture16.htm
Neural Crest Cells
4.
Neural crest cells from cranial region form:
a. Sensory components of cranial nerves V, IX, X
b. Schwann cells
c. Contribute to branchial cartilages
d. Contribute to membranous bones of skull
e. Dentin of teeth
f. Contribute to head mesenchyme
g. Cranial parasympathetic ganglia
h. Ciliary muscles of eye
http://www.nlm.nih.gov/medlineplus/ency/imagepages/19080.htm
i. Meninges of CNS (dura mater, arachnoid, pia mater)
http://vanat.cvm.umn.edu/neurHistAtls/pages/men1.html
Neural Crest Cells
5.
Neural crest cells from truck region form:
a. Parasympathetic and sympathetic ganglia
b. Dorsal root ganglia
c. Meninges of CNS (dura mater, arachnoid, pia mater)
d. Schwann cells
e. Adrenal medulla
6.
How can they become so many different things?
7.
Multiple inductions along their paths of migration.
Neural crest cell migration in the chick hindbrain.
Neural crest cells leave from near the midbrain (m),
midbrain/hindbrain boundary (m/h) and rostral rhombomeres (r1
and r2) and spread out to cover a wide region adjacent to the
neural tube. Duration: 7 hrs Time interval between images: 3
min
http://authors.library.caltech.edu/12902/
https://embryology.med.unsw.edu.au/embryology/index.php/Chicken_Neural_Crest_Migration_Movie_1
Cranial Nerves & Ganglia
Mnemonic
“On Old Olympus’ Towering Top A Finn And German
Vault And Hop”
Olfactory (I)
Optic (II)
Oculomotor (III)
Trochlear(IV)
Trigeminal (V)
Abducens (VI),
Facial (VII)
Acoustic (VIII)
Glossopharyngeal (IX)
Vagus (X)
Accessory (spinal accessory) (XI)
Hypoglossal (XII)
Cranial Nerves & Ganglia
vestibulo-acoustic
petrosal (distal)
Froriep’s ganglion is temporary group of nerve cells associated with the
hypoglossal nerve (CN XII) of the embryo. It does not persist into the adult stage.
Origin of the Cranial Ganglia
Origin of the Cranial Ganglia
Neural crest contribute
sensory neurons to the
ganglia of CN V, IX, and X
Neural crest
Dorsolateral placode
The dorsolateral
placodes
contribute to the
“special” sense
organs (olfactory,
optic (lens), otic).
Epibranchial placode
Contribute sensory
neurons to the ganglia
of cranial nerves V,
VII, IX and X.
Know the cranial ganglia, their origins, and
the nerves they are associated with.
geniculate
superior (proximal) neural crest (superior)
& petrosal (distal) & epibranchial placode
(petrosal)
jugular (proximal)
& nodose (distal)
Froiep’s ganglion
Froriep’s
The Neural Tube (primordium of the CNS)
Shape change of cells as the neural plate forms
P. 236, Carlson
Central nervous system development
Shape change in cells
P. 239, Carlson
Central nervous system development
http://courses.temple.edu/neuroanatomy/lab/embryo/histo.htm
Later cell division in the neural tube
Establishes cells
the ependymal, mantle and
layers
marginal
Establishes the ependymal, mantle and marginal layers
Outside
Establishment of ependymal, mantle and marginal
layers in spinal cord
Similar to P. 439, Carlson
PATHFINDING BY AXONS
A. General neuron structure
Myelination
In CNS - oligodendrocytes
In PNS - Schwann cells
B. Neurons and synapses
Mauthner neuron
Initiates tail-flick escape
response by activating groups
of motor neurons in the spinal
cord.
Hibbard, 1965 - pathfinding by axons in amphibian and fish hindbrain (medulla).
Anterior
Anterior
Posterior
Posterior
Hibbard, 1965 - pathfinding by axons in amphibian and fish hindbrain (medulla).
Anterior
Anterior
Posterior
Posterior
Hibbard, 1965 - pathfinding by axons in amphibian and fish hindbrain (medulla).
REVERSED ORIENTATION
Anterior
Anterior
Posterior
Posterior
Anterior
Anterior
Posterior
Posterior
Mechanisms for axon pathfinding
1.
2.
Stereotropism (contact guidance)
a.
Singer et al., 1979 - stereotropic pathfinding in
neuroepithelial matrix of a newt embryo (amphibian)
b.
Silver and Sidman, 1980 - stereotropic pathfinding in
mouse retina
Differential adhesion (integrins, cadherins)
a. Letoureau, 1975 - diff. Adhesive pathfinding in vitro.
3.
Galvanotropism
a. Patel et al., 1984 - pathfinding along charge
differential pathways in vitro.
4.
Chemotropism (netrin, connectin, nerve growth
factor)
a. Gunderson & Barrett, 1979, 1980 - pathfinding in
response to chemical signals in vitro.
Axon Pathfinding - chemotropism
http://web.sfn.org/content/Publications/BrainBriefings/
axon.html#fullsize
Blue - attractant molecules
Orange - repellent molecules
“Axons locate their target tissues by using chemical attractants (blue) and repellants (orange)”
Either diffusable substances released by cells or molecules embedded in the
plasmalemma
Surfaces of target tissue cells can also display attractant or repellent molecules.
Illustration by Lydia Kibiuk, Copyright © 1995 Lydia Kibiuk.
The Growth Cone
http://journal.frontiersin.org/article/10.3389/fnana.2011.00062/full
The growth cone
Axons that extend from the retina to the tectum of the midbrain.
http://www.anat.cam.ac.uk/pages/staff/academic/holt/images.html
Optic chiasma
As growth cones reach a point in axon growth where a decision
must be made, they change shape and speed of growth and
become more active. Filopodia appear to be searching for the
right signal.
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=0mVzBEg1E
X_YMkzpc2FN_TNTHWJTpj89DoajBTA4
(Optic chiasma)
(Optic chiasma)
One possible signal at the optic chiasma is a glycoprotein called
CD44. If CD44 is not expressed or if the cells that express it are
eliminated, the growing axons from the sensory retina ganglion cells
do not cross the chiasma.
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=0mVzBEg1EX_YMkzpc2FN_TNTHWJTpj89DoajBTA
4
Examples of the effect of molecular signals on growth cone
progress - Research by David Sretavan M.D., Ph.D., Doctor of Opthalmology
University of California, San Francisco
Growth of retinal axons in mice in response to specific retinal proteins
Eph (Ephrin) type proteins - can modulate axon
pathfinding. Have attractive, repulsive and other
effects on axon growth.
“The following QuickTime movies show retinal axon
growth cones responding to gradients of Eph type
proteins.
Videos presented at 420 times normal axon growth
rate.” i.e., 1.5 min = about 10 hr
http://ucsfeye.net/dsretavanresearch.shtml
The human brain
By the sixth prenatal month, nearly all of the billions of neurons (nerve
cells) that populate the mature brain have been created, with new
neurons having been generated at an average rate of more than
250,000 per minute.
http://www.futureofchildren.org/information2827/information_show.htm?doc_id=79339
The human brain
At birth - 100 billion neurons in brain = 100,000,000,000
1000 billion glial cells
= 1,000,000,000,000
However, the wiring of the brain is not yet complete at birth.
As a baby starts to experience life, connections are made between neurons - the more
connections there are, the more the brain can do. Much of the brain’s growth after birth is due
to the development of numerous dendrites that receive synaptic connections from growing
axons.
A baby's brain develops so fast that by age two a child who is developing normally has the
same number of connections as an adult. By age three, a child has TWICE as many brain
connections as an adult.
http://www.preschoolrainbow.org/brain-growth.htm
Adult - an average of 10,000 synapses per cortical neuron
Very rough estimate,
10,000 synapses/neuron X 100,000,000,000 neurons
= 1,000,000,000,000,000 synapses
= 1000 trillion synapses in the adult brain
How big is 1000 trillion?
There is nowhere near enough information in your DNA to code for the
specific locations of all these synapses.
Much of this “wiring” is completed after birth and results from the child’s interactions with
his/her environment.
Developing coordination
By age three, a child has TWICE as many brain
connections as an adult = 2000 trillion
Use it or lose it! Synaptic connections are winnowed
as children grow. Those not used are lost while those
that are used are retained. (Neurotrophic factors
produced by the cell that grew the axon are necessary
to maintain the synapse).
The more you do something, the better you get (e.g.,
practice improves your hand-eye coordination).
Practice makes perfect. This is true for young children
and also, to a certain extent, when you get older
(however, a child’s brain has much more “plasticity”).
Does this apply to other aspects of nervous
development?
Influence of Inheritance and Environment on ability
Environmentally determined outcome
Virtuoso
Totally incompetent
Genetically set limits
WHY IS THIS IMPORTANT?
BECAUSE YOU WANT
YOUR CHILDREN TO BE
ALL THAT THEY CAN
BE!!!!!
Inductions in the peripheral nervous system
Inductions in the peripheral nervous system
Olfactory epithelium
1. Formation of olfactory placode
a. Induction #1 - presumptive head endoderm
b. Induction #2 - presumptive head mesoderm
c. Induction #3 - telencephalon - seals fate
2. Cells of olfactory epithelium form stem cells,
neurons and supportive cells
3. Olfactory neurons extend axons to regions in the
developing telencephalon that will form the olfactory
lobes.
4. Pathfinding by axons
5. Synapse on other neurons in olfactory lobes
6. Neurons in the olfactory epithelium have a life-span
of about 1 month
7. Must be replaced from stem cells
8. Axons are constantly extending from these new
neurons into the olfactory lobes where new synapses
are formed.
Inductions in the peripheral nervous system
Otic vesicle/inner ear
1.Induction #1 - Chordamesoderm passes near
presumptive otic placode tissue
2. Induction #2 - nearby paraxial (somitomere)
mesoderm further conditions cells
3. Induction #3 - Lateral wall of myelencephalon seals fate and causes placode to form and
invaginate - forming the otic vesicle
Utricle - semi-circular canals, utriculus
Saccule - cochlea, sacculus
DEVELOPMENT OF THE EYE
Lens placode
Lens placode
Human - 7 - 8 wks
Choroid fissure
choroid fissure
Human ~ 10 wks
opticoel
posterior
chamber
iridica
(muscles)
vitreous body
pars iridica
ciliary body (muscles)
lens
ligament
opticoel
optic stalk (optic nerve)
ora serrata
pars caeca retinae
pars ciliaris
pars iridica (iris)
pars optica retinae
pigmented retina
sensory retina
vitreous body
Development of the Human Retina
neuroblastic
iridica
(muscles)
Development of the Human Retina
neuroblastic
neuroblastic
Development of the
Human Retina
neuron
neuroblastic
Development of the Human Retina
neuron
ganglion
cells (neurons)
Innermost neuron layer
Outer neuroblastic layer
Inner neuroblastic layer
lens
the
lens
added