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Ectodermal Organs
Developmental Biology – Biology 4361
November 22, 2005
Germinal neuroepithelium external limiting membrane
neuroepithelium neural tube
Figure 13.3
(stem cells)
Neuroepithelial derivatives
Figure 13.4
Figure 13.5
Myelination
Figure 13.7
Human spinal cord development
neurons in alar and basal plate
glial cells in floor and roof plate
mantle (gray matter) – neurons bodies
marginal layer (white matter) – myelinated axons
in
out
Figure 13.7
Human spinal cord development
afferent – receive impulses from skin, muscles, organs
efferent – send signals to muscles and glands
commissural axons – connect afferent and efferent signaling centers
Establishment of dorsoventral pattern in spinal cord
Figure 13.8
notochord graft experiment floor plate induction efferent neuron induction
notochord removal experiment no floor plate/efferent neurons extended midrange
floor plate graft experiment additional floor plate additional efferent neurons
notochord/floor plate
= efferent neurons, no afferent neurons
= ventralize neural tube
Establishment of dorsoventral pattern in spinal cord
Figure 13.10
NOTE antagonistic signals!
sonic hedgehog (Shh)
Inductive signals:
chordamesoderm notochord floor plate
Ventral (floor plate & efferent neurons)
Dorsal (roof plate, afferent neurons, neural crest)
BMPs, Wnt family
epidermal ectoderm
roof plate
Figure 13.10
Shh morphogen gradient increasing concentration of Shh induces
distinct types of neurons in vitro
Shh and dorsalizing signals interact to specify a dorsoventral pattern of motor neurons, interneurons & spinal cord
Figure 13.11
Spinal cord
Nervous system
Vertebrate brain development
brain develops from cranial part of neural tube
dorsoventral pattern is induced by same molecular mechanisms as spinal cord
however, organanization is different
central canal forms fluidfilled spaces (ventricles)
brain regions have different structures/functions
further differentiation of gray and white matter
Figure 13.13
Vertebrate brain development
4 week human embryo
Brain development in vertebrates 2
Figure 13.12
Vertebrate brain development
5 week human embryo
Figure 13.12
Vertebrate brain development
Figure 13.13
Vertebrate brain development
Figure 13.14
Myelencephalon medulla oblongata
human – 6 wk
Figure 13.15
Mesencephalonmetencephalon cerebellum
human – 8 wk human – 4 month
Figure 13.16
Brain development cerebellum
8 wk 12 wk
13 wk
15 wk
5 week human embryo
Figure 13.12
Vertebrate brain development
prosencephalon: diencephalon
telencephalon cerebrum (2 hemispheres)
optic cup
Figure 13.17
Diencephalon &
telencephalon
human – 8 wk
Brain architecture: nuclei – areas with specific functions
gray matter migration/stratification vertical – neurons move outward horizontal (6 layers)
Human brain development
Figure 13.13 spinal cord
main portion of the cerebrum; covers most other parts of the brain
extends into olfactory bulb, sense of smell gateway for sensory fibers from spinal cord regulatory center for visceral functions
forms posterior lobe of pituitary gland endocrine organ circadian rhythm, annual repro.
relay station for visual and auditory reflexes
coordination center for posture and movement
pathway for nerve fibers controls reflexes of neck, throat, tongue
mediates reflexes of trunk and appendages
Vertebrate brain development
Figure 13.19
4 month human fetus all major brain areas developed
Human brain development
Peripheral nerves
Cranial Nerves
Figure 13.22
Neural crest origins
only found only in vertebrates
originate from cells located between epidermal and neural ectoderm
Neural crest cells:
migrate to different positions within the body
variety of fates head cartilage pigment cells neurons hormoneproducing gland cells smooth muscle – cardiovascular system
Figure 13.24
Embryonic origin of neural crest cells
Juxtaposition hypothesis: NC cells arise at boundary between neural plate and epidermis
both grafts in this experiment will give rise to NC cells
NC cells are induced by local interactions between neural plate and epidermis
Figure 13.25
Neural crest cell fate mapping
fluorescent dyes & immunostaining
genetic labels: e.g. transplantation between quail and chicken
homotopic transplantation from radiolabeled donor to nonlabeled host
Methods used to monitor NC cell migration:
Neural crest transplantation
chick / quail chimera
Figure 13.27
Neural crest cell migration
slug may activate other regulatory genes involved in migration
slug expression causes dissociation of desmosomes
onset of migration controlled by regulatory gene slug+
NC cells lose epithelial connections, cell adhesion properties migrate
Migration
Migration routes
dorsolateral – skin melanocytes, xanthophores ventral – neurons, glial cells, visceral nervous system
Trunk NC cells
Figure 13.27
Neural crest cell fates
Trunk melanocytes, xanthophores neurons, glial cells visceral nervous system
sympathetic parasympathetic
Schwann cells adrenal medulla
Cranial
hormoneproducing cells parasympathetic ganglia sensory cranial ganglia pigment cells
bones, connective tissue
Cardiac (overlapping head and trunk)
melanocytes neurons
cartilage and other connective tissue
connective tissue, muscle of large blood vessels
Are NC cells pluripotent ?
all other NC derivatives can be formed by NC cells from anywhere along the anteriorposterior axis
NC Cell Potency:
cranial cartilage only from head NC cells some cardiovascular structures limited to
cardiac NC
Figure 13.28
NC fate and potency
(determined by heterotopic transplantation):
Figure 13.29
Neural crest cell determination
each NC cell has the potential to form many or all derivatives
external signals cause their determination
all NC regions contain mixed populations of determined cells, each of which has just one fate
external signals limit NC cells to certain migration pathways and differentiation patterns
Pluripotency hypothesis
Selection hypothesis
Clonal analysis in vitro:
NC cell determination
many NC cells are originally pluripotent
NC cell fate becomes restricted in a stepwise process
NC cells proliferate and form clones
some clones differentiate into only one or two cell types
most clones differentiate into several cell types
Figure 13.30
Doublelabeling experiments:
descendants of a single NC cell can be located in different tissues
dorsal root ganglion ventral root sympathetic ganglion
NC cell determination
Clonal analysis in vivo: clones from premigratory NC cells usually contain several cell types clones from older, migratory NC cells often contain more than one cell type
most NC cells are pluripotent
Figure 13.27
NC cells follow defined migration routes:
Spatial restrictions on NC cell migration
‘repulsive guidance’ by ephrins and their receptors
inhibition by somitic mesoderm
ventrally migrating trunk NC cells pass through anterior halves of somites, not through posterior halves
inhibitory signal from notochord chondroitinsulfate containing glycoprotein
avoid notochord area
Figure 13.23
Temporal restrictions to NC cell migration
NC cells ‘behave’ according to their age: chicken NC cells enter ventral pathway first, then dorsolateral pathway
isolated NC cells ‘aged’ in vitro, then transplanted into hosts at various stages of NC cell migration
transplanted aged NC cells behave according to their age, not according to surrounding host NC cells
ECM influence on NC cell determination
Figure 13.32
nitrocellulose microcarriers coated with ECM components from: dorsolateral pathway (pigment cell route) ventral pathway (dorsal root ganglia route)
dorsolateral ECM components induce NC cells into pigment cells
ventral ECM components induce NC cells into neurons
Extracellular matrix (ECM) ECM – fibrous and gelatinous material released from cells
amorphous ground substance (attracts water; forms gel) fibers (form meshwork; resist expansion)
provide multiple binding domains
ECM functions: basement membrane matrix for bones and teeth tendons – tensile strength cornea – forms transparent layer influences cell division, shape, movement, differentiation
(binding sites for growth factors, etc.)
Ground substance: glycosaminoglycans, proteoglycans amorphous, hydrophilic
hyaluronic acid heparin
Fibrous components: glycoproteins collagen fibronectin laminins
ECM influence on NC cell determination
Figure 13.32
nitrocellulose microcarriers coated with ECM components from: dorsolateral pathway (pigment cell route) ventral pathway (dorsal root ganglia route)
dorsolateral ECM components induce NC cells into pigment cells
ventral ECM components induce NC cells into neurons
Migration:
subpopulation of NC cells
diffusible signals from notochord, somites and potentially other tissues
‘age’ of NC cells
Determination:
subpopulation of NC cells
contact signals provided by ECM components
regionspecific growth factors ( endothelins, TGFβ superfamily & others)
Factors affecting NC cell migration & determination
Figure 13.33
areas of ectoderm in head region induced by underlying parts of the brain
epibranchial placodes contribute to sensory ganglia of cranial nerves
dorsolateral placodes contribute to sensory ganglia of cranial nerves form parts of ear, eye and nose
Ectodermal placodes squamous
columnar
forms otic pit à otic vesicle à inner ear
induced by rhombencepahlon & mesoderm
otic vesicle expands unequally into complicated shape; forms the labyrinth
Figure 13.34
Otic placode
Labyrinth: (higher vertebrates)
squamous and columnar epithelia form sensory epithelia
registration of gravity & acceleration in semicircular canal
perception of sound in cochlea
transmission of sounds to inner ear by tiny bones and membranous window of middle ear
Figure 13.35
Otic placode – labyrinth formation
Figure 1.16
induced by complex interactions of head ectoderm with pharyngeal endoderm, heart mesoderm, neural crest & optic vesicle
invaginates to form the lens vesicle
lens vesicle cells differentiate into lens fibers
synthesis of crystallins
Lens placode
eye development requires simultaneous development of lens vesicle and optic cup
outer layer of the optic cup forms the pigment layer of the retina
inner layer of optic cup: neural layer of retina converging axons form the optic nerve
opening of the optic cup forms the pupil
Figure 13.36
Lens placode – eye development
Figure13.37
Neural retina
human 25 wk
Figure 13.39
5 weeks 6 weeks 7 weeks 10 weeks
Nasal placode
induced by underlying endoderm and telencephalon forms lateral & medial nasal swellings & nasal pit (placode forms floor)
fusion defects: harelip
fusion forms nose, lip (partial), jaw (partial), palate (partial)
increase in size of maxillary swellings pushes nasal swellings towards center placodes surrounded by swellings (ridges)
Figure 13.40
6 weeks
7 weeks
9 weeks
Nasal placodes – nose formation
epithelium of the nasal pit forms the olfactory epithelium which lines the roof of the nasal chambers
nasal chambers elongate while secondary palate & secondary choanae form
oronasal membrane between nasal pit and oral cavity ruptures to form the primary choanae
Epidermis
largest ectodermal derivative outer layer of the skin
periderm temporary outer layer
germinative layer or basal layer progenitor cells differentiate into epidermal cells
keratin synthesis in granular layer
cornified layer dead keratin sacs
mesenchymal dermis supports the epidermis and induces formation of hair, feathers, scales & glands
Figure 13.41
differentiation
Figure 13.42
Hair development in humans:
dermal mesenchyme cells induce formation of epidermal hair buds
dermal mesenchyme cells are enclosed by base of hair bud = hair papilla
hair papilla and differentiated epidermal cells form the = hair follicle
core cells of hair follicle are keratinized and pushed outside = hair shaft
differentiation of blood vessels, nerve endings & associated glands
Epidermis – hair development
Figure 13.42
core cells of the hair follicle are keratinized and pushed outside
= hair shaft
melanocytes transfer pigment to hair
secretes the oily sebum sebum + shed peridermal cells
= vernix caseosa
bulb containing pluripotent hair follicle stem cells
root sheath is formed by epidermal and mesencymal cells
Epidermis – hair development
Figure13.43
Mammary gland development
7 wk human (generalized mammal)