1
2
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In contrast to amphibians, mammals have developed different type of terrestrial adaptations:
1. Mammals do not accumulate huge amount of yolk. Instead, embryo resides in the specialized portion
of the oviduct, uterus.
2. In the uterus, the nutrition and gas/metabolite exchange is ensured via communication of the
embryo with the maternal circulation via placenta.
Oogenesis comprises following steps: growth of the oocyte, meiosis and formation of membranes by the
follicle cells surrounding the egg.
The process of oogenesis and ovulation is regulated by pituitary hormones.
Fertilization occurs in the upper portion of the mammalian oviduct, the Fallopian tubes.
There are further differences in comparison to the amphibian early embryogenesis:
1. After fertilization, the peace of cell division is much slower in comparison to amphibians. In mice and
humans, the first division takes approximately a day and the next division occur every 10-12 hours.
The morula reaches the uterus after 5-7 days, consisting thus from approximately 100 cells.
2. On the other way, zygotic transcription initiates at the very early, 2 cell stage.
3. There is limited maternal control of egg or embryo organization.
4. Very early, the formation of extraembryonic membranes begins.
5. There might be differentially expressed genes in the males and females due to differential
methylation, imprinting.
Recent cell staining experiments, however, suggest that different portions of egg develop into different
portions of the embryo. Thus, even mammals might be affected by egg organization.
4
In most of the mammals, early embryogenesis events are conserved.
At the 8-cell stage, blastomeres, the individual cells of the early cleaving embryo,
the morula, become a true polarized epitehlium, the cells are flatten and become
tightly adherent-this is called compaction.
The glycoproteins called cell adhesive molecules (CAMs) are responsible for the
contact of individual blastomeres.
The early cleaved embryo is called morula.
5
Besides the anticlinal divisions (with plane of divison perpendicular to the cell
layer surface) that adds new cells to the surface, periclinal divisions (paralell to
the cell layer surface) produce internal cells, the inner cell mass (ICM).
ICM will produce the embryo proper and the amnion.
The outer cells are called trophoblast and these will develop into placenta.
6
After the sixth round of cell divisions (at stage of 64 cells), the cells of trophoblast
and ICM become specialized-as shown by surgical experiments of cell separation,
ICM cells are no longer capable of giving rise to trophoblast, nor trophoblast to
ICM.
7
Polarized epithelium of the trophoblast has assymetrically distributed sodium
pumps on the apical surface.
Activity of these pumps leads to the accumulation of sodium ions in the internal
space, which causes increase of the internal turgor (osmotic pressure) due to
water import. Cl-/HCO3- exchange occurs in order to maintain the electrical
neutrality.
That leads to the filling of the inner space by water and its swelling, a process
called cavitation.
8
The embryo at the stage of 128 cells is called blastocyst (NOT blastula).
The blastocyst is unique structure that forms exclusively in mammals and it is
incomparable to blastula.
9
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Near the end of the day four, the surface cell layer called zona pelucida, produced
by follicle cells in the ovary, is lysed due to the protease secretion by trophoblast
located opposite ICM (antipodal trophoblast) and the embryo implantates into
the highly vascularized uterus.
The ICM is covered with a layer of visceral endoderm, the blastocyst cavity with
parietal endoderm.
The trophoblast cells prolifereate and invade into the uterine wall.
The anatomical designation trophectoderm equals to trophoblast-that might lead
to confusion.
11
Visceral endoderm secretes siganlling ligands (BMP2 and BMP4, BONE
MORPHOGENIC PROTEINS, involved in the bone formation), which are crucial for
the initiation of complex processes of cell death, leading to the cavity formation
inside the ICM.
12
Process of the human embryo invagination. The process of the blastocyst
invagination slightly differs from that of mice (discussed before), particularly in
the shape of ICM (blastoderm).
The shape of blastoderm (blastodisc) in human embryos is more resembling the
situation in chicken “flat” blastoderm.
In mammals, the visceral endoderm forms what is called yolk sac, a fluid filled
sac, partially resembling yolk in the birds. However, there is no yolk as such in
mammals.
However most of the major events are fully comparable.
13
The seveth-day old embryo in the process of implantation. The polar trophoblast
cells are invading uterine endometrium.
14
On the eight day, trophoblast is more proliferated and is composed of both,
cellular and outer syncitial portion. The amniotic cavity started to form in ICM.
15
Late on the eight day, the amniotic cavity is well formed, embryo is completely
implanted and the epiblast and primitive endoderm is apparent. The primitive
endoderm later proliferates, giving rise to parietal endoderm adjoining mural
trophectoderm and visceral endoderm that covers ICM.
16
Formation of ICM is unique to mammals and it is connected with a very early cell
specification. Recently it has been shown that the cell polarity occurs already
at the two-cell stage of cleaving embryo (!).
Richard Gardner in Oxford and Karolina Piotrowska in Cambridge have shown
that one of the first two cells give rise to the upper portion of the embryo,
including ICM and polar trophectoderm while the other one to the lower
embryo portion, consisting of mural trophectoderm and primitive endoderm.
Hence, the plane that is orthogonal to the plane of the ICM is approximately
orthogonal to the plane of the first mitotic spindle.
Very probably, neither maternal mRNA or protein localization, nor gravitational
influence or sperm entry do affect this process.
The molecular mechanism of the asymmetric cells division that might be
underlying this early cell speciation will be discussed later.
17
18
Formation of individual germ layers is similar as those in chicken. I.e., the cells
from the posterior epiblast migrate to the midline, forming thus a primitive
streak that posses an anteriorly localized organizing centre (equivalent to
Hensen’s node, see the small inset above). In contrast to mice, the human
embryos are more similar to flatten chick embryo and its gastrulation disscused
ijn the previous lesson.
The function of the node is conserved, as shown in experiments, where the node
from mammals was transplanted to the area pellucida in chicken and newly
formed axis was apparent. Similarly transplantation to other mammal embryo
will cause another neural axis formation, except that anterior brain regions are
missing in the newly formed axis. Thus, very probably there are other factors
and/or second organizing centre necessary for the head development in
mammals.
Recently, genes expressed in the node and/or necessary for the node function or
secreted from other tissues, e.g. visceral endoderm and necessary for specific
developmental events, e.g. head formation are of interest and are being
identified (e.g. HEX, CERBERUS, ARCADIA, etc.).
Finally, the processes of the germ layers, i.e. ectoderm, entoderm and mesoderm
formation during gastrulation, i.e. the cell migration and invagination through the
node are similar as in the chicken.
19
Similarly to other vertebrates, invaginations along the midline of the node and
cell migrations form neural plate and lateral portions, where epidermis will form.
There is somatic and splanchnic mesoderm separated by a coelom (could be
better observed on later slides). The visceral endoderm (old hypoderm) is pushed
aside to become extraembryonic endoderm.
There is no substantial amount of yolk in mammals, the yolk sac enclosed by the
(mostly extraembryonal) endoderm and contains fluid.
20
The extraembryonic somatic mesoderm covers together with the extraembryonic
ectoderm the amniotic caviity and forms amnion. The inner extraembryonic
ectoderm of the amnion was produced by the ICM (see before).
21
Extraembryonic tissue forms membranes enclosing the embryo and secretes amniotic fluid, in which the
embryo is bathed. Occasionally, cells shed from amnion that could be a target of specific procedure,
amniocentesis, which allows e.g. genetic diagnosis of the developing fetus.
Some mesoderm near the developing posterior end remains connected to the trophoblast, creating thus
an isthmus that allows vascularization of the embryo portion of the placenta. This is equivalent to the
splanchnic mesoderm of the allantois in birds (compare with chicken embryo, inset).
Trophoblast invades into the maternal tissue and forms the placenta. Trophoblast forms, as previously
mentioned, syncitial form (syncitiotrophoblast) and cellular form (cytottrophoblast), that is more proximal
to the embryo.
Developing placenta remains connected to the embryo via mesodermal stalk, the body stalk, that will
become the umbilicus.
The degree of intimacy of the anatomical association between embryo and maternal tissue varies in
different organisms.
In humans and in mice, the maternal tissue of uterus erodes, the maternal capilaries open to form a
system of sinuses and maternal blood gets in the direct contact with endothelial layers of the embryo.
This allows direct exchange of metabolites and nutritions between embryo and its mother.
Placenta consist of both maternal and embryonal tissue and has important functions. It allows nutrition,
respiration and excretion of the embryo and overtakes the function of pituitary and ovary before
pregnancy, i.e. hormone production (chorionic gonadotropins and steroids).
Placenta also provides immunological protection for both mother and fetus, as the fetus carries paternally
inherited antigens. However, the mechanism of this protection is not known.
22
Scheme of the experiments that allowed to specify the number of cells necessary
for the embryo proper formation.
Two embryos from two, distinctly genotyped (i.e. coloured) mice are fused (after
the zona pelucida is removed). If the only one cell would be sufficient, then the
mice would be of one of the originally used genotypes. If two cells would be
involved, one quarter of the progeny would be of either genotype and one half
would be mixed i.e. they will become chimeras (if both genotypes would become
the precursor of the embryo proper with the same probability).
Beatriz Minz has found that the number of chimeras is about 75%, which
corresponds to the number of three precursor cells.
23
24
Individula ICM cells of the embryo could be isolated and later re-introgressed
into the new embryo. These ICM cells are called embryonic stem (ES) cells. It is
very important technique that allows production of transgenic mice.
The isolated ES cells are transformed via foreign DNA construct and it is injected
within the embryo. The transformed cell becomes a part of the embryo and
might result into formation of different tissue types, among them the
spermatogonia or oogonia. i.e. the tissue that provides progenitor for sperm or
egg cells in the resulting chimera. Thus, the progeny of those chimeras will
inherit the modified cell with certain probability and these individuals will carry
the transgene in every cell of their body. Thus, the trangenic mice will be
produced.
This is very important mainly with regard of the knockout mutant (K.O.)
production. In the modified ES, the genes might be specifically eliminated via
DNA recombination. In that way, function of many of the mice genes was
identified.
E.g. the gene NODAL is expressed in the anterior portion of the primitive streak
that is equivalent to the Hensen’s node. nodal/nodal embryos are lethal, they do
not undergo gastrulation and form almost no mesoderm.
25
26
Sections through the young larva of amphibians showing the ectoderm
derivatives:
In the head region, there is brain, forming eye and migrating neural crest cells, in
the trunk region, there is apparent neural tube and neural crest cells, forming
ganglia and adrenal.
In both sections, there is epidermal portion of the integument.
Myotomes refer to upper (dorsal) portion of the somites that will develop into
muscles (will be discussed in the next lecture).
27
What are the factors that induce neural plate formation during early
gastrulation?
Plenty of substances, among them even pH or different ions concentration were
identified. Recent hypothesis suggests that neural development is a default state.
There are in principal two possible ways of the neural tissue induction: (i)
induction of epidermis to form neural tissue (upper figure) or, (ii) vice versa,
factors inhibiting default developmental programme that is epidermis formation
by ectoderm (lower figure).
28
It has been found that by default, the neural cell development occurs. Thus, to
allow epithelial cell development, this default developmental program must be
inhibited .
Recently, factors inhibiting neural tissue and thus promoting epidermis formation
were identified. These are BMPs (BONE MORPHOGENIC PROTEINS, also
mentioned previously in the fifth chapter as inducers of the cell death secreted
by endodermis) and allowing cavity formation on the ICM).
BMPs are secreted by the prospective ventral and marginal portions of the
embryo and form a gradient. They interact with their receptors and induce
epithelial fate formation.
In the prospective neural plate region, NOGGIN and CHORDIN are expressed.
Their protein products inhibit the interaction of BMPs with their receptors and
this allow running the default, neural developmental program.
29
A molecular complex involving BMP, CHORDIN, and TSG regulates
the D-V activity gradient of BMP4 in Xenopus.
CHORDIN binds BMP4 through cysteine-rich domains CR1 and CR3.
After cleavage of CHORDIN by the XOLLOID-RELATED (XLR) zinc
metalloprotease, the affinity between CR modules and BMP4 is greatly
reduced, and BMP4 protein is able to bind to and activate BMP
receptor (BMPR) on the cell surface (De Robertis and Kuroda, 2004).
30
However, as recently found, both inhibition of BMP4 and activation via FGF (fibroblast
growth factor) and IGF (insulin-like growth factor) are necessary for the neural tissue
induction.
Integration of multiple signaling pathways at the level of Smad1 phosphorylation during
neural induction. Neural induction requires extremely low levels of Smad1 activity that
are reached through the combination of two signaling systems.
One is inhibition of binding between BMP4 and its serine/threonine kinase (RS/TK)
receptor by anti-BMP molecules such as Chordin and Noggin. In the absence of those
inhibitors, the Smad proteins are phosphorylated at the carboxy-terminal SXS
sequences, thus triggering nuclear translocation. In the nucleus, Smad proteins inhibit
neural gene expression.
The other is inhibitory phosphorylation of Smad1 in the linker region by receptor
tyrosine kinase (RTK) signals such as FGF, IGF, HGF, and EGF mediated by activation of
MAPK. That phosphorylation prevents nuclear translocation of Smad1 and has an
inhibitory effect on Smad activity. That allows neural gene expression and thus
acquisition of neural tissue identity.
Thus, both of these regulations, i.e. inhibition of the BMPs via CHORDIN and NOGGIN
and negative regulation of Smads via MAPK activity is necessary for the neural tissue
initiation.
MH1 and MH2 are evolutionarily conserved globular Mad-homology domains; MH1
contains the DNA-binding domain and MH2 multiple protein interaction sites.
31
32
During development of the neural plate, the epidermal cells first elongate in the
animal-vegetal axis, forming a “pavement” of columnar cells.
The entire region of the prospective neural plate lengthens as the cells rearrange
(see the arrows in the figures) that is called convergent extension.
The neural plate elongation is at least partially driven by the elongation of the
notochord. When the notochord is surgically removed, the neural plate does not
elongate.
As the cell columnarization is completed, the neural plate forms tear-like
structure leading finally to the neural tube formation.
33
Early in the neural plate formation, the developmental faith of the cells becomes
predetermined in the arterioposterial axis.
If the cells from the anterior portion of the neural plate are explanted, eye-like
tissue forms. However, this will never happen when the posterior portion of the
neural plate is removed and cultivated in the cell culture medium.
Homologues of Drosophila ‘s HOM genes, involved in the specification of
Drosophila segments, were identified along the anteroposterior axis in the neural
plate region. The molecular nature of the process will be discussed later in more
detail.
34
There are two organizing centers, important for the dorsoventral patterning of
the neural plate: Notochord and floor plate, the ventral portion of the neural
plate.
When another notochord from donor embryo is ectopically placed close to the
existing neural plate, secondary ventral region is formed leading to severely
disturbed developmental aberrations with two “bottoms” of the resulting
embryo.
35
Both notochord and floor plate produce SHH, protein (homologous to the
Drosophila’s HEDGHEHOG, see later).
The SSH protein is autocatalytically cleaved and cholesterol binds to the Cterminal
portion of the N-terminal peptide. This peptide then acts as a signalling
molecule. Binding of the cholesterol allows tethering of the peptide to the cell
membranes, thus restricting it’s diffusion.
The underlying molecular mechanisms of the SHH action is not completely
known, however, there is experimental evidence suggesting that:
1. SHH is necessary for the proper dorsoventral patterning of the neural plate
2. SHH activates expression of ventral-specific genes
3. Vice versa, SHH suppresses activity of BMP4 in the ventral portion of the
neural tube
4. SHH induces formation of interneurons and motor neurons
SHH was cloned by Italian researcher Valeria Marigo in 1995 (Marigo et al.,
Genomics, 1995) and designated according to the popular videogame.
36
SHH seems to induce different types of neurons in the ventral spinal chord based
on it’s concentration. That is, SHH probably acts as a morphogen.
Different gene expression domains mutually interact and are of spatial
interaction during neural tube formation.
BMPs are expressed in the lateral portion of the neural plate that will give rise to
the dorsal spinal chord, while SHH is expressed by notochord.
37
As the neural folds are formed and the neural groove gets closed, the neural
plate is still under the influence of BMPs, emitted by the non neural ectoderm
adjacent to the lateral portion of the neural plate and SCC, emitted by both
notochord and floor plate.
38
Finally, as the neural tube closes, the dorsal portion (roof of the neural plate) is
characterized by the expressionn of BMPs, while the ventral part by the
expression of SHH.
39
Because of its origin, the most dorsal portion of the spinal chord express high
level of BMPs that subsequently activate expression of specific TFs (e.g. MSX,
PAX3, PAX7).
In the region of the dorsal spinal chord where the expression of SHH is low or no,
commisurral (neurons with neurites across the midline) and dorsal-type
interneurons are formed here.
The high activity of SHH suppresses BMP4 and induces ventral differentiation.
CHORDIN, an inhibitor of BMP4 action in the neural tissue initiation (see before)
is also involved in the SHH-mediated inhibition of BMPs in the neural tube
differentiation.
The above described lateral portion of the neural plate give rise to subpopulation
of cells called neural crest, which will be discussed in the following slides.
40
41
Neutral crest cells are specific cell subpopulation that originates in the lateral
portion of the neural plate during closure of the neural tube. These cells migrate
through specific paths to specific locations in the embryo, where it will develop
into a plenty of tissue types (see following slides).
The molecular mechanisms of the cell movement and their paths specification is
one of the fundamental questions of the developing biology and will be
discussed in more detail in following chapters.
42
The cells that migrate under epidermis will develop into pigment cells
(melanocytes), others migrate more ventrally, giving rise to sensory and
autonomic ganglia and adrenal medulla (see the figures).
The neural crest of the brain region contributes to sensory ganglia of cranial
nerves, cartilage, bones, teeth and other mesenchymal tissue.
Mesenchyme, or mesenchymal connective tissue, is a type of undifferentiated
loose connective tissue that is derived mostly from mesoderm, although some is
derived from other germ layers; e.g. neural crest cells and thus originates from
the ectoderm. Most embryologists use the term mesenchyme only for those cells
that develop from the mesoderm (Wiki).
43
In the trunk, the neural crest cells migrate through the anterior portion of
somite. Molecular nature of this spatial preference is not known.
44
The developmental fate of neural crest cells is largely specified by their final
localization in developing embryo.
That could be demonstrated by the grafting experiments using quail and chicken
embryonic tissue.
The nuclei of these two species are differentially stained, allowing thus
identification of their origin.
45
In normal development, neural crest cells migrating through the area of thoraic
region (somite 1-7) and the sacral region (somite after 28) form sympathetic
ganglia that secrete acetylcholin.
While the neural crest cell from the midbody region (somite 18-24) form adrenal
medulla, which secretes norepinephrine.
46
Crest cells from the thoraic region of young quail donor transplanted to the older
(more differentiated) chick midbody region, produce adrenal medula, making
norepinephrine.
Vice versa, the midbody from older chicken graft transplanted to the younger
thoraic quail embryo region will result into enteric ganglia producing
acetylcholine.
Enteric ganglia occur in the gut and are part of the so called enteric nervous
system (ENS). The ENS is a subdivision of the peripheral nervous system (PNS),
that directly controls the gastrointestinal system
However, the multipoteciality of the neural crest cells is not absolute. Posteriorly
transplanted cells from the head region form sensory and autonomy neurons in
accordance to their new location while the cells from the trunk implanted to the
head fail to form skeletal structures.
47
48
During development of the CNS, the epithelial cell layer forming the neural tube
gets stratified. That is reflected in the morphological and functional
diversifications in individual layers.
49
Closest to the lumen of neural tube, ependymal zone is formed. This zone is
formed of cuboidal cells and these cells participate on the production of
cerebrospinal fluid inside the lumen.
The cells stretch to the periphery, the so called marginal zone. When the cell
enters G2 phase, their nuclei migrate to the ependymal zone, undergo mitosis
and the daughter cells stretch again.
As the development proceeds, cells tend to inhabit the mantel zone and form
neurons and supporting neuroglial cells (glia).
Cell bodies accumulate in the mantel zone while neurites extend through the
peripheral (marginal) zone, which has fewer cell bodies and more neurites (gray
vs. white matter, see figure).
This anatomical arrangement is largely preserved in spinal cord and hindbrain, in
the midbrain and forebrain, there are chracteristic further rearrangements due to
further cell migrations and divisions (see the next slide).
50
51
The size of the spinal cord varies along the body axis, with extended number of
neural cell bodies in the region of limbs. This is due to increased number of the
peripheral targets innervations, which has been clearly shown by the limb
removal and/or transplantation experiments.
When the limb is removed, the number of motor neurons was not increased.
However, after supernumerary limb transplantation, the number of motor
neurons extended in this position (see the figures).
At the cellular level, this is due to inhibition of apoptosis (programmed cell
death) that occurs in the developing spinal cord. In the regions where the limbs
are developing, the newly formed connections with their muscle cells protect
neurons from the apoptosis, extending thus their number in that region.
For the growth and proper development of neurons, the so called NEURAL
GROWTH FACTORS (NGFs) are necessary. There is a whole family of these factors,
specific for different neuron types. The first NGF was identified in the chicken
tumors and it was identified as a small dimeric glycoprotein. This glycoprotein is
recognized by the receptor tyrosine kinase and stimulates neurons proliferation
and outgrowth of neurites from somatic sensory and sympathetic neurons.
52
53
During development of the CNS, segmental dilatations are formed along the
anteropostreior axis. These are called neuromeres or sometimes also brain
vesicles.
These will develop in to for- mid- and hindbrain, called prosencephalon,
mesencephalon and rhombencephalon, respectively.
54
Prosencephalon forms the forebrain (cerebral hemispheres) and the diencephalon
(hypothalamus).
Mesencephalon forms midbrain.
Rhombecephalon forms metencephalon (cerebellum and pons area) and the
myelencephalon, which forms the hindbrain (brain stem).
The different brain vesicles reveal specific gene expression very early in the
development. E.g. PAX6 (discussed previously in connection with the ventral neural cord
formation, see slide # 40) is expressed in diencephalon while PAX2 and PAX5 in
mesencephalon.
Isthmus, the strip of tissue between mesencephalon and rhombencephalon, produces
FIBROBLAST GROWTH FACTOR 8 (FGF8), which inhibits expression of HOX gene in the
anterior rhombencephalon. Vice versa, retinoic acid, possibly produced by the
surrounding mesoderm tissue, induces HOX expression. Thus, the mutual interplay of
two counteracting factors establishes HOX expression pattern.
Some vertebrates form the very distal vesicles, called rhombomeres. Rhombomeres are
borders of specific gene expression, as will be discussed later. Rhombomeres also
represent developmental boundaries, that prevent cell migration.
55
The spinal cord has a segmental organization, too, consisting of individual
neuromeres. Each neuromere is connected with two somites, each of them on
the opposite lateral side.
The crest cells migrate, as discussed before (see the inset on the left-hand side),
through the anterior portions of the somites and some of them form dorsal root
sensory ganglia, one on each side of the neuromere.
The somites form cartilagious precursors of vertebrae, which are segmented in
register with ganglia and neuromeres, too.
The somites are crucial for the segmental development of the spinal cord.
Removal or transplantation of somites leads to absence or ectopic ganglia
formation, respectively. This suggests communication between somites and
developing spinal cord.
56
During development, brain develops in a segmental way, i.e. the migratorial
behavior of the neurons originating at the ependymal border varies according to
the area of developing CNS.
That results in a different anatomy of formed layers or congregations of cell
bodies, called nuclei.
Neurites connecting different nuclei form tracts.
57
As a remnant of the brain formation from the neural tube, hollow multipocketed
chambers are formed inside the brain, which are called ventricles. These
ventricles communicate with the spinal canal that goes through the spinal cord,
another remnant of the neural tube.
58
Along the anterioposterior axis, the changes in motility, cell division and
apoptosis result into segmental differentiation of individual brain portions, called
vesicles.
Formation of individual vesicles is a result of complex events, leading to acquiring
of identity of individual vesicles, their wiring and final sculpting of the brain. The
molecular mechanisms driving those events will be discussed later.
59
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The eyes develop from diencephalon, which forms two outpocketings on each side. These
outpocketings develop into optic vesicles, which further proliferate and evaginate through the
head mesenchyme (that is mostly of the neural crest origin (see previous slide # 43) and abut
the surface ectoderm.
Then the optical vesicle invaginates, forming thus optical cup. As the optical cup forms, the
overlying ectoderm is induced and changes from cuboidal to columnar and invaginates, allowing
thus lens placode, a future lens, formation.
The outer layer of the optical cup develops photoreceptors (rods and cones). The efferent
neurons of the retina project back to the brain via the optic nerve that develops from the stalk
of the optical vesicle.
The inner surface of the optic cup becomes pigmented retina, while the outer surface becomes
the photoreceptors and neurons of the neural retina and also generates an epithelial iris.
The surrounding head mesenchyme derived from mesoderm develops into hard scleral covering
of the eye ball and eye muscles.
The early transplantation experiments (see above) suggested that lens cup is able (similarly to
Spemann organizer) to induce ectopically lens formation. However, more recent experiments
have shown that the newly induced lens originates from the contaminant ectoderm of the
donor, adjacent to the lens cup.
Recent model proposes that the lens-induction signals of the optical cup interact with multiple
signals produced earlier in the development by surrounding tissue, e.g. from prospective head
mesoderm and from anterior endoderm. The nature of individual receptors and signals remains
to be identified.
61
The what is called master genes were identified to be critical for eye formation.
In Drosophila, mutants in EYELESS gene are unable to form eye. When ectopically
expressed, EYELESS is able to induce eye formation (see the images).
EYELESS encodes TF that very probably triggers the downstream cascade leading
to the eye formation. These genes are called “master” or “executive” genes.
In mouse, homologue of the EYELESS is PAX6 (see previous slides # 40). PAX6 is
able to complement eyeless mutation, suggesting thus functional conservation of
the whole developmental cascade which had to evolve approximately 500 mil.
years ago, before the arthropods separated from the vertebrate lineage.
62
The development of the eye in vertebrates and Drosophila differs tremendously.
Thus, it is very surprising that the eyeless mutation could be complemented with
the mice PAX6.
In vertebrates, the eye development was just discussed.
In Drosophila, the eye develops from the specific imaginal disc, one of the
patches of stem cells in the embryo that were discussed in the second lesson.
The Drosophila eye (what is called “compound eye”) is composed of approx. 800
identical facets, each of them having its own lens and a set of eight
photoreceptors. Each facet sends an efferent neurite to the ventrally located
anterior ganglion of the fly’s nervous system.
Just before and during metamorphosis, the differentiation of the imaginal disc
cells is apparent as a wave (morphogenic furrow) moving from the posterior
towards the anterior pole. The homologue of vertebrate BMPs,
DECAPENTAPLEGIC (DPP) a Drosophila TGF-beta homologue, is secreted in the
morphogenic furow, while HEDGHEOG (HH) is expressed in cells posterior to the
furrow.
The identity of the disc is established by the action of EYELESS.
63
64
Multiple autonomous and sensory neurons in cranial ganglia and sensory organ
epithelia (e.g. nasal epithelium) are formed from the what is called placodes.
These are patches of ectoderm that, similarly to neural plate, elongates to form
columnar epithelium.
The major placodes include the nasal and otic placodes that form portions of the
nose and inner ear, respectively. Several ganglia receive neural crest as well as
placodal contributions.
65
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67
Ectoderm and mesoderm form integument.
Ectoderm forms basal germinative layer, where the ongoing mitosis ensures
supply of new cells. Epidermal cells filling with keratin undergo apoptosis,
resulting in formation of outer, cornified layer that is several cells thick.
The ectoderm also forms specialized structures of the skin, e.g. hairs, feathers,
sweat and sebaceous glands, gills and mammary glands.
Mesoderm gives rise to the underlying dermis, comprised of mesenchyme and a
vascular supply.
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