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Histogenesis is the process by which cells and tissues acquire functional specialization

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Title: Histogenesis is the process by which cells and tissues acquire functional specialization


1
Histogenesis is the process by which cells and
tissues acquire functional specialization
In humans Cleavage ? gastrulation ?
organogenesis ? histogenesis (2 weeks) (1
week) (4 weeks) (7
months) Organogenesis the formation of organ
rudiments to establish the basic body
plan. Histogenesis differentiation of cells
within the organs to form specialized tissues.
Tissues are composed of cells and extracellular
material that perform a specific function. Each
specific tissue develops mainly from one germ
layer.
embryo
fetus
2
(No Transcript)
3
The neural tube gives rise to the central nervous
system and contributes to the peripheral nervous
system
Central nervous system brain and spinal
cord Peripheral nervous system all nervous
tissue outside of the skull and vertebral column.
From the time of neural tube closure to birth,
approximately 250,000 neurons are formed each
minute. The CNS contains over 100 billion neurons
when complete. How do neural cells grow and
differentiate? The early neural epithelium
contains a pseudostratified layer of stem cells.
The basement membrane is at the outer edge and
tight junctions form at the inner surface.
Different cell types are formed.
4
Development of neuroepithelium
Neuroepithelial cells differentiate to form two
major types 1. Stem cells have a unlimited
capacity for self renewal 2. Committed progenitor
cells these divide to produce 2 differentiated
types A. Neuroblasts develop into neurons B.
Glioblasts form glial cells that can develop into
astrocytes (glue that holds neurons together),
and oligodendrocytes (form myelin around
neurons). The microglia are macrophages derived
from mesenchyme.
Mantle the inner area where neurons and glial
cells accumulate to form gray matter. Marginal
layer area containing axons of neurons that
transmit signals to other organs. They are
white due to the presence of myelin white
matter. Ependymal cells form the ependymal
layer that lines the cavities. May be stem cells
5
The spinal cord develops a dorsoventral pattern
All nervous functions depend on development of
complex connections between neurons. These
circuits start to develop in the embryo. Alar
plate as neurons and glial cells accumulate in
the mantle, they form ridges on either side of
the neural tube (dorsal or afferent columns).
These will develop into afferent nerves that
conduct signals to the brain. Basal plate
accumulation of cells in the ventral region
produces basal or efferent columns. These will
develop into efferent neurons that carry signals
to muscles and organs (motor neurons). The gray
matter in the mantle layer is composed of cell
bodies of neurons and the white matter in the
marginal layer is composed of myelinated axons.
The floor and roof plates are composed of glial
cells. What causes this pattern?
6
The notochord and floor plate induce
the dorsoventral pattern of neural development
To examine whether ventral columns were induced
by adjacent tissue, an extra notochord was
grafted to the side of the developing neural
plate.
This created an extra floor plate and ventral
column, suggesting that it induced these
structures. If the notochord was removed, no
ventral column or floor plate developed,
consistent with the above idea. It is believed
that the notochord first induces the floor plate
and the floor plate then induces ventral columns.
This was confirmed by grafting a floor plate,
which also induced efferent nerves. What is the
molecular nature of this inducer of dorsoventral
patterning?
7
Sonic hedgehog (shh) induces the dorsoventral
pattern
Sonic hedgehog (shh) is a gene that is expressed
in the notochord at first and later in the floor
plate. Mice that lack shh fail to develop floor
plates in the CNS. Shh is a secreted
glycoprotein that induces a gradient that is high
near the floor plate and progressively lower in
dorsal regions. Shh initially induces neural
plate cells to form floor plate. Other signals
from the dorsal ectoderm direct the dorsal
columns and the roof plate. Different levels of
shh appear to specify different types of neuron
differentiation.
8
Different concentrations of shh induce distinct
types of neurons
In the developing spinal cord, the floor plate
produces shh and creates a concentration
gradient. Motor neurons develop closest to the
floor plate, type 2 interneurons are next,
followed by type 1 interneurons. To see whether
the shh gradient is really important, isolated
cells from neural tubes were cultured in various
concentrations of shh. Cells were stained with
antibody specific for floor plate, motor neurons,
or type 1 or 2 interneurons.
Bone morphogenetic protein released by the dorsal
ectoderm and roof plate has an analogous function
in generating the dorsal columns
9
How does the brain develop?
The initial stages of brain development are
similar to spinal cord (neurulation and
establishment of the dorsoventral pattern. The
brain becomes more complex as the central canal
expands to form four fluid filled ventricles.
After 4 weeks, the human brain has 3 regions, the
prosencephalon, mesencephalon, and
rhombencephalon. By 5 weeks the prosencephalon
divides into telencephalon (2 outpockets that
will become the cerebral hemispheres surrounding
the 1st and 2nd ventricles) and the diencephalon
which forms around the 3rd ventricle. The
rhombencephalon forms the metencephalon in the
anterior and the myelencephalon in the posterior.
The myelencephalon surrounds the 4th ventricle.
10
Flow chart showing brain development
11
Brain of a four month old fetus
Telencephalon forms the cerebral hemispheres
with 1st and 2nd ventricles Diencephalon forms
the posterior pituitary gland (infundibulum), the
thalamus (sleep), and hypothalamus (homeostasis)
with the 3rd ventricle Mesencephalonmidbrain Mete
ncephalon forms the cerebellum (balance and
muscle tone) and pons Myelencephalon forms the
medulla (reflexes)
12
Neural crest cells arise during neurulation
Neural crest cells These cells arise from both
dorsal epidermis and neural plate. They migrate
throughout the body. Neural crest cells form a
variety of cell types including cartilage,
pigment cells of skin, neurons, smooth muscle
cells, and adrenal medulla.
Migration staging area the cells originate at
the crests of the neural folds during
neurulation. Both epidermal tissue and neural
tissue contribute to this lineage. Slug a
regulatory gene that is expressed as neural crest
cells start to leave the staging area. Slug
appears to alter expression of cell adhesion
molecules (cadherins) and it causes dissociation
of desmosomes on neural crest cells.
13
Neural crest cells form a variety of tissues
The fate of neural crest cells has been mapped by
a number of techniques (radioactive tracers,
transplants from pigmented species to albinos).
There are 2 patterns of migration in the trunk
region Dorsolateral path enter skin and form
melanocytes Ventral path form afferent neurons
of dorsal route ganglia, sympathetic and
parasympathetic ganglia, and adrenal medulla
Neural crest cells help to form addition
structures in the head such as bones, connective
tissue, eyes, ears, and teeth. They also help
to form blood vessels and connective tissue in
the trunk
14
How do neural crest cells differentiate into many
tissues?
Pluripotency hypothesis each neural crest cell
has the potential to form any or all structures.
Inductive signals from adjacent tissue determines
their fate. Selection hypothesis the neural
crest contains a mixed population of
predetermined cells. Each cell has only one
possible fate and it migrates according to this
fate.
The real truth may lie between these two
extremes. Clonal analysis when individual
neural crest cells are placed in culture, it is
clear that a single cell can give rise to others
that differentiate into multiple cell types
(pigment cells and neurons). Premigratory cells
have a wider potential than do the cells that
have already started to migrate. They may become
partially differentiated as they migrate.
15
Pluripotency of neural crest cells in vivo
By injecting migrating neural crest cells with
red fluorescent dye, it was possible to trace
their fate. Multiple cells were injected and
their fate was analyzed after several days. The
structures that they formed were determined by
staining with specific antibodies (ie dorsal
route ganglia). Some red neural crest cells
participated in dorsal route ganglia, others
formed parts of sympathetic ventral route
ganglia, pigment cells, or adrenal medulla. The
cells appeared to be pluripotent in
vivo. Premigratory cells had a greater potential
to form different structures than migrating
cells. Thus, differentiation appears to accompany
migration.
16
What are the molecular signals that control
differentiation of neural crest cells?
Extracellular matrix (ECM) neural crest cells
constantly extend filopodia to feel the
ECM. Pieces of filter were placed in an embryo
at the dorsolateral or ventral pathways. After
the filters absorbed ECM, they were removed to a
culture dish and allowed to interact with neural
crest cells. Dorsolateral ECM induced
melanocytes and yellow pigment cells. Ventral ECM
induced neurons. No matrix allowed the cells to
remain undifferentiated.
17
Polypeptide growth factors also
stimulate Differentiation of neural crest cells
1. Steel factor a polypeptide growth factor
produced by skin cells that acts with the c-kit
receptor on neural crest cells. Mutant mice that
produce less steel factor appear steel gray
rather than black because they have fewer
melanocytes. Loss of the c-kit receptor leads
to areas of skin that lack pigmentation, white
spotting in animals. The same condition leads to
similar symptoms in humans, called Piebaldism.
18
2. Endothelins growth factors that are important
in recruiting development of melanocytes and
parasympathetic nerves in the gut. Mice that are
deficient in endothelin-3 or its receptor, EDNRB,
show unpigmented regions of skin and distention
of the large intestine. The latter is due to lack
of parasympathetic neurons that induce
peristaltic movements. A similar condition
occurs in humans due to mutation of the EDNRB
receptor. Hirschsprungs disease is characterized
by irregular skin pigmentation and chronic severe
constipation.
3. Transforming growth factor beta family
(TGF-bs) members of the TGF-b family selectively
inhibit specific types of differentiation by
neural crest cells. When neural crest cells are
grown in culture, they form colonies that contain
cells with many patterns of differentiation.
When cultured with bone morphogenetic
protein-2, 50 of cells become neurons, 25
become muscle, and 25 are mixed. If the same
cells are cultured in TGF-b, all clones develop
into smooth muscle. The colonies of cells are
very large in the presence of TGF-b, suggesting
that this cytokine inhibits differentiation of
other phenotypes and enhances growth. Neural
crest cells are important in formation of
multiple tissues and there are several important
factors that induce their differentiation.
19
Head ectoderm is induced to form placodes
Placodes the epidermis covering the head is
induced by underlying brain to form dense areas
composed of columnar epithelium. Epibranchial
placodes these form along the ventral lateral
region and develop into sensory ganglia of
cranial nerves. Dorsolateral placodes contribute
to sensory ganglia and also form parts of the
eye, ear, and nose.
Ectodermal placodes and neural crest cells have
common properties, such as their ability to form
sense organs, neurons, and cartilage.
20
The otic placode forms the inner ear
In humans, the otic placode appears by the third
week on both sides of the rhombencephalon. The
otic vesicle is induced by the underlying neural
tissue in the rhombencephalon (one example of
reciprocal interaction). It invaginates to form
the otic pit and then pinches off to form the
otic vesicle. Ganglion cells develop from its
medial surface. Labyrinththe otic vesicle
expands unequally and constricts in other areas
to form a complex shape. The cochlea develops to
sense sound and the semicircular canals form to
serve as an organ for balance and body position.
21
Formation of the eye involves reciprocal
interactions
Lens placode the ectoderm invaginates in
response to signals from the optic cup
underneath. It then pinches off as a lens
vesicle. Cells elongate to fill the vesicle and
start to synthesize crystallins. Optic cup
forms from the neural tube by invagination. The
opening (choroid fissure) closes forming a round
optic cup, an extension of the brain. Optic
stalk connection to the brain that is filled
with neurons to form the optic nerve. Reciprocal
interaction the lens induces the formation of
the optic cup and the cup regulates formation of
the lens. When the lens from a species with large
eyes is transplanted, it induces an extra large
optic cup and it also does not grow as large as
usual.
22
Paradoxical arrangement of rods and cones
The pigmented retina is the outer layer and the
neural retina is the inner lining of the optic
cup. These cells form the neurons and the rod and
cone cells that detect light. It is a
paradoxical arrangement because the light
sensitive cells are actually in the rear of the
eye behind the neurons and facing away from
light! The rods and cones are covered by layers
of bipolar nuclei and ganglion cells.
23
Nasal placodes form the olfactory epithelium
In humans, the nasal placodes appear after the
4th week. They are induced by the underlying
telecephalon. During the fifth week, nasal
swellings appear around the placodes which now
become the two nasal pits. The pits start out
far apart, but the 2 maxillary swellings grow
large and push the pits to the center. The medial
parts of the nasal swellings fuse to form part of
the upper lip. Olfactory epithelium the
original lining of the nasal pit comes to rest on
the roof of the nasal cavity. It forms the
epithelium that senses smell and connects to
neurons in the telencephalon. The nasal cavity
becomes continuous with the pharynx.
What can go wrong?
24
Cleft lip and palate
When the nasal swellings that form the upper lip
and/or palate fail to fuse properly a cleft
occurs (4th to 8th weeks of gestation). You can
feel the fusion junction with your finger
(indentation in upper lip) or tongue (fusion line
on roof of mouth).
Usually occurs bilaterally The cause is
unknown Occurs in 1 of 700 babies. Smoking, too
much vitamin A or too little folic acid in the
mother may be a factor A parent with cleft has a
minimum 5 chance of passing the cleft along. An
autosomal dominant genetic condition causes a 50
chance of cleft palate
25
How does skin develop and differentiate?
Epidermis the largest derivative of ectoderm
forms the outer layer of the skin. It is an
epithelium and cells are connected by desmosomes
and tight junctions. The epidermis consists
initially of two layers periderm is a temporary
outer layer and the germanative layer lies below.
The germanative layer is composed of stem cells
that divide actively to produce differentiated
progeny. The basal layer develops from the
germanative layer (it contains stem cells in the
adult).
Spinous layer forms as cells are squeezed out of
the basal layer. They become large and
differentiate. Granular layer starts making
keratin granules Cornified or horny layer is
composed of dead cells that are filled with
keratin It takes cells about 7 days for each
cell to journey through the skin
26
Hair development in humans
The underlying mesenchyme in skin forms a dermis,
a layer of connective tissue just below the
epidermis. The dermis induces a variety of
epidermal structures depending on the species
(feathers, hair, scales).
Hair bud hair formation begins as a small bud
that that penetrates the dermis. It is induced by
a group of mesenchymal cells and the hair bud
then envelopes these cells to form a hair
papilla. Hair follicle is the entire
organ Sebaceous glands are induced to form on
the side. The hair shaft is formed when the
inner cells of the follicle start to
differentiate and produce keratin in the form of
hair. The continued production of keratin by the
cells at the base of the shaft causes the hair to
grow longer.
27
Development of mammary glands
Mammary glands these glands develop from 2
bandlike swellings in the epidermis called the
mammary ridges. Depending on the species, one or
more of the segments of these ridges persist on
each side. In 7 week human embryos, the mammary
ridge extends from the armpit to the groin.
Mammary ridge this sprouts buds that penetrate
down into the mesenchyme to form the lactiferous
ducts. The actual milk producing glands develop
prior to the first pregnancy. The lactiferous
ducts open into a small pit at the surface which
is transformed into a nipple.
28
Mammary gland development in humans mimicks
ancestral patterns
In normal development of humans, only one pair of
segments in the ridge survives, and the remaining
precursors degenerate. In some individuals, the
other segments of the mammary ridge fail to
degenerate, so that accessory nipples or breasts
are formed.
Primates evolved from small creatures that nursed
multiple offspring. An extended mammary ridge
would have given human ancestors a survival
advantage. Two breasts are obviously more
adaptive for humans who normally have only one
offspring at a time. Atavism the occasional and
abnormal persistance of a primitive adult feature
in an evolved species (multiple mammary glands).
It is easier to modify an older pattern of
development than to develop a totally new
pattern. This idea is a pervasive in
developmental biology.
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