Download Morphogenesis

Morphogenesis
the process by which bodily form is
established
Morphogenesis
The key embryonic processes that contribute to
morphogenesis are:
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Cell division
Changes in cell shape
Folding of cell layers
Cell migration
Cell death (apoptosis)
Cell-to-cell communication
Cell differentiation
Embryonic disc
The cells of the inner cell mass of the blastocyst
become organized into two hollow spheres in contact
with each other and attached to the inside of the
trophoblastic shell.
One of the inner spheres encloses
the amniotic cavity, and the other
encloses the yolk sac cavity.
Where the two inner spheres
contact each other there is a twolayered embryonic disc.
Cell division
• Cell division continues throughout the
prenatal period.
• Most of these divisions are mitotic
• Dividing cells - and hence the embryo - are
very sensitive to a variety of chemical and
physical agents.
Changes in cell shape
Shape changes have a central role in morphogenesis.
• They are produced by the co-ordinated activity of
the cytoskeleton. The cytoskeleton consists of
microtubules and different types of microfilaments.
• Microtubules assist in elongation of cells and
movement of structures such as the nucleus and
chromosomes.
• Microfilaments can draw structures closer together.
Images of cells
stained to show the
cytoskeleton
Microfilaments and microtubules
Epithelium
• one of the four basic types of animal tissue,
along with connective tissue, muscle tissue
and nervous tissue.
• Epithelial tissues line the cavities and surfaces
of structures throughout the body, and also
form many glands.
• Functions of epithelial cells include secretion,
selective absorption, protection, transcellular
transport and detection of sensation
Folding of cell layers
When shape changes occur within cells forming
an epithelium, the epithelium will buckle and
fold.
This occurs frequently during morphogenesis,
and can give rise to tubular and spherical forms.
Examples are seen in the formation of the
central nervous system and the lens of the eye.
Folding of an epithelium
Cell migration
During morphogenesis, many cells migrate
within the embryo.
• Mesoderm cells move and aggregate to form
structures such as the somites (paired
structures).
• Neural crest cells arise from the ectoderm
alongside the developing neural tissue and
migrate amongst the mesodermal cells.
Cell death
Some embryonic cells have to die for normal
development to occur.
• Eg. normal development of the fingers and
toes depends on death of the cells forming
webs between them.
• The cells die without initiating an
inflammatory response
• The process of this selective cell death is
called apoptosis.
Cell-to-cell communication
Cell-to-cell interactions co-ordinate
development and influence gene-selection.
• Some regions of the embryo are induced to
develop in the correct way by chemical signals
from neighbouring tissues, eg: the lens of the
eye.
Differentiation
All the cells of the embryo are derived from the
zygote, and they have the same genetic
information in their nuclei.
• However, during development differences
arise between cells.
• Different genetic characteristics are expressed
in each cell type.
• As cells differentiate, they lose developmental
potential and become more specialized.
Embryonic disc
Initially, the embryonic disc has two layers ectoderm and endoderm. Then a third layer
called the mesoderm is formed by invagination
of ectodermal cells through the primitive streak.
The three layers are
referred to as the germ
layers - they give rise to
all the organs and
systems of the baby’s
body
Germ layers
Contributions by germ layers to parts
of the body
• Some of the mesodermal cells aggregate to
form the midline notochord, a precursor of
the axial skeleton.
• The ectoderm over the notochord thickens to
form the neural plate.
• The embryonic disc remains two-layered in
the region of the oral and cloacal membranes.
– The cloacal membrane covers the urogential
region of the embryo
Folding of neural plate
The neural plate begins to fold, forming the neural groove.
Closure to form the neural tube begins in the future midbrain
region, and then extends cranially and caudally.
Neural crest cells migrate away from the site of closure and later
take numerous roles within the body, forming peripheral
neurons, Schwann cells, melanocytes, and head and neck
structures.
Sections through the
neural plate as it
closes to become the
neural tube
Formation of somites
Mesodermal cells form up alongside the neural
tube and notochord to form segmental somites.
Later, the somites will contribute to skeletal
elements, muscles, and dermis. This segmental
origin is apparent in the pattern of spinal nerves,
vertebrae, ribs, nerve supplies, and dermal
layers in the completed body.
The three main zones of somites –
• sclerotome (yellow),
• myotome (brown),
• dermatome (purple)