Developmental biology is the study of the process by which animals and
plants grow and develop.
Developmental biology also encompasses the
biology of regeneration, asexual reproduction, metamorphosis, and the
growth and differentiation of stem cells in the adult organism.
In the late 20th century, the discipline has largely transformed into
evolutionary developmental biology.
2 Developmental processes
2.1 Cell differentiation
3 Embryonic development of animals
4 Plant development
4.2 Morphological variation
4.3 Evolution of plant morphology
5 Developmental model organisms
6 See also
8 Further reading
9 External links
The main processes involved in the embryonic development of animals
are: regional specification, morphogenesis, cell differentiation,
growth, and the overall control of timing explored in evolutionary
Regional specification refers to the processes that create spatial
pattern in a ball or sheet of initially similar cells. This generally
involves the action of cytoplasmic determinants, located within parts
of the fertilized egg, and of inductive signals emitted from signaling
centers in the embryo. The early stages of regional specification do
not generate functional differentiated cells, but cell populations
committed to develop to a specific region or part of the organism.
These are defined by the expression of specific combinations of
Morphogenesis relates to the formation of
three-dimensional shape. It mainly involves the orchestrated movements
of cell sheets and of individual cells.
Morphogenesis is important for
creating the three germ layers of the early embryo (ectoderm, mesoderm
and endoderm) and for building up complex structures during organ
Cell differentiation relates specifically to the
formation of functional cell types such as nerve, muscle, secretory
epithelia etc. Differentiated cells contain large amounts of specific
proteins associated with the cell function. Growth involves both an
overall increase in size, and also the differential growth of parts
(allometry) which contributes to morphogenesis. Growth mostly occurs
through cell division but also through changes of cell size and the
deposition of extracellular materials. The control of timing of events
and the integration of the various processes with one another is the
least well understood area of the subject. It remains unclear whether
animal embryos contain a master clock mechanism or not.
The development of plants involves similar processes to that of
animals. However plant cells are mostly immotile so morphogenesis is
achieved by differential growth, without cell movements. Also, the
inductive signals and the genes involved are different from those that
control animal development.
The Notch-delta system in neurogenesis.(Slack Essential Dev Biol Fig
Cell differentiation is the process whereby different functional cell
types arise in development. For example, neurons, muscle fibers and
hepatocytes (liver cells) are well known types of differentiated cell.
Differentiated cells usually produce large amounts of a few proteins
that are required for their specific function and this gives them the
characteristic appearance that enables them to be recognized under the
light microscope. The genes encoding these proteins are highly active.
Typically their chromatin structure is very open, allowing access for
the transcription enzymes, and specific transcription factors bind to
regulatory sequences in the DNA in order to activate gene
expression. For example,
NeuroD is a key transcription factor
for neuronal differentiation, myogenin for muscle differentiation, and
HNF4 for hepatocyte differentiation.
Cell differentiation is usually the final stage of development,
preceded by several states of commitment which are not visibly
differentiated. A single tissue, formed from a single type of
progenitor cell or stem cell, often consists of several differentiated
cell types. Control of their formation involves a process of lateral
inhibition, based on the properties of the Notch signaling
pathway. For example, in the neural plate of the embryo this system
operates to generate a population of neuronal precursor cells in which
NeuroD is highly expressed.
Regeneration indicates the ability to regrow a missing part. This
is very prevalent amongst plants, which show continuous growth, and
also among colonial animals such as hydroids and ascidians. But most
interest by developmental biologists has been shown in the
regeneration of parts in free living animals. In particular four
models have been the subject of much investigation. Two of these have
the ability to regenerate whole bodies: Hydra, which can regenerate
any part of the polyp from a small fragment, and planarian worms,
which can usually regenerate both heads and tails. Both of these
examples have continuous cell turnover fed by stem cells and, at least
in planaria, at least some of the stem cells have been shown to be
pluripotent. The other two models show only distal regeneration of
appendages. These are the insect appendages, usually the legs of
hemimetabolous insects such as the cricket, and the limbs of
urodele amphibians. Considerable information is now available
about amphibian limb regeneration and it is known that each cell type
regenerates itself, except for connective tissues where there is
considerable interconversion between cartilage, dermis and tendons. In
terms of the pattern of structures, this is controlled by a
re-activation of signals active in the embryo. There is still debate
about the old question of whether regeneration is a "pristine" or an
"adaptive" property. If the former is the case, with improved
knowledge, we might expect to be able to improve regenerative ability
in humans. If the latter, then each instance of regeneration is
presumed to have arisen by natural selection in circumstances
particular to the species, so no general rules would be expected.
Embryonic development of animals
Main article: Embryogenesis
Generalized scheme of embryonic development. Slack "Essential
Developmental Biology" Fig.2.8
The initial stages of human embryogenesis.
The sperm and egg fuse in the process of fertilization to form a
fertilized egg, or zygote. This undergoes a period of divisions to
form a ball or sheet of similar cells called a blastula or blastoderm.
These cell divisions are usually rapid with no growth so the daughter
cells are half the size of the mother cell and the whole embryo stays
about the same size. They are called cleavage divisions. Morphogenetic
movements convert the cell mass into a three layered structure
consisting of multicellular sheets called ectoderm, mesoderm and
endoderm, which are known as germ layers. This is the process of
gastrulation. During cleavage and gastrulation the first regional
specification events occur. In addition to the formation of the three
germ layers themselves, these often generate extraembryonic
structures, such as the mammalian placenta, needed for support and
nutrition of the embryo, and also establish differences of
commitment along the anteroposterior axis (head, trunk and tail).
Regional specification is initiated by the presence of cytoplasmic
determinants in one part of the zygote. The cells that contain the
determinant become a signaling center and emit an inducing factor.
Because the inducing factor is produced in one place, diffuses away,
and decays, it forms a concentration gradient, high near the source
cells and low further away. The remaining cells of the embryo,
which do not contain the determinant, are competent to respond to
different concentrations by upregulating specific developmental
control genes. This results in a series of zones becoming set up,
arranged at progressively greater distance from the signaling center.
In each zone a different combination of developmental control genes is
upregulated. These genes encode transcription factors which
upregulate new combinations of gene activity in each region. Among
other functions, these transcription factors control expression of
genes conferring specific adhesive and motility properties on the
cells in which they are active. Because of these different
morphogenetic properties, the cells of each germ layer move to form
sheets such that the ectoderm ends up on the outside, mesoderm in the
middle, and endoderm on the inside. Morphogenetic movements
not only change the shape and structure of the embryo, but by bringing
cell sheets into new spatial relationships they also make possible new
phases of signaling and response between them.
Growth in embryos is mostly autonomous. For each territory of
cells the growth rate is controlled by the combination of genes that
are active. Free living embryos do not grow in mass as they have no
external food supply. But embryos fed by a placenta or extraembryonic
yolk supply can grow very fast, and changes to relative growth rate
between parts in these organisms help to produce the final overall
The whole process needs to be coordinated in time and how this is
controlled is not understood. There may be a master clock able to
communicate with all parts of the embryo that controls the course of
events, or timing may depend simply on local causal sequences of
Developmental processes are very evident during the process of
metamorphosis. This occurs in various types of animal. Well known are
the examples of the frog, which usually hatches as a tadpole and
metamorphoses to an adult frog, and certain insects which hatch as a
larva and then become remodeled to the adult form during a pupal
All the developmental processes listed above occur during
metamorphosis. Examples that have been especially well studied include
tail loss and other changes in the tadpole of the frog
Xenopus, and the biology of the imaginal discs, which generate
the adult body parts of the fly Drosophila melanogaster.
Further information: Plant development
Plant development is the process by which structures originate and
mature as a plant grows. It is studied in plant anatomy and plant
physiology as well as plant morphology.
Plants constantly produce new tissues and structures throughout their
life from meristems located at the tips of organs, or between
mature tissues. Thus, a living plant always has embryonic tissues. By
contrast, an animal embryo will very early produce all of the body
parts that it will ever have in its life. When the animal is born (or
hatches from its egg), it has all its body parts and from that point
will only grow larger and more mature.
The properties of organization seen in a plant are emergent properties
which are more than the sum of the individual parts. "The assembly of
these tissues and functions into an integrated multicellular organism
yields not only the characteristics of the separate parts and
processes but also quite a new set of characteristics which would not
have been predictable on the basis of examination of the separate
A vascular plant begins from a single celled zygote, formed by
fertilisation of an egg cell by a sperm cell. From that point, it
begins to divide to form a plant embryo through the process of
embryogenesis. As this happens, the resulting cells will organize so
that one end becomes the first root, while the other end forms the tip
of the shoot. In seed plants, the embryo will develop one or more
"seed leaves" (cotyledons). By the end of embryogenesis, the young
plant will have all the parts necessary to begin in its life.
Once the embryo germinates from its seed or parent plant, it begins to
produce additional organs (leaves, stems, and roots) through the
process of organogenesis. New roots grow from root meristems located
at the tip of the root, and new stems and leaves grow from shoot
meristems located at the tip of the shoot. Branching occurs when
small clumps of cells left behind by the meristem, and which have not
yet undergone cellular differentiation to form a specialized tissue,
begin to grow as the tip of a new root or shoot. Growth from any such
meristem at the tip of a root or shoot is termed primary growth and
results in the lengthening of that root or shoot. Secondary growth
results in widening of a root or shoot from divisions of cells in a
In addition to growth by cell division, a plant may grow through cell
elongation. This occurs when individual cells or groups of cells grow
longer. Not all plant cells will grow to the same length. When cells
on one side of a stem grow longer and faster than cells on the other
side, the stem will bend to the side of the slower growing cells as a
result. This directional growth can occur via a plant's response to a
particular stimulus, such as light (phototropism), gravity
(gravitropism), water, (hydrotropism), and physical contact
Plant growth and development are mediated by specific plant hormones
and plant growth regulators (PGRs) (Ross et al. 1983). Endogenous
hormone levels are influenced by plant age, cold hardiness, dormancy,
and other metabolic conditions; photoperiod, drought, temperature, and
other external environmental conditions; and exogenous sources of
PGRs, e.g., externally applied and of rhizospheric origin.
Plants exhibit natural variation in their form and structure. While
all organisms vary from individual to individual, plants exhibit an
additional type of variation. Within a single individual, parts are
repeated which may differ in form and structure from other similar
parts. This variation is most easily seen in the leaves of a plant,
though other organs such as stems and flowers may show similar
variation. There are three primary causes of this variation:
positional effects, environmental effects, and juvenility.
Evolution of plant morphology
Transcription factors and transcriptional regulatory networks play key
roles in plant morphogenesis and their evolution. During plant
landing, many novel transcription factor families emerged and are
preferentially wired into the networks of multicellular development,
reproduction, and organ development, contributing to more complex
morphogenesis of land plants.
Developmental model organisms
Much of developmental biology research in recent decades has focused
on the use of a small number of model organisms. It has turned out
that there is much conservation of developmental mechanisms across the
animal kingdom. In early development different vertebrate species all
use essentially the same inductive signals and the same genes encoding
regional identity. Even invertebrates use a similar repertoire of
signals and genes although the body parts formed are significantly
Model organisms each have some particular experimental
advantages which have enabled them to become popular among
researchers. In one sense they are "models" for the whole animal
kingdom, and in another sense they are "models" for human development,
which is difficult to study directly for both ethical and practical
Model organisms have been most useful for elucidating the
broad nature of developmental mechanisms. The more detail is sought,
the more they differ from each other and from humans.
Thale cress (Arabidopsis thaliana)
Xenopus (X.laevis and tropicalis). Good embryo supply.
Especially suitable for microsurgery.
Zebrafish: Danio rerio. Good embryo supply. Well developed
Chicken: Gallus gallus. Early stages similar to mammal, but
microsurgery easier. Low cost.
Mouse: Mus musculus. A mammal with well developed genetics.
Fruit fly: Drosophila melanogaster. Good embryo supply. Well
Nematode: Caenorhabditis elegans. Good embryo supply. Well
developed genetics. Low cost.
Also popular for some purposes have been sea urchins and
ascidians. For studies of regeneration urodele amphibians such as
the axolotl Ambystoma mexicanum are used, and also planarian worms
such as Schmidtea mediterranea. Organoids have also been
demonstrated as an efficient model for development. Plant
development has focused on the thale cress
Arabidopsis thaliana as a
Cell signaling networks
Gene regulatory network
Plant evolutionary developmental biology
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