"Views of a Fetus in the Womb",
Leonardo da Vinci
, ca. 1510-1512. The subject of prenatal development is a major subset of developmental biology.
Developmental biology
is the study of how organisms grow and develop.
Modern developmental biology studies the
genetic
control of
cell growth
,
differentiation
and
morphogenesis
.
[1]
These are the
processes
which turn a
zygote
into an adult animal.
Differentiation
is the formation of cell types, from what is originally one cell ? the
zygote
or
spore
. The formation of cell types like
nerve cells
occurs with a number of intermediary, less differentiated cell types. A cell stays a certain cell type by maintaining a particular pattern of
gene expression
.
[2]
293-5
This depends on regulatory genes.
The initial stages of human embryogenesis.
Embryogenesis
is the step in the
life cycle
after
fertilisation
? the development of the
embryo
, starting from the
zygote
(fertilised egg). Organisms can differ drastically in the how embryo develops, especially when they belong to different
phyla
.
Growth is the enlargement of a tissue or organism. Growth continues after the embryonic stage, and occurs through
cell division
, enlargement of cells or accumulation of extracellular material. In plants, growth results in an adult organism that is strikingly different from the embryo. The dividing cells tend to be distinct from differentiated cells (see
stem cell
).
In some tissues dividing cells are restricted to special areas, such as the
growth plates
of
bones
.
[2]
467-482
But some stem cells move to where they are needed, from the
bone marrow
to form muscle, bone or adipose (fat) tissue.
[3]
Frog
lifecycle
Many animals have a
larval
stage, with a body plan different from that of the adult organism. The larva abrubtly develops into an adult in a process called
metamorphosis
. For example,
caterpillars
(butterfly larvae) are specialized for feeding whereas adult butterflies (
imagos
) are specialised for flight and reproduction. When the caterpillar has grown enough, it turns into an immobile
pupa
. Here, the imago develops from
imaginal discs
found inside the larva.
[4]
575-585
Regeneration
is the reactivation of development so that a missing body part grows back. This phenomenon has been studied particularly in
salamanders
, where the adults can reconstruct a whole limb after it has been amputated.
[4]
Researchers hope to one day be able to induce regeneration in humans.
[5]
There is little spontaneous regeneration in adult humans, although the
liver
is a notable exception. Like for salamanders, the regeneration of the liver involves reversing some cells to an earlier state.
[4]
592-601