Introduction To
Development
Development
Development,
in this journey from a single cell to a complex multicellular organism.
Development does not end at birth, but continues with childhood and puberty to
early adulthood.
We must
describe how a cell from the father and a cell from the mother combine to form
a new genetic individual, and how this new cell forms others, how they become
organised to form new shapes, specialised interlinked structures, and grow.
With this knowledge we become able to understand how these processes can be
interfered with, and how abnormalities arise.
Growth may
be described as the process of increasing in physical size, or as development
from a lower or simpler form to a higher or more complex form.
In
embryology, growth with respect to a change in size may crease in cell number,
an increase in cell size or an increase in extracellular material (Figure 3.1).
Increasing
cell number occurs by cells dividing to produce daughter cells by proliferation.
Proliferation is a core mechanism of increasing the size of a tissue or
organism, and is also found in adult tissues in repair or where there is an
expected continual loss of cells such as in the skin or gastrointestinal tract.
Stem cells are particularly good at proliferating.
An increase
in cell size occurs by hypertrophy. In adults, muscle cells respond to
weight training by hypertrophy, and this is one way in which muscles become
larger. During development, hyper- trophy of cartilage cells during
endochondral ossification is an important part of the growth of long bones. Be
aware that the term hypertrophy can also be used to describe a structure that
is larger than normal.
Cells may
surround themselves with an extracellular matrix, particularly in connective
tissues such as bone and cartilage. By accretion these cells increase
the size of the tissue by increasing the amount of extracellular matrix, either
as part of development or in response to mechanical loading.
Cells may
also die by programmed cell death, or apoptosis. This might be considered an
opposite to growth, and in development is an important method of forming
certain structures like the fin- gers and toes.
During
development, cells become specialised as they move from a multipotent stem cell
type towards a cell type with a particular task, such as a muscle cell, a bone
cell, a neuron or an epithelial cell. When the cell becomes more specialised it
is considered to have differentiated into a mature cell type. If that
cell divides, its daugh- ter cells will also be of that mature cell type.
In humans, a
mature cell is unlikely to dedifferentiate back into a stem cell, but the
process by which this can occur is being exploited in the laboratory with the
aim of producing stem cells from adult tissues. These stem cells could then be
pushed to dif- ferentiate into the cell type needed to grow new tissue or treat
a disease.
A signal
from one group of cells influences the development of another (adjacent, nearby
or distant) group of cells. Hormones act as signals, for example. For a cell to
be affected by a signal it must possess an appropriate receptor.
In the
embryo the signalling of a vast array of different proteins by different groups
of cells allows those cells to gain information about their current and future
tasks, be that migration, proliferation, differentiation or something else.
Early in
development the ball of cells or simple sheets of the embryo do not give much
clue about which cells will form which structures. It is difficult to determine
which part will become the head and which will become the tail. However, the
cells are aware of their position and the roles that they will have and we can
see this by looking at the signalling proteins and connections between cells.
For example,
the upper limb begins to develop as a simple bud of cells. The cells in that
bud must be organised to produce the structures of the arm, the forearm and the
hand. The ulna bone must form in the right place relative to the radius, and
the thumb must form appropriately in relation to the fingers. This may occur
partly because a group of cells on the caudal aspect of the limb bud produces a
morphogen that diffuses across the early limb bud (Figure 3.2). Cells near the
site of morphogen production experi- ence a high concentration, and cells
further away on the cranial side of the bud experience a lower concentration.
Development of these cells progresses differently as a result. If
experimentally you trans- plant some of the morphogen‐producing cells to the
cranial part of the limb bud, duplicate digital structures form. See Chapter 25
for more about limb development.
This is one
example of how cells organise themselves and others during development. With
organisation, structure follows.
The
formation of shape during development is morphogenesis.
Cells are
able to change the ways in which they adhere to one another, they can extend
processes and pull themselves along, migrating to new locations, and they can
change their own shapes. In a tissue there may be a change in cell number, cell
size or accretion of extracellular material. In these ways a tissue gains and
changes shape.
An early
example of morphogenesis in embryonic develop- ment occurs with the change from
simple flat sheets of cells to the rolled up tubes of the embryo and
gastrointestinal tract (Figure 3.3). A simple structure has become more
complex. Chapter 14 covers this in more detail.
Interruptions
of signalling, proliferation, differentiation, migration, and so on, cause
congenital abnormalities. Teratogens that affect development during key
periods may have significant effects. For example, if the drug thalidomide is
taken during early limb development it can cause phocomelia (hands and feet
attached to abnormally shortened limbs). Other environmental factors and
genetic mutations can cause abnormal development. The embryo is most sensitive
during weeks 3–8.
Dysmorphogenesis is a term used for the
abnormal development of body structures. It may occur because of malformation
or deformation. If the processes required to normally form a structure fail to
occur the result is a malformation. If the neural tube fails to close, for
example, the resulting neural tube defect is a malformation. A deformation
occurs if external mechanical forces affect development. For example, damage to
the amniotic sac can cause nds that may wrap around developing limbs and cause mputation
of limbs or digits.
