8
1 Introduction
1.4.2 Development
Mechanobiology is ubiquitous in the developing embryo. During development, the
embryo and its component parts undergo dramatic changes in size, mechanical properties, and shape through the processes of growth, remodeling, and morphogenesis.
Chapter 8 focuses on animal morphogenesis. Time-lapse movies give the impression that the embryo merely grows into its final shape, but embryogenesis actually
is a much more complex process. One reason for this complexity is that embryos
need to stay alive as they develop. The embryonic heart, for example, must continue
to pump blood even while undergoing dramatic changes in shape.
The following paragraphs merely skim the surface of a highly complex subject.
The book by Wolpert (2008) offers a wonderful introduction to embryonic development. For a more in-depth treatment, interested readers are encouraged to consult the
classic textbook Developmental Biology by Gilbert (2010). Supplemental references
for specific topics are included below.
Embryonic Cells Embryos contain two types of cells. Mesenchymal cells move
and deform as individual cells or 3D aggregates, while epithelial cells move and
deform together in sheets. Mesenchyme is a mixture of mesenchymal cells and
extracellular matrix that creates bones, muscles, connective tissues, and blood
vessels. During the early stages of heart, brain, and lung development, epithelia fold
into tubes that undergo further changes in morphology. Mesenchymal and epithelial
cells combine to create the final form of almost all organs.
Embryonic cells perform a limited number of activities. They divide, migrate,
change shape, differentiate, and communicate with other cells through molecular,
chemical, and mechanical signals. However, there is an almost unlimited number of
ways these activities can be combined to build an embryo.
In the embryo, genes control cell behavior such as pattern formation (spatial
organization), changes in form (morphogenesis), and cell differentiation (specialization). However, epigenetic (environmental) factors play major roles in determining
which genes are expressed (turned on), as well as when and where they are
expressed. Epigenetic factors include signaling molecules called morphogens, as
well as mechanical stress and strain.
How genetic, chemical, and mechanical processes are integrated to produce an
embryo is perhaps the fundamental problem in developmental biology. Although
significant progress has been made in this area, solving this problem likely will
keep researchers busy for many years to come.
Early Embryonic Development Fertilization of the egg produces the single-cell
zygote, which then undergoes sequential divisions that yield an aggregate of cells
called blastomeres. A fluid-filled cavity, the blastocoel, then forms inside this
aggregate to create a blastula. The form of the blastula depends on the species
(Fig. 1.2).
Blastomeres are sometimes used as a source of embryonic stem cells, which
have the capacity to differentiate into specialized cells of any type, e.g., heart
1 Introduction
1.4.2 Development
Mechanobiology is ubiquitous in the developing embryo. During development, the
embryo and its component parts undergo dramatic changes in size, mechanical properties, and shape through the processes of growth, remodeling, and morphogenesis.
Chapter 8 focuses on animal morphogenesis. Time-lapse movies give the impression that the embryo merely grows into its final shape, but embryogenesis actually
is a much more complex process. One reason for this complexity is that embryos
need to stay alive as they develop. The embryonic heart, for example, must continue
to pump blood even while undergoing dramatic changes in shape.
The following paragraphs merely skim the surface of a highly complex subject.
The book by Wolpert (2008) offers a wonderful introduction to embryonic development. For a more in-depth treatment, interested readers are encouraged to consult the
classic textbook Developmental Biology by Gilbert (2010). Supplemental references
for specific topics are included below.
Embryonic Cells Embryos contain two types of cells. Mesenchymal cells move
and deform as individual cells or 3D aggregates, while epithelial cells move and
deform together in sheets. Mesenchyme is a mixture of mesenchymal cells and
extracellular matrix that creates bones, muscles, connective tissues, and blood
vessels. During the early stages of heart, brain, and lung development, epithelia fold
into tubes that undergo further changes in morphology. Mesenchymal and epithelial
cells combine to create the final form of almost all organs.
Embryonic cells perform a limited number of activities. They divide, migrate,
change shape, differentiate, and communicate with other cells through molecular,
chemical, and mechanical signals. However, there is an almost unlimited number of
ways these activities can be combined to build an embryo.
In the embryo, genes control cell behavior such as pattern formation (spatial
organization), changes in form (morphogenesis), and cell differentiation (specialization). However, epigenetic (environmental) factors play major roles in determining
which genes are expressed (turned on), as well as when and where they are
expressed. Epigenetic factors include signaling molecules called morphogens, as
well as mechanical stress and strain.
How genetic, chemical, and mechanical processes are integrated to produce an
embryo is perhaps the fundamental problem in developmental biology. Although
significant progress has been made in this area, solving this problem likely will
keep researchers busy for many years to come.
Early Embryonic Development Fertilization of the egg produces the single-cell
zygote, which then undergoes sequential divisions that yield an aggregate of cells
called blastomeres. A fluid-filled cavity, the blastocoel, then forms inside this
aggregate to create a blastula. The form of the blastula depends on the species
(Fig. 1.2).
Blastomeres are sometimes used as a source of embryonic stem cells, which
have the capacity to differentiate into specialized cells of any type, e.g., heart