Development of Multi Cellular Organisms
• Genome of nearly every cell is identical. Cells just differ in their expression of the
different parts of the genome.
• Selective gene expression controls 4 essential properties: 1) Cell proliferation, producing
many cells by which the embryo is constructed, 2) cell specialization, creating cells with
different characteristics at different positions, 3) cell interactions, coordinating the
behaviour of one cell with that of its neighbours, and 4) cell movement, rearranging the
cells to form structured tissues and embryos.
• Cells have a memory: the genes a cell expresses and the way it behaves depend on the
cell’s past as well as its present environment.
Universal Mechanisms of Animal Development
• Most of the basic machinery of development is essentially the same across all vertebrates
• The 4 essential processes by which a multicellular organism is made: cell proliferation,
cell specialization, cell interaction, and cell movement.
• Plants are a separate kingdom: they have evolved their multicellular organization
independently of animals.
Animals Share Some BasicAnatomical Features
• Similarities due to evolutionary common ancestor.
• Features common to all animals – epidermal cells forming a protective outer layer, gut
cells to absorb nutrients, muscle cells to move, neurons and sensory cells to control
movements – correspond to a basic anatomical scheme of development.
• Cleavage: 1) Physical splitting of a cell into two. 2) Specialized type of cell division seen
in many early embryos whereby a large cell becomes subdivided into many smaller cells
• Ectoderm: Embryonic epithelial tissue that is the precursor of the epidermis and the
• Endoderm: Embryonic tissue that is the precursor of the gut and associated organs.
• Mesoderm: Embryonic tissue that is the precursor to muscle, connective tissue, skeleton,
and many of the internal organs.
• Steps:Afertilized egg divides to produce a blastula – a hollow sphere of epithelial cells
surrounding a cavity. Then, in the process of gastrulation, some of the cells tuck into the
interior to form the gut and other internal tissues.Agroup of cells break loose from the
epithelium to become the mesoderm. These cells then crawl over the inner face of the
wall of the blastula. Epithelium is continuing to tuck inward to become the endoderm.
The invaginating endoderm extends into a long gut tube. The end of the gut tube makes
contact with the wall of the blastula at the site of the future mouth opening. Here the
ectoderm and endoderm will fuse and a hole will form.
• Gastrulation: Stage in animal embryogenesis during which the embryo is transformed
from a ball of cells to a structure with a gut (a gastrula).
Multicellular Animals Are Enriched in Proteins Mediating Cell Interactions and Gene Regulation • Recognizable versions of at least 50% of all human genes were already present in the
common ancestor of worms, flies and humans.
• Genes that are more specific to humans, are actually likely not to be as important, as they
have been observed t have mutations to them without any major consequences to the
• Most organisms therefore have a similar set of proteins available at their disposal.
• 2 classes of proteins are especially important for multicellular organization:
transmembrane molecules used for cell adhesion and cell signaling, and gene regulatory
Regulatory DNADefines the Program of Development
• Proteins encoded in the genome can be viewed as the components of a construction kit.
Many things can be built from the same kit. Some pieces necessarily go together, others
not so much.
• The instructions needed to produce a multicellular animal are determined by the non-
coding regions of DNAin a gene.
• Non-coding DNAmay contain multiple elements, or enhancers – short DNAsegments
that serve as binding sites for specific complexes of gene regulatory proteins.
• The presence of a given regulatory module of this sort leads to expression of a gene
whenever the complex of proteins recognizing that segment of DNAis appropriately
assembled in the cell.
• The DNAsequences of many of the individual modules have been well conserved and are
recognizably homologous in the different animals.
Manipulation of the Embryo Reveals the Interactions Between Its Cells
• Descriptive embryology is describing the anatomical changes that convert the egg into
the mature organism,
• Cell lineage tracking involves explaining development in terms of cell behaviour
• Experimental embryologists have tried to understand development in terms of the ways in
which cells and tissues interact to generate multicellular structure. (Fish with 2 sides)
• Developmental geneticists have tried to analyze development in terms of the actions of
Studies of MutantAnimals Identify the Genes That Control Development
• Developmental genetics irradiates germ cells of a parent animal and studies the mutant
offspring. Do this to study which genes are involved in proper formation of features.
ACell makes Developmental Decisions Long Before it Shows a Visible Change
• Cell fate: In developmental biology, describes what a particular cell at a given stage of
development will normally give rise to.
• Determination: In developmental biology, an embryonic cell is said to be determined if it
has become committed to a particular specialized path of development. Determination
reflects a change in the internal character of the cell, and it precedes the much more
readily detected process of cell differentiation.
• Acell’s state of determination can be tested by implanting it to altered environments.
• There is a spectrum of determination.Acell may be specialized for its normal fate, with a
strong tendency to develop in that direction, but still be able to change due to
environmental influences. Some all these cells specified, or committed.
Cells Have Remembered Positional Values That Reflect Their Location in the Body • Positional value: A cell’s internal record of its positional information.
• Positional information: Information supplied to or possessed by cells according to their
position in a multicellular organism.
• Long before cells become committed, they become regionally determined – they switch
on and maintain expression of genes that can best be regarded as markers of position or
region of the body.
Inductive Signals can Create Orderly Differences Between Initially Identical Cells
• Most straightforward way to make cells different is by exposing them to different