10.0 Introduction to development - Biologie ENSmthomas/L3/intro_biologie/10... · 10.1 Differential Gene expression in Development Four processes of development: • Determination

10.0 Introduction to development

Related Sadava’s chapters:

•  19) Differential Gene expression

in Development •  20) Development and

Evolutionary Change

10.1 Differential Gene expression in Development

A child preformed in the sperm, according to Harsoeker (1694)


Four processes of development:

• Determination sets the fate of the cell

• Differentiation is the process by which different types of cells arise

• Morphogenesis is the organization and spatial distribution of differentiated cells

• Growth is an increase in body size by cell division and cell expansion


Determination and differentiation occur largely because of differential gene expression.

Cells in the early embryo arise from repeated mitoses and soon begin to differ in terms of which of which genes are expressed.


Morphogenesis also involves differential gene expression and can occur in several ways:

• Cell division • Cell movements • Apoptosis (programmed cell death) • Growth occurs by increasing the number of

cells or enlargement of existing cells.







Cell fate: Type of cell it will ultimately become; a function of differential gene expression and morphogenesis.

Experiments in which specific cells of an early embryo are grafted to new positions on another embryo show the role of morphogenesis.

As development proceeds, the potential of cells becomes more restricted.

Cell fate is also influenced by the extracellular environment, as well as changes in gene expression.



A zygote is totipotent: It can give rise to every cell type in the adult body.

Later in development, the cells lose totipotency and become determined.

Determination is followed by differentiation.

For a long time, it has been thought that differentiation is irreversible. However, most cells retain the entire genome.



In mammals, stem cells occur in tissues that require frequent replacement—skin, blood, intestinal lining.

Adult stem cells are multipotent: They produce daughter cells that differentiate into a few cell types:

•  Hematopoietic stem cells produce red and white blood cells.

•  Mesenchymal stem cells produce bone and connective tissue cells.


Pluripotent cells in the blastocyst embryonic stage retain the ability to form all of the cells in the body.

In mice, these embryonic stem cells (ESCs) can be removed from the blastocyst and grown in laboratory culture almost indefinitely.


Pattern formation: The process that results in the spatial organization of tissues.

Linked with morphogenesis.

Programmed cell death—apoptosis—is also important. Many cells and structures form and then disappear during development.


In early human embryos, connective tissue links the fingers and toes. Later, the cells between the digits die.



Polarity was demonstrated using sea urchin embryos.

If an eight-cell embryo is cut vertically, it develops into two small larvae.

If the eight-cell embryo is cut horizontally, the bottom develops into a larva, the top remains embryonic.

Cytoplasmic determinants are distributed unequally in the egg cytoplasm.

These materials play a role in development of many animals.








Segmentation genes determine properties of the larval segments.

Three classes of genes act in sequence: •  Gap genes organize broad areas •  Pair rule genes divide embryo into

units of two segments each •  Segment polarity genes determine

boundaries and anterior-posterior organization in individual segments



How to establish a protein gradient?


Maternal effect genes are transcribed in the cells of the ovary that surround the egg.

Bicoid and Nanos determine the anterior-posterior axis. Their mRNAs become localized at both ends of the embryo.

Bicoid protein diffuses away from the anterior end, establishing a gradient.




How to establish regional domains of expression


At sufficient concentration, bicoid stimulates transcription of the Hunchback gene. A gradient of that protein establishes the head.

Nanos mRNA is transported to the posterior end. Nanos protein inhibits translation of Hunchback.


Fate of a cell is determined by its history and where the cell is.

Positional information may come in the form an inducer, a morphogen, which diffuses from one group of cells to another, setting up a concentration gradient.






How to establish a stripped pattern?




How to specify regional identity


• Homeotic genes are expressed in overlapping patterns along the length of the embryo.

• Each segment expresses a specific combination of genes. They will determine what each segment will become.

• Hox genes are on chromosome 3, in two clusters, in the same order as their anterior limit of expression



Antennapedia gain-of-function

Bithorax loss-of-function


Homeotic genes share a 180-bp sequence, the homeobox, that encodes a 60-amino acid sequence called the homeodomain.

The homeodomain binds to a specific DNA sequence in the vicinity of target genes and homeotic proteins are transcription factors

10.2 Development and Evolutionary change

•  The molecular pathways that determine different developmental processes operate independently from one another. This is called modularity

•  Changes in location and timing of expression of particular genes are important in the evolution of new body forms and structures

•  Much of morphological evolution occurs by modifications of existing development genes and regulatory sequences

•  Mechanisms of development have often evolved to be responsive to environmental conditions


• The Hox gene cluster is an example of homology in genes.

• They provide positional information and control pattern formation in early Drosophila embryos.

• Hox genes have homologs in mammals, and the genes are arranged in similar clusters and expressed in similar patterns in the embryos.



• The number of Hox genes is limited in each species (<40).

• Corresponding genes in two clusters in the same species are called paralogous

• Homologous genes between two species are called orthologous

• In addition, the homeobox is found in numerous other transcription factors, including some from plants.




10.2 Development and Evolutionary change Genetic switches that determine where and when

genes are expressed underlie both development and the evolution of differences among species.

In centipedes, Ubx protein activates expression of the Dll gene to promote the formation of legs.

In insects, a change in the Ubx gene results in a modified Ubx protein that represses Dll expression in abdominal segments, so leg formation is inhibited.



Similar processes govern development of the vertebral column.

Vertebral column has anterior-to-posterior regions (cervical, thoracic, lumbar, caudal). The regions are controlled by Hox genes.

The characteristic numbers of vertebrae in different species result from genetic changes that expand or contract the expression domain of different Hox genes.



Modularity also allows the timing of developmental processes to be independent—heterochrony.

The neck vertebrae of giraffes are much longer than those of other mammals.

Bone growth is stopped by a signal that results in death of chondrocytes (cartilage-producing cells) and calcification of the bone matrix.

In giraffes this signaling process is delayed in the neck vertebrae, and they grow longer.

Thus, the evolution of longer necks resulted from changes in the timing of gene expression.


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10.0 Introduction to development - Biologie ENSmthomas/L3/intro_biologie/10... · 10.1 Differential Gene expression in Development Four processes of development: • Determination