Chapter 10 Development and Differentiationcsls-text.c.u-tokyo.ac.jp/pdf/Chap_10.pdfChapter 10 Development and Differentiation 10 CSLS / THE UNIVERSITY OF TOKYO 191 Part III Organization

Chap t e r 10 De v e l opmen t a nd D i f f e r e n t i a t i o n

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CSLS / THE UN IVERS ITY OF TOKYO 191

Part III

Organization of Cell Populations

Chapter 10Development and Differentiation

Since ancient times, people have wondered how organisms are formed during

the developmental process, and many researchers have worked tirelessly in

search of the answer. Although large amounts of data have been accumulated

along the way, a full understanding of the developmental phenomena has not yet

been achieved. However, with the development of techniques to analyze these

phenomena at the gene level, our knowledge in the area has rapidly increased

in recent years. Now, we are becoming increasingly aware that the seemingly

diverse biological developmental phenomena involved are in fact related to a

number of common basic mechanisms.

By referring to specific case examples, this chapter discusses the basic mechanisms

of biological development that have been revealed at the molecular level.

I . Oogenesis

In many animals other than mammals, the materials necessary for continued

development to a certain stage need to be pre-stored in the egg. These materials,

which are derived from the mother and therefore known as maternal factors, include

many substances that play important roles in the early stages of development. The main

constituents of maternal factors are mRNA and special proteins. These factors are

involved in many of the important events that occur during the early stages of

development, such as determining which way the embryo faces (e.g., head-tail

direction and dorsal-ventral-side direction), determining the fate of embryonic cells

(i.e., deciding which tissues or organs they will become), regulating cell growth, etc.

Many maternal factors are stored in the oocyte during oogenesis, and many are

stored unevenly in the cytoplasm of the oocyte (Fig. 10-1). The uneven distribution

of these factors is closely associated with the determination of the direction of the

embryo and the fate of embryonic cells. As an example, maternal factors

localized in the egg will become unevenly distributed over embryonic cells

(blastomeres) through the division of the egg (cleavage). As a result, they become

deeply involved in the determination of these variables.



I I . Ferti l ization and Cleavage

Besides the important task of combining two sets of genes from the father and

mother into one, fertilization plays a number of other significant roles. As an

example, in frog embryos, as discussed later, the direction of the body is

determined by the site of the oocyte that the fertilizing sperm enters. The entry of

the sperm also sets off the process of cleavage.

The cell cycle of cleavage during the early stages of development is very rapid. As

an example, the average cell cycle in the cell division of frogs is approximately 16

hours, while early embryonic cells complete their cycle in 30 minutes. This is

because the cell cycle lacks phases equivalent to the extent of the G1 and G2

phases. In other words, cell division takes place through repetition of the DNA

synthesis period (S phase) and the mitotic period (M phase), making the cell cycle

very short (see Chapter 9). In the early stages of development, a certain number of

cells are thus created by the repetition of cell division within a short space of time.

Cleavage patterns are diverse, and include a type with unique cleavage

directions as well as one with uneven blastomere sizes (Fig. 10-2). This diversity

is due to the uneven distribution of nutrients in the embryo, or may reflect the need

to unevenly distribute maternal factors localized in the cytoplasm (see IV in Chapter

9). Among these, the uneven distribution of maternal factors plays a particularly

important role in the developmental process. This can be easily understood by

taking the destiny of the blastomeres in nematodes as an example (Fig. 10-3). In

this case, in each cell division, germ cell granules (maternal factors necessary for

the embryo to form germ cells) are unevenly allocated to just one of the two cells.

Those that receive the granules remain in the germ cell line and finally become

Figure 10-1 Oogenesis in fruit f l iesOogenesis in fruit flies occurs with the help of cells known as nurse cells. A germ cell divides four times to form sixteen cells, of which only one becomes an oocyte (an immature egg in the process of oogenesis). The rest become nurse cells and facilitate the creation of the oocyte. The cytoplasm of the oocyte and nurse cells is connected, and many proteins and mRNA (i.e., the mRNA of maternal factors such as nanos and bicoids) that are synthesized in nurse cells are transported through this pathway to the oocyte. Many of these materials are unevenly stored in the oocyte. → indicates the direction of transport of materials synthesized in nurse cells.


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germ cells. All others become somatic cells that form various tissues and organs.

Cell lineage describes which cells become the cells of which tissues during the

process of a fertilized egg becoming an adult. (see the Column on p.194).

I I I . Determination of Embryo Directionality

The direction of the body is determined immediately after fertilization in terms of,

for example, which side of the embryo becomes the head/tail, and maternal

factors pre-stored in the oocyte play a pivotal role in this process. Since the

importance of these factors in development has been studied in depth for fruit flies

and frogs, these organisms are used as examples in the discussions below.

Figure 10-3 Cleavage and the fate of blastomeres in nematodesIn each cleavage, germ cell granules are unevenly allocated to one of the two cells. Those that receive the granules remain the germ cell line and become germ cells, while all others become somatic cells.

Figure 10-2 Cleavage patternsThere are various types of cleavage, which are classified by the direction of division and the size of blastomeres.



Cell lineage, like a family tree, is a line that shows the history of a fertilized

egg going through cleavage and subsequently being differentiated into

various parts of the body. Since humans consist of an enormous number of

cells (60 trillion in an adult), it is impossible to trace the history of all cells

from fertilized egg to adult body. However, in nematodes, which measure

only 1 mm in length, the number of cells in an adult body is only 959. The

initial number is 1,090, but 131 cells are programmed to die during the

developmental process (this is known as apoptosis; see Chapter 9). It is

therefore easy to create a cell lineage for nematodes, as they consist of such

small numbers of cells. Indeed, a cell lineage map for nematodes, from the

fertilized egg to all cells that make up a mature individual, has already been

created (Column Fig. 10-1). Furthermore, the complete DNA sequence of

nematodes (consisting of approx. 100 million base pairs) has been

determined, and it is estimated that approximately 17,500 genes are

distributed over six chromosomes. As multicellular organisms on an easily

manageable scale, nematodes have therefore been widely used as a

material suitable for analyzing the basic mechanisms of organisms at the

molecular level, including developmental mechanisms, functional analysis of

the neural network and the causes of diseases.

Column Cell Lineage of Nematodes

Column Figure 10-1 Cell l ineage of nematodesIn nematodes, the lineage of all cells – from the fertilized egg (zygote) to the 959 somatic cells that make up an adult individual – has been determined.


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Development of Fruit Flies

In fruit flies, as shown in Figure 10-1, a number of maternal factor types that

determine the directionality of the embryo (i.e., the head and tail sides) are

stored in the oocyte during oogenesis. Among these, the major factors are mRNA

for two protein – bicoid and nanos. These mRNAs are stored disproportionately

in both sides of the oocyte. After fertilization, they are translated and the

concentration gradient of bicoid and nanos is created along the lengthwise axis

of the embryo (Fig. 10-4).

In the early stages of development in fruit flies, only the nucleus divides, and the

plasma membrane that envelops it is not newly created; the embryo therefore takes

on the appearance of a multinucleated cell. At this stage, all the nuclei are in the

same cytoplasm. Under these circumstances, when the concentration gradient of

bicoid and nanos is created in the embryo, the germ cell nuclei are directly

exposed to the gradient. Since bicoid and nanos are factors that regulate gene

expression and protein synthesis, the expression of new genes and protein synthesis

take place in accordance with their concentration gradients (Fig. 10-4).

The expression of gap genes occurs in accordance with the concentration

gradient patterns of bicoids and nanos, and the proteins translated from gap

genes are known as transcription factors. As a result, based on the expression

pattern of the gap genes, pair-rule genes are expressed in a way that forms

seven stripes in the embryo (this mechanism is discussed later). The proteins

translated from the pair-rule genes are also transcription factors. Then, based on

the expression pattern, segment-polarity genes are expressed in a way that

forms 14 stripes in the embryo. As a result of the cascade expression of these

gene groups, 14 regions (or compartments) are formed in the embryo, which is

Figure 10-4 The process from determination of directionality in the fruit - f ly embryo to segmentation of the embryo into 14 partsThe gene expression pattern observed during the process of bicoid and nano protein expression and the subsequent segmentation of the embryo into 14 parts is shown here. See Column Figure 10-2 on homeobox genes.



the origin of the 14 basic regions of fruit flies (i.e., the segmental structure of

insects). The cascade expression of these gene groups takes place in accordance

with the concentration gradient patterns of transcription factors (Fig. 10-5), which

are the proteins that regulate the expression of target genes by binding to their

regulatory regions (see Chapter 4). Such regulation may enhance or suppress the

expression of target genes. Figure 10-5A shows an example of the expression of

a target gene being enhanced only in regions where the transcription factor

concentration is within a certain range, and otherwise being suppressed. In this

case, the expression of target genes is induced when the concentration is within

a certain range. Figure 10-5B shows an example of the expression of a target

region occurring in an area where the action of enhancing transcription factors

dominates that of inhibitory transcription factors.

During the process from the formation of the concentration gradients of bicoids

and nanos to the expression of segment polarity genes, 14 regions are formed

in the embryo based on the expression pattern of the genes. As the next step, the

fate of the 14 regions (in terms of the organs to be created from each) must be

determined. A group of genes called homeobox genes play an important role in

this process (Column Fig. 10-2).

Figure 10-5 Patterns of a new gene being expressed in accordance with the transcription factor concentration gradient(A) An example of a gene being expressed only in a region where the transcription factor concentration is within a certain range. In this case, upper and lower limits (thresholds) of the transcription factor concentration causing gene expression exist. (B) An example of a gene being expressed in a region where the concentration balance of transcription factors that inhibit gene expression and those that enhance it is within a certain range. The enhancement of gene expression is based on the combined effects of the different promoting factors.

(A)

(B)


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During the long history of research into fruit flies, many mutants with structural

abnormalities have been reported. Recent gene-level analysis has identified

a series of genes that cause structural abnormality in the body of fruit flies.

These are called homeotic gene complexes (HOM-C), and consist of a

group of eight genes (Column Fig. 10-2A). It has also been found that gene

groups equivalent to these exist in four clusters (HoxA-D) in mammals.

In addition to this correspondence in the sequence of genes in chromosomes

between HOM-C in fruit flies and Hox in vertebrates, their expression

patterns in the body are also similar. As an example, the expression pattern

of HOM-C in fruit flies (which is expressed in an anteroposterior direction

along the body) and that of Hox in vertebrates (which is expressed along the

head-tail axis) are very similar (Column Fig. 10-2B).

The series of genes contained in HOM-C and Hox are called homeobox

genes, as all proteins translated from such genes have a region consisting of

60 amino acids called a homeodomain. The homeodomain is a region

with a special tertiary structure for binding to DNA (Column Fig. 10-2C).

The proteins translated from homeobox genes are transcription factors that

regulate the expression of other genes by attaching to them.

It is believed that homeobox genes determine which region becomes which

organ after the rough regional division that takes place in the embryo.

Column Homeobox Genes

Column Figure 10-2 Homeobox genes(A) HOM-C in fruit flies and HoxA–D in mammals are shown here. ↔ indicates the correspondence between the two. (B) The similarity of gene expression between HOM-C in fruit flies and Hox in vertebrates. The homeobox genes in fruit flies are expressed in the body in accordance with the gene sequence on the chromosomes. This is also the case with homeobox genes in vertebrates. As an example in mice, a part of the homeobox genes expressed in the brain and the spinal cord is shown. The arrow shows the directionality of homeobox gene expression along the head-tail axis. (C) The tertiary structure of the region known as the homeodomain, which all proteins translated from homeobox genes have, is shown here. It has three α-helices with which it binds to DNA.

(A) (B)

(C)



Development of Frogs

Next, we discuss development of frog embryo as an example of vertebrate

development. The upper hemisphere of the frog egg, which is rich in pigment

and therefore dark, is called the animal pole, while the opposite hemisphere,

which is white, is called the vegetal pole. Before fertilization, the egg has only

animal-vegetal-pole directionality. However, upon fertilization, the future dorsal

and ventral sides are determined in the embryo. This is because the surface part

of the cytoplasm in the egg moves in a particular direction after fertilization. This

in turn causes the movement of the maternal factors that determine the dorsal side

of the embryo from the vegetal pole to near the equator on one side of the

embryo (Fig. 10-6). As a result, the side opposite the sperm entry point becomes

the future dorsal side. Thus, in the development of frogs too, maternal factors

stored locally in the egg play an important role in determining the directionality

of the embryo.

A protein called Dishevelled is among the important maternal factors that are

stored in the vegetal pole of the embryo and turn the region to which they move

into the dorsal side. This protein moves to a region where it causes the expression

of new genes, thereby inducing that side of the embryo to become the dorsal side

(Column Fig. 10-3). As a result, a region called the organizer, which determines

the dorsal side of the embryo, is created on the side that the Dishevelled moves

to. The basic structure of the frog’s body is formed around this region.

Figure 10-6 Determination of dorsal-ventral directionalityPost-fertilization movement in the surface part of the frog egg is shown here. Upon fertilization, part of the cytoplasm in the vegetal pole moves to a point near the equator on the side opposite the sperm entry point.


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Dishevelled suppresses the degradation of a transcription factor called

β–catenin. This means that β–catenin increases in the cytoplasm on the

side of the embryo to which Dishevelled moves. The increased amount of

β–catenin moves into the nucleus and causes the expression of its target,

the Siamois gene. Siamois (a transcription factor) causes the expression of

its target (the goosecoid gene) in cooperation with other transcription factors

known as Smads. The goosecoid gene has a strong effect in inducing the

formation of the dorsal side in the embryo.

IV. Cell Differentiation and Stem Cells

During the developmental process, cells derived from one fertilized egg become

units with various functions such as muscle cells, neurons and epithelial cells. This

phenomenon is called cell differentiation.

Embryonic cells in the early developmental stage have the potential to become

various cell types. These cells, which have not yet undergone differentiation, are

referred to as undifferentiated cells. However, as development proceeds, most

cells are differentiated into units with particular functions. Each differentiated cell

Column Figure 10-3 Process of Dishevelled determining the dorsal side of the embryoThe process of maternal factors determining the dorsal side of the embryo takes place mainly through the regulation of gene expression by transcription factors. During the process, maternal factors and extracellular signals collaborate.

Column Roles of Maternal Factors in Determining the Dorsal Side of the Frog Embryo



type shows the different gene expression pattern and level. (Fig. 10-7). In other

words, in differentiated cells, the expression of certain genes necessary to fulfill

their cellular functions is enhanced, whereas the expression of unnecessary genes

is suppressed.

In differentiated cells, the suppression of unnecessary genes is performed through

chemical modification of the genes themselves (or of proteins that bind to them)

or through the binding of special proteins to them. Such suppression may be

temporary or semi-permanent (see Chapter 4). If the suppression is unlocked –

thus allowing gene expression – the cells regain the potential to be transformed

into other cells.

Cells that retain the ability to differentiate into many cell types exist in the various

tissues of the human body. These are known as stem cells, and are believed to

be involved in the repair of damaged tissues. Attempts to collect, grow and

differentiate these cells in order to artificially create tissues and organs have

recently been made. The aim is to create tissues and organs using stem cells

obtained from patients and implant them back into the body. The method of

implanting the patient’s own tissues avoids immunological rejection and raises

fewer ethical issues than other methods (such as techniques that use human

embryonic cells).

Figure 10-7 Cell DifferentiationA model showing changes in the gene expression pattern and level during cell differentiation. The gene expression patterns occurring in three types of differentiated cell are schematically outlined. Differentiated cells have the same gene set as the fertilized egg, but the gene expression pattern and level differ.


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V. Induction and Morphogenetic Movement

Induction is a phenomenon that strongly influences the fate of adjacent cells and

tissues through its action on them. Induction occurs throughout the course of

embryonic development.

When induction (which is found in various phases of the developmental process)

is considered at the cellular level, a number of common mechanisms are found.

The three main methods by which one cell influences and changes the fate of

other cells are via secretory substances, molecules on the plasma membrane

surface, and gap junctions (Fig. 10-8). Whichever method is taken, one cell can

influence many adjacent cells.

In triploblastic animals, whose bodies consist of the three basic layers of

endoderm, mesoderm and ectoderm (including many types of animal from

planarians to humans), the embryo is roughly divided into three regions in the

early stages of development. This determines the fate of each region. By way of

example, the respiratory and digestive organs are formed from the endoderm,

muscle and connective tissues are formed from the mesoderm, and the central

nervous system and skin are formed from the ectoderm. Induction plays a role in

the differentiation into the three germ layers. This mechanism is discussed below

using mesoderm induction in frogs as an example.

In mesoderm induction, the mesoderm is formed in a certain area of the embryo

(the equatorial region) as a result of influence from various other parts of the

embryo (Fig. 10-9). This induction is caused by many types of substance secreted

from the organizer on the dorsal side, the animal pole, the vegetal pole and

ventral side. Through their interaction, the mesoderm region is induced in the

intermediate area of the embryo (the equatorial region).

Morphogenetic Movement

After the determination of the rough arrangement of embryonic regions – such as

the prospective mesoderm and ectoderm areas – following the formation of the

organizer (the center of embryogenesis), major morphogenetic movement is seen.

First, the formation of three germ layers and morphogenetic movement occurs to

create archenterons, which later become the gastrointestinal tract. Second, further

morphogenetic movement occurs to create the neural tube. The distribution of

embryonic cells is rearranged through these morphogenetic movements, thereby

allowing the interaction of embryonic cells that previously existed separately.

Figure 10-8 Induction in adjacent cells(A) There are three methods of causing induction in adjacent cells: a) influence via secretory substances such as growth factors and hormones; b) influence through cell adhesion via proteins and carbohydrate chains that exist on the surface of the plasma membrane; and c) influence from the formation of gap junctions between the cell and adjacent cells. (B) Methods of transmitting induction to surrounding adjacent cells: a) through the diffusion of secretory substances; b) by extending cell protrusions and attaching them to surrounding cells; and c) intercellular transmission via gap junctions. → indicates the direction of induction.

(B)

(A)



Figure 10-9Mesoderm induction in frogsMesoderm induction in frogs is caused by the action of inducers secreted from the surrounding area. → indicates the direction of induction from each of the four regions. As a result of the combined induction effects, the mesoderm region is induced in the central area.

Figure 10-10 Morphogenetic movement(A) Morphogenetic movement occurring in the early embryo. This takes place mainly through the movement and deformation of the epithelial cells that constitute the embryo, and includes: a) the movement of cells that break away (or migrate) from epithelial tissues; b) the bending or invagination movement of the epithelium; and c) the extension movement caused by the rearrangement or flattening of epithelial cells. (B) Examples of morphogenetic movement in sea-urchin and chicken embryos: a) bending and invagination movement in the sea-urchin embryo, and b) locomotion of mesodermal cells in the chicken embryo. In chicken embryos, cells that separate from the central part of the ectoderm move inwardly in the embryo and become mesodermal cells, which then move in the direction of the arrows between the ectoderm and endoderm. The part from which cells break away appears as a streak, which is referred to as a primitive streak.

(A)

(B)


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Mesoderm formation, gastrulation and neurulation all take place through a number

of basic cell movements (Fig. 10-10). Since the early embryo is made of epithelial

tissue (see Chapter 11), morphogenetic movement during this stage mainly involves

the movement and deformation of epithelial cells. This includes the bending

movement caused by contraction on one side of epithelial cells, invagination

movement in which bent epithelial tissues extend inward in the embryo, locomotive

movement caused by cells separated from epithelial cells, and extension movement

caused by the flattening and rearrangement of epithelial cells.

Among these movement types, one that plays a particularly important role in

subsequent organogenesis is the large-scale movement of mesodermal cells. This

movement in vertebrates is so great that the cells move to the opposite side of the

embryo. Taking the chicken embryo as an example, cells that break away from

the epithelium of the ectoderm enter the embryo, form mesodermal cells and move

to the sides and the anterior part between the ectoderm and endoderm (Fig. 10-

11). The prospective heart mesoderm regions, which later become the heart, are

located at the tip of the moving mesoderm on both sides of the embryo.

Neural Induction

After mesoderm induction, neural induction occurs. By this process, the neural

tube (which later becomes the brain and the spinal cord) is created from the

ectoderm. Neural induction is an important event in the development of animals,

since the neural tube plays the central role in the construction of the body and

Figure 10-11Movement of mesodermal cells in the chicken embryoA schematic diagram showing the movement of mesodermal cells, seen from the dorsal side (the ectoderm is not shown). The prospective heart mesoderm region is located at the tip of the moving mesoderm. → indicates the direction of movement of mesodermal cells.



forms the brain and the spinal cord – the most important components in the

bodies of animals.

Neural induction is brought about by the organizer and the mesoderm. First, the

organizer sends induction signals to the ectoderm. Then, induction signals are

sent from the mesoderm, which has moved below the ectoderm, to the ectoderm

itself. Both induction effects are exerted by substances secreted from cells. In

response to this induction, which destines the ectoderm to become nerve tissue,

the neural tube – the origin of the brain and the spinal cord – is formed from the

ectoderm. The brain is created from the anterior part of the tube, and the spinal

cord is created from the posterior part (Fig. 10-12).

Figure 10-12Neural induction in a frog embryoNeural induction exerted by the mesoderm to the ectoderm is shown here. Inducers are secreted from the anterior and posterior parts of the mesoderm that have moved inside the embryo, which respectively induce the formation of the brain and the spinal cord from the ectoderm.


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As with animals, organogensis in plants is also regulated by various genes.

In this column, the formation of flower organs is discussed in connection with

homeotic genes. Generally, flowers consist of four organs: calyxes, petals,

stamens and pistils (carpels) (Column Fig. 10-A). These organs can be

depicted as a concentric ring structure of four layers (whirls) as in the floral

diagram (Column Fig. 10-4B). It has gradually become clear that the

formation of these organs is regulated by the combined expression of the

three homeotic gene groups of A, B and C (the ABC model, Column Fig.

10-4C). In the first whirl, only gene group A is expressed, forming calyxes.

In the second whirl, gene groups A and B are expressed, forming petals. In

the third whirl, gene groups B and C are expressed, forming stamens. In the

fourth whirl, only gene group C is expressed, forming pistils (carpels)

(Column Fig. 10-4C). Antagonism exists between gene groups A and C; if

the functions of gene group A are lost, the gene-group-C functions dominantly

over the entire flower, and if those of gene group C are lost, the gene-

group-A functions dominantly. Therefore, if the functions of gene group B are

lost, only gene group A is expressed in the first and second whirls, and only

gene group C is expressed in the third and fourth whirls, thereby forming a

flower consisting of calyxes, calyxes, carpels and carpels (Column Fig. 10-

4D). If gene C is lost, only gene group A is expressed in the first whirl, only

gene groups A and B are expressed in the second and third whirls, and only

gene group A is expressed in the fourth whirl, thereby forming a flower

consisting of calyxes, petals, petals and calyxes (Column Fig. 10-4E).

Column The Mechanism of Flower Organ Formation

Column Figure 10-4 The ABC model in f lowers(A) An Arabidopsis flower, and (B) an Arabidopsis floral diagram. The flower consists of four calyxes, four petals, six stamens and two carpels. (C) Different organs are formed through the combined effects of the three gene groups A, B and C. (D) A mutant that has lost the functions of gene group B. (E) A mutant that has lost the functions of gene group C.

(A)

(B)

(C)(D) (E)



VI. Organogenesis

The triploblastic structure is formed through a process in which the mesoderm

enters between the ectoderm and the endoderm. In concurrence with the

formation of the triploblastic structure, interaction between the germ layers occurs,

thereby initiating organogenesis. Induction phenomena like those shown in Figure

10-8 are seen when organs are formed through interaction between germ layers.

Here, organogenesis is discussed using the heart as an example.

The formation of the heart (the first of all organs to begin expressing its functions)

is initiated by mesoderm movement. This organ is formed from embryonic regions

collectively known as the prospective heart mesoderm. There are two such

regions located in the posterior part of the embryo. Together with the mesoderm,

these two regions move a long way from the posterior to the anterior part of the

embryo. They then merge in the anterior part, creating a tubular heart that

gradually changes shape to eventually form a heart consisting of atria and

ventricles (Fig. 10-13A).

While moving to the anterior part of the embryo, the prospective heart mesoderm

regions are influenced by inducers secreted from the endoderm and ectoderm,

thereby being induced to take the path to generate the heart. Through the

subsequent expression of homeobox genes, the regions then become destined to

form the heart.

A number of interesting findings have been made from the identification of the

inducers involved in heart formation and the genes that determine embryonic

regions to become the heart. Such findings include a structural similarity between

the homeobox gene that controls heart formation in vertebrates (the Nkx gene)

and the homeobox gene that controls the formation of dorsal blood vessels

(equivalent to the heart in mammals) in fruit flies (the Tinman gene) (Fig. 10-13B).

Another such finding is a similarity between the inducers (secretory substances)

that cause the expression of these genes in both fruit flies and vertebrates. These

findings indicate that the basic mechanism used in the formation of dorsal blood

vessels in fruit flies has been well conserved throughout evolution and is found in

heart formation in vertebrates. Similar cases have been confirmed in many

organs, including the formation of limbs and the nervous system.

Homeotic genes also play an important role in plant organogenesis. One such

example is the formation of flower organs (see the Column on p.205).


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Figure 10-13 Cardiogenesis(A) The heart is formed by two prospective heart mesoderm regions moving from the left and right sides of the embryo and finally merging. First, a tubular heart is formed, which is then twisted clockwise in a sigmoid curve (this is reversed in the photo, since it was taken from the ventral side) to form the atria and ventricles, thus becoming the heart. The photo shows an amphibian (newt) heart consisting of two atria and one ventricle. (B) The similarities between vertebrates and fruit flies are shown with regard to inducers involved in cardiogenesis, the homeobox genes that determine heart formation, and the timing of expression for these genes. Bone morphogenetic factors and decapentaplegic, which induce the mesoderm to form the prospective heart mesoderm region, belong to the same growth factor group.

(B)

(A)



• While the developmental processes of organisms are diverse, a number

of common basic mechanisms are found.

• In the various phenomena that occur during the early stages of development

(such as the determination of embryo directionality and the destiny of

germ cells), substances derived from the mother (maternal factors) stored

in the egg play important roles.

• As development proceeds, the embryo is roughly compartmentalized into

regions. The fates of these regions (i.e., determination of which ones will

form the various tissues and organs) are then decided by the expression

of homeobox genes.

• The proteins translated from homeobox genes are transcription factors,

which have been well conserved in the evolutionary process. These play

an important role in forming the bodies of organisms.

• Throughout the development process, induction occurs in which cells in

certain regions influence those in adjacent regions. There are several

induction patterns, but in each case, induction causes the expression of

new genes in target cells, thereby determining the fate of those cells.

• Germ layers are formed in many animals, and interaction among these

layers causes the formation of tissues and organs. The large-scale

morphogenetic movement that occurs in the early stages of development

allows the formation of these germ layers and the subsequent interaction

among them.

• It is now possible to compare the genes involved in the development of

nematodes and fruit flies with those involved in human development. As a

result, it has been shown that genes with a similar structure play similar

functions in each. This indicates that the basic mechanisms forming the

bodies of organisms have been continuously passed on during the course of

evolution and function in a similar way in the formation of the human body.

Summary Chapter 10


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[1]

The embryo of triploblastic organisms consists of three basic

structures. Name these structures and the organs and tissues

that develop from them.

[2]

Using fruit flies and frogs as examples, explain how the

anteroposterior axis of animals is determined in the early

stages of development.

[3]

Explain the concept of induction using examples.

[4]

Using the ABC model, explain how double flowers are

formed. Also outline which organs in a normal flower form

which organs in these cases.

Problems

(Answers on p.258)

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Chapter 10 Development and Differentiationcsls-text.c.u-tokyo.ac.jp/pdf/Chap_10.pdfChapter 10 Development and Differentiation 10 CSLS / THE UNIVERSITY OF TOKYO 191 Part III Organization