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 this page).

Fig. 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.


The Mechanism of Flower Organ Formation

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. 10-4A). 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 Fig. 10-4. The ABC model in flowers

A) An Arabidopsis flower.
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.

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