Development of the left-right axis

The asymmetrical development of vertebrates can be considered as occurring around three main axes, the anteroposterior, dorsoventral and left-right axes.

Figure 2.5-1: Anteroposterior/ dorsoventral and left right axes

dorso/ventral

antero/posterior

left/right

/

Figure 2.5-1: A schematic diagram illustrating the anatomical asymmetry o f man and the relationship of the three axes to each other.

The left right axis is defined by its relationship to the anteroposterior and

dorsoventral axes. These two axes have been extensively studied and many of the important signalling pathways identified. Until recently less was known about the determination of left right asymmetry or laterality but this is currently an active field of research.

The normal asymmetry of an adult vertebrate can be considered as arising from three particular patterns of development (Kosaki and Casey 1998). The unpaired organs of both the thoracic and abdominal cavity, which includes the heart, stomach, intestine, liver, gallbladder, spleen and pancreas, are originally formed in the midline. The first of these to show asymmetry is the heart which begins its rightward looping on day 23. Study of embryological development within chick embryos confirms that this dextral bending of the heart loop is the first gross anatomical feature to show asymmetry. This is closely followed by rotation of the embryo. The second pattern of development concerns unpaired arteries and veins such as the superior and inferior vena cavae. These are originally formed as paired arteries or veins during early development and then one of the pairs undergoes embryonic regression. The third pattern of asymmetrical development is illustrated by the lungs, which develop from asymmetrical primordia. The resulting left-right asymmetry of adult vertebrate organisms can subsequently be identified by imaging and anatomical studies.

It is an interesting observation that in human conjoined twins joined at the abdomen or chest one of the twins has an increased probability of complete situs inversus compared to those joined at the head or pelvis (Levin, Roberts et al. 1996). This observation has also been confirmed in chick embryos. The prevalence of laterality defects in one of chick twins joined at the chest or abdomen approached 50%. This has led to morphogenic gradients being

suggested as candidates in the determination of laterality in a similar way to their role in the determination of the anteroposterior axis (Galloway 1990).

evidence of the development of left-right asymmetry the exact origin of these events are not yet fully understood. It has been proposed that the asymmetrical distribution of an essential cytoskeletal component may be the initiation event. To further understand the theories of the development of left-right asymmetry and to compare the studies of this in various vertebrate organisms an

understanding of the first stages of development is necessary. It is also important to be aware that although the mechanisms of establishing left-right asymmetry may be conserved between organisms there may also be significant differences.

Following fertilisation in the vertebrate a single celled diploid zygote is formed. In the Xenopus embryo microtubule dependent cortical rotation occurring in the first cell cycle establishes the three embryonic axes (Gerhart, Danilchik et al. 1989). Disruption of the microtubules at this stage results in disruption of dorsoanterior development and abnormalities of laterality. These initial cell cycle events are governed by signalling factors encoded by maternal genes (Hyatt, Lohr et al.

1996; Hyatt and Yost 1998) (Yost 1998). The two cell zygote undergoes further cell cleavage to produce the 16 cell morula at day 3 in humans:

Figure 2.5-2: Early fetal development.

Outer cell mass

\

Inner cell mass

Two cell stage Morula Blastocyst

Figure 2.5-2: A diagram illustrating the development o f the two-cell zygote into a morula and blastocyst

Studies within chimeric mice suggest that left-right development is fixed by this early stage (Harvey 1998). The cells of the morula are already showing signs of differentiation and can be divided into the inner cell mass or embryoblast and the outer cell mass or trophoblast. The cells of the embryoblast will eventually

develop into the embryo and the trophoblast cells develop into the placenta. Subsequent development involves further cell cleavage and the formation of an inner fluid filled cavity. This stage of development is now known as a blastocyst and occurs at about day 6 in humans. It is possible to influence laterality within the frog embryo up until this point.

The cells of the inner cell mass or embyroblast then undergo differentiation into the two layers of hypoblast and epiblast that form the bilaminar germ disc.

During the third week of development the process of gastrulation occurs which creates the three germ layers of embryonic ectoderm, mesoderm and endoderm. This begins with the formation of the primitive streak. The cephalic end of the primitive streak is known as the primitive node and surrounds the primitive pit. Cells of the epiblast migrate towards the primitive streak and invaginate to displace the hypoblastic cells. This creates the new layers of mesoderm and intra-embryonic endoderm and the ectoderm arises from the cells that remained in the epiblast.

A midline axis is defined by the development of the notochord that arises by invagination of cells within the primitive pit of the primitive streak. Determination of left right asymmetry within the chick embryo has experimentally been

observed during the process of gastrulation. It is now established that

asymmetrical gene expression in the axial structures adjacent to the notochord is involved in the establishment of laterality. These genes and their expression patterns are discussed in the next section.