Regional differentiation
In the field of developmental biology, regional differentiation, or regional specification is the process by which different areas are identified in the development of the early embryo. The process by which the cells become specified differs between organisms.
Cell fate determination
In terms of developmental commitment, a cell can either be specified or it can be determined. Specification is the first stage in cellular differentiation. A cell that is specified can have its commitment reversed while the determined state is irreversible. There are two main types of specification: autonomous and conditional. A cell specified autonomously will develop into a specific fate based upon cytoplasmic determinants with no regard to the environment the cell is in. A cell specified conditionally will develop into a specific fate based upon other surrounding cells or morphogen gradients. Another type of specification is syncytial specification, characteristic of most insect classes.Specification in sea urchins uses both autonomous and conditional mechanisms to determine the anterior/posterior axis. The anterior/posterior axis lies along the animal/vegetal axis set up during cleavage. The micromeres induce the nearby tissue to become endoderm while the animal cells are specified to become ectoderm. The animal cells are not determined because the micromeres can induce the animal cells to also take on mesodermal and endodermal fates. It was observed that β-catenin was present in the nuclei at the vegetal pole of the blastula. Through a series of experiments, one study confirmed the role of β-catenin in the cell-autonomous specification of vegetal cell fates and the micromeres inducing ability. Treatments of lithium chloride sufficient to vegetalize the embryo resulted in increases in nuclearly localized b-catenin. Reduction of expression of β-catenin in the nucleus correlated with loss of vegetal cell fates. Transplants of micromeres lacking nuclear accumulation of β-catenin were unable to induce a second axis.
For the molecular mechanism of β-catenin and the micromeres, it was observed that Notch was present uniformly on the apical surface of the early blastula but was lost in the secondary mesenchyme cells during late blastula and enriched in the presumptive endodermal cells in late blastula. Notch is both necessary and sufficient for determination of the SMCs. The micromeres express the ligand for Notch, Delta, on their surface to induce the formation of SMCs.
The high nuclear levels of b-catenin results from the high accumulation of the disheveled protein at the vegetal pole of the egg. disheveled inactivates GSK-3 and prevents the phosphorylation of β-catenin. This allows β-catenin to escape degradation and enter the nucleus. The only important role of β-catenin is to activate the transcription of the gene Pmar1. This gene represses a repressor to allow micromere genes to be expressed.
The aboral/oral axis is specified by a nodal homolog. This nodal was localized on the future oral side of the embryo. Experiments confirmed that nodal is both necessary and sufficient to promote development of the oral fate. Nodal also has a role in left/right axis formation.
Tunicates
have been a popular choice for the study of regional specification because tunicates were the first organism in which autonomous specification was discovered and tunicates are evolutionary related to vertebrates.Early observations in tunicates led to the identification of the yellow crescent. This cytoplasm was segregated to future muscle cells and if transplanted could induce the formation of muscle cells. The cytoplasmic determinant macho-1 was isolated as the necessary and sufficient factor for muscle cell formation. Similar to Sea urchins, the accumulation of b-catenin in the nuclei was identified as both necessary and sufficient to induce endoderm.
Two more cell fates are determined by conditional specification. The endoderm sends a fibroblast growth factor signal to specify the notocord and the mesenchyme fates. Anterior cells respond to FGF to become notocord while posterior cells respond to FGF to become mesenchyme.
The cytoplasm of the egg not only determines cell fate, but also determines the dorsal/ventral axis. The cytoplasm in the vegetal pole specifies this axis and removing this cytoplasm leads to a loss of axis information. The yellow cytoplasm specifies the anterior/posterior axis. When the yellow cytoplasm moves to the posterior of the egg to become posterior vegetal cytoplasm, the anterior/posterior axis is specified. Removal of the PVC leads to a loss of the axis while transplantation to the anterior reverses the axis.
''C. elegans''
In the two cell stage, the embryo of the nematode C. elegans exhibits mosaic behavior. There are two cells, the P1 cell and the AB cell. The P1 cell was able make all of its fated cells while the AB cell could only make a portion of the cells it was fated to produce. Thus, The first division gives the autonomous specification of the two cells, but the AB cells require a conditional mechanism to produce all of its fated cells.The AB lineage gives rise to neurons, skin, and pharynx. The P1 cell divides into EMS and P2. The EMS cell divides into MS and E. The MS lineage gives rise to pharynx, muscle, and neurons. The E lineage gives rise to intestines. The P2 cell divides into P3 and C founder cells. The C founder cells give rise to muscle, skin, and neurons. The P3 cell divides into P4 and D founder cells. The D founder cells give rise to muscle while the P4 lineage gives rise to the germ line.
- Axis specification
- Localization of cytoplasmic determinants
- Specification of germ line
- Specification of EMS and P1 cells
- Specification of C and D founder cells
- Specification of E lineage
- Specification of ABa and ABp
''Drosophila''
Anterior/posterior axis
The anterior/posterior patterning of Drosophila come from three maternal groups of genes. The anterior group patterns the head and thoracic segments. The posterior group patterns the abdominal segments and the terminal group patterns the anterior and posterior terminal regions called the terminalia.The anterior group genes include bicoid. Bicoid functions as a graded morphogen transcription factor that localizes to the nucleus. The head of the embryo forms at the point of highest concentration of bicoid and the anterior pattern depends upon the concentration of bicoid. Bicoid works as a transcriptional activator of the gap genes hunchback, buttonhead, empty spiracles, and orthodentical while also acting to repress translation of caudal. A different affinity for bicoid in the promoters of the genes it activates allows for the concentration dependent activation. Otd has a low affinity for bicoid, hb has a higher affinity and so will be activated at a lower bicoid concentration. Two other anterior group genes, swallow and exuperantia play a role in localizing bicoid to the anterior. Bicoid is directed to the anterior by its 3' untranslated region. The microtubule cytoskeleton also plays a role in localizing bicoid.
The posterior group genes include nanos. Similar to bicoid, nanos is localized to the posterior pole as a graded morphogen. The only role of nanos is to repress the maternally transcribed hunchback mRNA in the posterior. Another protein, pumilio, is required for nanos to repress hunchback. Other posterior proteins, oskar, Tudor, vasa, and Valois, localize the germ line determinants and nanos to the posterior.
In contrast to the anterior and the posterior, the positional information for the terminalia come from the follicle cells of the ovary. The terminalia are specified through the action of the Torso receptor tyrosine kinase. The follicle cells secrete Torso-like into the perivitelline space only at the poles. Torso-like cleaves the pro-peptide Trunk which appears to be the Torso ligand. Trunk activates Torso and causes a signal transduction cascade which represses the transcriptional repressor Groucho which in turn causes the activation of the terminal gap genes tailless and huckebein.
Segmentation and homeotic genes
The patterning from the maternal genes work to influence the expression of the segmentation genes. The segmentation genes are embryonically expressed genes that specify the numbers, size and polarity of the segments. The gap genes are directly influenced by the maternal genes and are expressed in local and overlapping regions along the anterior/posterior axis. These genes are influenced by not only the maternal genes, but also by epistatic interactions between the other gap genes.The gap genes work to activate the pair-rule genes. Each pair-rule gene is expressed in seven stripes as a result of the combined effect of the gap genes and interactions between the other pair-rule genes. The pair-rule genes can be divided into two classes: the primary pair-rule genes and the secondary pair-rule genes. The primary pair-rules genes are able to influence the secondary pair-rule genes but not vice versa. The molecular mechanism between the regulation of the primary pair-rule genes was understood through a complex analysis of the regulation of even-skipped. Both positive and negative regulatory interactions by both maternal and gap genes and a unique combination of transcription factors work to express even-skipped in different parts of the embryo. The same gap gene can act positively in one stripe but negatively in another.
The expression of the pair-rule genes translate into the expression of the segment polarity genes in 14 stripes. The role of the segment polarity genes is to define to boundaries and the polarity of the segments. The means to which the genes accomplish this is believed to involve a wingless and hedgehog graded distribution or cascade of signals initiated by these proteins. Unlike the gap and the pair-rule genes, the segment polarity genes function within cells rather than within the syncytium. Thus, segment polarity genes influence patterning though signaling rather than autonomously. Also, the gap and pair-rule genes are expressed transiently while segment polarity gene expression is maintained throughout development. The continued expression of the segment polarity genes is maintained by a feedback loop involving hedgehog and wingless.
While the segmentation genes can specify the number, size, and polarity of segments, homeotic genes can specify the identity of the segment. The homeotic genes are activated by gap genes and pair-rule genes. The Antennapedia complex and the bithorax complex on the third chromosome contain the major homeotic genes required for specifying segmental identity. These genes are transcription factors and are expressed in overlapping regions that correlate with their position along the chromosome. These transcription factors regulate other transcription factors, cell surface molecules with roles in cell adhesion, and other cell signals. Later during development, homeotic genes are expressed in the nervous system in a similar anterior/posterior pattern. Homeotic genes are maintained throughout development through the modification of the condensation state of their chromatin. Polycomb genes maintain the chromatin in an inactive conformation while trithorax genes maintain chromatin in an active conformation.
All homeotic genes share a segment of protein with a similar sequence and structure called the homeodomain. This region of the homeotic proteins binds DNA. This domain was found in other developmental regulatory proteins, such as bicoid, as well in other animals including humans. Molecular mapping revealed that the HOX gene cluster has been inherited intact from a common ancestor of flies and mammals which indicates that it is a fundamental developmental regulatory system.