The development is a process starting with fertilization and forming a multicellular organism from a single cell based on the four cellular principles; (i) cell proliferation, (ii) cell specialization, (iii) cell interaction and (iv) cell movement. The underlying richness and astonishing outcome of development are facilitated by the cooperation of these four features, together with genetic material, that make up cell memory.
The entire animal kingdom shares common anatomical features of development that occurs through conserved mechanisms. The body plan consists of anterior-posterior, dorsal-ventral and left-right body axis. After fertilization, the zygote usually divides rapidly, or cleaves, to form many smaller cells; during this cleavage, the embryo, which cannot yet feed, does not grow. This phase of development is initially driven and controlled entirely by the material deposited in the egg by the mother. The embryonic genome remains inactive until a point is reached when maternal mRNAs and proteins rather abruptly begin to be degraded. The embryo’s genome is activated, and the cells cohere to form a blastula—typically a solid or a hollow fluid-filled ball of cells. These cells are often at the totipotent and or pluripotent state- they have the potential to give rise to all or almost all of the cell types of the adult bodies.
In gastrula, complex cell rearrangements and determination, called gastrulation (from the Greek “gaster,” mean- ing “belly”), transform the blastula into a multilayered structure containing a rudimentary internal gut. Some cells of the blastula remain external, constituting the ectoderm, which will give rise to the epidermis and the nervous system; other cells invaginate, forming the endoderm, which will give rise to the gut tube and its appendages, such as lung, pancreas, and liver. Another group of cells moves into the space between ectoderm and endoderm and forms the mesoderm, which will give rise to muscles, connective tissues, blood, kidney, and various other components. Thus, cell determination starts with gastrulation and progressively narrows the options as the cell steps through a programmed series of intermediate states- guided at each step by its genome, its history, and its interactions with neighbors. The process reaches its limit when a cell undergoes terminal differentiation to form one of the highly specialized cell types of the adult body. Although there are cell types in the adult that retain some degree of pluripotency, their range of options is generally narrow. Further cell movements and accompanying cell differentiation create and refine the embryo’s architecture.
Genes involved in cell–cell communication and transcriptional control are especially important for animal development
Two classes of genes are essential for development which are
- Cell-cell adhesion and cell signaling: hundreds of human genes encode signal proteins, cell-surface receptors, cell adhesion proteins, or ion channels that are either not present in yeast or present in much smaller numbers.
- Transcription and chromatin structure: Along with transcription regulators and chromatin structure defining genes, this category includes non-coding RNAs such as miRNAs. Of note, the loss of individual miRNA genes in C. elegans, rarely leads to obvious phenotypes, suggesting that the roles of miRNAs during animal development are often subtle, serving to fine-tune the developmental machinery rather than to form its core structures.
Regulatory DNA seems largely responsible for the differences between animal species
Each gene in a multicellular organism is associated with many thousands of nucleotides of noncoding DNA that contains regulatory elements. These regulatory elements determine when, where, and how strongly the gene is to be expressed, according to the transcription regulators and chromatin structures that are present in the particular cell. Consequently, a change in the regulatory DNA, even without any change in the coding DNA, can alter the logic of the gene-regulatory network and change the outcome of development.
Small numbers of conserved cell-cell signaling pathways coordinate spatial patterning
Spatial patterning of a developing animal requires that cells become different according to their positions in the embryo, which means that cells must respond to extracellular signals produced by other cells, especially their neighbors. This process is called inductive signaling. Generally, the inductive signal is limited in time and space so that only a subset of the cells capable of responding—the cells close to the source of the signal. Some inductive signals depend on cell-cell contact; others act over a longer range and are mediated by molecules that diffuse through the extracellular medium or are transported in the bloodstream.