In the kingdom Animalia, organisms are primarily grouped according to their development. Early development is conserved across the members of this kingdom. This means that two animals can appear to be very similar when in an early developmental stage but be very different as adults.
Development is defined as those processes that are irreversible and occur from zygote formation to death. The word "irreversible" is key and limits the definition of development. Cellular differentiation is defined as how a cell diverges from its early morphology into a more specialized morphology, in an irreversible manner. Morphogenesis is how an organism's "shape" is acquired, and pattern formation describes how cells, tissues, and organs are arranged in an organism.
The embryonic cells of protostomes and deuterostomes have different potentials for future development. Protostomes have blastomeres whose developmental fate is determined very early; hence, their development is termed determinate development. Animals that have a radial cleavage pattern (deuterostomes) have blastomeres that can potentially form all tissue types (i.e., their developmental fate is not predetermined); hence, this type of early development is termed indeterminate development.
Totipotency is the capability of certain embryonic cells to form any type of adult cell. The early embryonic cells (blastomeres) of deuterostomes can be totipotent. Totipotency is required for two individuals to develop from a single zygote that is split in half (e.g., identical twins, also called monozygotic twins). Totipotency is typically lost during early embryo development, but scientists can reprogram some cells to become totipotent once again in the laboratory.
The development of a diploid organism begins with fertilization. Shortly after the gametes unite to form the single-cell zygote, cell division commences. These early cell divisions are called cleavage. Cell division is not a random process, and various patterns of cleavage are observed. In some cases (e.g., protostomes), a spiral pattern is observed in the first few cell divisions, whereas in other cases (e.g., deuterostomes), the blastomeres (early cells) have a radial configuration.
The figure at right illustrates how the first collection of cells (blastomeres) form a hollow ball, the blastula. However, blastulae aren't always hollow spheres; they can be flattened (e.g., the blastulae of birds and mammals). From this hollow embryonic stage, cells start migrating into the interior and simultaneously begin to differentiate; the cells that migrate inward express a different set of genes. This sets up the first two tissue layers, ectoderm and endoderm. Gastrulation, this movement of surface cells inward, is accompanied by induction (the process by which certain embryonic cells trigger the differentiation of other embryonic cells). During gastrulation, cells change their developmental pathway. The gastrula stage, in which tissue types arise, is only observed in the eumetazoans.
At the conclusion of gastrulation, the endoderm and ectoderm have been defined, and the mesoderm then forms. In addition, the archenteron and blastopore arise. The archenteron is a pouch of cells created by invagination during gastrulation, and the blastopore is the opening to the pouch. (In the simplest scenario, think of pushing into a lump of clay with your finger; the area that your finger tip forms (the pouch) represents the archenteron, and its opening represents the blastopore.) The blastopore's fate is determined by the type of organism. If the blastopore gives rise to a mouth, it is a protostome. If the blastopore gives rise to an anus, it is a deuterostome. The gut forms from the archenteron.
Animals I Part 2 VoiceThread Transcript
Hox genes are found in animals and determine the pattern formation of the body during the development of the embryo. These genes code for transcription factors, which control the expression of other genes. In animals, these genes are found in a linear sequence along the chromosome, starting with the genes that affect the structure at the head of the organism, and moving down the body.
These genes were first discovered in the fruit fly (an invertebrate model organism) which has one cluster of hox genes, and are now known to be in all animals. They are also well-studied in the mouse (a vertebrate model organism) which has four clusters of hox genes (humans also have four clusters). These additional clusters have taken on new functions, including determining the patterning of the limbs. Mutations in these genes can lead to a change in the postion of body parts, and the results include flies that have legs where their antennae should be!