Human Cell Atlas (maps every cell of the human body)

See also types of cells

See also fertilization and embryonic development

Introduction:

Cell development starts as a fertilized egg (Zygote) that must undergo cell division to produce the new individual. Thus extensive cell divisions is required. However, in many cases it does not incuded much growth as the egg cell itself is quite large.

As cells divide, precise changes in gene expression result in differences between cells that ultimately result in cell specialization. In differentiated cells, certain genes are expressed at particular times, but other genes may not be expressed at all.

Pattern formation involves cells abilities to detect positional information that guides their ultimate fate. In animal embryos, the timing and number of cell divisions are species specific, and this period of rapid cell division following fertilization is called cleavage. During cleavage, the enormous mass of the zygote is subdivided into a larger and larger number of smaller and smaller cells, called blastomeres. Cleavage is not done by an increase in the overall sie of the embryo. The G1 and G2 phases of the cell cycle, during which a cell increases its mass and size, are greatly shortened or climinated alltogether during cleavage. This is controlled by cyclins and clin-dependent kinases. For exaple, zebrafish blastomeres divide once every 15 minutes during cleavage to create an embryo with a thousand cells in just under 3 hours. As a comparison, cycling adult human intestinal epithelial cells divide on average once every 19 hr.

As development continues the form of the body, its organs and anatomical features is generated. Morphogenesis may invovle cell death, cell division, cell migration, changes in cell shape and differentiation.

Cell Differentiation:

A human contains about 300 ytes of differentiated cells. They can be distinguised based on the particular porteins they synthesize, their morphologies and their specific functions. During development, cells become different from one another because of the differential expression of subsets of genes, both at different time and in different locations of the devleoping embryo.

Before differentiation takes place, cells make a molecular decision to become a particular cell type. The standard procedure to test whether a cell or group of cells is determined is to move the donor cell(s) to a different location in a host embryo. If the cells of the transplant develop into the same type of cell that they would have if left undisturbed, they they are considered to be determined. For example, a cell in the prospective brain region of an amphibian embryo at the early gastrula stage has not yet been determined if transplanted elsewhere in the embryo. It will develop according to the site of transplant. By the late gastrula stage, however, additional cell interactions have occurred and determination has taken place. The cell will develop as neural tissue o matter where it is transplanted.

Determination usually takes places in stages, with a cell first becoming partially committed. In a chicken embryo, for example, tissue at the base of the leg bud normally gives rise to the thigh. I this tissue is transplanted to the tip of the identical looking wing bud, which would normally give rise to the wing tip, the transplanted tissue will develop into a toe rather than a thigh. The tissue has already been determened, but it is not yet committed to being a particular part of the leg. Thus, it can be influenced by the positional signaling at the time of the wing bud to form a tip but in this case, a tip of leg.

Cells initiate development by using transcription factors to change patterns of gene expression. When genes encoding these transcription factors are activated, one of their effects is to reinforces their own activation. This reinforcement makes the developmental switch deterministic, initiating a chain of events that leads down a particular developmental pathway.

Cells in which a set of regulatory genes have been activated may not actually undergo differentiation until some time later, when other factors interact with the regulatory proteins and cause it to activate still other genes. Nevertheless, once the initial “switch” is thrown, the cell is fully committed to its future developmental path.

Cells become committed to follow a particular developmental pathway by differential inheritance of cytoplasmic determinants, which are maternally produced and deposited into the egg during oogenesis or via cell-cell interaations.

Plant Development:

A major difference between animals and plants is that most animals are mobile. Plants, in contrast, are anchored in position and most therefore endure whatever environment they experience.

Instead of creating a body in which every part is specified to have a fixed size and location, a plant assembles its body throughout its life span from a few types of modules, such as leaves, roots, branch nocdes and flowers. Each module has a rigidly controlled structure and organization, but how the modules are utilized is flexible; it can be adjusted to environmental condtiions.

Plasnts develop by building their bodies outward, creating new parts from groups of stem cells that are contained in structures called meritems. As meristomatic stem cells cotinually divide, they produce cells that can differentiate into teh tissues of the plant. The plant cell cycle is regualted by the same mechanisms as animal cells and yeast by cyclins and cyclin-dependent kianses. For example, overexpression of a Cdk inhibitor can result in strong inhibition of cell division in leaft meristems, leading to significant changes in leaf size and shape.