complexity and development

Epigenesis

The theory that new structures arise progressively during embryonic development.

Preformation

The historical belief that all embryos existed since the beginning of time, with some fully formed structures present from the start.

Homunculi

The idea that tiny human-like forms exist in sperm.

Totipotent

The capability of a cell to develop into any type of cell in the embryo, including the placenta.

Pluripotent

The ability of a cell to differentiate into almost any cell type in the body, as seen in the inner cell mass (ICM) of the blastocyst.

Multipotent

Stem cells that can differentiate into a limited number of cell types, such as hematopoietic stem cells.

Unipotent

The capacity of a cell to differentiate into only one type of cell, such as epidermal cells.

Morphogenesis

The biological process that causes an organism to develop its shape, requiring cell movement and changes in cell adhesion.

Gastrulation

The developmental process where an embryo transforms from a one-dimensional layer of cells into a three-dimensional structure with three germ layers: ectoderm, mesoderm, and endoderm.

Induction

The process where a group of cells signals another group in the embryo, influencing their development.

Zone of Polarizing Activity (ZPA)

A region that acts as an organizer for the anterior-posterior axis of limb development.

Morphogen

A substance that establishes a concentration gradient and can cause different effects in target cells based on its concentration.

HOX Code

Genes that determine the identity of different body segments in an embryo, conserved across species.

Homeotic transformations

Abnormal developments caused by mutations in homeotic selector genes, such as legs growing in the place of antennae in flies.

Gene expression regulation

The process by which gene expression is turned on or off by transcription factors binding to regulatory regions.

Cleavage divisions

The early cell divisions in an embryo that occur without growth, resulting in a 1-16 cell stage.

Terminally differentiated cells

Cells that have developed into their final form with specific functions, such as blood cells and skin.