14.9 Ontogeny (Development)

209

Table 14.6 Summary of ontogenetic paths (see text for further explanation)

Rate

Effect

Morphological

result

Name

Soma

Gonads

Fast

–

Acceleration

Recapitulation

Acceleration

–

Fast

Truncation

Paedomorphosis

Progenesis

Slow

–

Retardation

Paedomorphosis

Neoteny

–

Slow

Prolongation

Recapitulation

Hypermorphosis

and standard shape is defined as left parenthesis y divided by x right parenthesis Subscript normal upper C(y/x)C, retardation implies that this ratio occurs at

larger xx and acceleration that it occurs at smaller xx. Another form of acceleration

is “recapitulation”—previously adult features are pushed into progressively earlier

stages of descendent ontogenies. Table 14.6 summarizes ontogenetic paths. Devel-

opment has been seen as a manifestation of the wider phenomenon of symmetry

breaking. ⁶¹

14.9.1

Stem cells

Multicellular organisms begin life as a single cell, which divides, and the offspring,

in turn, grow and divide and ultimately differentiate to create the variety of cells

that constitute the organism’s cellular repertoire. Stem cells may be defined as cells

that can both self-renew (i.e., reproduce themselves) and differentiate into multiple

cell types (lineages). The “ultimate” stem cell is totipotent and has the ability to

form all cell types. In mammals, the fertilized egg, zygote, and the cells from the

first four divisions (up to 16 blastomeres) are totipotent. Note, however, that strictly

speaking these cells cannot self-renew (e.g., a zygote cannot divide to make two

zygotes), and hence should not perhaps be called stem cells. Pluripotent stem cells

are able to differentiate into the three fundamental types of embryonic germ layer,

namely ectoderm, mesoderm, and endoderm (see footnote aa to Table 14.7 for more

explanation), from which all the more specialized cell types are derived. Lower down

in the hierarchy are multipotent stem cells, which can form a small number of more

specialized cells derived from a particular germ layer and constituting the somatic

tissues. Fully differentiated cells are typically unable to divide.

The phrase “reprogramming stem cells” is often used to describe the remarkable

discovery that by adding just four new genes to a differentiated (skin) cell, after 2–3

weeks they reverted to pluripotent stem cells (induced pluripotent stem cells, iPS). ⁶²

“Reprogramming” does not imply that cells operate like digital computers, but the

61 Li and Bowerman (2010), Saxena (2015).

62 Takahashi and Yamanaka (2006).