Gene control of differentiation in ontogeny

Gene control

Fertilization produces a zygote from the two gametes, and a morula from the grooving one. As the cells travel out, a blastula is formed and then a gastrula (three germ layers) is formed. This is followed by the development of the primitive streak, organogenesis and histogenesis.

During ontogeny, the following mechanisms apply:

• cell growth, cell division, migration cells,
• differentiation cells,
• apoptosis.

All cells in the body have the same set of genes, but differ in their expression.

Ontogenesis = sequence of events that is initiated by fertilization - regulatory cascade (local mediators, hormones, receptors, transcription factors, etc. are used here)

Morphogenes = control the differentiation and predetermination of cells, their action depends on the concentration of their products - they create a concentration gradient where the effect occurs only from places where the concentration reaches a certain threshold level. Their cascade is already activated before fertilization

Maternal Way Genes

• They are part of the mother's genome,
• expressed in follicular bb and their mRNA and proteins transported to the egg,
• in their mutations, the zygote dies independently of its genotype,
• they determine the anterioposterior and dorsoventral polarity of the embryo,
• this includes bicoid (determines where the front of the embryo is) and nanos (determines the back of the embryo),
• works in collaboration with hunchback gene.

Genes of the zygote

Segmentation genes

• In vertebrates, the segments are distinct only in the initial stages of development.

Gap genes

• E.g. hunchback, knirps, giant a krüpl,
• genes of maternal origin are activated,
• their mutation: developmental disorder of part of the segments,
• they are transcription factors,
• affect the basic differentiation of the embryo.

HOX genes

• They contain a homeodomain (homeobox)
• Their mutations can cause one organ to be confused with another,
• bithorax and antenapedia complex genes,
• mutation of the antenapedia gene in Drosophila melanogaster causes the development of a leg on the head instead of a antennae,
• in humans, HOX1 mutation causes craniosynostosis.

Pair-rule genes (PAX)

• They also contain a homeodomain (homeobox)
• They specify the nature of the segments,
• regulated by gap genes,
• expressed in 7 stripes along the anterioposterior axis,
• they divide the embryo into 15 parasegments,
• their mutations reduce the number of segments by half (fushi tarazu - development of odd parasegments, even-skipped - development of even parasegments),
• PAX3 mutation = Waardenburg syndrome (deafness, white strand of hair, iris heterochromia).

(Para)segment polarity genes

• They influence the anterioposterior polarity of parasegments, defined by pair-rule genes,
• the embryo is gradually divided into smaller and smaller developmental sections,
• e.g. engrailed gen.

Tissue-specific genes

• A cascade of hundreds of genes,
• eyeless gene - its mutation in Drosophila causes the development of a rudimentary eye, in mice microophthalmia and in humans aniridia,
• when the eyeless gene is linked to the promoter of a gene typical for another tissue, an eye develops in this tissue,
• with interspecies transfer of the eyeless gene, a species-specific eye is created → basic tissue-specific genes are developmentally old,
• other, developmentally younger and species-specific genes also decide on the definitive form of the organ.

The spectrum of genes that is expressed in a certain type of cells is determined by RNA - DNA saturation.

Links

Used literature

• KAPRAS, Jan – KOHOUTOVÁ, Milada. Kapitoly z lékařské genetiky III.. 1. edition. Karolinum, 2009. vol. 1. ISBN 80-246-0001-3.