SGN24 The Genetic Basis of Development

Development – division of cells, beginning with the zygote, to form in most cases, tissues, organs, organ systems and a complete multicellular organism Of particular interest is the connection between genes, cell specialization/differentiation and organism development How do cells become differentiated? During growth, development, and maintenance

Development involves cell and tissue differentiation (becoming specialized in structure and function); occurs during embryonic development Morphogenesis - arranging of cells and tissue in the embryo that gives the organism its shape

Animals – movement of cells and tissues are important during embryonic development Very early embryonic stem cells have totipotency (ability to differentiate into any cell type), as embryos mature cells become pluripotent (ability to develop into several cell types) while in later development organism retains stem cells that can typically divide to make several cell types (multipotent), for example different types of blood cells

Animal development – fair amount of variation on this theme in animal kingdom Egg (oocyte), within maternal tissue; sperm (spermatocyte) delivers its genetic material Zygote in many cases developing within maternal tissue 8-celled embryo – unequal division produces different sized cells at north and south pole Blastula – hollow ball of at least 128 cells Gastrula – embryo undergoes gastrulation to form 3 germ layers and segmentation

Gastrulation – dramatic rearrangement of the cells of the blastula to fprm the gastrula; cells migrate to form endoderm, ectoderm and mesoderm, and future orifice, the blastopore Ectoderm – epidermis, nervous system and sensory organs Endoderm – digestive tract, liver, several other organs and glands including reproductive system Mesoderm – skeletal and muscular system; circulatory system

Organogenesis is process of tissue and organ development from 3 germ layers of gastrula

Fetus, etc. Fetus - an unborn or unhatched vertebrate especially after attaining the basic structural plan of its kind; specifically : a developing human from usually two months after conception to birth

Larva and metamorphosis Larva - the active immature form of an insect, especially one that differs greatly from the adult and forms the stage between egg and pupa, e.g. a caterpillar or grub. Metamorphosis - a process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation.

All cells are offspring of the original zygote – how do cells “know” what kind of cells they should become? During development regulatory mechanisms turn specific genes on and off, leading to differences in gene expression in different types of cells, even though all cells of an organism have the same genome

Animal cells have genomic equivalency but during development and as an organism ages cells express different transcription factors or related molecules; this results in cell differentiation (process of becoming a particular cell type) But what causes an undifferentiated cell to become a differentiated cell?

The process that initiates differentiation is called cell determination (CD) CD occurs during embryonic development when critical activation/inactivation of master regulatory genes (code for transcription factors) sends cells down pathway of differentiation to produce different cell lines These transcription factors produced by MRG turn on genes that produce tissue-specific proteins (cell differentiation) So determination leads to differentiation

Cell lines undergo change from totipotent to pluripotent to multipotent to differentiated (unipotent)

Example - Muscle cells and myoD Embryonic precursor cells (undifferentiated – totipotent) undergo determination and differentiation to form myoblasts (premature muscle cell - multipotent) which continue to differentiate through several cell divisions to become a mature muscle cells Determination turns on master regulatory genes that commit cell to becoming skeletal muscle One MRG is myoD, which produces transcription factors that bind to specific control elements in DNA and stimulate production of additional transcription factors that initiate cascade of signals that differentiate cell into muscle

??But what induces a master regulatory gene in a specific cell type (like myoD) to turn on?? We must look to the very first instances of determination and differentiation during early embryonic development??

Determination and differentiation starts in the embryo Embryonic differentiation is directed by… Unequal distribution of cytoplasmic determinants in ovum and therefore in zygote Chemical signals from adjacent embryonic cells (induction), which provide positional information

Unequal distribution of cytoplasmic determinants (CyD) in ovum and therefore in zygote Cytoplasmic determinants are generally RNA and proteins coded for by maternal effect genes (genes of female genome; proteins are produced in the egg or imported from surrounding nurse cells) and are unevenly distributed in egg/zygote

CyD that determine axes of embryo are produced by egg-polarity genes (they control orientation of the blastula, gastrula and fetus)

CyDs are transcription factors that turn on genes in zygote and embryo

Chemical signals from adjacent embryonic cells, during first divisions, provide positional information Positional information (the molecular cues that control pattern formation in embryo) tells a cell its location relative to other cells and the body axes Induction – cells signal nearby cells to change in some specific way bringing about cell differentiation

Role of signal induced apoptosis in development (vs self-induced apoptosis in response to cell/DNA damage)

An example of development - Drosophila Drosophila egg is very large; maternal effect mRNAs and proteins (CyDs) exist in numerous gradients throughout egg and zygote Multiple cell divisions occur without daughter cells growing in size As division occurs daughter cells will have different concentrations of the CyDs due to their original distribution in the egg

Earliest divisions establish body plan based upon differing presence and concentration of CyD’s in different cells So head and tail position, left and right are established early; many embryos also begin process of segmentation (dividing the body up into repetitive segments that will further differentiate) Body axis development and segmentation controlled by egg-polarity genes (a type of maternal effect gene) and induction  

Example of egg-polarity gene – bicoid gene; produces a transcription factor bicoid mRNA concentrates in the anterior end of egg but is kept untranslated; upon fertilization the mRNA is translated, creating a bicoid protein gradient in the zygote

The presence or absence, and concentration, of bicoid protein leads to a cascading production of morphogens (proteins that are important in determining axes orientation and body shape) that are differentially produced in different cells and lead to cell and segment differentiation bicoid = very early master regulatory gene and product involved in establishing body plan

Continued development of embryo One group of morphogen is produced by segmentation genes, which determine the segmentation pattern

Following segmentation, within each segment specific homeotic genes are then turned on, based upon concentration of morphogens Homeotic genes (code for transcription factors that develop body pattern and differentiate segments) are differentially expressed in each segment, determining the types of characteristics each segment will have (appendages, internal characteristics, etc.)

Homeotic genes fall into a category of genes called homeobox genes Homeobox genes, and therefore all homeotic genes, include a sequence (the homeobox) that codes for a 60 amino acid homeodomain Homeodomain of protein binds to DNA when protein functions as transcription factor Other variable segments of the transcription factor protein give the protein specificity by allowing interaction with other transcription factors (transcription activation domain)

Homeodomain is highly conserved in virtually all animal groups; in mammals typically referred to as Hox genes; versions even found in prokaryotes and fungus

Maternal effect genes determine polarity (also called Maternal effect genes determine polarity (also called egg polarity genes) EPG induce segmentation genes to produce morphogens that continue to establish body plan by dividing body into segments Hox genes differentiate the segments to establish different parts of body