Chapter 19

BiologySixth Edition

Raven/Johnson

(c) The McGraw-Hill Companies, Inc.

How is development from a single cell to a highly organized organism accomplished?

Although there are differences among the development of these four organisms, it is surprisingly similar!

Cleavage – the initial developmental period of cell division.

Although the number of cells increase, the overall size of the cell mass does not increase.

Vertebrate Development

Mammalian Development

Na+ is pumped out of cells

Cells move (lamellipodia)

Insect Development

Produce two types of bodies. The change from a larva to an adult is called metamorphosis.

Special nurse cells release their own mRNA into the developing embryo – this orients the development!

Plant Development

Early cell division separate the embryo from the suspensor.

Soon, three tissue types (epidermal, ground, and vascular) are evident in the embryo.

Plant cells can’t move – new tissues are made at meristems.

Plants grow in response to their environment.

Consists of 959 somatic cells.

The same 131 cells are programmed to die.

Fate of each cell is the same in every Caenorhabditis elegans individual.

Basic Mechanisms of Development

• Cell Movement • Induction• Determination• Pattern Formation• Expression of Homeotic Genes• Program Cell Death

Cell Movement

Cells migrate during many stages of animal development by pulling themselves along using cell adhesion molecules.

Important molecules involved:

Cadherins span the plasma membrane and attach to the same kind of cadherins of other cells (over a dozen known to date; imaginal discs of fruit fly).

Integrins span the plasma membrane and ‘grab’ the matrix of intracellular collagen or fibronectin.

Induction

Drosphila:

Cells created by initial cleavage contain different developmental signals (determinants) mosaic development

Mammals:

Cells created by initial cleavage contain equivalent determinants; body form is determined by cell-to-cell interactions regulative development

Induction – when a cell switches from one developmental path to another as a result of interactions with adjacent cells.

Organizers – groups of cells that secrete a signaling molecule (morphogen) that conveys positional information.

Relative distance from organizers influences development.

Determination

Up to the 8-cell stage, cells are totipotent – any one of them can produce a complete individual if separated from the others.

Reverse works as well – chimera:

Chimera

Beyond the 8-cell stage each cell’s fate has been determined.

Determination – the commitment of a particular cell to a specialized developmental path.

Differentiation – the specialization that occurs at the end of a developmental path.

Activation of gene regulatory proteins is the mechanism of determination.

Determination is reversible!

Pattern Formation- free diffusion of morphogens

Positional information is used to determine the basic pattern of body compartments.

In Drosphila, maternal mRNA (from bicoid gene) is at one end of egg and translation begins at fertilization – this establishes an anterior end. Bicoid protein then activates the first transcription of hunchback genes, which is spread throughout, but transcription is blocked by nanos.

W/in 2 hours – gap genes are transcribed – map out the coarsest subdivision of the embryo.

W/in 3 hours, a cascade of segmentation gene activity produce’s the basic body plan.

Homeotic genes – code for proteins that act as transcription factors – activates a particular module of the genetic program.

Antennapedia Bithorax

Order of the genes mirrors the order of the body parts they control!

Homeotic genes typically contain the homeobox – 180 nucleotides for 60 amino acids.

Ensures that genes are transcribed at the appropriate time.

Homeobox genes (Hox) have been found also in mice and humans.

This indicates that they arose very early in animal history.

In mammals, homeotic genes appear to be lined up in the same order as the segments they control.

Apoptosis – programmed cell death – your fingers have to separate!

The same 131 cells always die during development of the nematode worm – under genetic control.

Same in humans.

Transfer human bcl-2 gene to nematode with defective ced-9 gene – apoptosis is avoided.

Four-theories on aging:

Accumulated Mutation Hypothesis

Telomere Depletion Hypothesis

Wear-and-tear hypothesis

Gene Clock Hypothesis

The End.

Divided into 17 segments, which are established very early in development.

Two clusters: Antennapedia and Bithorax.

Hox genes in mice are very similar to those in Drosophila and control body part development in mice

It appears that genes that determine organ development in animals operate in the same manner in plants.