Induction Mechanisms

• Development is the formation of specialized cells, tissues and organs from a single cell, known as a zygote.
• Embryonic Developmental Factors include Differentiation, Proliferation, Determination, Apoptosis, Migration, Morphogenesis, Genes, Gravity, and Position.
• Determination is the commitment of a cell to undergo differentiation and is an irreversible process but is not accompanied by morphological changes.
• Determinants are the cytoplasmic effector molecules that mediate determination.
• Induction is the stimulation of a cell to differentiate in response to a stimulus produced by another cell and is mediated by inducer substances that diffuse from one cell to another.
• Once determined cells are no longer responsive to other inducer molecules.
• Males are sterile because the flagella of their spermatozoa are nonmotile.
• Less severe cases of syndactyly may exhibit only a web of skin between the digits.
• Syndactyly is a condition where the bony elements of digits 2 and 3 are fused, even in severe cases.
• Neural crest cells migrate to ganglia, tooth pulp, medulla of suprarenal glands, and melanocytes in the skin.
• The amount of apoptotic cell death that occurs in many developing and adult vertebrate tissues is astonishing: at least a million cells die this way each second in a healthy adult human (and are replaced by cell division).
• Apoptosis depends on an intracellular proteolytic cascade mediated by caspases.
• Apoptosis, or programmed cell death, eliminates unwanted cells.
• Cell determination starts early and progressively narrows the options as the cell steps through a programmed series of intermediate states, guided at each step by its genome, its history, and its interactions with neighbors.
• The process of cell determination reaches its limit when a cell undergoes terminal differentiation to form one of the highly specialized cell types of the adult body.
• There are three classes of molecules that are important during the development of an organism: Transcription factors, Signalling molecules, and Cell adhesion molecules.
• Transcription factors act in the cells that produce them, bind to DNA and controls transcription of other genes in the cell, and initiate patterns of gene expression.
• Transcription factors bind to a specific DNA sequence (known as cis regulatory sequence) to influence the transcription of a gene.
• Transcriptional control is regulated by a cell of gene expression by controlling when and how often a given gene is transcribed.
• Examples of transcription factors include Homeodomain or Hox genes, Zinc finger Transcription Factors, and Basic helix-loop-helix proteins.
• The Sox genes comprise a large family (>20 members) and bind to seven nucleotides on the minor instead of the major groove on the DNA helix and cause a pronounced conformational change in the DNA.
• Sonic hedgehog is one of three proteins in the mammalian signaling pathway family called HEDGEHOG.
• Hox genes are crucial targets of retinoic acid receptors in development.
• Shh is secreted at high levels by the notochord, and therefore the concentration of Shh is highest in the floor plate of the neural tube.
• Affected individuals have serious respiratory problems, since the cilia of their respiratory epithelium are nonmotile.
• Shh stimulates the expression of the growth factor called nodal.
• Shh is essential for organ patterning during development.
• Activin is expressed unilaterally on the right, and inhibits Shh expression.
• The signalling molecule known as Sonic hedgehog (Shh) is initially expressed uniformly throughout the embryo.
• Individuals with Kartagener's syndrome may have their heart on the right side of the body - a condition called situs inversus.
• Kartagener's syndrome is an inherited disease that affects embryonic development.
• Recent studies have shown that Shh may also mediate communication with stem cell niches and may play a role in cell proliferation and differentiation.
• Sonic hedgehog plays a key role in regulating vertebrate organogenesis, such as in the growth of digits on limbs and organization of the brain.
• Shh is also important for the polarization of the anterior-posterior axis of the developing limb-bud.
• Retinoic acid binds to its receptors (transcription factors) inside the cell.
• Alterations in the level of vitamin A (retinol) in the diet can lead to the development of congenital malformations.
• Most internal organs are asymmetrical, for example, the heart is on the left side, the right lung has more lobes than the left, the stomach and spleen lie to the left, the liver has a single right lobe.
• There are other genes that are expressed unilaterally.
• Shh is expressed in the developing limb, lung, gut, CNS, teeth and hair follicles.
• Nodal is expressed only on the left and this results in asymmetric development.