BIOLOGY:ORIGIN OF LIFE::PART 4 ,SEQUENCIAL DEVELOPMENT IN METAZOANS

The history of man for the nine months preceding his birth would be far more interesting than all the three ten scores that follow it-Samuel Taylor Coleridge 

Formation of blastula by cell assymetry signalling

The blastula is the first structure to arise from the zygote. The zygote divides repeatedly to form a spherical ball of cells called the blastula. Thereafter the blastula develops a fluid filled cavity called the blastocoel. The question is how this structure is formed. The origin of this activity lies in asymmetry of cell in blastula. Cell on the surface do not have neighbours and behave differently. Cells inside have neighbours on all sides and this leads them to activate cis genes which cause secretion of fluid and cavity formation.

  Later the process of cavity formation is repeated again to form another fluid cavity amniotic cavity. Now the cells lying between the two cavities will give rise to the future embryo.

Gastrulation and cell cell signalling

Whereas the previous structure formation was based on assymetry of cell layers future development of embryo depends on cell cell interactions and cross talk. The cells between the two cavities form two layers epiblast above and hypoblast below. The hypoblast cells secrete a morphogen which causes its cells to proliferate at one end of the embryonic disc. This causes a break in symmetry of the embryo as it will form the head end. The hypoblast cells at the head end now secrete a morphogen AVE. This morphogen diffuses to upper epiblast cell layer. The cells of epiblast in the meantime will form the anal end due to morphogens from the placenta. The AVE from hypoblast layer prevents the entire epiblast to become the anal end by inhibiting the morphogens anteriorly where it has high concentration.The structure formed now is a linear proliferating streak of cells called primitive streak. Soon the cells divide and migrate to form three layers ectoderm mesoderm and endoderm

   Another remarkable symmetry which arises is bilateral symmetry at this time. The cells lining the undersurface of developing embryo have cilia which are angled behind and show rotation. This sweeps more body fluids including morphogns towards one side. The morphogens now accumulate more on one side and create different organs

Formation of organ structure by cell morphogenesis

Cells signal between them to now form morphological structure.The first structure to form is the notochord which forms by infolding of the ectoderm to form a solid rod dorsally Once formed the cells of the notochord secrete different morphogen anteriorly and posteriorly. These morphogens induce the ectoderm mesoderm to form various organs.

    The notochord induces the ectoderm dorsally to form a  tube the neural tube. Again it induces the mesoderm on both sides to form muscles and endoderm below  to form a hollow gut.The mesoderm also forms vascular tubes on both sides. These fuse below to form a heart

Formation of segmental structures ,somites by cyclical morphogens

All vertebrates .arthropods.annelids are made of mesodermal segments with repeated structures as vessels.kidneys.muscles and nerves. The formation of segments in mesoderm requires a boundary to form in a gradient of morphogen. The mesodermal cells have a molecular clock inside the cells consisting of a network of proteins, The protein and the RNA responsible for their synthesis have different lifetimes. As a result the proteins rise cyclically and fall cyclically. The mesoderm has a morphogenetic gradient which falls from anterior to posterior. The mesodermal cells form a boundary when the protien inside them has a certain value and this differs physically and intermittently from anterior to posterior.

   Another important mechanism is the hox gene. This gene has a number of parts arranged linearly in the chromosome. These genes are activated sequentially in the mesoderm by cis epigenetic mechanism involving histone methylation. The genes in the homeobox control a host of genes downward. Thus one hox gene will control limb formation and another wings.

Formation of organ structures by cell migration

  There are two types of migration of cells to form organs. The first involves actual migration of entire cells. This occurs throughout development starting from gastrulation. One specialised form of migration is neural crest cell migration. This occurs from sides of neural tube and the cells migrate inside the body to locate precisely in adrenal glands,pigment cells of skin ,over vast distances. The second type is seen in nervous system where the neural outpouches  grow and terminate at definite points in the brain. 

     Both phenomena depends on cell adhesive  properties of extra cellular space where migration takes place. The migrating cells have differential adhesive properties while moving and take a substratum which has a gradient of more to less adhesive  gradient.During development cell signalling leads to different paths having gradient of adhesives to these migrating cells. In case of neural growth the extracellular space of nervous system is similarly endowed with attractant and repellants. These direct the growing cell processes to correct sites.

Ack: Life unfolding