Campbell Biology Chapt 47 Part Ii

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Organogenesis

Back 1

Various regions of germ layers develop into rudimentary organs

Adoption of developmental fates cause cells to change shape or even migrate to a new location

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Neurulation

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Formation of brain and spinal cord

Cells from dorsal mesoderm form notochord

Signal molecules secreted by notochord and other tissues cause ectoderm above to form neural plate ( example of induction)

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Induction

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When cells or tissues cause a developmental change in nearby cells

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Neural tube

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Formed by Neural plate curving inward

Becomes the central nervous system (brain and spinal cord)

Notochord disappears before birth but some of it will make up disks btw vertebrae in spinal cord

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Neural crest cells

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Develop along the neural tube of vertebrates and migrate in the body

Eventually form various parts (nerves, parts of teeth, and skull bones)

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Somites

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Mesoderm lateral to the notochord forms blocks

Parts of somites dissociate to form mesenchyme cells which form the vertebrate, ribs, and muscles associated with vertebral column

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Mesenchyme cells

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A loosely organized, mainly mesodermal embryonic tissue which develops into connective and skeletal tissues (including blood and lymph)

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Organogenesis in Chicks and insects

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Similar to frogs

By time embryo is 3 days old, rudiments of major organs are readily apparent

ie: folic acid before pregnancy: helps to enhance neuro tube formation -spina bifeda- no closure of neural tube

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Organogenesis in chicks and insects is similar how

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Neurolation

Pattern and appearance can be different

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Cytoskeleton in morphogenesis

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In animals, movements of parts of a cell bring about cell shape changes or can enable a cell to migrate to a new location

The microtubles and microfilaments of the cytoskeleton are essential to these events

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Cell shape changes in morphogenesis

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Reorganizing the cytoskeleton is a major force in changing cell shape

Contraction of actin filaments at the apical end of cells causes them to become edge shape

Common mechanism for invaginating cell layer

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Convergent extension

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Directed by cytoskeleton

Sheet of cells undergoes rearrangement to form longer and narrower shape

Cells elongate and wedge btw each other to form fewer columns of cells

(Merging on highway)

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Cell migration morphogenesis

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Cytoskeleton responsible

Trans membrane glycoproteins (cell adhesion molecules) play a role in migration

Also involves extracellular matrix (mesh work of secreted glycoproteins and other molecules lying outside the plasma membrane of cells)

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Apoptosis

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Programmed cell death

Individual cells, sets of cells, or whole tissues stop developing or die (skin btw fingers or tadpole tail)

ie: many more neurons are produced in developing embryos than will be needed, extra are eliminated

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Determination vs differentiation

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Determine:Process by which a cell or a group of cells becomes committed to a particular fate

Differentiation: the resulting specialization in structure and function

Cells in multicellular organisms share the same genome

Differences in cell types are the result of the expression of different sets of genes

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Fate maps

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Diagrams showing organs and other structures that arise from each region of an embryo

Classic studies using frogs indicated that cell lineage in germ layers is traceable to blastula cells (dyed cell and then tracked it throughout development)

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Study for fate mapping

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Studied nematode

Used ablation of single cells to determine the structures that normally arise from each cell

Researchers were able to determine lineage of each 959 somatic cells in worm

Also discovered there were 131 cells that go through apoptosis. Found mutations that prevented apoptosis which encouraged study of apoptosis

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Fate mapping

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Germ cells are specialized cells that give rise to eggs or sperm

Complexes of RNA and protein are involved in the specification of germ cell fate

Such complexes, called P granules persiste through development and canbe detected in adult worm

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P granules

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Distributed throughout the newly fertilized egg and move to the posterior end before the first cleavage

With each subsequent cleavage, PGs are partitioned into the posterior most cells

PGs act as cytoplasmic determinants, fixing germ cell fate at earliest stage of development

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Axis formation

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Bilateral symmetry is found across many animals

Asymmetry across the dorsal-ventral and anterior-posterior axes

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Axis formation in frog

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Anterior-posterior is determined during oogenesis

Animal-vegetal asymmetry indicates where the anterior- posterior axis forms

Dorsal-ventral axis is not determined until fertilization

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Cortical rotation

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Fusion of the egg and Sperm rotated according to where sperm entered

Brings molecules from vegetal cortex to molecules in the inner cytoplasm of animal hemisphere

Leads to dorsal and ventral specific genes

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Axis formation in chicks

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Gravity is involved in establishing anterior-posterior axis

Later pH difference btw the 2 sides of the blastoderm establish the dorsal-ventral axis

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Mammals axis formation

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Orientation of eggs and sperm nuclei may help establish embryonic axes

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Insect axis formation

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Morphogenesis gradients establish the anterior-posterior and dorsal ventral axes

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Restricting developmental potential

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Hans speman

Experiments to determines a cell develop potential

First two nblastmeres are totipotent

Changed cleavage and changed development. If you didn’t have gray crescent, then didn’t form

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Restrictive development in mammals

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Animal cells remain totipotent until 8th cell stage (longer than other organisms) this is also where twins can be formed.

Tissue-specific gates of cells are fixed by late gastrula stage

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Totipotent

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Can develop into all the possible cell types

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Cell fate determination and pattern formation by inductive signals

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As embryonic cells acquire distinct fates, they influence each others fate by induction

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The organizer

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Spemann n mangold transplanted tissues between early gastrula and found that the transplanted dorsal lip of the blastopore triggered a second gastrulation in the host

The dorsal lip functions as an organizer of the embryo body plan inducing changes in surrounding tissues to form notochord, neural tube, and so on

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Pattern formation

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Development of spatial organization

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Positional information

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Molecular cues that control pattern formation

This informs a cell where it is with respect to the body axes

Determines how the cell and its descendants respond to future molecular signs

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Formation of vertebrate limb

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Wings and legs begin as bumps of tissue (limb buds)

Embryonic cells in a limb bud respond to positional information indicating location along three axes

-proximity-distal, anterior-posterior, dorsal-ventral

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Apical ectoderm also ridge AER

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Limb bud regulating region

AER is thickened ectoderm at the buds tip

Fibroglass growth factor that promotes our growth so if you don’t have it, it will not grow

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Zone of polarizing activity (ZPA)

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Second region near armpit

Mesodermal tissue under ectoderm where the posterior side of the bud is attached to the body

Regulates anterior-posterior patterning of the limb

Secretes a signal called Sonic hedgehog that hits cells to make posterior structures (pinkies) but cells getting less sonic make anterior parts (like thumbs)

Implanting these cells elsewhere can adds extra fingers or toes

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Cilia and cell fate

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Needed for proper specification of embryo

Motile: play roles in left/right specification

Monocilia: act as antenna on cell surface, receive signals from proteins (like sonic hedgehog).

When mono are defective signaling is disrupted

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Kartagners syndrome

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A set of medical conditions that appear together (immotial sperm, infections of nasal sinuses, situs inversus reversal of normal left-right asymmetry)

Due to defect that makes cilia immotile

Cilia generate a fluid flow in early development which is what disrupts asymmetry

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Metamorphosis

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Involves major transition from one form to another. Does not happen in mammals.

ie: tadpole

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Stages of embryogenesis

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Zygote (undergoes cleavage until it reaches blastula), blastula, gastrula (dramatic rearranging of cells

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Meroblastic cleavage

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Cell division is restricted to a small disk of yolk-free cytoplasm at the animal pole of the zygote

Yolk impedes cell division. In cells with large yolk cleavage is often restricted to the animal pole

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Difference btw blastula and gastrula...

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Opening to the archenteron in the cavity called blastopore

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During organogenesis cells..

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Differentiate into tissues

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Involution

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Along blastopore where future endoderm and mesoderm cells roll over edge of the lip into interior of embryo

Equivalent to dorsal blastopore lip in frog

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2 main genetic mechanisms that underlie differentiation

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Uneven distribution of cytoplasmic determinants

Induction

These differences set stage for distinct programs of gene expression

Send cells down specialized paths