Development of the embryo

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Embryonic development

• Ovum Fertilised ovum

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Cell Division
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Development of the embryo

• Arm where an arm should be and not from the top
  of your head
• HOW?
• Fertilised egg ? fully formed neonate
• HOW?

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All nuclei are the same

• All cells contain the same genes
• a complete copy of the genome
• except gametes
• Every cell with a nucleus can create every other
  cell in the body! nuclear totipotency.

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Differential gene expression
How's it controlled?

• Different cell types express (transcribe) only
  those genes needed to produce that tissue
• i.e. only synthesises proteins needed e.g. muscle
  is only site of myoglobin production.
• During development, genes are needed only at
  certain times, then switched off e.g. foetal
  haemoglobin
• SPATIAL TEMPORAL differential gene expression
  in development

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Differential gene expression

• Gene expression is regulated by
• PROMOTERS INHIBITORS (transcription factors)
• Bind to regulatory sites near the genes and
  control transcription

Animation
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Differential gene expression

• During development need to ensure correct
  promoters and inhibitors are present
• Studied in drosophila

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The importance of the egg

• Within the egg (before fertilization) a gradient
  of mRNAs is established
• They code for proteins, that are transcription
  factors (known as morphogens)
• Locate at either ends (the poles)

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Distribution of proteins after fertilisation

• Fertilisation stimulates the translation of
  bicoid and nanos mRNAs
• The proteins diffuse
• Set up a concentration gradient

Egg Egg
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First cell division

• More bicoid than nanos protein
• More nanos than bicoid protein

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bicoid nanos are transcription factors

• bicoid and nanos regulate transcription of
  another set of genes
• The segmentation genes (a class of genes which
  produce segments GAP, PAIR RULE, SEGMENT
  POLARITY genes))
• They are also transcription factors
• GAP controls PAIR RULE which control expression
  of SEGMENT POLARITY genes.
• The SEGMENT POLARITY genes regulate expression of
  the homeotic genes the final set of
  transcription factors.
• Homeotic genes regulate expression of genes
  producing different parts of the body (i.e.
  structural proteins) This one gene controls
  many.

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GAP GENE EXPRESSION

• Brief signals from a cascade of genes then split
  the fly embryo into ever smaller and many more
  specialized regions. In the photograph the
  embryo is divided into large blocks by proteins
  from so-called gap genes - Krüppel (red) and
  hunchback (green), which is turned on by bicoid
  2½ hours after fertilization. The region where
  the two proteins overlap is yellow. The colors
  come from fluorescent dyes in antibodies that
  bind to these proteins.

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PAIR RULE genes

• About a half hour later (3½ hrs), hairy a
  "pair-rule" gene that is regulated by the gap
  genes, switches on and produces seven transient
  stripes. These stripes act like boundaries,
  dividing the embryo into seven segments

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SEGMENT POLARITY genes

• Finally, "segment-polarity" genes, divide each of
  the previous units into anterior and posterior
  compartments.
• The narrow compartments correspond to specific
  segments of the embryo.
• three head segments (H, top right),
• three thoracic segments (T, lower right),
• eight abdominal segments (a, from bottom right to
  upper right).

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Segmentation genes divide the embryo into regions
The drosphila embryo ends up with 17 segments
Each segment will produce a different part of
the body The instructions for the body parts are
controlled by the HOMEOTIC GENES
Animation
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Homeotic gene expression determines ultimate
function of segment

• Bithorax mutant

Mutant bithorax gene(s) Inappropriately expressed
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• Antennapodia complex mutant

Mutant antennapedia complex gene(s)
inappropriately expressed
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In utero diethylstilbestrol (DES) exposure alters
Hox gene expression in the developing mullerian
system.Block K, Kardana A, Igarashi P, Taylor
HS.Department of Obstetrics and Gynecology,
Yale University School of Medicine, New Haven,
Connecticut 06520, USA.Diethylstilbestrol (DES)
was widely used to treat pregnant women through
1971. The reproductive tracts of their female
offspring exposed to DES in utero are
characterized by anatomic abnormalities. Here we
show that DES administered to mice in utero
produces changes in the expression pattern of
several Hox genes that are involved in patterning
of the reproductive tract. DES produces posterior
shifts in Hox gene expression and homeotic
anterior transformations of the reproductive
tract. In human uterine or cervical cell
cultures, DES induces HOXA9 or HOXA10 gene
expression, respectively, to levels approximately
twofold that induced by estradiol. The
DES-induced expression is not inhibited by
cyclohexamide. Estrogens are novel morphogens
that directly regulate the expression pattern of
posterior Hox genes in a manner analogous to
retinoic acid regulation of anterior Hox genes.
Alterations in HOX gene expression are a
molecular mechanism by which DES affects
reproductive tract development. Changes in Hox
gene expression are a potential marker for the
effects of in utero drug use that may become
apparent only at late stages of development.
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Summary

• Maternal co-ordinate genes differentially
  distributed in the egg they are transcription
  factors.
• They regulate transcription of another set of
  genes
• The segmentation genes (a class of genes which
  produce segments)
• They are also transcription factors
• After a cascade of 3 different types of
  segmentation genes (GAP, PAIR RULE, SEGMENT
  POLARITY), the homeotic genes are expressed
• Homeotic genes are transcription factors they
  regulate expression of genes producing different
  parts of the body
• Each homeotic gene determines the anatomic fate
  of the area in which it is expressed.

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• http//7e.devbio.com/contents.php?sub1art1

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Vertebrate Development
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VERTEBRATE DEVELOPMENT

• In addition to differential gene expression, cell
  cell communication and cell movements are
  important in the development of the vertebrate
  embryo.
• Cells talk to neighbouring cells organise the
  differentiation of their neighbours.
• Cells migrate widely over the embryo.

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CELL MIGRATION

• Cells migrate towards diffusible chemical signals
  chemotaxis
• Along pathways of insoluble chemical - haptotaxis

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• Glycoproteins allow cells to adhere to each other
  and to the extracellular matrix