Download Development of the embryo - Hyndland Secondary School

Embryonic development
Ovum
Fertilised ovum
Cell Division
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?
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.
Dolly the sheep
Differential gene
expression
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
Differential gene
expression
Gene expression is regulated by

PROMOTERS & INHIBITORS
(transcription factors)
Bind to regulatory sites near the genes
and control transcription
Animation
Differential gene
expression
During development need to ensure
correct promoters and inhibitors are
present
Studied in drosophila
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)
bicoid mRNA
nanos mRNA
Distribution of proteins
after fertilisation
bicoid mRNA
bicoid
nanos
mRNA
nanos
Egg
Egg
Fertilisation
stimulates the
translation of bicoid
and nanos mRNAs
The proteins diffuse
Set up a
concentration
gradient
First cell division
More bicoid than
nanos protein
More nanos than
bicoid protein
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.
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.
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
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).
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
Homeotic gene expression determines
ultimate function of segment
Bithorax mutant
Mutant bithorax gene(s)
Inappropriately expressed
Antennapodia complex mutant
Mutant antennapedia complex
gene(s) inappropriately
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.
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.
http://7e.devbio.com/contents
.php?sub=1&art=1
Vertebrate Development
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.
CELL MIGRATION
Cells migrate
towards diffusible
chemical signals –
chemotaxis
Along pathways of
insoluble chemical
- haptotaxis
Glycoproteins allow cells to adhere to
each other and to the extracellular
matrix