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 tissuei.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 haemoglobinSPATIAL & 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

bicoid mRNA

The importance of the eggWithin 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)

nanos mRNA

Distribution of proteins after fertilisation

Fertilisation stimulates the translation of bicoid and nanos mRNAs

The proteins diffuse

Set up a concentration gradient

bicoid

nanos

bicoid mRNA

nanos mRNA

Egg Egg

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 EXPRESSIONBrief 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 expressed

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.

SummaryMaternal 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