MOLECULAR EMBRYOLOGY 3

Activin as a morphogen

Morphogen is a diffusible molecule controlling the differentiation of other cells in a dosage-dependent manner

Gurdon experiment: Beads releasing activin were surrounded by Xenopus animal cup cells. No activin induces the cells to become ectoderm. Low Activin activates Brachyury and the surrounding cells became Ventral Mesoderm. High levels of Activin induces Goosecoid and the cells became Dorsal Mesoderm

Dorsal-Ventral Axis in the Neural Tube

The notochord produces Shh and induces the neural tube to become floor plate and to produce Shh The ectodermal cells produce BMP4, 7, 5, Dorsalin and Activin and induce the dorsal neural tube to become roof plate and to start to produce the same proteinsTwo gradients are created the TGF-B family and the Shh gradientsDifferent concentrations of these proteins activate the expression of different sets of genesThese genes are responsible for the different cell fate of the neural cells along the D/V axis in sensory neurons, interneurons and motorneurons

Neural crest cells are formed at the Neural crest cells are formed at the juxtaposition between the prospective neural juxtaposition between the prospective neural plate and the ectodermplate and the ectoderm

They migrate to different locations They migrate to different locations

They originate many cell types such as neurons They originate many cell types such as neurons and glia of the sensory, sympathetic, and glia of the sensory, sympathetic, parasympathetic nervous system and the parasympathetic nervous system and the pigment containing cells of the epidermispigment containing cells of the epidermis

Neural crest cells are formed at the Neural crest cells are formed at the juxtaposition between the prospective neural juxtaposition between the prospective neural plate and the ectodermplate and the ectoderm

They migrate to different locations They migrate to different locations

They originate many cell types such as neurons They originate many cell types such as neurons and glia of the sensory, sympathetic, and glia of the sensory, sympathetic, parasympathetic nervous system and the parasympathetic nervous system and the pigment containing cells of the epidermispigment containing cells of the epidermis

Neural Crest CellsNeural Crest Cells

FoxD3, Slug

Wnt6, ectoderm

BMPs

Neural Crest Cell Induction

Slug activates factors inducing the Slug activates factors inducing the dissociation of tight junctionsdissociation of tight junctions

Migrating cells follow clues from the EM Migrating cells follow clues from the EM One set of proteins (fibronectin, laminin) One set of proteins (fibronectin, laminin)

promote migration while ephrins impede promote migration while ephrins impede migrationmigration

Slug activates factors inducing the Slug activates factors inducing the dissociation of tight junctionsdissociation of tight junctions

Migrating cells follow clues from the EM Migrating cells follow clues from the EM One set of proteins (fibronectin, laminin) One set of proteins (fibronectin, laminin)

promote migration while ephrins impede promote migration while ephrins impede migrationmigration

Neural Crest Cell MigrationNeural Crest Cell Migration

Induction means that a cell or a group of Induction means that a cell or a group of cells secrete a substance that can affect cells secrete a substance that can affect the fate and the behavior of neighboring the fate and the behavior of neighboring cellscells

Membrane proteins on one cell interact Membrane proteins on one cell interact with receptor proteins on adjacent cell with receptor proteins on adjacent cell surfaces (justacrine interactions)surfaces (justacrine interactions)

Proteins synthesized by one cell diffuse over a small distance to induce changes in neighbouring cells (paracrine interaction)

Induction means that a cell or a group of Induction means that a cell or a group of cells secrete a substance that can affect cells secrete a substance that can affect the fate and the behavior of neighboring the fate and the behavior of neighboring cellscells

Membrane proteins on one cell interact Membrane proteins on one cell interact with receptor proteins on adjacent cell with receptor proteins on adjacent cell surfaces (justacrine interactions)surfaces (justacrine interactions)

Proteins synthesized by one cell diffuse over a small distance to induce changes in neighbouring cells (paracrine interaction)

Induction during DevelopmentInduction during Development

Paracrine Interaction

Paracrine Factors

Proteins synthesized by one cell diffuse over a small distance to induce changes in neighbouring cells.

Embryos have a complex tool kit of paracrine pathways: the FGF, the Wnt, the Hedgehog families and the TGF-β superfamily

NUCLEUS-TRANSCRIPTION

A Paracrine Pathway

When a ligand (paracrine factor) binds its receptor induces a conformational change in its structureThis in turn uncovers a Kinase activity The active receptor phosphorilates downstream proteinsThe cascade of phosphorilations activates a transcription factor that activates or represses a set of target genes

Activation of the Casein Gene during Milk Production

Prolactin binds to its receptors causing them to dimerize

Two Jack2 proteins are brought together and they can phosphorilate the receptors

The active receptor phosphorilates inactive STAT proteins and cause them to dimerize

STAT dimers move to the nucleus and they activate the expression of the casein gene

The Hh PathwayIn the absence of Hh, the Ci protein is tethered to the MTs by the Cos2 and Fused proteins

This binding allow PKA and Slimb to cleave Ci into a transcriptional repressor

Ci enters the nucleus and it represses Hh-responsive genes

In the absence of Hh, Patched inhibits Smoothened

The Hh Pathway

When Hh binds to its receptor Patched, induces a conformational change in the receptor

In so doing, it releases the inhibition of Smoothened protein and Smoothened inactivates the cleavage proteins, PKA and Slimb This also releases Ci from the MTs probably because of the additon of more phosphate groups to Fused and Cos2

Ci enters the nucleus, binds to CBP, a co-activator, and activates the transcription of Hh-target genes

The Shh pathway is important in vertebrate The Shh pathway is important in vertebrate limb development and neural differentiation limb development and neural differentiation

Mice mutants for Shh pathway have major Mice mutants for Shh pathway have major limb abnormalities as well as Cyclopia, a limb abnormalities as well as Cyclopia, a single eye in the middle of the foreheadsingle eye in the middle of the forehead

Cholesterol binds to the N-terminus of the Cholesterol binds to the N-terminus of the Shh protein and allows this paracrine factors Shh protein and allows this paracrine factors to diffuse over a range of few micrometersto diffuse over a range of few micrometers

The Shh pathway is important in vertebrate The Shh pathway is important in vertebrate limb development and neural differentiation limb development and neural differentiation

Mice mutants for Shh pathway have major Mice mutants for Shh pathway have major limb abnormalities as well as Cyclopia, a limb abnormalities as well as Cyclopia, a single eye in the middle of the foreheadsingle eye in the middle of the forehead

Cholesterol binds to the N-terminus of the Cholesterol binds to the N-terminus of the Shh protein and allows this paracrine factors Shh protein and allows this paracrine factors to diffuse over a range of few micrometersto diffuse over a range of few micrometers

The Sonic Hedgehog PathwayThe Sonic Hedgehog Pathway

“The amazing thing about development is not that

sometimes it goes wrong, but that it ever succeeds”

Teratology is the branch of embryology Teratology is the branch of embryology and pathology that deals with abnormal and pathology that deals with abnormal development and congenital (present at development and congenital (present at birth) malformationsbirth) malformations

Congenital abnormalities can have Congenital abnormalities can have intrinsic and extrinsic causesintrinsic and extrinsic causes

Abnormalities caused by genetic events Abnormalities caused by genetic events (mutations, aneuploidies, translocations) (mutations, aneuploidies, translocations) are called malformations.are called malformations.

Teratology is the branch of embryology Teratology is the branch of embryology and pathology that deals with abnormal and pathology that deals with abnormal development and congenital (present at development and congenital (present at birth) malformationsbirth) malformations

Congenital abnormalities can have Congenital abnormalities can have intrinsic and extrinsic causesintrinsic and extrinsic causes

Abnormalities caused by genetic events Abnormalities caused by genetic events (mutations, aneuploidies, translocations) (mutations, aneuploidies, translocations) are called malformations.are called malformations.

TeratologyTeratology

Our molecular understanding of Our molecular understanding of mammalian development has expanded mammalian development has expanded exponentially over the last few years exponentially over the last few years Congenital abnormalities can have Congenital abnormalities can have intrinsic and extrinsic causesintrinsic and extrinsic causes

It is now possible to link a malformation It is now possible to link a malformation to a genetic defect to a genetic defect

This raises the possibility of finding a This raises the possibility of finding a therapytherapy

Our molecular understanding of Our molecular understanding of mammalian development has expanded mammalian development has expanded exponentially over the last few years exponentially over the last few years Congenital abnormalities can have Congenital abnormalities can have intrinsic and extrinsic causesintrinsic and extrinsic causes

It is now possible to link a malformation It is now possible to link a malformation to a genetic defect to a genetic defect

This raises the possibility of finding a This raises the possibility of finding a therapytherapy

TeratologyTeratology

Teratogenesis and Shh

This lamb has the Cyclopia phenotype as the ewe has eaten a plant Veratrum californicum

This plant contains chemical products such as jervine and cyclopamine that can block cholesterol synthesis

This blocks the Shh pathway and causes Cyclopia phenotype

Holoprosencephaly is a human disease where humans are born with the Cyclopia phenotypePatients affected by this disease have mutations in the Shh gene

Teratogens ( Greek, monster-formers) are Teratogens ( Greek, monster-formers) are environmental factors that can disrupt environmental factors that can disrupt developmentdevelopment

Most teratogens produce their effects Most teratogens produce their effects during critical periods of development during critical periods of development

Teratogenic agents can be naturally Teratogenic agents can be naturally present in the environment or they can be present in the environment or they can be artificial compoundsartificial compounds

Teratogens ( Greek, monster-formers) are Teratogens ( Greek, monster-formers) are environmental factors that can disrupt environmental factors that can disrupt developmentdevelopment

Most teratogens produce their effects Most teratogens produce their effects during critical periods of development during critical periods of development

Teratogenic agents can be naturally Teratogenic agents can be naturally present in the environment or they can be present in the environment or they can be artificial compoundsartificial compounds

Teratogenesis and EnvironmentTeratogenesis and Environment

Heavy methals (Zinc, lead and mercury) are Heavy methals (Zinc, lead and mercury) are potent teratogenspotent teratogens

Lead and mercury damage the developing Lead and mercury damage the developing nervous system (small brains and eyes)nervous system (small brains and eyes)

In highly industrialized societies heavy In highly industrialized societies heavy metals are found in drinking water and metals are found in drinking water and vegetables. Mercury can be transmitted vegetables. Mercury can be transmitted through the placenta and the mother’s milkthrough the placenta and the mother’s milk

Heavy methals (Zinc, lead and mercury) are Heavy methals (Zinc, lead and mercury) are potent teratogenspotent teratogens

Lead and mercury damage the developing Lead and mercury damage the developing nervous system (small brains and eyes)nervous system (small brains and eyes)

In highly industrialized societies heavy In highly industrialized societies heavy metals are found in drinking water and metals are found in drinking water and vegetables. Mercury can be transmitted vegetables. Mercury can be transmitted through the placenta and the mother’s milkthrough the placenta and the mother’s milk

Teratogenesis and EnvironmentTeratogenesis and Environment

RA can become a teratogen if present in large RA can become a teratogen if present in large amounts or at particular times during amounts or at particular times during developmentdevelopment

It is a secreted molecule involved in A/P axis It is a secreted molecule involved in A/P axis formation in mammals and in forming the jawsformation in mammals and in forming the jaws

It affects Hox gene expression in A/P axis It affects Hox gene expression in A/P axis determination and it inhibits neural crest cell determination and it inhibits neural crest cell migration from the cranial region of the neural migration from the cranial region of the neural tubetube

RA can become a teratogen if present in large RA can become a teratogen if present in large amounts or at particular times during amounts or at particular times during developmentdevelopment

It is a secreted molecule involved in A/P axis It is a secreted molecule involved in A/P axis formation in mammals and in forming the jawsformation in mammals and in forming the jaws

It affects Hox gene expression in A/P axis It affects Hox gene expression in A/P axis determination and it inhibits neural crest cell determination and it inhibits neural crest cell migration from the cranial region of the neural migration from the cranial region of the neural tubetube

Retinoic acid as a teratogenRetinoic acid as a teratogen

RA as a Teratogen

RA causes a characteristic pattern of anomalies: extra or missing limbs or misplaced eyes, deformed jaws and heart and gut malformations

In the US methoprenoic acid used to fight mosquitos induced RA-like pattern of malformations in frogs. In the picture a frog and a tadpole (B) with eye abnormalities. MA is structurally similar to RA

TF is a type of male pseudo-hermafroditism TF is a type of male pseudo-hermafroditism characterized by female external genitalia characterized by female external genitalia incompletely developed and testes present incompletely developed and testes present within the abdomen within the abdomen

Androgens and estrogens are formed but Androgens and estrogens are formed but target tissues are unresponsive to target tissues are unresponsive to androgens androgens

It is due to mutations in the androgen It is due to mutations in the androgen receptor gene on the chromosome Xreceptor gene on the chromosome X

TF is a type of male pseudo-hermafroditism TF is a type of male pseudo-hermafroditism characterized by female external genitalia characterized by female external genitalia incompletely developed and testes present incompletely developed and testes present within the abdomen within the abdomen

Androgens and estrogens are formed but Androgens and estrogens are formed but target tissues are unresponsive to target tissues are unresponsive to androgens androgens

It is due to mutations in the androgen It is due to mutations in the androgen receptor gene on the chromosome Xreceptor gene on the chromosome X

Testicular FeminizationTesticular Feminization

Benzimidazole compounds are used as Benzimidazole compounds are used as anthelmintics to cure parasite infections anthelmintics to cure parasite infections of the intestineof the intestine

They are teratogens in domestic and They are teratogens in domestic and experimental animalsexperimental animals

Many embryos will be aborted and the Many embryos will be aborted and the survivors exhibit skeletal, muscle and survivors exhibit skeletal, muscle and renal malformationsrenal malformations

Benzimidazole compounds are used as Benzimidazole compounds are used as anthelmintics to cure parasite infections anthelmintics to cure parasite infections of the intestineof the intestine

They are teratogens in domestic and They are teratogens in domestic and experimental animalsexperimental animals

Many embryos will be aborted and the Many embryos will be aborted and the survivors exhibit skeletal, muscle and survivors exhibit skeletal, muscle and renal malformationsrenal malformations

Anthelmintics as TeratogensAnthelmintics as Teratogens

FreemartinismIt is one of the most severe forms of sexual abnormalities in cattle. When a female is twin with a male they share the placental membrane connecting the foetuses with the mother A joining of the placental membranes occurs at fortieth day of pregnancy and the fluids of the foetuses are mixed

When hormones are mixed, every twin develops with characteristics of the other sex

The male exhibit only reduced fertility while the female is completely infertile. A freemartin is genetically a female but it has an abnormal reproductive anatomy with female external genitalia while internal genitalia consist of a variable array of phenotypically male organs