Semester 1 *

Question 1

Spemann organization center is a _______________ in the developing embryo of an ________ that induces development of _____.

Located in the _______, and can direct the differentiation of cells around it and __________________________

Answer 1

Spemann organization center: cluster of cells in the developing embryo of an amphibian that induces development of CNS.

Located in the dorsal lip (dorsal side of the lip of amphibian gastrula), and can direct the differentiation of cells around it and organize body structures & axes.

i.e. it acts not only as a natural inducer but also as an “organizer” of the entire body axis.

Question 2

The equivalent to the spemann organizer center center in mammals is the ____________.

Answer 2

The equivalent to the spemann organizer center center in mammals is the primitive node.

As the primitive streak elongates, migration of cells of the epiblast join the tip of the streak, and a dynamic mass of cells, called the primitive node, becomes evident at the tip of the primitive streak.

Question 3

Cells of the node express many genes, including three classic molecular markers of the organizer region in many vertebrates:

Answer 3

Chordin - a signalling molecule associated with the node. Involved with neural induction and expression of nodal on the left side of the embryo is a key element in the setting of left-right asymmetry. Also binds to BMP-4 to prevent BMP-4 from binding to ectoderm receptors, therefore instead of becoming skin cells, the ectodermal tissue in the area of the organizer become CNS tissue.

Goosecoid - encoded by GSC gene. It defines neural crest cell fate specification and contributes to dorsal-ventral patterning.

Hepatic nuclear factor-3𝞫 (HNF-3𝞫) - required for the formation of the node itself and the initiation of notochord function. If absent, both the notochord and the floor plate of the neural tube fails to form.

Question 4

Transcription factors are proteins that bind to _______ or ______ regions and interact to _______ or ______ the transcription of a particular gene.

Transcription factors have three major domains:

Answer 4

Transcription factors are proteins that bind to enhancer or promoter regions and interact to activate or repress the transcription of a particular gene.

Transcription factors have three major domains:

• DNA-binding domain
• Trans-activating domain
• Protein-protein interaction domain

Question 5

Transcription factors have three major domains:

• DNA-binding domain
• Trans-activating domain
• Protein-protein interaction domain

Describe each.

Answer 5

DNA-binding domain - recognizes a particular DNA sequence. There are several structural motifs present in the DNA binding domains:

• Leucine Zipper - the dimeric Y shaped protein contains two extended alpha-helices that “grip” the DNA at two adjacent major grooves separated by about half a turn of the double helix. The motif contains the hydrophobic amino acid leucine at every seventh position in the sequence.
• Helix turn Helix (HTH) - two alpha helices, one of them is called a recognition helix. Is present in many bacterial repressor proteins.
• Zinc finger - DNA recognition, transcriptional activation.
• Helix loop helix (HLH) - two alpha helices connected by a loop.

Trans-activating domain - activates or suppresses the transcription of the gene whose promoter or enhancer it has bound.

Protein-protein interaction domain - allows the transcription factor’s activity to be modulated by other regulatory proteins.

Question 6

HOX genes a group of related genes that control ________ of an embryo along the ________ axis.

After the embryonic segments have formed, the _________ determine the type of segment structures that will form on a given segment.

Answer 6

HOX genes: a group of related genes that control the body plan of an embryo along the cranio-caudal axis.

After the embryonic segments have formed, the HOX proteins determine the type of segment structures that will form on a given segment.

Question 7

Signaling molecules: the same signalling molecules can be used at many different times and places as the embryo takes shape.

Locally controlled factors such as ____________ or _______ of exposure to a signalling molecule are often important determinants of the fate of a group of responding cells.

Answer 7

Signaling molecules: the same signalling molecules can be used at many different times and places as the embryo takes shape.

Locally controlled factors such as concentration or duration of exposure to a signalling molecule are often important determinants of the fate of a group of responding cells.

Question 8

The signalling pathway is:

Answer 8

Ligand → receptor → signal transduction → gene expression changes (induced by the signal).

Question 9

Give some examples of signalling molecules

Answer 9

TGF-beta family members are involved in dorso-ventral patterning and the formation of some organs

• Activin - mesodermal induction
• Inhibin - inhibition of gonadotropin secretion from hypophysis.

Sonic Hedgehog (Shh) - essential to early patterning in the developing embryo.

Retinoic acid - metabolite of vitamin A, is used to stimulate the growth of the posterior end of the organism. It binds to retinoic acid receptors that act as transcription factors to regulate the expression of HOX genes.

Question 10

Epigenetics is the study of ________________ that does not involve ___________________________ -

it is a change in the ________ without a change in the _________, they do not involve a change in the ______________.

Answer 10

Epigenetics is the study of changes in gene expression that does not involve changes to the underlying DNA sequence -

it is a change in the phenotype without a change in the genotype, they do not involve a change in the nucleotide sequence.

Question 11

DNA methylation - a process by which ________ are added to DNA segments. This changes the activity of a DNA segment without changing the sequence. When located in gene promoter, it typically acts to ____________________. DNA methylation is essential for ______________________ as is associated with key processes e.g. ___________________ and __________________

Answer 11

DNA methylation - a process by which methyl groups are added to DNA segments. This changes the activity of a DNA segment without changing the sequence. When located in gene promoter, it typically acts to repress gene transcription. DNA methylation is essential for normal development as is associated with key processes e.g. Genomic imprinting and X-chromosome inactivation

Question 12

Genomic imprinting - a phenomenon by which certain genes are expressed in a ________________ manner. Therefore, if the allele inherited from the father is imprinted, it is silenced and only the allele from the mother is expressed and vice versa.

These epigenetic marks are established/imprinted in the _________ of the _________ and are maintained through mitotic cell divisions in the somatic cells of an organism.

Answer 12

Genomic imprinting - a phenomenon by which certain genes are expressed in a parent-of-origin specific manner. Therefore, if the allele inherited from the father is imprinted, it is silenced and only the allele from the mother is expressed and vice versa.

These epigenetic marks are established/imprinted in the germline of the parents and are maintained through mitotic cell divisions in the somatic cells of an organism.

Question 13

Embryonic stem cells - found in ________.

ES cells are __________, that is, they can differentiate into all derivatives of the three primary germ layers: ectoderm, endoderm and mesoderm.

Pluripotency distinguishes embryonic stem cell from adult stem cells (found in adults); while embryonic stem cells can generate all cell types in the body, adult stem cells are ________ and can only produce a limited number of cell types.

Answer 13

Embryonic stem cells - found in blastocyst.

ES cells are pluripotent, that is, they can differentiate into all derivatives of the three primary germ layers: ectoderm, endoderm and mesoderm.

Pluripotency distinguishes embryonic stem cell from adult stem cells (found in adults); while embryonic stem cells can generate all cell types in the body, adult stem cells are multipotent and can only produce a limited number of cell types.

Question 14

Under defined conditions, embryonic stem cells can propagate themselves indefinitely.

This allows embryonic stem cells to be employed as useful tools for both _________ and _________ ________ because they produce limitless numbers of themselves for continued _________ or _______ use.

Because of their ________ and potentially _______ _______ for self-renewal, ES cell therapies have been proposed for regenerative medicine and tissue replacement after injury or disease.

Other potential uses of embryonic stem cells include:

Answer 14

Under defined conditions, embryonic stem cells can propagate themselves indefinitely.

This allows embryonic stem cells to be employed as useful tools for both research and regenerative medicine because they produce limitless numbers of themselves for continued research or clinical use.

Because of their plasticity and potentially unlimited capacity for self-renewal, ES cell therapies have been proposed for regenerative medicine and tissue replacement after injury or disease.

Other potential uses of embryonic stem cells include investigation of early human development, study of genetic disease and as in vitro systems for toxicology testing.

Question 15

Induced pluripotent stem cells: a type of ________ stem cell artificially derived from a ___________, typically an ________ cell, by inducing a “forced” expression of specific genes. iPSC are like natural pluripotent stem cells, such as embryonic stem cells.

Answer 15

Induced pluripotent stem cells: a type of pluripotent stem cell artificially derived from a non-pluripotent cell, typically an adult somatic cell, by inducing a “forced” expression of specific genes. iPSC are like natural pluripotent stem cells, such as embryonic stem cells.

Regeneration?

Question 16

List the Functions of ECM

Answer 16

• Functions as adhesive substrate
• Provides structure
• Presents growth factors to their receptors
• Sequesters and stores growth factors
• Senses and transduces mechanical signals

Question 17

Basement membranes are ________ ____________ _________ that coat the ______ aspect of _______ and _______ cells and surround ________, _______ and _______ cells.

They are first expressed in _____ ____________ and are _________ on competent surfaces through binding interactions among _______, type ___ _______, _________ and _________.

Answer 17

Basement membranes are distributed extracellular matrices that coat the basal aspect of epithelial and endothelial cells and surround muscle, fat and Schwann cells.

They are first expressed in early embryogenesis and are self-assembled on competent surfaces through binding interactions among laminins, type IV collagens, nidogens and proteoglycans.

Question 18

The basal lamina has several functions and can be modified for specialised functions.

During development, it guides migrating ________ cells to their destinations, which is important during early development.

Basement membranes play a role in _______ and _______ morphogenesis and help ________ function in adults.

All basement membranes contain:

Answer 18

The basal lamina has several functions and can be modified for specialised functions.

During development, it guides migrating primordial cells to their destinations, which is important during early development.

Basement membranes play a role in tissue and organ morphogenesis and help maintain function in adults.

All basement membranes contain laminin, type IV collagen, Nidogens and proteoglycans.

Question 19

Explanation of the role of basal membrane in cell migration, branching of epithelia using embryonic development of salivary glands as an example:

• Epithelial cells of developing embryonic organs, such as salivary glands, can display _______ _______ during branching morphogenesis.
• ______ of individual salivary gland epithelial cells differs by ______ during branching morphogenesis.
• Migration speed is _______ in outer bud cells close to the basement membrane and ______ in inner bud and duct cells.
• ________ interactions are necessary for both motility and morphology of the outer bud cells, which are in contact with the basement membrane.
• ________, but not E-cadherin, is needed for outer bud cell migration.
• These findings identify crucial roles for epithelial cell interactions with the basement membrane for both ________ and _____ __________.

The ___________________ that occurs transiently during salivary gland branching morphogenesis is developmentally regulated, and it ceases when glands mature to form the stable epithelial cell-cell adhesions characteristic of adult organisms in fact, the cell-cell adhesion complexes that comprise the classical adherens, tight and desmosome-based junctions of very early epithelia are lost when oral epithelial cells undergo branching morphogenesis to form buds.

Answer 19

Explanation of the role of basal membrane in cell migration, branching of epithelia using embryonic development of salivary glands as an example:

• Epithelial cells of developing embryonic organs, such as salivary glands, can display substantial motility during branching morphogenesis.
• Motility of individual salivary gland epithelial cells differs by region during branching morphogenesis.
• Migration speed is highest in outer bud cells close to the basement membrane and low in inner bud and duct cells.
• Integrin interactions are necessary for both motility and morphology of the outer bud cells, which are in contact with the basement membrane.
• Myosin II, but not E-cadherin, is needed for outer bud cell migration.
• These findings identify crucial roles for epithelial cell interactions with the basement membrane for both high motility and tissue organization.

The epithelial cell motility that occurs transiently during salivary gland branching morphogenesis is developmentally regulated, and it ceases when glands mature to form the stable epithelial cell-cell adhesions characteristic of adult organisms in fact, the cell-cell adhesion complexes that comprise the classical adherens, tight and desmosome-based junctions of very early epithelia are lost when oral epithelial cells undergo branching morphogenesis to form buds.

Question 20

Germ cells are any biological cells that _________________ of an organism that reproduces sexually.

The germ cells originate in the ______ _______ and migrate via the _____ of an embryo to the developing gonads. There, they undergo _______, followed by cellular differentiation into mature gametes.

In mammals, germ cell specification begins in the ________ during _________, and after transport which involves passive movements and active migration, germ cells arrive at the developing gonads.

Answer 20

Germ cells are any biological cells that give rise to gametes of an organism that reproduces sexually.

The germ cells originate in the primitive streak and migrate via the gut of an embryo to the developing gonads. There, they undergo meiosis, followed by cellular differentiation into mature gametes.

In mammals, germ cell specification begins in the epiblast during gastrulation, and after transport which involves passive movements and active migration, germ cells arrive at the developing gonads.

Question 21

There are two mechanisms to establish the germ cell lineage in an embryo.

• Pre-formistic: involves that the cells destined to become germ cells inherit the specific germ cell determinants present in the germ plasm (specific area of the cytoplasm) of the ovum. The unfertilised egg of most animals is asymmetrical – different regions of the cytoplasm contain different amounts of mRNA and proteins. Hence, germ cells obtained by the first divisions of the fertilised egg are characterised by specific molecules of a particular region of the egg cytoplasm.
• Zygotic gene control: Found in birds and mammals, where germ cells are not specified by such determinants, but by signals controlled by zygotic genes. In mammals, a few cells of the early embryo are induced by signals of neighbouring cells to become primordial germ cells. Mammalian eggs are somewhat symmetrical and after the first divisions of the fertilized egg, the produced cells are all totipotent. This means that they can differentiate in any cell type in the body and thus germ cells.

Answer 21

There are two mechanisms to establish the germ cell lineage in an embryo.

• Pre-formistic: involves that the cells destined to become germ cells inherit the specific germ cell determinants present in the germ plasm (specific area of the cytoplasm) of the ovum. The unfertilised egg of most animals is asymmetrical – different regions of the cytoplasm contain different amounts of mRNA and proteins. Hence, germ cells obtained by the first divisions of the fertilised egg are characterised by specific molecules of a particular region of the egg cytoplasm.
• Zygotic gene control: Found in birds and mammals, where germ cells are not specified by such determinants, but by signals controlled by zygotic genes. In mammals, a few cells of the early embryo are induced by signals of neighbouring cells to become primordial germ cells. Mammalian eggs are somewhat symmetrical and after the first divisions of the fertilized egg, the produced cells are all totipotent. This means that they can differentiate in any cell type in the body and thus germ cells.

Question 22

Migration: _______ ______ _____, germ cells that still have to reach the gonads, divide repeatedly on their migratory route through the ____ and into the developing gonad. In mammals, migration begins with ___ gonocytes and approximately ____ PCG’s arrive at the gonads.

Proliferation also occurs during migration and lasts for ___ weeks in humans. PGCs come from the _______ and migrate subsequently into the ________, the _________ and the posterior of the _______.

Migration then takes place from the ______ along the gut and across the _____________ to reach the gonads (4.5 weeks in human beings).

Fibronectin maps here also a polarized network together with other molecules. The somatic cells on the path of germ cells provide them attractive, repulsive, and survival signals, but germ cells also send signals to each other.

Answer 22

Migration: Primordial germ cells, germ cells that still have to reach the gonads, divide repeatedly on their migratory route through the gut and into the developing gonad. In mammals, migration begins with 50 gonocytes and approximately 5000 PCG’s arrive at the gonads.

Proliferation also occurs during migration and lasts for 3-4 weeks in humans. PGCs come from the epiblast and migrate subsequently into the mesoderm, the endoderm and the posterior of the yolk sac.

Migration then takes place from the hindgut along the gut and across the dorsal mesentery to reach the gonads (4.5 weeks in human beings).

Fibronectin maps here also a polarized network together with other molecules. The somatic cells on the path of germ cells provide them attractive, repulsive, and survival signals, but germ cells also send signals to each other.

Question 23

The SRY (Sex-determining Region of the Y chromosome) directs ____ development in mammals by inducing the somatic cells of the _______ _______ to develop into a testis, rather than an ovary. Sry is expressed in a small group of somatic cells of the gonads and influences these cells to become Sertoli cells (supporting cells in testis). _____ _______ are responsible for sexual development along a male pathway in many ways.

One of these ways involves stimulation of the arriving primordial cells to differentiate into sperm.

In the absence of the ____ ______, primordial germ cells differentiate into ______.

Answer 23

The SRY (Sex-determining Region of the Y chromosome) directs male development in mammals by inducing the somatic cells of the gonadal ridge to develop into a testis, rather than an ovary. Sry is expressed in a small group of somatic cells of the gonads and influences these cells to become Sertoli cells (supporting cells in testis). Sertoli cells are responsible for sexual development along a male pathway in many ways.

One of these ways involves stimulation of the arriving primordial cells to differentiate into sperm.

In the absence of the Sry gene, primordial germ cells differentiate into eggs.

Question 24

Development: Gametogenesis, the development of ______ germ cells into either ______ _____ or _____, is different for each species but the general stages are similar.

Oogenesis and spermatogenesis have many features in common, they both involve:

• _______
• ________
• ________

Despite their homologies they also have major differences:

• __________
• __________

Answer 24

Development: Gametogenesis, the development of diploid germ cells into either haploid eggs or sperm, (respectively oogenesis and spermatogenesis) is different for each species but the general stages are similar.

Oogenesis and spermatogenesis have many features in common, they both involve:

• Meiosis
• Extensive morphological differentiation
• Incapacity of surviving for very long if fertilization does not occur

Despite their homologies they also have major differences:

• Spermatogenesis has equivalent meiotic divisions resulting in four equivalent spermatids while oogenic meiosis is asymmetrical: only one egg is formed together with three polar bodies.
• Different timing of maturation: oogenic meiosis is interrupted at one or more stages (for a long time) while spermatogenic meiosis is rapid and uninterrupted.

Question 25

Gastrulation: is an early phase in embryonic development, during which the single layered _______ is reorganised into a ______ structure known as the ______. These three layers are known as the ectoderm, mesoderm and endoderm.

Answer 25

Gastrulation: is an early phase in embryonic development, during which the single layered blastula is reorganised into a trilaminar structure known as the gastrula. These three layers are known as the ectoderm, mesoderm and endoderm.

Question 26

What are the stages of gastrulation are?

Answer 26

The stages of gastrulation are:

1. Loss of symmetry: In preparation for gastrulation, the embryo must become asymmetric along both the proximal-distal axis and the anterior-posterior axis. Many signalling pathways contribute to this reorganisation, including BMP, FGF, nodal and Wnt.
2. Formation of the Primitive Streak: is reliant upon nodal signalling within the cells contributing to the primitive streak, and BMP4 signalling from the extraembryonic tissue. Furthermore, Cer1 and Lefty1 restrict the primitive streak to the appropriate location.
3. Epithelial to Mesenchymal transition and ingression: needed for the cells to move from the epithelium of the epilblast through the primitive streak to form a new layer.
   
   • FGF signalling is necessary for proper EMT.
   • FGFR1 is needed for the up-regulation of Snai1, which down-regulates E-cadherin, causing a loss of cell adhesion.
   • Following the EMT, the cells ingress through the primitive streak and spread out to form a new layer of cells or join existing layers.
   • FGF8 is implicated in the process of this dispersal from the primitive streak.

Question 27

Following gastrulation, each layer gives rise to specific tissues and organs in the developing embryo:

Ectoderm – epidermis structures, neural crest and neural tissues which give rise to the nervous system.

Mesoderm – somites, notochord, blood and blood vessels, bone and connective tissue.

Endoderm - epithelium of the digestive system and respiratory system, organs associated with the digestive system.

Answer 27

Ectoderm –

• epidermis structures, neural crest and neural tissues which give rise to the nervous system.

Mesoderm –

• somites, notochord, blood and blood vessels, bone and connective tissue.

Endoderm -

• epithelium of the digestive system and respiratory system, organs associated with the digestive system.

Question 28

All vertebrates share a conserved body plan. In the anterior-posterior (A-P) axis they form head, trunk, and tail, and in the dorsal-ventral (D-V) axis the backbone and belly of the animal.

During development:

• _________ gives rise to neural plate and eventually to the entire central nervous system (CNS).
• __________ gives rise to epidermis and its derivatives.
• ________ differentiates–from dorsal to ventral— into prechordal plate and notochord, somite (which forms skeletal muscle, vertebral column, and dermis), kidney, lateral plate mesoderm, and ventral blood islands

Patterning of the vertebrate anteroposterior (AP) axis proceeds temporally from anterior to posterior. How dorsoventral (DV) axial patterning relates to AP temporal patterning is unknown.

• D-V patterning starts under the influence of a dorsal nuclear ______ signal
• At mid-blastula two signaling centers are present on the dorsal side:
  
  1. The prospective neuroectoderm expresses _____________________ antagonists
  2. The future dorsal endoderm secretes _______________________

Answer 28

All vertebrates share a conserved body plan. In the anterior-posterior (A-P) axis they form head, trunk, and tail, and in the dorsal-ventral (D-V) axis the backbone and belly of the animal.

During development:

• Dorsal ectoderm gives rise to neural plate and eventually to the entire central nervous system (CNS).
• Ventral ectoderm gives rise to epidermis and its derivatives.
• Mesoderm differentiates–from dorsal to ventral— into prechordal plate and notochord, somite (which forms skeletal muscle, vertebral column, and dermis), kidney, lateral plate mesoderm, and ventral blood islands

Patterning of the vertebrate anteroposterior (AP) axis proceeds temporally from anterior to posterior. How dorsoventral (DV) axial patterning relates to AP temporal patterning is unknown.

• D-V patterning starts under the influence of a dorsal nuclear β-Catenin signal
• At mid-blastula two signaling centers are present on the dorsal side:
  
  1. The prospective neuroectoderm expresses bone morphogenetic protein (BMP) antagonists
  2. The future dorsal endoderm secretes Nodal-related mesoderm-inducing factors

Question 29

When dorsal mesoderm is formed at gastrula, a cocktail of growth factor antagonists is secreted by the Spemann organizer and further patterns the embryo.

A ventral gastrula signaling center opposes the actions of the dorsal organizer, and another set of secreted antagonists is produced ventrally under the control of BMP4. BMP signaling patterns rostral DV cell fates at the onset of gastrulation, while progressively more caudal DV cell fates are patterned at progressively later intervals during gastrulation. Increased BMP signal duration is not required to pattern more caudal DV cell fates, rather distinct temporal intervals of signaling are required. This progressive action is regulated downstream of, or in parallel to BMP signal transduction at the level of Smad1/5 phosphorylation. A temporal cue regulates a cell’s competence to respond to BMP signaling, allowing the acquisition of a cell’s DV and AP identity simultaneously. A gradient of BMP signaling specifies cell fates along the DV axis, with the highest levels specifying the ventral-most tissues, while increasing BMP inhibition leads to more lateral tissues and an absence of BMP signaling to dorsal tissues in the ectoderm and mesoderm.

Answer 29

When dorsal mesoderm is formed at gastrula, a cocktail of growth factor antagonists is secreted by the Spemann organizer and further patterns the embryo.

A ventral gastrula signaling center opposes the actions of the dorsal organizer, and another set of secreted antagonists is produced ventrally under the control of BMP4. BMP signaling patterns rostral DV cell fates at the onset of gastrulation, while progressively more caudal DV cell fates are patterned at progressively later intervals during gastrulation. Increased BMP signal duration is not required to pattern more caudal DV cell fates, rather distinct temporal intervals of signaling are required. This progressive action is regulated downstream of, or in parallel to BMP signal transduction at the level of Smad1/5 phosphorylation. A temporal cue regulates a cell’s competence to respond to BMP signaling, allowing the acquisition of a cell’s DV and AP identity simultaneously. A gradient of BMP signaling specifies cell fates along the DV axis, with the highest levels specifying the ventral-most tissues, while increasing BMP inhibition leads to more lateral tissues and an absence of BMP signaling to dorsal tissues in the ectoderm and mesoderm.

Question 30

Patterning of cell fates along the orthogonal anteroposterior (AP) axis occurs in a clear temporal fashion from anterior to posterior.

Ventrolateral ectodermal and mesodermal cell fates are not patterned simultaneously over the entire AP extent of the embryo.

Rather, BMP signaling acts in a temporally progressive manner, required earliest in anterior ventrolateral cells and subsequently in progressively more posterior ventrolateral cells.

Cells along the AP axis in the head and trunk respond to the BMP signaling gradient during progressively later, distinct critical intervals, a mechanism allowing cells to adopt both an AP and DV identity simultaneously, thus coordinating patterning of these two axes.

Answer 30

Patterning of cell fates along the orthogonal anteroposterior (AP) axis occurs in a clear temporal fashion from anterior to posterior.

Ventrolateral ectodermal and mesodermal cell fates are not patterned simultaneously over the entire AP extent of the embryo.

Rather, BMP signaling acts in a temporally progressive manner, required earliest in anterior ventrolateral cells and subsequently in progressively more posterior ventrolateral cells.

Cells along the AP axis in the head and trunk respond to the BMP signaling gradient during progressively later, distinct critical intervals, a mechanism allowing cells to adopt both an AP and DV identity simultaneously, thus coordinating patterning of these two axes.