Embryology

Question 1

What type of cell forms all embryonic and extra embryonic cell types?

Answer 1

Totipotent cells

Question 2

What type of cell only forms embryonic cells types?

Answer 2

Pluripotent

Question 3

What is a multipotent cell?

Answer 3

Forms multiple cell types typically from one germ layer

Question 4

What are tri/bipotent cells?

Answer 4

Cell that forms 2/3 different cell types

Question 5

What cells only form one type of cell?

Answer 5

Unipotent

Question 6

What is the definition of specification?

Answer 6

Commitment to a particular fate that can be changed if those cells are moved to a new environment.

Question 7

What is the definition of determination?

Answer 7

Commitment to forming a particular cell type.

Question 8

What is the definition of differentiation?

Answer 8

When cells acquire their functional characteristics for that cell types.

Question 9

What type of cell commitment is localised determinants?

Answer 9

Localised determinants are a type of cell that is made depending on the distribution of certain molecules.

Question 10

Define regulative development

Answer 10

This is when a single embryo cell is capable to give rise to all cell types. This suggests that cells communicate with each other to restrict their potency.

Question 11

Define embryonic induction

Answer 11

This is the process by which a cell’s fate is changed by signals from an adjacent group of cells. This requires a signalling and responding cell.

Question 12

Describe competence

Answer 12

The ability of a cell to respond to a signal

Question 13

How are embryonic signals transferred?

Answer 13

Embryonic signals are released from one cell population and they diffuse across the short space to act on nearby cells. The signals bind to cell receptors, and this causes a cascade of intracellular signalling which results in changes in gene expression (secondary messenger model).
Important signalling proteins include Transforming Growth Factor Beta, Fibroblast Growth Factor, Wingless/Int1 and Hedgehog.

Question 14

What is epigenesis?

Answer 14

When organs and tissues are added progressively during development with complexity increasing over time.

Question 15

What are morphogens?

Answer 15

Molecules that specify different cell types at different concentrations.

Question 16

Outline the experiment carried out by Hans Dreisch that showed that cells are not filled with determinants but can form all cell types through regulation

Answer 16

Hans Dreisch isolated blastomeres from urchin embryos. He then divided then when they were in their cleavage stage. The embryos still went on to develop into larvae (although smaller) which shows that their development is conducted by regulation. eg. monozygotic twins are also an example of this
Overall, cells are divided and polarity determined by determinants, but is told what cell type to become by inductive proteins.

Question 17

What is the function of the SHH (Sonic HedgeHog) gene?

Answer 17

The gene codes for the Sonic Hedgehog protein, which acts as a chemical signal essential for cell growth, specialisation, and normal shaping(patterning) of the body. It changes fate within cell types.

Question 18

What can mutated versions of this gene cause?

Answer 18

They can cause congenital abnormalities and disrupt anterior/posterior polarity in each segment of the embryo. For example in Drosphila fruit flies it causes a shortened embryo with denticles (tooth projections) on the ventral (underside) surface.

Question 19

What happens if there is not SHH?

Answer 19

The receptor PATCHED1 cannot bind to its ligand of SHH. This inhibits fusion of Smoothened receptor to the cell surface membrane (this receptor is needed for embryonic development and adult tissue homeostasis). The Gli transcription factors that activate and inhibit transcription by binding to Gli responsive genes that are important for growth, are turned into repressors so these are not transcribed. Leads to denticles.

Question 20

What happens when SHH is present?

Answer 20

SHH can bind to the receptor PATCHED1. This stops inhibition of the Smoothened receptor so the Gli factors become transcriptional activators that can transcribe Gli proteins.

Question 21

Outline the function of SHH in the development of the spine of the embryo (1)

Answer 21

In the ventral(lower) part of the neural tube there are many different types of neuronal cells. Their cell fate is specified by the notochord (embryonic midline structure that provides mechanical and signalling cues to pattern (shape) surrounding tissue in the developing embryo). The notochord induces ectopic floor plate (midline glial structure to provide structure and support) and motor neurones when grafted to the side of the neural tube. The floor plate induces motor neurone formation itself. They are both signalling centres that act on adjacent cells to change cell fate. Neither is formed if there is no notochord. The notochord expresses SHH and acts on the ventral neural plate, causing it to become the floor plate. Floor plate cells then begin producing SHH and acts on adjacent cells to change their cell fate. Without SHH certain neurones don’t form or are in the incorrect place.

Question 22

Outline the function of SHH in the development of somites

Answer 22

In somites it induces the formation of sclerotome, a transient embryonic tissue made of pluripotent, mesenchymal stem cells located in the ventromedial region of the somite. This induces the formation of different cell types. The SHH and WNT signalling pathway induces the epaxial myotome (myotome above axis in the dorsal area ~from motor fibres). The WNT and BMP signalling induces the hypaxial myotome (myotome beneath the axis, ventral). WNT signalling pathway also induces dermatomes.

Question 23

Outline the function of SHH as a morphogen

Answer 23

It can induce multiple different cell fates in a concentration dependant manner - there is a higher concentration ventrally and lower concentration dorsally. The varying concentration produces different cell types.

Question 24

What are blastulae?

Answer 24

They are hollow spheres of cells, or blastomeres, produced during the development of an embryo by repeated cleavage of a fertilised egg. The cells of the blastula form an epithelial (covering) layer, called the blastoderm, enclosing a fluid-filled cavity, the blastocoel.

Question 25

Outline the Animal Cap Assay experiment that can be carried out to see the effect of different signal inducers on mesoderm induction

Answer 25

Blastulae from organisms such as amphibians have vegetal and animal poles. Animal pole is where cells divide rapidly, and is above the vegetal pole which is not as active. Mesoderm can be formed when signals pass from the vegetal hemisphere to the animal hemisphere, and the mesoderm is only formed from the animal hemisphere. These signals can be detected by mis blastula fragments being incubated with candidate factors and assayed for mesoderm induction. If mesoderm inducing factors are present then the animal cap elongates and differentiates to mesodermal tissues. This induction requires the NODAL gene. This was found out as a vegetal pole from an embryo that lacked a functional NODAL gene could not induce mesoderm in the animal cap - indicating that NODAL signalling is needed for mesoderm induction. Same experiment can be repeated with other signal inducers to see their effect on mesoderm induction.

Question 26

Define an oocyte

Answer 26

A cell capable of dividing by meiosis to form an ovum

Question 27

How does the egg become fertilised and move into the uterus?

Answer 27

There are finger like projections called fimbriae at then ends of the fallopian tube that can sense when the egg is going to be released, so can capture the egg and pass it into the fallopian tube. The sperm will meet the egg in the upper region (ampulla) of the Fallopian tube. The fertilised egg then slowly passes down the oviduct. The first division will take place 1-2 days after fertilisation. The embryo is surrounded by the zone pellucida membrane, which stops the egg from implanting in the Fallopian tube. Division continues until a call of cells with a fluid filled cavity is formed = blastocyst. This will enter the uterus 4-5 days after fertilisation.

Question 28

Define oogenesis

Answer 28

Production of the egg

Question 29

How does oogenesis occur?

Answer 29

Firstly, the primordial germ cells (germline stem cells that give rise to gametes in vertebrates) move to the front of the gonad (organ that produces gametes) and divides to produce more cells. These cells are surrounded by a layer of granulosa cells (critical somatic component of the ovary that are essential for follicle development). Granulosa cells have LH receptors on their CSM, and aid production of progesterone and oestrogen - steroid hormones. Once entering the ovary it becomes an oogonium, which undergoes mitosis to increase cell number. Then it undergoes meiosis in the gonad, however this only occurs up to PROPHASE 1. After reaching puberty some primary oocytes have meiosis reinitiated and the first division is completed. This produces a large secondary oocyte and a small polar body which contains discarded chromosomes. The secondary oocyte will begin meiosis 2 but will only reach METAPHASE 2. This is when the egg is ovulated. Only after ovulation meiosis is completed and it is fertilised. This results in a fertilised egg and another polar body. Many primary oocytes are produced however only a fraction of these actually reach the stage of puberty.

Question 30

How does spermatogenesis occur?

Answer 30

In the testes meiosis is not initiated until after puberty. The spermatogonium (early sperm cell) undergoes the first division of meiosis, which produces one primary spermatocyte. This then divides into two spermatocytes(same size). Then in meiosis 2, this becomes 4 spermatids are produced, which undergo differentiation to produce 4 spermatozoa.

Question 31

Outline the process of fertilisation

Answer 31

Sperm cells become more active when they reach near an egg cell. There is a sperm receptor on the zone pellucida (ZP3) of the egg. After interacting with this receptor the acrosome of the sperm bursts, releasing enzymes that can digest the zone pellucida. A specific protein on the head of the sperm interacts with a specific protein on the CSM of the egg, allowing the sperm to fuse with the egg and for the nucleus to pass into the egg. This interaction also causes an increase in intracellular calcium levels, and this causes the egg nucleus to restart meiosis, and for the cortical granules to fuse with the plasma membrane. These release enzymes that interact and modify proteins on the zone pellucida so that no more sperm can fuse with the egg. When the male and female DNA comes together it is now called a zygote.

Question 32

What is parthenogenesis?

Answer 32

This is when the mother produces clones of her off spring (doesn't happen in mammals). If only female chromosomes present = small yolk and small trophoblast. If only male chromosomes = small yolk and large trophoblast. The reason for this is genomic imprinting. 80 genes are methylated during oogenesis or spermatogenesis, which means that they are not transcribed due to being inaccessible to transcriptional factors. However these genes are needed for embryonic development, and are only activated when both male and female copies of chromosome are present. (so if cannot be fertilised these genes are not activated).

Question 33

What are cleavage divisions?

Answer 33

The mitotic divisions that the fertilised egg undergoes as it moves from Fallopian tube to the uterus. These are not accompanied by cell growth. The large egg is split into smaller cells of around 10-20 micrometres. This takes 12-24 hours, and will remain like this for 5 days. These then undergo compaction, where cells maximise contact with each other. This creates 2 cell populations.

Question 34

What is the outside population of cells of a blastocyst called?

Answer 34

Trophoblast

Question 35

What is the inside population of cells of a blastcyst called?

Answer 35

Inner cell mass

Question 36

What is the fluid filled cavity inside the embryo and how does it form?

Answer 36

It is called the blastocoel and is formed by the embryo pumping salts into the centre of the embryo. This moves the inner cell mass to one pole of the embryo. Outer cells are polarised, inner cells are non polar = blastocyst

Question 37

Can cells in the 2 cell or 4 cell stage of the blastocyst be divided individually and still grow to normal mass?

Answer 37

Yes, they have regulative development. They are totipotent so can produce all tissues from the fertilised egg. This is also responsible for formation of monozygotic twins, as 30% of them are formed during cleavage stage. This demonstrates that blastomeres are totipotent. (Pluripotent cells can only make embryonic tissues.)

Question 38

Define tetragametic chimera

Answer 38

These are made up of 2 genetically different types of cell. There are two fertilised eggs (fraternal twins) that are fused to form a single chimeric embryo. In mice this is shown as a brown and white mouse formed from a brown mouse and a white mouse.

Question 39

What is implantation?

Answer 39

Process by which the blastocyst burrows into the uterus wall.

Question 40

List the 4 functions of the trophoblast

Answer 40

1. Implantation 2. Differentiate the chorion (the outer most membrane of the embryo) 3. Immunosuppression 4. Endocrine gland

Question 41

Outline the function of the HCG (human chorionic gonadotropic) hormone

Answer 41

Necessary to maintain pregnancy. Helps produce oestrogen and progesterone from the ovary in the first trimester. In the second trimester the production of HCG shifts from the ovary to the trophoblasts themselves.

Question 42

Outline the process of implantation

Answer 42

The zona pellucida prevents the embryo from implanting in the Fallopian tube. In the uterus the embryo secretes enzymes to create a hole in the zone pellucida so that the blastocyst can pass through. The blastocyst attaches to the uterine wall with the side containing the epiblast., so the amnion formed is facing towards the uterus wall. Once attached, the trophoblast undergoes rapid division. As they divide the cells stop making cell membranes or remove cell membranes once they are made. This results in the format of a big cytoplasm with multiple nuclei = SYNCYTIOTROPHOBLAST. The remaining trophoblast is called the CYTOBLAST. The syncytiotrophoblast is very invasive and enters finger like projections into the epithelium of the uterine walls.

Question 43

Describe the appearance and structures of an implanted blastocyst

Answer 43

When the syncytiotrophoblast invades the uterine walls it expands and forms lacunae. These are spaces which fill with maternal blood. This is the beginning of the utero-placental circulation. The amnion (fluid filled sac) forms in the epiblast. The hypoblast (opposite side of the uterus wall) spreads along the cytotrophoblast and forms Heuser's membrane. (The amniotic cavity is above the epiblast) This enclosed space is known as the yolk sac. An extracellular reticulum is secreted by Heuser's membrane and the cytotrophoblast. This structure will connect to the connecting stalk, which will form part of the umbilical cord.

Question 44

Describe the components of stem villi

Answer 44

After 14 days, the syncytiotrophoblast, cytotrophoblast and extra embryonic mesoderm protrude into the lacunae, which forms stem villi. The mesoderm forms the blood vessels of the chorion (surrounds the whole embryo). This increases the efficiency of oxygen, carbon dioxide and waste products being transferred between maternal and embryonic blood supplies.

Question 45

List substances that can be transferred from the foetus to the mother across the placenta

Answer 45

CO2, water, urea, uric acid(component of urine), creatinine (break down product of creatinine phosphate), bilirubin (breakdown product of red blood cells), hormones, RBC antigens.

Question 46

List substances that can be transferred from the mother to the foetus across the placenta

Answer 46

O2, water, nutrients, hormones, antibodies, vitamins, drugs, alcohol, viruses

Question 47

Describe the appearance of a 3 month old foetus

Answer 47

Developing extraembryonic tissues provide a survival capsule for the foetus. The foetus is bathed in amniotic fluid (from the initial amnion.) The foetus is now connected to the chorion via the umbilical cord and its blood vessels. As the embryo goes it will bulge into the uterine cavity, and the chorion will become restricted to the side of the embryo closest to the umbilical cord. This will form the discoid (disc shaped) placenta.

Question 48

What is an ectopic pregnancy?

Answer 48

When implantation occurs in the cervis, Fallopian tube, ovary or abdomen due to failed function of the fimbriae. The tissue outside the uterus does not have sufficient thickness or blood supply to allow development of the embryo when this happens, so can cause internal bleeding.

Question 49

What is pre implantation genetic testing?

Answer 49

Involves in vitro fertilisation. Taking one of the blastomeres and testing for Tay-Sachs, muscular dystrophy and common X linked disorders(males). If not disorders are found, the blastocyst from which the blastomere was isolated can be implanted back into the mother.

Question 50

What is trophoblast disease?

Answer 50

Uncontrollable growth can form hydatidiform moles. These can cause miscarriage and tumours. (Complete moles hide the embryo, incomplete moles partially hide the embryo.) Caused by the embryo only containing paternal chromosomes (full set) or 2 sets of paternal chromosomes and 1 set of maternal chromosomes (partial). Embryos require both maternal and paternal chromosomes as 2 maternal chromosomes result in no trophoblast forming so no placenta.

Question 51

What are the three germ layers that form from the epiblast around 9 days after fertilisation?

Answer 51

The ectoderm, mesoderm and endoderm | also forms extraembryonic mesoderm and the amniotic membrane

Question 52

What does the ectoderm (outer layer) give rise to?

Answer 52

Forms the epidermis (keratinocytes, sweat glands, hair follicles), the entire nervous system (neurones, glia, Schwann cells, PNS and CNS), and the bone and cartilage of the face, retina, cornea, lens and mammary glands.

Question 53

What does the endoderm (inner layer) give rise to?

Answer 53

Forms the epithelium of digestive tract (oesophagus, stomach, intestines, rectum), the lungs, liver, gall bladder, pancreas. As well as this the epithelial layer surrounding the lumen is derived from endoderm (apart from digestive tract).

Question 54

What does the mesoderm (middle layer) give rise to?

Answer 54

Tbxt hormone is expressed in all mesoderm tissues undergoing gastrulation. Forms most cartilage and bone, connective tissue, all skeletal muscle, kidneys, blood vessels, ovary/testes. Forms the heart and smooth muscle.

Question 55

What is gastrulation?

Answer 55

The process by which the blastula is organised into a multilayered structure and cells move into the interior of the embryo. Converts the single epiblast layer into multiple layers of the foetus. Defects in this process can cause miscarriages.

Question 56

Define the word caudal

Answer 56

At the posterior part of the body near the "tail"

Question 57

Define the word rostral

Answer 57

At the front part of the body near the "mouth"

Question 58

What is the primitive node?

Answer 58

An area through which migrating cells are channelled into a rod like mass of mesechymnal cells called the notochord.

Question 59

What does mesenchymal mean?

Answer 59

Adult stem cells isolated from different cells that can differentiate to other types of cells.

Question 60

What is the primitive streak?

Answer 60

An elongating curve on the blastula wall that is a source of extra embryonic mesoderm of the yolk sac wall. The node is a centre that releases signalling molecules that can give instructions to adjacent cells what tissues to form. It is possible to take a node from one embryo and move it to another embryos epiblast.

Question 61

What is an organiser?

Answer 61

It is a region of the embryo that is responsible for specifying cell fates. This could affect the whole embryo of just a part of the embryo. It is a signalling centre that secretes diffusible signalling molecules that may act as morphogens (they specify fates of cells depending on the concentration of the molecule).

Question 62

Outline the process of gastrulation

Answer 62

14 days after fertilisation a ridge appears on the caudal half of the embryo on the midline of the epiblast. This is the primitive streak. 15 days after fertilisation, a mound of cells called the primitive node is formed in the middle of the upper end of the primitive streak. Gastrulation itself is the phase in which the single layered blastula becomes a three layered gastrula (trilaminar disc).

Question 63

Outline the process of ingression ( in gastrulation)

Answer 63

The cells of the epiblast move towards the primitive streak and node. When in the primitive streak these cells break away from the epiblast and become mesenchymal. During this the adherent junctions, tight junctions and the basement membrane are lost so cells can move freely in the sub germinal space. The first cells that go through the primitive streak push the hypoblast to the edge. These cells form the endoderm. A day later the cells migrating through the primitive streak that express the Tbxt hormone will form mesoderm. These will fill up the space between the endoderm and ectoderm. The cells that do not migrate through the primitive streak form the ectoderm. (remains closest to the primitive streak.) The epiblast and hypoblast are tightly attached. These membrane will eventually form the mouth (oropharyngeal) and anus (cloacal).

Question 64

Outline the process of axial mesoderm formation (in gastrulation)

Answer 64

Some epiblast cells that go through the node migrate to the anterior (front) direction to form the prechordal plate and the notochord. (this is axial mesoderm, along the central axis under the neural tube). Prechordal mesoderm underlies the forebrain and midbrain and is important in establishing neuronal identities.

Question 65

Describe the notochord and its function

Answer 65

The notochord runs through the midline of the embryo. It is composed of large vacuolated cells encased in a sheath of collagen fibres.(Type II). This provides axial support. Eventually this degenerates and is replaced by the axial skeleton (vertebral column, skull and rib cage). The nucleus pulposus of intervertebral discs is the only remaining part of the notochord. (if retained in other part it tends to become a chordoma - type of cancer). The notochord releases signals that help to control cell differentiation. (For example motor neurones are derived as a single signal from the notochord - dermatome)

Question 66

Outline the process of paraxial, intermediate and lateral plate mesoderm - somite development (in gastrulation)

Answer 66

Remaining mesoderm cells that do not form the notochord move down either side, and this is paraxial mesoderm the is adjacent to axial mesoderm. A morphogen gradient generated by the axial mesoderm forms the intermediate and lateral plate mesoderm from this group of mesoderm cells. The paraxial mesoderm forms paired blocks called somites that progress from the anterior to the posterior direction. These somites are formed from a simple epithelium surrounding a central cavity.

Question 67

What are the 3 types of tissue that somites give rise to?

Answer 67

Dermatome (on top) Myotome (in the middle) Sclerotome (on the bottom)

Question 68

Define the sclerotome and its function (from paraxial mesoderm)

Answer 68

These are the cells closest to the notochord (central). These cells break away and migrate around the neural tube. Here it differentiates to cartilage and bone of the axial skeleton. Forms the occipital bone, vertebrae, ribs, sacrum and coccyx.

Question 69

Define the dermatome and its function (from paraxial mesoderm)

Answer 69

Rises from the dermamyotome tissue. Differentiates into the dermis (connective tissue) of the dorsal skin. It is an area of skin where the sensory nerves are innervated from a single spinal nerve root.

Question 70

Define the myotome and its function (from paraxial mesoderm)

Answer 70

Rises from dermamyotome tissue. Differentiates to skeletal muscle. It is an area of skin where the motor nerves (muscle) are innervated by a single spinal root.

Question 71

What does the intermediate mesoderm give rise?

Answer 71

Forms the kidneys, the male or female reproductive system.

Question 72

What tissues does the lateral plate mesoderm give rise to?

Answer 72

Forms two layers: The somatopleuric layer The splanchnopleuric layer These surround the coelom cavity.

Question 73

What does the somatopleuric layer give rise to? (from lateral plate mesoderm)

Answer 73

Forms connective tissues of body wall and limbs, aswell as limb bones.

Question 74

What does the splanchnopleuric layer give rise to? (from lateral plate mesoderm)

Answer 74

Forms connective tissue and smooth muscle layers of the gut, cardiac muscles and the circulatory system.

Question 75

What does the coelom cavity give rise to? (from lateral plate mesoderm)

Answer 75

Forms the pericardial, pleural and peritoneal cavities.

Question 76

What is neurulation?

Answer 76

This is the formation of the neural tube from the dorsal ectoderm. It is a coordinated series of cell movements and shape changes that put the nervous system inside the embryo. This is where it forms the neurones and glia of the central and peripheral nervous systems. Defects of this process give rise to neural tube defects such as anencephaly and spina bifida.

Question 77

Outline the process neurulation (in gastrulation)

Answer 77

The cells on the outside of the neural plate (primitive streak and notochord) are cuboidal cells, and cells on the midline are columnar cells. As the epiblast increases in size the primitive streak moves back in order to rain the same size. This is regression. The notochord secretes proteins that induce the neural plate to form ectoderm rather than allowing differentiation of cells. The neural plate folds along the midline at 20 days and forms a neural groove, which causes the lateral edges of the neural plate to rise. Folds form when cells constrict their apical (exposed to the outside)surfaces. This makes them become wedge shaped, which forms hinge points. The lateral edges move towards each other and fuse to form the neural tube. It closes at the middle first the up and down at the same time. the ectoderm then fuss and the neural tube becomes embedded within the mesoderm layer of the embryo.

Question 78

Explain the condition of the anencephaly neural tube defect

Answer 78

This is a failure to close the neural tube at the rostral end (cranium). The open forebrain remains in contact with the amniotic fluid which causes degeneration. The foetus lacks bones of the skull.

Question 79

Explain the condition of spina bifida neural tube defect

Answer 79

This is failure to close the neural tube at the caudal end (tail). May not be fatal but can result in severe paralysis below the lesion. Incomplete closing of the backbones and membrane around the spinal cord.

Question 80

What can help prevent neural defects from occurring?

Answer 80

Folic acid if taken during the first 4 weeks of pregnancy.

Question 81

How does the neural crest help the differentiation of nervous cells?

Answer 81

The neural crest (multipoint cells) forms at the 'roof' of the neural tube with the epidermis. Crest cells divide into layers from the neural tubes and migrate around the body to form a number of different cells. It is only found in vertebrates. The cranial crest cells: In the ventral (underside) region of the head. Forms ganglia, neurones, thymus, cartilage/bone, teeth. The trunk crest cells: Form ganglia, neurones, Schwann cells, melanocytes and smooth muscle for the heart.

Question 82

How does the vertebrate body plan give rise to the rest of the foetus?

Answer 82

The neural tube is divided into forebrain (telencephalon and diencephalon), the midbrain (mesencephalon), the hindbrain (rhombencephalon) and the spinal cord, which forms 6 somites. The hindbrain is further divided into 7 transient segments known as rhombomeres.

Question 83

What does the telencephalon give rise to? (neural tube)

Answer 83

Cerebral cortex, Hippocampus, Basal ganglia, Olfactory bulb.

Question 84

What does the diencephalon give rise to? (neural tube)

Answer 84

Thalamus, sub thalamus, Hyptothalamus, Epithalamus.

Question 85

What does the mesencephalon give rise to? (neural tube)

Answer 85

Tectum, Tegmentum, Ventricular mesocoelia, Cerebral peduncles.

Question 86

What does the rhombencephalon give rise to? (neural tube)

Answer 86

Medulla, Pons, Cerebellum