Development Lecture 2 Flashcards
Describe gastrulation in sea urchin embryos?
FIRST PHASE OF GASTRULATION
1. some of the cells at the vegetal pole elongate to form the vegetal plate
- apical pole of some of these cells constricts so they form bottle cells
- bottle cells lose adhesion with hyaline membrane and other cells
SECOND PHASE OF GASTRUALTION = inward buckling of the monolayer at the vegetal plate
- constriction of the apical surface of vegetal plate cells due to action of actin and myosin filaments at the apical surface
- convergent-extension causes the elongation of the invaginating monolayer and the archenteron (tubular structure) is generated
- in order to elongate the archenteron further, secondary mesonchyme cells produce filopodia and make contact with them to the basal lamina of the animal pole
- the filopodia of the SMCs pull the the archenteron towards the animal pole
Describe the cleavage patterns of sea urchins
- very stereotypical for the first eight divisions
What are the two main stages of gastrulation of sea urchins?
- Ingression of primary mesenchyme cells
2. invagination of the archenteron
What does the endoderm in sea urchins form?
The gut, mouth and anus
What three adhesion changes that primary mesenchyme cell smuts undergo in order to leave from the epithelium of the vegetal plate?
- lose affinity for neighbouring epithelial cells
- lose affinity fro the hyaline layer on the exterior of the embryo
- gain affinity for the basal lamina
The blastopore will form the _
anus of the sea urchin
How does the vegetal plate buckle?
Apical constriction:
- the apical (the tip) ends of some cells in the vegetal plates would actively constrict due to myosin and actin movement
- these cells become bottle cells
What do primary mesenchyme cells form?
the larva skeleton
What change occurs so that the bottle cells that form vegetal plate can lose adhesion with the hyaline membrane and becomes migratory loose clusters primary mesenchyme cells?
PMCs lack calcium dependent adhesion molecules, cadherins, from their epithelium
What is the site of invagination of the vegetal plate called?
the blastopore
What is convergent-extension?
- cells become elongated perpendicular to the axis of elongation
-at the other end the cells form lamellipodia, rich in actin and myosin
-cells use the lamellipodia to exert traction on each other to force intercalation
(they move closer together, like fingers being pushed into each other)
the tube therefore becomes narrowed (convergence) and longer (elongation)
How is the archenteron pulled towards the ventral surface?
- The SMCs filopodia have a greater affinity for the parts of the animal pole that will form the ventral surface
- Filopodia remain in contact with this area for longer than other parts of the animal pole
How is GFP incorporated into a mouse genome?
- introduce foreign genes into the mouse genome
- e.g. Green Fluorescent Protein (GFP) first identified in jellyfish, and the resultant mice are known as transgenics
- The gene for GFP is placed behind promoter/enhancer elements that allow transcription in either all or a subset of cells
- then added to ES cells in culture
- It is taken up by a small number of cells and incorporated into the mouse genome
- Those expressing GFP, which give off green light under UV illuminescence, can be added to the blastocoel of a host blastocyst
- becoming incorporated into the ICM
- creating a chimeric mouse in which some of the cells produce GFP
- If they are incorporated into the germ line (future gametes) then breeding experiments will produce a mouse line in which all cells express GFP
How are knockout mice created to test for genes involved in embryonic development?
- a gene is isolated and mutated
- e.g. replacing a exon with a foreign sequence
- ES cells with the mutation are added to the blstocoel of a host blastocyst
- ES cells are incorporated into the inner cell mass
- mice with these mutations are inbred to produce homozygous offspring
- offspring physiology is studied
How does the mammalian blastocyst form from a 16- to 32-cell embryo?
- compaction
- forms an outer epithelium, the trophoblast
- an internal inner cell mass - trophoblast pumps sodium ions and water follows into the centre of the embryo to form the blastocoel
What does the trophoblast layer form in mammals?
chorionic layers of the placenta
What does the inner cell mass form?
The foetus tissues
What potency do inner cell mass cells have?
Pluripotent (form all metal tissue types)
What happens after compaction, once the blastocyst is formed?
- The ICM divides into the hypoblast (touching blastocoel) and the epiblast
- hypoblast cells spread around the trophoblast, lining the blastocoel to form an extra embryonic endoderm layer called Heuser’s membrane
What does hypoblast cells form?
- hesuer’s membrane - the extraemryonic layer between blastcoeol and trophoblasts
What is the cavity enclosed by Heuser’s membrane called?
The yolk sac
it is equivalent o the yolk sac of reptilian embryos, which helps to transport nutrients from the yolk to the embryo
What is Meckel’s diverticulum?
a remnant of the yolk sack which is a small bulge on the small intestine
What extra embryonic tissue does the epiblast form?
the amniotic membrane, extra embryonic ectoderm, this is a layer of flattened dpeithelial cells which line a fluid filled cavity called the amnion
What does the amniotic membrane do and what forms it?
Forms from the epiblast
- surrounds the entire embryo/fetus and provides it with a protective cushion
Which of these are amniotes and which are anamniotes?
birds, fish, reptiles, amphibians, mammals
amnitoes - reptiles, birds, mammals
anamniotes - fish amphibians
What does the epiblast layer form?
the three germ layers
What potency does epiblast cells have?
pluripotenet
What forms the vasculature of the placenta and the first cells of embryonic blood?
a layer of extra embryonic mesoderm which migrates from the epiblast between Heuser’s membrane and the trophoblast
How long can an embryo survive on nutrition from the egg only?
one week
How does a mammalian embryo establish a nutrition source?
implantation
1. in the uterus, the embryo ‘hatches’ from the zona pellucida
- the trophoblast attach to the uterine wall
- trophoblast in contact with the uterus undergo rapid proliferation and the cells fuse to form a cynctium composed of many nuclei, it is the SYNCTIOTROPHOBLAST
- the synctiotrophoblast penetrate the uterine cell wall
- lacunae (cavities) form in the sycntiotrophoblast
- the synctiotrophblast rupture maternal capillaries, so maternal blood fills the lacunae (maternal-placental circulation)
Who’s cell make up the placenta
maternal and embryonic
Where does implantation occur and why?
in the uterus
the zona pellucida stope the embryo egg implanting in the fallopian tubes
What is an ectopic pregnancy?
When the egg implants in the fallopian tubes (tubal pregnancy), cervix, ovary or abdominal cavity
Why are ectopic pregnancies terminated?
- embryo does not implant properly
- embryo still attracts a large blood supply
- embryo may easily break away from implantation site
- extensive bleeding is threat to mother
What is the synctiotrophoblast?
In implantation, when the embryo attaches to the uterus, the trophoblasts that are in contact with the uterine wall, rapidly proliferate and then all the cells fuse to form a synctium
What is the cytotrophoblast?
Single layer of epithelium cells which line the blastocoel and are in contact with the inner cell mass
How does the synctiotrophoblast penetrate the uterine wall?
- secretes enzymes that digest the the extracellular matrix which usually surrounds and connects cells
What are lacunae?
cavities in the scyntiotrophoblatst that are filled with maternal blood
What does human chorionic gonadotrophin do?
Maintains progesterone level production by the corpeus luteum, without hCG endometrium breaks down
What do human pregnancy test identify?
hCG
What induces the production of hCG?
implantation
What cells produce hCG?
trophoblast cells
What is progesterone needed for and what produces it?
- maintain uterine endometrium
- luteal body and then placenta
What other hormones do the trophoblasts product and what do they do?
Chorionic Somatomammotropin, Chorionic Thyrotropin, and Chorionic Corticotropin, which maintain placental growth, regulate the mother’s metabolism and the growth of mammary glands
How do trophoblast cells ensure that an embryo has food supply before and after birth
- secrete hormones that stimulate growth of mammary glands and maintain placental growth
- forms the synctiotrophbolast which penetrate uterine wall
What reaction does the trophoblast block?
trophoblast blocks maternal immune response to paternal antigens ont he embryo
What is the first sign of gastrulation in mammals and birds?
the primitive streak = a ridge of cells forming at the posterior half of the epiblast layer
What does the rostral end of the epiblast and hypoblast form?
buccopharyngeal membrane
later this forms the mouth
What does the caudal end of the epiblast and hypoblast form?
cloacal membrane
this later forms the anus
What is the primitive node called in gallus gallus?
Henson’s node
what is the anaamniote equivalent of the primitive node and streak?
the blastopore
What does the primitive node and streak do?
prefigure the the future rostral-caudal and left-right axes of the embryo
What embryonic tissues do the endoderm form?
-epithelial lining of gut
-lungs, liver, pancreas
(germ layer closest to the blastocoel)
What embryonic tissues do the mesoderm form?
muscle, bone, connective tissue, heart, kidney, gonads
How are the notochord and prechordal plate formed?
Cells ingressing though the node, along the midline, in a rostral direction
What is the prechordal plate?
- loose population of mesoderm and endoderm cells
- underlie the developing forebrain
- contributes to the rostral endoderm
- contributes to the mesodermal population of head and acts as important signalling centre for establishing different neuronal populations in the developing forebrain
What is the notochord?
a flexible rod of cells that extends throughout most of the rostral-caudal axis of the embryo
it does NOT extend to to the rostral edge of the embryonic disc
What adaptations does the notochord have in lower vertebrates (fish and amphibians)?
-large diameter because cells fill it with lipid filled vacuoles
- skeletal muscle is anchored to the notochord
and provides the right amount of flexibility required for swimming
- the notochord in amniotes is smaller because they don’t swim
What is the function of the notochord?
- source of signalling molecules that patterns the adjacent nervous system, mesoderm, endoderm
What is the eventuality of the mammalian notochord?
The mammalian notochord will degenerate and be replaced by the vertebral column
what part of the notochord is retained in mammals?
the nucleus pulposus of the inter- vertebral discs is the notochord retained
How are somites formed?
- The paraxial mesoderm forms a compact rod of cells, the segmental plate
- At the rostral end of the segmental plate, balls of epithelial cells bud off, these are somite
- new cells are continually added to the caudal end of the segmental plate
What is the rate of somite generation
3 -4 somites per day
When are somites formed?
Day 20-30 of development
How many somites are formed?
in humans, chicks, mice and snakes?
42 somites are formed, this is reduced to 38
Chicks produce 50 somites
mice produce 65 somites
snakes produce 500 somites
What do mesodermal cells leaning against the notochord form?
- paraxial mesoderm (closest to notochord)
- intermediate mesoderm
- lateral plate mesoderm
What do paraxial mesoderm cells form and in which direction?
somites
rostral to caudal
Which organism is invagination clearly seen in?
seen in sea urchin embryos, involves the infolding of the surface epithelium of the blastula, usually occurs at the vegetal pole, forms an archenteron, occurs at the blastopore
Which organism is involution clearly seen in?
seen in amphibians, forms an archenteron, extends towards the animal pole from the blastopore, involuting cells fold bac onto the inner surface of the blastula
Which organism is ingression clearly seen in?
seen in chicks and mice (and humans), migration of individual cells from the surface epithelium into the interior of the embryo, cells ingress at the primitive streak
A new somite is formed every _ in humans?
5 hours
A new somite is formed every _ in chicks?
90minutes
A new somite is formed every _ in zebrafish?
30 minutes
What three populations of cells are formed by somites and what are their fates?
Sclerotome - forms axial skeleton (vertebrate + ribs), the base of the skull (occipital bone) to tail
Myotomes - forms all skeletal muscle
Dermatome - connective tissue of dermal layer of dorsal skin
What does the intermediate mesoderm form?
blood cells, kidney, bladder, reproductive organs
What does the lateral plate mesoderm form?
connective tissue
smooth muscle layers of digestive tract
connective tissue of the dermal layer of the ventral skin
membranes lining the body cavities
skeleton and connective tissue of the limbs
blood vessels
heart
From what does the neural plate form?
The ectoderm anterior to node
What is the neural plate?
Precursor to the nervous system
cells of the neural plate elongate to form _
pseudostratified columnar epithelium
Cells of the epidermis remain _
cuboidal
What is regression?
As the embryo expands, through cell proliferation, the primitive streak becomes restricted to the posterior margin of the epiblast
When does neural tube formation occur in humans?
fourth week of development
What is the neural plate
A thickened region of ectoderm on the dorsal side of the embryo
What neural specific genes are already transcribed at this stage?
Sox2 and Sox3
What causes the elongation of neural plate cells?
changes in the structure of microtubules - they become polarised and elongated along the apical (external) – basal (internal) axis
Neurulation:
- neural plate cells elongate into columnar cells
- lateral edges of the neural plate fold upwards due to changes in cell shape
- the neural plate folds around the midline of the neural plate (the median hinge point)
- neural plate cells at the hinge point contract their apical surfaces to create ‘wedge shaped cells’ due to myosin/actin action
- convergent extension - expansion of the rostral-caudal width, and convergence of the medial-lateral width
- apical constriction and convergent-extensions allow the lateral edges of the neural plate to fuse and form the neural tube
- the epidermis will also fuse to complete the surface layer
What mechanisms cause neural tube closure?
convergent-extension
apical constriction
What regions of the neural tube close first?
first in the hindbrain region and progresses in both directions until ti is fully closed
Failure to close the anterior (rostral) neural tube causes
anencephaly - the brain degenerates and the cranium fails to form
Failure to close the posterior (caudal) neural tube results in
spina bafida
- can cause severe paralysis
Where does the ventral body wall and gut tube remain open in birds and humans?
at the umbilical cord
At the end of neurulation, the embryo is a _ disc with _, _, and _ layers
trilaminer disc upper ectoderm middle mesoderm lower endoderm
The midline of the embryo is located at the _
lateral edges of the disc
Define embryonic folding
when the lateral edges of the disc (its future midline) are brought together
Embryonic folding is when
- folding closes of the gut tube
- gut tube is surrounded with splanchnic mesoderm (from the lateral plate)
splanich mesoderm forms the dorsal and ventral mesentery - The epidermis and somatic mesoderm (also from the lateral plate) close off the ventral body wall and form the body cavities (coelom)
What is the umbilical cord?
The umbilcal cord contains the vasculature connecting the embryo to the placenta, as well as the yolk sac
On what side of the embryo is the neural tube?
the dorsal (back)
What divisions are present in the neural tube?
forebrain (prosencephalon), midbrain (mesencephalon), hindbrain (rhombencephalon), and spinal cord.
ON either side of the notochord are _
somites
