Developmental biology 2 (Prof. Dale) Flashcards
How is the yolk distributed in sea urchin eggs ?
Evenly.
What are micromeres in the sea urchin egg ?
Micromeres = small blastomeres produced at the vegetal pole.
What is the structure of the blastula generated by the sea urchin egg ?
At the 62-cell stage, the blastula consists of a simple epithelium surrounding a fluid filled blastocoel.
How does gastrulation take place in the sea urchin embryo ?
- vegetal pole of the sea urchin blastula thickens to form a the vegetal plate –> releases primary mesenchyme cells into the blastocoel
- vegetal plate buckles inwards to form the archenteron + a group of secondary mesenchyme cells form at the tip –> these cells produce pseudopodia that attach to the animal pole + pull the archenteron towards it
- archenteron fuses with the animal pole to form a continuous tube (the gut) with the blastopore forming the anus
What causes the buckling of the vegetal plate in the sea urchin embryo ?
It is caused by localised contraction of actin-myosin microfilaments near the apical surface of the blastula epithelium –> apical contraction narrows the apical surface relative to the basal surface, bending the epithelium inwards
What is invagination in the see urchin embryo ?
Invagination = inward movement of cells at the vegetal pole following apical contraction
How is elongation of the archentron mediated in the sea urchin embryo ?
What is the combined effect on the archentron in this process ?
By convergent-extension, in which cells acquire lamellipodia (made of β-actin) perpendicular to the invaginating vegetal plate and intercalate with each other.
This narrows the archenteron (convergence) + at the same time elongates it (extension).
What are the different steps of cleavage divisions in mammalian embryos ?
- mitosis = initiated ~ 24 hours after fertilisation + occurs every 12-24 hours
- at the morula stage (~16 cells) they cells undergo compaction (maximize their contacts)
- cells at the centre = inner cell mass (ICM)
- peripheral cells = the trophoblast
- blastocoel cavity that forms in the centre of the embryo (now called the blastocyst)
What are the different fates in the mammalian blastocyst ?
Does this vary between animal kingdoms (phyla) ?
- trophoblast –> forms the chorion (placenta)
- the ICM –> divides into epiblast + hypoblast layers
- hypoblast –> differentiates as extraembryonic endoderm
- epiblast –> forms the amniotic membrane, extraembryonic mesoderm + all tissues (ectoderm, mesoderm and endoderm) of the fetus
Each germ layer forms similar tissues in all animal phyla.
To which tissue types does the ectoderm give rise to ?
Epidermis + nervous system (cartilage/bone in the head)
To which tissue types does the mesoderm give rise to ?
Cartilage/bone, connective tissue muscle, kidney, blood, vasculature, heart + gonads
To which tissue types does the endoderm give rise to ?
Digestive tract (epithelium), lungs, liver, gall bladder + pancreas
How does implantation take place ?
What happens to the ICM ?
- mammalian blastocyst attaches to uterine wall + trophoblast proliferates to form syncytiotrophoblast = highly invasive tissue that penetrates the uterine wall
- formation of lacunae that will fill with maternal blood from uterine capillaries
- ICM divides into epiblast + hypoblast layers
- epiblast –> forms the amniotic membrane
- hypoblast –> forms Heuser’s membrane
What is ingression and when does it happen ?
Ingression = process where epiblast cells migrate through the primitive streak and node as individual cells, happens during gastrulation
What are the different stages of gastrulation ?
- ingression
- the 1st cells to ingress invade + displace the hypoblast –> forming embryonic endoderm
- subsequent cells migrate into the space between the epiblast / endoderm –> forming the mesoderm
- cells remaining in the epiblast layer form ectoderm
What are the 5 types of mesoderm ?
How are these and the notochord formed ?
- cells migrating through the node = axial mesoderm + prechordal mesoderm + notochord
- mesoderm from primitive streak condenses either side of the notochord = paraxial mesoderm, intermediate mesoderm + lateral plate mesoderm
How are somites formed and along which axis ?
What are they made of ?
How many are formed ?
- paraxial mesoderm –> forms pairs of somites, sequentially from rostral to caudal
- the number formed varies between species
- somites = formed of a simple epithelium surrounding a central cavity
What do somites subsequently divide into ?
In response to signals from surrounding tissues, they subsequently divide into sclerotome, myotome and dermatome layers.
How is the lateral-plate mesoderm further subdivided ?
- Sometopleuric (most lateral)
- Splanchnopleuric (more medial)
To what organs/tissues do the mesodermal tissues give rise to ?
- Dermatome: dermal (deep) layer of skin
- Myotome: skeletal (striated) muscle
- Sclerotome: vertebrae, ribs, sacrum, coccyx
- Intermediate: kidneys, gonads, reproductive tracts
- Somatopleuric: limb bone, connective tissues, vasculature, blood
- Splanchnopleuric: cardiac mesoderm, smooth muscle,
connective tissues, vasculature, blood
What is the neural plate ?
From what does it form ?
- the neural plate = precursor of the NS
- the ectoderm anterior to node forms the neural plate
What is the fate of neural plate cells compared to epidermal cells ?
- cells of the neural plate –> elongate to form a
pseudostratified columnar epithelium - cells of the epidermis –> remain cuboidal
What is regression ?
Regression = a phenomenon where, as the embryo
expands through cell proliferation, the primitive streak
becomes restricted to the posterior margin of the epiblast
When does the neural tube form ?
4th week of human development.
How does the neural tube close ?
Which parts of the tube close last ?
- the edges initially meet at only a single site –> at the boundary of the hindbrain and spinal cord (but in reality there may also be a sites in the midbrain and at the anterior boundary)
- the rest of the tube closes like a “zipper” from the initial sites, the caudal neuropore closing last
What drives neural tube closure ?
- cell shape changes drive neural tube closure
- prior to neurulation, neuroepithelial cells = cuboidal
- during neurulation –> cells elongate along apical-basal axis to become columnar
What forces neuroectodermal cells to adopt a wedge shape ?
Apical constriction –> occurs when the apical actin-myosin cable contracts, forcing cells to adopt wedge shape. This generates the physical force that causes the neural plate to bend.
What are hinge points ?
Hinge point = apical contraction sites causing neural plate to bend
What are the most common neural tube defects (NTDs) ?
How frequent are they ?
How lethal are they ?
- Anencephaly = rostral neuropore fails to close
- Spina bifida when caudal neuropore fails to close.
- Anencephaly + spina bifida account for up to 95% of all NTD, with equal prevalence
- affects ~1400 UK/year, 90% are terminated
Where are neural crest cells formed ?
- neural crest cells –> formed at the lateral edges of the neural plate –> become the roof of the neural tube
- they delaminate from the neural tube to become a mesenchymal population that migrates away form the neural tube to form many different cell types
Which cells will mesodermal neural crest cells give rise to ?
- smooth muscle
- osteoblasts
- osteoclasts
- adipocytes
- chondrocytes
Which cells will ectodermal neural crest cells give rise to ?
- melanocytes
- Schwann cells
- PNS neurons
In which species is the neural crest found ?
Why is this important ?
In vertebreate embryos.
Considered crucial for vertebrate evolution.
At what point do mammalian embryos display the basic vertebrate body plan ?
Which organs can we thus see ?
- at the end of neurulation
- we can make out the forebrain (fb), midbrain (mb), hindbrain (hb), trunk (t), a large heart (h), aorta, and umbilical cord
What is the phylotypic stage ?
What happens after this stage ?
- Phylotypic stage = the point where vertebrate embryos are most similar to each other at the end of neurulation and share a common body plan
- After –> morphology increasingly diverges
What evidence supports that, at the phylotypic stage, vertebrate embryos share the same “molecular anatomy”
Using in situ hybridization, we can see that the mRNA of a gene called krox-20 is localised to two stripes in the hindbrain of mouse, chick, frog, and fish embryos.
By which point are most of the organs formed in human embryos ?
When do all these organs become functional ?
- most of the organs are formed during a 4 week period after the formation of the body plan = organogenesis
- most of the organs and tissues –> become fully functional during the fetal period (~30 weeks)
- even then only a very immature infant is born
