Study Questions 4 Flashcards
- How does amount of yolk influence cleavage in birds? How do we call this type of cleavage?
a. The egg is telolecithal in that it has a small amount of cytoplasm sitting atop a large yolk (at the animal pole) – called blastodisc.
b. The bird egg undergoes discoidal meroblastic cleavage.
c. Cleavage occurs only in the blastodisc and doesn’t extend into the yolk⇒ formation of the tissue 5-6 layers thick
- How is hypoblast formed during gastrulation in birds? (Keep in mind there are two hypoblasts.)
a. Formation of the primary hypoblast occurs through delamination and migration of epiblast cells forming islands (=primary hypoblast).
b. The secondary hypoblast is then formed by a sheet of cells from the posterior marginal zone (PMZ) of the blastoderm migrating anteriorly and merging with the primary hypoblast cells anteriorly. (Koller’s sickle is a visible localized thickening of the posterior marginal zone.)
c. This process forms the blastocoel between (merged primary and secondary) hypoblast and epiblast
- Compare and contrast gastrulation in amphibians and birds. (Think about different cell movements embryonic cells have to “use” due to the amount of yolk present; this will dictate the way blastocoel will be formed.)
a. Difference:
i. Amphibians – mesolecital egg – blastocoel forms through holoblastic cell divisions; gastrulation starts through INVAGINATION and INVOLUTION of cells at organizer
ii. The chicken egg is telolecithal ⇒ small amount of cytoplasm sitting atop a large, dense yolk – cell divisions cannot easily go through yolk and they cannot start gastrulation through involution/invagination).
iii. The chicken solves this by having another method of gastrulation – blastocoel formation starts through DELAMINATION
b. Similar:
i. hypoblast is formed in subgerminal space
ii. hypoblast (in subgerminal space) with epiblast (on top of subgerminal space) form blasotocoel and blastula.
iii. Gastrulation – through formation of primitive streak; cells go through INTERCALATION and CONVERGENT EXTENSION
- What is the relationship between primitive streak, primitive groove and Hensen’s node?
a. The Primitive Streak is the major structural characteristic of avian, reptilian and mammalian gastrulation.
b. The streak is initially visible as a thickening of the epiblast at the posterior marginal zone. Moves from P to A (and eventually retracts)
c. The presumptive streak cells become globular and motile, and they appear to digest away the extracellular matrix underlying them.
d. This process allows them to move: intercalation (mediolaterally) and convergent extension.
e. Convergent extension allows for the progression of the streak- a doubling in streak length is accompanied by an simultaneous halving of its width
f. Eventually a depression forms within the streak called the primitive groove and it serves as an opening through which migrating cells pass into the blastocoel.
g. At the anterior end of the streak is a regional thickening called the primitive knot (node) or Hensen’s node. The center of this node contains a funnel shaped depression through which cells can pass into the blastocoel.
- What are the very first cells to pass through the primitive streak during gastrulation? (i.e. what will they form?)
a. The first cells to pass through the streak are endodermal precursors. They undergo an epithelial to mesenchymal transition as the streak elongates toward the putative head region (towards A).
- Compare and contrast formation of Nieuwkoop center in amphibians and birds. What is a chicken equivalent to Spemann organizer? What is a chicken equivalent to blastopore lip? Explain your reasoning (in respect to their role in gastrulation). You can use diagrams.
b. Amphibians
i. The Nieuwkoop center is created by cytoplasmic rotation following fertilization
ii. Before fertilization β-catenin is expressed from maternal mRNA throughout the embryo
iii. After fertilization - β-catenin is in the dorsal part of the embryo; protein Vg1 is in the vegetal part (also coded by maternal mRNA); both important in defining the Nieuwkoop center.
iv. Accumulation covers both the Nieuwkoop center and the organizer regions.
c. Review: in Xenopus, site of sperm entry determines the placement of β-catenin in the dorsal region (through regulation of GSK-3). The vegetal cell secrete Vg1. The overlap of the two becomes the Nieuwkoop center
d. In chicken, β-catenin (induced by Wnt8c; inhibited by GSK-3) is in the outer cells of the blastodisc. The Vg1 producing cells are restricted, by gravity, to the presumptive posterior cells. Again the overlap (PMZ) becomes the chicken “Nieuwkoop center”.
e. Hensen node, is the structural analog to the amphibian Spemann’s organizer (=dorsal blastopore lip) cells can pass into the blastocoel.
f. primitive streak is equivalent to amphibian blastopore lip(s) as it is the site of invagination of (future) endoderm and mesoderm cells;
- What are characteristics of mammalian early cleavage?
a. Isolecital, holoblastic, rotational cleavage
b. Early cleavage in mammals differentiates itself from our other examples in five primary ways;
c. Divisions are very slow.
d. The orientation of blastomere cleavage varies relative to each other (rotational).
e. Divisions are not necessarily synchronic/symmetrical.
f. Unlike most embryos we have discussed, the mammalian genome is activated early in cleavage (mouse’s- after 2 divisions, human’s- after 4).
g. Finally, and potentially most important, is the phenomenon of compaction.
h. In mouse, cells are loosely packed until about the eight cell stage.
i. Following the third cleavage the blastomeres undergo a radical change; blastomeres huddle together, forming a compact ball;
j. Cells acquire polarity – microvilli on the apical end, tight junctions between the cells. The inside of the sphere is sealed. Internally the cells form gap junctions to allow molecules and ions to pass.
k. Dependent upon adhesion molecules such as e-cadherin.
l. The compacted 8 cell embryo divides to form the 16-cell-morula.
m. The morula consists of a large group of surface cells and a small group of internal cells.
n. Most external cells become part of the trophoblast (or trophectoderm), they don’t contribute to embryonic structures. They form the chorion, the embryonic portion of the placenta.
o. The mouse embryo proper (= the actual embryo) is derived from the cells at the center of the 32-cell-morula. These cells give rise to the inner cell mass (ICM) which will develop into the embryo, but also into the extraembryonic membranes yolk sac, allantois and amnion.
v
- Define blastocyst. Draw blastocyst. Label ICM and trophoblast.
a. Blastocyst – mammalian blastula, specific for mammalian cleavage, some differentiation has occurred
- What was the role of zona pellucida during fertilization? What is the role of zona pellucida during implantation of mammalian embryo? When and why does it have to shed?
The zona pellucida is essential in early embryo development to keep the embryo from implanting within the oviduct.
b. It must be shed upon entering the uterus for embryo to implant.
c. Blastocyst does this by digesting a small hole in the zona pellucida and squeezing out as it expands. Accomplished by the enzyme trypsin on the surface of the trophoblast cell membrane.
d. Once out, the blastocyst binds to the uterine epithelium (endometrium). The blastocyst will digest and imbed itself into the uterine wall - IMPLANTATION
- What is the role of trophoblast during embryonic implantation (and in general during pregnancy)? What are the two types of trophoblast cells and what is their role in implantation? Is trophoblast going to give rise to any embryonic tissue? What is chorion?
a. Trophoblast starts to embed the embryo into uterine tissue through the uterine endometrium
b. Cytotrophoblast and syncytiotrophoblast (from the trophoblast) contribute to further digestion of the uterus and the formation of the placenta (embryonic tissue).
c. cytotrophoblast and the syncytiotrophoblast secrete proteolytic enzymes that remodel the uterine wall.
d. They also secrete paracrine factors that attract maternal blood vessels and remodel them to be lined with trophoblast cells.
e. Extraembryonic mesoderm then extends to the trophoblast cells forming the umbilical cord.
f. The fully developed organ consisting of trophoblast tissue and extraembryonic mesoderm containing blood vessels is the chorion.
g. The chorion fuses with the uterine wall to create the placenta.
- What is formed during the first segregation of cells within the ICM? How do we call this structure? Which part of the structure will develop into the embryo proper?
a. The fist segregation of cells within the inner cell mass forms the hypoblast (=primitive endoderm; green) and the epiblast (=primitive ectoderm; brownish purple).
b. The epiblast and the hypoblast form the bilaminar germ disk.
c. embryonic epiblast part which will develop into embryo proper
- What will be formed from the epiblast layer during mammalian early development? What will be formed from the hypoblast layer?
a. Epiblast – embryonic ectoderm, primitive streak, embryonic endoderm and mesoderm
b. Hypoblast – extraembryonic endoderm – yolk sac , extraemcryonic mesoderm
- What are parietal and visceral hypoblasts? What do they give a rise to?
a. some cells delaminate to line the blastocoel cavity and they give rise to the parietal endoderm;
b. those hypoblast cells in contact with epiblast will form visceral endoderm
c. they form the extraembryonic endoderm
- What does DVE stand for and what is its role during mammalian embryonic development. (Alternative question could be about AVE.)
a. Distal visceral endoderm (DVE) is extraembryonic endoderm (originates from hypoblast) laying over the distal most part of the epiblast
i. Nodal signaling from the epiblast induces expression of specific genes in distal visceral endoderm, DVE
ii. DVE cells move to one side of the epiblast cup, and induce anterior visceral endoderm, AVE
b. AVE (Anterior Visceral Endoderm – EXTRAEMBRYONIC structure)
i. AVE promotes anterior differentiation by secreting Wnt inhibitors Cerberus and Dikkopf, Nodal inhibitor Lefty1, and some BMP inhibitors; this prevents Nodal activity in the anterior (i.e prevents formation of primitive streak in the anterior)
- What are the signaling centers and morphogens involved in mammalian anterior-posterior axis formation? Are the signaling center embryonic or extraembryonic in origin?
a. AVE (Anterior Visceral Endoderm – EXTRAEMBRYONIC structure)
b. Node (equivalent to Hensen’s node, which is anterior end of the primitive streak - EMBRYONIC structure).
c. AKA both in origin
d. Morphogens
e. Transient BMP signals from amnionic ectoderm (= extraembryonic ectoderm) induce Wnt3 and Nodal in the epiblast;
f. Epiblast cells become competent to respond to different (new) signals, depending on position in the epiblast cup
g. Nodal signaling from the epiblast induces expression of specific genes in distal visceral endoderm, DVE
