lec 18- early development in mammals Flashcards
why is studying embryonic development in mammals challenging?
-mammal eggs are small making them hard to manipulate
-they are produced in small numbers
-development is inside the mother
why is the mouse an important model organism?
-short life cycle
explain mouse life cycle and turning:
-fertilized egg in oviduct and cleavage occurs
-5 days after fertilization, the blastocyst implants in the uterine wall where gastrulation occurs
-gastrulation is followed by a complicated process called turning in which the ectoderm on the inside of the cup-shaped embryo is brought to the outside and the mesoderm on the outside is brought inside
-organogenesis
-growth and birth
where does fertilization and the first cleavage occur in mammals?
in the oviduct
is the cleavage in mammals the slowest in the animal kingdom?
yes, in mice it takes 24 hours after fertilization for cleavage to occur
what happens when the embryo reaches the uterus?
it implants in the uterine wall and the remainder of the embryonic development occurs there
how many days after fertilization does implantation occur in humans?
7 to 11 days
mammals exhibit rotational cleavages, what are they?
the 2nd division of the two blastomeres are oriented perpendicular to the first division but at right angles in different planes to each other
what does the asynchronous cleavages in mammals mean?
the blastomeres do not all divide at the same time, many stages when the embryo contains uneven number of blastomeres
what are the steps to cleavage in the mouse embryo?
-fertilized mouse egg before 1st cleavage
-the polar body is a haploid cell formed during the meiotic events that generate the egg
-after the 3rd cleavage, at the 8-celled stage the blastomeres huddle together
-contact between balstomeres is maximized and they form a compact ball of cells = morula
what is compaction?
uncompacted embryos form a loose arrangement with space between the blastomeres
explain blastocyst parts:
-after compaction, further cleavages produce a morula that has a small group of internal cells and a large group of external cells
what does the internal cells do in the blastocyst?
the internal cells give rise to the embryo proper while most of the descendants of the other cells become the trophoblast
what does the trophoblast form in the blastocyst?
it forms the embryonic portion of the placenta and it is required for implantation of the embryo in the uterine wall
what are trophoblast cells responsible for?
responsible for filling the interior of the compacted morule with fluid to create blastocoel inside, the inner cell becomes positioned on one side and the embryo is now a blastocyst
how does water flow into the interior space of the embryo during blastocyst formation?
sodium creates a gradient that allows water to be pumped in and enlarge the cavity resulting in the ball shaped embryo
what are the two regions the inner cell mass divides into during implantation and what do they do?
-the epiblast and the primitive endoderm
-the epiblast develops into embryo proper and the primitive endoderm contributes to the formation of extra-embryonic tissues
what surrounds the embryo during implantation?
trophoblast cells which form a trophectoderm
what does the polar trophectoderm make contact with?
the epiblast
where is the mural trophectoderm located?
on the opposite side of the polar trophectoderm
explain implantation in mice:
-the polar trophoectoderm in contact with he epiblast proliferates and forms extra embryonic tissues that contribute to the placenta
-in mouse, the epiblast elongates and develops a cavity (proamniotic cavity), this gives the embryo a cup-shaped form, this cavity forms in a different way from the blastocyst cavity, in mice it forms by a process called programmed cell death
-the mural trophectoderm undergoes DNA replication without cell division, these cells become the trophoblast giant cells, they invade the uterine wall and help anchor the blastocyst to it
what happens after implantation in mice?
the mouse epiblast elongates and develops an internal cavity (proamniotic cavity), giving it a cup shaped form
what marks the beginning of gastrulation?
-the embryo enlarges, the appearance of the primitive streak at the posterior of the epiblast marks the start of gastrulation
-the primitive streak extends towards the anterior
what happens during gastrulation?
-cells converge and move through the primitive streak then migrate outwards to form a layer of prospective mesoderm
-the migrating cells move under the endoderm, gradually displacing it
-the primitive streak lengthens and reaches the bottom of the cup, the node is located at the anterior end of the primitive streak
-at this stage the embryo is inside out, ectoderm is on the inner surface and the endoderm is on the outer, unlike other embryos
how does the embryo develop in mice?
-inside out embryo
-embryo grows in size, definitive endoderm forms a layer on the ventral surface of the embryo and the notochord begins to form
-further development result in the notochord, head, gut, and somites to form
what is turning in the embryo of mice?
-the ventral side is now curled around the amniotic cavity
-the ectodermal tissues are brought to the outside of the embryo and the green endodermal tissues are brought to the inside
explain development similarities and differences between mouse and human embryos?
-the early stages of development till the formation of the epiblast are similar
-the only difference is that the epiblast remains flat shaped
-gastrulation is similar to chicks
what type of cell populations do es the chick enbryo have during the morula stage?
distinct outer and inner cell populations
what defines the embryonic-abembryonic axis in chick embryos?
the position of the inner cell mass
what can the inner cell mass be isolated to produce?
embryonic stem cells (ES cells)
what do ES cells give rise to?
any of the cell types
does the AP axis form before the primitive streak is visible?
yes
what do the inhibitory signals from the extra embryonic ectoderm restrict?
anterior fate in the embryo to the distal region
what is the endoderm specified as from signals from the epiblast?
distal visceral endoderm (DVE)
how does the primitive streak form?
-the DVE extends and moves to one side where it becomes the anterior visceral endoderm (AVE)
-AVE specifies the end and prevents primitive streak from forming there and signals to epiblast to induce anterior ectoderm
-the primitive streak forms on opposite side of epiblast
what is left-right symmetry?
body plan development with respect to the left and right sides
what is situs inversus?
when organs are located on the opposite side of that to where they normally are
how does the left right axis form?
-the node is lined by cilia, the cilia in the center beat in a coordinated manner in the clockwise direction causing a leftward flow of extracellular fluid
-the immotile cila act as mechanoreceptors that release Ca2+ in response of movement of the fluid (to the left means more Ca2+ at the left cilia)
-more Ca2+ means more nodal expression
-or the leftward flow results in accumulation of morphogen which causes more up regulation of nodal
