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Axis Formation 1

Organisation of the Body

QuestionAnswer
What occurs during days 6-14 Inner cell mass forms the bilaminar germ disc Contains hypoblast and epiblast (cell monolayer that forms the embryo) Bilaminar disc surrounded by cavities Trophoblast invades endometrium and makes HCG
Decidual reaction Endometrium responds to trophoblast and starts to generate maternal blood invasions Allows later placental development
Formation of syncytiotrophoblast Cytotrophoblast is mitotic - continues dividing and fuses to form the syncytiotrophoblast This produces hydrolytic enzymes which invade the endometrium and allow for placental development This also produces HCG to maintain the corpus luteum
Formation of the bilaminar disc Inner cell mass contains the hypoblast and epiblast (starts prior to implantation) These make up the bilaminar germ disc The epiblast is a sheet of cells destined to form the embryo
Formation of the amniotic cavity The first of several new cavities that eventually surround the whole embryo Forms above epiblast Formed from epiblast and amnioblast cells
Formation of primary yolk sac Embryo embeds into endometrium The coagulation plug forms - endometrium seals the structure Vacuoles in syncytiotrophoblast fuse to form lacunae Cells from hypoblast form Heuser's membrane lining primitive yolk sac (blastocoel)
Formation of chorionic cavity Extraembryonic mesoderm formed by second migration of cells from edges of hypoblast and epiblast generates acellular material between the trophoblast and amnion and primitive yolk sac Chronic cavity forms here
Formation of the secondary yolk sac Growth of trophoblast much faster than bilaminar disc They separate to form the chorionic cavity and secondary yolk sac Connecting stalk forms the umbilical cord Amniotic cavity will eventually surround embryo and lie inside chorionic cavity
Further development of yolk sac Endometrium - decidua reaction Oedematous tissue forms containing cells loaded with glycogen and lipids This spreads through endometrium Lacunae fill with maternal blood forming sinusoids Cellular columns of cytotrophoblast cells - primary villi
What affect can bleeding at 2 weeks have As the surface defect (coagulation plug) heals bleeding can occur from lacunae Can lead to inaccuracies in due date as it appears a period has occured
Embryonic axis Top - rostral Bottom - caudal Back - dorsal Front - ventral
Development of bilaminar germ disc Ectoderm forms a cylindrical shape Epiblast becomes a multi-layered structure Top layer forms ectoderm - outer lining of fetus Inner layer contains mesoderm and endoderm - forms inner structures
Placental development Primary villi extend and form secondary/tertiary villi Villi begin to form blood vessels to allow transport to the fetus Maternal blood fills lacunae to allow exchange
What is Gastrulation A complex series of movements during which the three germ layers establish their appropriate topological positions and the basic body plan emerges This forms the 3D structure
Germ layers All form from the epiblast Ectoderm - outer surface, CNS and neural crest Endoderm - digestive tube, pharynx and respiratory tube Mesoderm - all other aspects Mesenchymal cells can derive from any germ layer - form a loosely organised structure
Formation of germ layers - Xenopus Ectoderm and endoderm specified Ectoderm not committed - if placed above endoderm will form mesoderm Shows there is signalling between cells telling them what to form
Molecules regulating mesodermal induction in Xenopus Beta catenin along with VegT and Vg1 trigger development of the organiser Only VegT and Vg1 leads to mesoderm Different signals are present on different sides of the embryo, so different regions develop
Organiser region - Xenopus Produces BMP inhibitors that trigger mesodermal cells to form Induced by dorsal endoderm (organiser function partly involves BMP signalling inhibition) Regulates mesodermal, neural and endodermal patterning Forms the axis
Gastrulation in Xenopus Cells on the outside of the embryo (endoderm and mesoderm) move inwards Ectoderm remains outside The organiser migrates inside Cells migrate and internalise as a sheet Mesoderm ends up between ectoderm and endoderm
Spemann's organiser graft Took an organiser (dorsal tip of blastopore) and transplanted it to the opposite side of another embryo As it developed, this embryo formed two embryos Almost all cells in the 2nd were derivatives of the 1st reprogrammed by the organiser
Temporal control of the organiser When transplanted early in development - leads to rostral development When transplanted late in development - caudal development The rostral/caudal fate of cells is determined by the time at which they pass through the organiser
Mammalian gastrulation - Primitive streak and Primitive node Reduced hypoblast signalling induces epiblast cells to pile up along primitive streak Cells detach and migrate individually through basement membrane - endoderm then mesoderm Ectoderm on outside Delaminating spreads laterally and rostrally
Buccopharyngeal and Cloacal membranes Adherent endoderm and ectoderm - bind ends of gut tube No mesoderm
Forms of mesoderm in human gastrulation Prechordal mesoderm - forms head and heart Prenotochordal (axial) mesoderm Paraxial, intermediate and lateral plate mesoderm
Primitive node as an organiser Primitive streak functionally equivalent to blastophore lip - acts as an organiser Primitive node can be thought of as a functional equivalent to organiser/dorsal lip of blastopore Nodal TGF-beta for primitive streak initiation and maintenance
Defects in primitive node/streak activity Regresses caudally - usually by 4th week Conjoined twins occurs when two primitive nodes joint to give two different axis Sacrococcygeal teratoma occurs when remnants of the primitive streak are left in sacrococcygeal region - not fully regressed
Human Stem-cell embryo models Human stem-cell-based embryo models with spatially organised morphogenesis can create both embryonic and extraembryonic tissues Combine artificially partially differentiated cells representing epiblast, hypoblast, extra-embryonic and trophoblast
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