GastrulationIN

MAMMALS

SWATIM.Sc. (Prev.)

BiotechnologyMDU Rohtak

"It is not birth, marriage, or death,but gastrulation, which is truly the most

important time in your life."

Lewis Wolpert (1986)

"It is not birth, marriage, or death,but gastrulation, which is truly the most

important time in your life."

Lewis Wolpert (1986)

•

• The process in which a gastrula develops from ablastula by the migration of cells is known asgastrulation.• Gastrulation produces a three-layered embryo• Gastrulation Second major phase of embryonic

development; adds more cells to the embryo; sortsthe cells into distinct cell layers.

• The process in which a gastrula develops from ablastula by the migration of cells is known asgastrulation.• Gastrulation produces a three-layered embryo• Gastrulation Second major phase of embryonic

development; adds more cells to the embryo; sortsthe cells into distinct cell layers.

In this process, the bilaminar embryonicdisc, consisting of epiblast and hypoblast,transforms into trilaminar embryonic discconsisting of three primary germ layers :- Ectoderm- Mesoderm- Endoderm.

In this process, the bilaminar embryonicdisc, consisting of epiblast and hypoblast,transforms into trilaminar embryonic discconsisting of three primary germ layers :- Ectoderm- Mesoderm- Endoderm.

• Inner Cell Mass (ICM)delaminates to formhypoblast and epiblast• Occurs just prior toimplantation &gastrulation• Epiblast (green cells) is2-layered (i.e., it isbilaminate) disc ofapproximately cuboidalcells & will form theembryo proper•Flatter hypoblast cellslie below the epiblastand will form yolk sac

• Inner Cell Mass (ICM)delaminates to formhypoblast and epiblast• Occurs just prior toimplantation &gastrulation• Epiblast (green cells) is2-layered (i.e., it isbilaminate) disc ofapproximately cuboidalcells & will form theembryo proper•Flatter hypoblast cellslie below the epiblastand will form yolk sac

HumanGastrulation

HumanGastrulation

• Very little work has been done on humangastrulation

• Most work on fixed and stained human embryos• No experimental work is being done on human

gastrulation because it is not ethical or legal• Therefore, difficulty getting specific stages• Originally used knowledge from chick gastrulation:

general morphology looks similar

• Very little work has been done on humangastrulation

• Most work on fixed and stained human embryos• No experimental work is being done on human

gastrulation because it is not ethical or legal• Therefore, difficulty getting specific stages• Originally used knowledge from chick gastrulation:

general morphology looks similar

Gastrulationin

Human Embryo

Gastrulationin

Human Embryo

• Embryo is bilaminate(i.e., epiblast andhypoblast)• Epiblast forms the

three germ layers(ectoderm, mesodermand endoderm)• Hypoblast forms the

extraembryonicmembranes

• Embryo is bilaminate(i.e., epiblast andhypoblast)• Epiblast forms the

three germ layers(ectoderm, mesodermand endoderm)• Hypoblast forms the

extraembryonicmembranes

•Initially cells move alongsurface (blue arrows) butupon reaching the centerline (primitive streak) willenter the embryo, turn thecorner and move internally(red arrows).• The moving surface cellsfirst pile up to form aprominent bump known asthe primitive node (also“node”). This occursbecause the cells movealong the top faster thanthey can separate off andmove internally.

•Initially cells move alongsurface (blue arrows) butupon reaching the centerline (primitive streak) willenter the embryo, turn thecorner and move internally(red arrows).• The moving surface cellsfirst pile up to form aprominent bump known asthe primitive node (also“node”). This occursbecause the cells movealong the top faster thanthey can separate off andmove internally.

•

CELL movementsCELL movements

- Invagination: a sheetof cells (called anepithelial sheet) bendsinward.

- Ingression: individualcells leave an epithelialsheet and becomefreely migratingmesenchyme cells.

- Involution: anepithelial sheet rollsinward to form anunderlying layer.

- Invagination: a sheetof cells (called anepithelial sheet) bendsinward.

- Ingression: individualcells leave an epithelialsheet and becomefreely migratingmesenchyme cells.

- Involution: anepithelial sheet rollsinward to form anunderlying layer.

• Epiboly: a sheet of cellsspreads by thinning. Themovement of epithelialsheets that spread as a unit,rather than individually, toenclose the deeper layers ofthe embryo.

• Intercalation: rows of cellsmove between one another,creating an array of cellsthat is longer (in one or moredimensions) but thinner.

• Convergent Extension: rowsof cells intercalate, but theintercalation is highlydirectional.

• Epiboly: a sheet of cellsspreads by thinning. Themovement of epithelialsheets that spread as a unit,rather than individually, toenclose the deeper layers ofthe embryo.

• Intercalation: rows of cellsmove between one another,creating an array of cellsthat is longer (in one or moredimensions) but thinner.

• Convergent Extension: rowsof cells intercalate, but theintercalation is highlydirectional.

Formationof

primitive node

Formationof

primitive node

primitive streak is afaint groove on thedorsal surface of theepiblast thatelongates from theposterior to theenterior part of theembryo.

primitive streak is afaint groove on thedorsal surface of theepiblast thatelongates from theposterior to theenterior part of theembryo.

•The primitive streakestablishes the headand tail ends of theembryo as well as itsright and left sides.•At the head end ofthe primitive streak asmall group ofepiblastic cells formsa rounded structurecalled the primitivenode.

•The primitive streakestablishes the headand tail ends of theembryo as well as itsright and left sides.•At the head end ofthe primitive streak asmall group ofepiblastic cells formsa rounded structurecalled the primitivenode.

• The structure closer to the head end is called theoropharyngeal membrane.

• The structure closer to the tail end is called the cloacalmembrane, which degenerates in the 7th week to form theopenings of the anus and the urinary and reproductivetracts.

• About 16 days after fertilization, mesodermal cells fromthe primitive node migrate towards the head end of theembryo and form a hollow tube of cells in the middle calledthe notochordal process.

• By 22-24 days, the notochordal process becomes a solidcylinder of cells called the notochord.

• The structure closer to the head end is called theoropharyngeal membrane.

• The structure closer to the tail end is called the cloacalmembrane, which degenerates in the 7th week to form theopenings of the anus and the urinary and reproductivetracts.

• About 16 days after fertilization, mesodermal cells fromthe primitive node migrate towards the head end of theembryo and form a hollow tube of cells in the middle calledthe notochordal process.

• By 22-24 days, the notochordal process becomes a solidcylinder of cells called the notochord.

neurulationneurulation

• Notochord also induces ectodermal cells over it to formthe neural plate.

• By the end of the 3rd week, the lateral edges of theneural plate become more elevated and form the neuralfold.

• The depressed mid region is called the neural groove.Generally, the neural folds approach each other andfuse, thus converting the neural plate into a neuraltube.

• Neural tube cells then develop into the brain and spinalcord.

• The process by which the neural plate, neural folds, andneural tube form is called neurulation.

• Notochord also induces ectodermal cells over it to formthe neural plate.

• By the end of the 3rd week, the lateral edges of theneural plate become more elevated and form the neuralfold.

• The depressed mid region is called the neural groove.Generally, the neural folds approach each other andfuse, thus converting the neural plate into a neuraltube.

• Neural tube cells then develop into the brain and spinalcord.

• The process by which the neural plate, neural folds, andneural tube form is called neurulation.

• After 4 weeks of fertilization, the head and neuraltube develops into three enlarged areas of braincalled: the prosencephalon or forebrain,mesencephalon or midbrain, and rhombencephalonor hindbrain.• After 5 weeks, the prosencephalon develops into

secondary brain vesicles called the telencephalonand diencephalon; and the rhombencephalondevelops into secondary brain vesicles called themetencephalon and myelencephalon.• The areas of the neural tube adjacent to the

myelencephalon develop into the spinal cord.

• After 4 weeks of fertilization, the head and neuraltube develops into three enlarged areas of braincalled: the prosencephalon or forebrain,mesencephalon or midbrain, and rhombencephalonor hindbrain.• After 5 weeks, the prosencephalon develops into

secondary brain vesicles called the telencephalonand diencephalon; and the rhombencephalondevelops into secondary brain vesicles called themetencephalon and myelencephalon.• The areas of the neural tube adjacent to the

myelencephalon develop into the spinal cord.

Genes involvedin

gastulation

Genes involvedin

gastulation

• shh - Sonic Hedgehog plays a role in cell growth, cellspecialization, and the normal shaping of the body. Thisprotein is important for development of the brain andspinal cord (central nervous system), eyes, limbs, andmany other parts of the body.

• Fgf8 - Fibroblast Growth Factor 8 plays an importantrole in the regulation of embryonic development, cellproliferation, celldifferentiation and cell migration. Required for normaldevelopment of the gonadotropin-releasing hormone(GnRH).

• Lrd - Left-right dynein protein selectively segregatessister chromatids based on their replication history i.e.determines the symmetry or left right axis of embryo.

• shh - Sonic Hedgehog plays a role in cell growth, cellspecialization, and the normal shaping of the body. Thisprotein is important for development of the brain andspinal cord (central nervous system), eyes, limbs, andmany other parts of the body.

• Fgf8 - Fibroblast Growth Factor 8 plays an importantrole in the regulation of embryonic development, cellproliferation, celldifferentiation and cell migration. Required for normaldevelopment of the gonadotropin-releasing hormone(GnRH).

• Lrd - Left-right dynein protein selectively segregatessister chromatids based on their replication history i.e.determines the symmetry or left right axis of embryo.

• Kif2A - Kinesin Heavy Chain Member 2A required fornormal congress of chromosomes at the metaphaseplate.

• Lefty2 - This gene encodes for the TGF-beta family ofproteins. The encoded protein is secreted and plays arole in left-right asymmetry determination of organsystems during development.

- Mutations in this gene have been associated with left-right axis malformations,particularly in the heart and lungs.

• Kif2A - Kinesin Heavy Chain Member 2A required fornormal congress of chromosomes at the metaphaseplate.

• Lefty2 - This gene encodes for the TGF-beta family ofproteins. The encoded protein is secreted and plays arole in left-right asymmetry determination of organsystems during development.

- Mutations in this gene have been associated with left-right axis malformations,particularly in the heart and lungs.

Thank You