The Early Development of Birds Mammals Gilbert Chapter

The Early Development of Birds & Mammals Gilbert - Chapter 11

Goals • Become familiar with the cleavage and gastrulation patterns in birds and mammals • Examine the adaptations specific to the amniote egg • Understand the specialized structures that develop in mammals to allow for embryonic implantation

Early Avian Development: Chick • Why Chick embryos as a model system? – Easily obtained – Control developmental stage needed by incubating when ready – Relatively large embryo • Good for surgical manipulation • Many similarities to human development – Inexpensive

Early Avian Development: Chick • Fertilization occurs internally prior to secretion of shell • Egg is telolecithal – VERY yolky! • Cleavage is discoidal

Cleavage in Chick embryo • 1 st cleavages occur in a small area of cytoplasm called the blastodisc – These do not penetrate through whole cell, so cells are continuous with each other • As cleavage continues, a small disc of cells about 4 -6 layers deep is created in the center of the blastodisc

• In the center of that disc – Cells in the deeper layers die off, forming a single celled layer of blastoderm called the AREA PELLUCIDA – This will form the actual embryo - these cells are also called the EPIBLAST • On the margins of that disc – Deeper cells do not die off, creating a thicker layer called the AREA OPACA • Underneath this disc is a fluid-filled cavity called the subgerminal space

Occurs while inside the hen!

Gastrulation in Chick Embryo • First step: form a 2 layered blastoderm – EPIBLAST (embryo proper) & HYPOBLAST (extraembryonic membranes) – 2 mechanisms • 1) Cells delaminate from the epiblast and migrate as “islands” into the subgerminal cavity – Primary hypoblast • 2) Cells near the posterior end migrate as a sheet toward the anterior end (inside the subgerminal cavity) – Secondary hypoblast

Avian Gastrulation • Second step: – Formation of germ layers • Avian embryos form a structure called the primitive streak • This is where the formation of germ layers begins • Thickening of the epiblast near the posterior end

• As the primitive streak extends, a depression forms at the midline, called the primitive groove • It is through the primitive groove that cells will migrate into the blastocoel – Analogous to the amphibian blastopore • At the anterior end, a thickening called Hensen’s node is a funnel shaped depression where cells begin to migrate by ingression into the blastocoel – Analogous to the organizer (dorsal blastopore lip)

The first cells that migrate through Hensen’s node are endoderm, then chordamesoderm

• Creation of the primitive streak establishes the anterior-posterior axis of the embryo – Begins at the tail end, moves toward the head end

• Cells continue to ingress through the primitive streak, laying down the mesoderm layer • At this point, the primitive streak regresses

Regression of the Primitive Streak • During regression, the dorsal axis of the embryo is laid down – Notochord, and neural precursors become established – Head ectoderm begins to develop • Hensen’s node regresses to the posterior end, becomes anus

• In this case, the result is actually a flat embryo with 3 germ layers, rather than a spherical embryo • The avian embryo is still atop the yolky mass – Endoderm separates the embryo from the yolk

The Chick Organizer • Hensen’s node! • Transplant to an ectopic location, form a secondary axis

Avian Gastrulation: Summary • • Complex! Initiated by Hensen’s node Sets up the body axes Establishes the germ layers

Mammalian Development • Difficult to study – Internal fertilization and development • Fertilization occurs IN the oviduct! – Few embryos produced – Lengthy development periods – Mice: mother must be sacrificed to harvest embryos – Some early development can be viewed in in vitro culture systems • Fertlization, cleavage, some of gastrulation (mice)

Mammalian Cleavage • Very tiny egg! • Meiosis not completed until fertilization • Different from other types of cleavage – – – Holoblastic, rotational Slower Asynchronous 1 st cleavage: normal, meridional cleavage plane 2 nd cleavage: • One blastomere meridional • One blastomere equitorial – Embryonic gene expression occurs right away

Cleavage and Compaction • Another major difference = Compaction – After the 3 rd cleavage (8 cells), blastomeres express large amounts of adhesion molecules, like cadherins – Cells become a tight, compact ball of cells • Inner cells sealed off from outside environment – At the 16 cell stage, embryo becomes a morula • Small group of cells inside surrounded by layer of external cells

Morula • Cells on the outside are called trophoblast and become extraembryonic membranes and embryonic portion of the placenta • Cells on the inside are called Inner Cell Mass (ICM) and produce the embryo proper This is the first differentiation event in mammalian cells

The Blastocyst • As the morula develops, a fluid filled cavity forms (blastocoel) • In this case the ICM is positioned on one side of the trophoblast, creating not a blastula, but a blastocyst

• The trophoblast will develop into specialized tissues that will allow for the embryo to implant into the placenta • The inner cell mass – Develops into two layers: Epiblast and Hypoblast – Gastrulation will occur here – Similar to chick embryo • ICM is the source of pleuripotent stem cells!

Early Mammalian Development: Summary • Very different cleavage pattern • Form a blastocyst – ICM, trophoblast • Gastrulation in humans is very similar to chick • The node is the organizer • Mouse - very unusual gastrulation pattern!