Chapter 9 – Early development in vertebrates (birds and mammals) Flashcards
Amniotes
vertebrates whose embryos form an amnion (water sac), i.e. reptiles, birds and mammals.
Birds and reptiles follow a very similar pattern of developmen
Amniote egg - characterized by a set of membranes that enable the embryo to survive on land:
- The amnion – formed early in embryonic development, enables the embryo to float in a fluid environment that protects it from desiccation.
- The yolk sac – enables nutrient uptake and the development of the circulatory system
- The allantois – developing at the posterior of the embryo, stores waste products
- The chorion – contains blood vessels that exchange gases with the outside environment
In birds and most reptiles - major distinctions
- Embryo and its membranes are enclosed in a hard or leathery shell, protecting the egg outside the mother’s body.
- Cleavage is meroblastic, with only a small portion of the egg cytoplasm being used to make the cells of the embryo, vast majority of egg is composed of yolk.
In most mammals - major distinctions
Holoblastic cleavage is modified to accommodate the formation of a placenta (enables embryo to develop inside another organism.
Placenta
organ containing tissues and blood vessels from both the embryo and the mother. Gas exchange, nutrient uptake and waste elimination take place through the placenta.
Domestic chicken as model (Gallus gallus)
- Accessible year-round and is easily maintained
- Its developmental stage can be accurately predicted at any particular temperature = large numbers of same-stage embryos can be obtained and manipulated
- One of the few organisms whose embryos are amenable to both surgical and genetic manipulations
Mouse as model (Mus musculus)
- Mammalian model organism of choice
- Mouse genome first to be sequenced
- ”Working on mouse models allows the manipulation of each and every gene to determine their functions”
- Amenable to both surgical and genetic manipulation
Bird cleavage
- Fertilization of the chick egg occurs in the hen’s oviduct, before the albumin and shell are secreted to cover it
- The chick egg is telolecithal
o Small disc of cytoplasm (Blastodisc) on top of large yolk
o Blastodisc: 2-3 mm in diameter, located at animal pole - Undergo discoidal meroblastic cleavage, like fish eggs
o Cleavage occurs only in the blastodisc
o First cleavage furrow appears centrally in disc; others follow to create single-layered blastoderm. Cleavages do not extend into yolky cytoplasm
o Thereafter, equatorial and vertical cleavages divide the blastoderm into a tissue 5-6 cell layers thick. Cells become linked by tight junctions.
Subgerminal cavity
space between the blastoderm and the yolk of avian eggs. Created when the blastoderm cells absorb water from the albumin (“egg white”) and secrete the fluid between themselves and the yolk.
Area pellucida
the deep cells in the centre of the blastoderm appear to be shed and die, leaving behind a 1-cell-thick area; this part forms most of the actual embryo.
Area opaca
the peripheral ring of blastoderm cells that have not shed their deep cells.
Marginal zone
thin layer of cells between area pellucida and area opaca; some become very important in determining cell fate during early chick development.
Avian gastrulation
Epiblast
an “upper layer” (pga they remain at the surface) formed of most of the cells of the area pellucida; around the time a hen has laid the egg.
Avian gastrulation
Koller’s sickle
local thickening of the epiblast at the posterior edge of the area pellucida, shortly after the egg is laid. Sheet of cells derived from here migrates anteriorly beneath the surface
Avian gastrulation
Posterior marginal zone (PMZ)
in between the area opaca and Koller’s sickle
Avian gastrulation
Primary hypoblast
cells that have migrated together, but come from the more anterior regions where they have delaminated (but stayed attached to epiblast) to form hypoblast “islands” (disconnected clusters of 5-20 cells each).
Avian gastrulation
Secondary hypoblast
the cells that grow anteriorly from Koller’s sickle
Avian gastrulation
Endoblast
the two hypoblast layers joined to form a complete layer
The resulting two layered blastoderm (epiblast+hypoblast) is joined together at the marginal zone of the area opaca, and the space between the layers forms a blastocoel.
Epiblast becomes:
- Entire embryo
- Amnion, chorion and allantois extraembryonic membranes
Hypoblast cells provide:
- None of the cells of the developing embryo
- Instead forms portions of the extraembryonic membranes, especially the yolk sac and the stalk lining the yolk mass to the endodermal digestive tube.
- Chemical signals that specify the migration of epiblast cells.
Primitive streak
equivalent of the blastopore lip of amphibian embryos, takes place in avian, reptilian and mammalian gastrulation.
First arises from Koller’s sickle and the epiblast above it
Primitive groove
depression formed within the primitive streak as the cells converge to form it.
- Serves as an opening through which migrating cells pass into the deep layers of the embryo.
- Homologous to the amphibian blastopore
Hensen’s node
– regional thickening of cells at anterior end of primitive streak (aka primitive knot)
- Functional equivalent of the dorsal lip of the amphibian blastopore (i.e. the organizer) and the fish embryonic shield.
- Primitive pit: centre of Hensen’s node containing a funnel-shaped depression through which cells can enter embryo to form the notochord and prechordal plate.
Primitive streak defines the major body axes of the avian embryo:
- It extends from posterior to anterior
- Migrating cells enter through its dorsal side and move to its ventral side
- It separates the left portion of the embryo from the right
- Axis of streak is equivalent to DV axis of amphibians.
Cells migrating through primitive streak becomes:
o Anterior end of streak (Hensen’s node) gives rise to prechordal mesoderm, notochord, and medial part of somites.
o Cells ingressing through the middle of the streak give rise to the lateral part of the somites, and to the heart and kidneys.
o Cells in posterior make the lateral plate and extraembryonic mesoderm.
o Epiblast cells remaining outside (after ingression of mesoderm cells) close to the streak will form medial (dorsal) structures, fx neural plate
o Epiblast cells farther from the streak will become epidermis.
Elongation of the primitive streak
Cells entering the primitive streak undergo an epithelial-mesenchymal transformation, and the basal lamina beneath them breaks down.
Elongation of streak appears to be coextensive with the anterior migration of the hypoblast cells (from posterior marginal zone of blastoderm), and the hypoblast directs the movement of the primitive streak.
The streak eventually extends to 60-75% of the length of the area pellucida.
Convergent extension
– responsible for the progression of the streak (elongating toward future head region as more anterior cells migrate toward centre of embryo), doubling in length is accompanied by a concomitant halving of its width.
- Cell division adds to this length
