Chapter 9 – Early development in vertebrates (birds and mammals)

Question 1

Amniotes

Answer 1

vertebrates whose embryos form an amnion (water sac), i.e. reptiles, birds and mammals.

Birds and reptiles follow a very similar pattern of developmen

Question 2

Amniote egg - characterized by a set of membranes that enable the embryo to survive on land:

Answer 2

1. The amnion – formed early in embryonic development, enables the embryo to float in a fluid environment that protects it from desiccation.
2. The yolk sac – enables nutrient uptake and the development of the circulatory system
3. The allantois – developing at the posterior of the embryo, stores waste products
4. The chorion – contains blood vessels that exchange gases with the outside environment

Question 3

In birds and most reptiles - major distinctions

Answer 3

• 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.

Question 4

In most mammals - major distinctions

Answer 4

Holoblastic cleavage is modified to accommodate the formation of a placenta (enables embryo to develop inside another organism.

Question 5

Placenta

Answer 5

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.

Question 6

Domestic chicken as model (Gallus gallus)

Answer 6

• 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

Question 7

Mouse as model (Mus musculus)

Answer 7

• 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

Question 8

Bird cleavage

Answer 8

• 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.

Question 9

Subgerminal cavity

Answer 9

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.

Question 10

Area pellucida

Answer 10

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.

Question 11

Area opaca

Answer 11

the peripheral ring of blastoderm cells that have not shed their deep cells.

Question 12

Marginal zone

Answer 12

thin layer of cells between area pellucida and area opaca; some become very important in determining cell fate during early chick development.

Question 13

Avian gastrulation

Epiblast

Answer 13

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.

Question 14

Avian gastrulation

Koller’s sickle

Answer 14

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

Question 15

Avian gastrulation

Posterior marginal zone (PMZ)

Answer 15

in between the area opaca and Koller’s sickle

Question 16

Avian gastrulation

Primary hypoblast

Answer 16

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).

Question 17

Avian gastrulation

Secondary hypoblast

Answer 17

the cells that grow anteriorly from Koller’s sickle

Question 18

Avian gastrulation

Endoblast

Answer 18

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.

Question 19

Epiblast becomes:

Answer 19

• Entire embryo

- Amnion, chorion and allantois extraembryonic membranes

Question 20

Hypoblast cells provide:

Answer 20

• 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.

Question 21

Primitive streak

Answer 21

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

Question 22

Primitive groove

Answer 22

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

Question 23

Hensen’s node

Answer 23

– 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.

Question 24

Primitive streak defines the major body axes of the avian embryo:

Answer 24

• 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.

Question 25

Cells migrating through primitive streak becomes:

Answer 25

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.

Question 26

Elongation of the primitive streak

Answer 26

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.

Question 27

Convergent extension

Answer 27

– 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