Human marginal zone b cells

Bird

Early Development

https://www.youtube.com/watch?v=w-M33PtwtM4

https://www.youtube.com/watch?v=PedajVADLGw

https://www.youtube.com/watch?v=vBGumRAWaa0

https://www.youtube.com/watch?v=uP7IHXAaJq4

https://embryology.med.unsw.edu.au/embryology/index.php/Hamburger_Hamilton_Stages

Parts of the Egg

A hen requires about 24 to 26 hours to produce an egg.

The Shell

The shell, is a hard, protective covering / made almost entirely of calcium carbonate (CaCO3) crystals.

The shell is porous (about 7,000 pores) / allow for the transfer of gases

Carbon dioxide and moisture are given off through the pores and are replaced by atmospheric gases, including oxygen.

The shell also has a thin outermost coating called the bloom or cuticle that helps keep out bacteria and dust.

Parts of the Egg

Immediately beneath the shell are two membranes:

outer shell membrane

inner shell membranes

Protect the contents of the egg from bacteria and prevent moisture from leaving the egg too quickly.

While the embryo is growing, the shell membranes surround and contain the white or albumen of the egg.

The albumen provides the liquid medium in which the embryo develops, and it also contains a large amount of the protein necessary for proper development.

Parts of the Egg

Because the body temperature of a hen is approximately 106° F, eggs are very warm at the time they are laid.

The temperature of the air is usually much lower than 106° F, and the egg cools to the temperature of its surroundings. As cooling takes place, the contents of the egg contract more than does the shell of the egg. This creates a vacuum and air is drawn through the pores of the egg.

Parts of the Egg

Chalazae - are made of twisted strands of mucin fibers. The chalazae hold the yolk in the center of the egg.

The yolk is the source of food for the embryo and contains all the fat in the egg. The small white spot on the yolk is call the germinal disc. The germinal disc is where the female's genetic material is found.

Early Development of Vertebrates: Birds

Cleavage in Bird Eggs

Fertilization of chick egg occurs in the oviduct, before albumen and shell are secreted to cover it.

Telolecithal egg – small disc of cytoplasm sitting atop a large yolk.

Discoidal meroblastic cleavage.

Cleavage only in blastodisc.

Initially get a single-layered blastoderm, eventually 5 to 6 layered blastoderm.

Cells linked by tight junctions.

Early Development of Vertebrates: Birds

Subgerminal cavity – space between the blastoderm and the yolk. Cells absorb water from the albumen and secrete the fluid between themselves and the yolk.

Early Development Birds

At this point deep cells in the center of blastoderm are shed and die, leaving behind a one cell thick area pellucida. This area forms most of embryo.

At sides get the area opaca –peripheral ring of blastoderm cells that have not shed their deep cells.

Between two areas have marginal zone cells some of which are important for determining cell fate during early chick development.

12.3 Formation of the three-layered blastoderm of the chick embryo

Gastrulation of the Avian Embryo

The Hypoblast

By the time egg is laid the blastoderm contains over 20,000 cells.

Area pellucida cells mostly epiblast, some have migrated into the subgerminal cavity to form the poly-invagination islands (primary hypoblast).

Shortly thereafter, a sheet of cells from the posterior margin of the blastoderm migrates anteriorly, pushes the primary hypoblast cells anteriorly, and forms the secondary hypoblast, also known as the endoblast.

Two layers now – joined together at marginal zone of the area opaca, and the space between = blastocoel.

Embryo will develop entirely from epiblast.

Hypoblast – portions of yolk sac, stalk linking the yolk mass to the endodermal digestive tube,

chemical signals that specifiy migration of epiblast cells.

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devbio8e-fig-11-15-1.jpg
The Primitive Streak

Eventually a depression forms = primitive grove, serves as an opening through which migrating cells pass into the blastocoel. Homologous to amphibian blastopore.

Anterior end get primitive knot or Hensen’s node – center has a funnel shaped depression = primitive pit.

Cells pass thru pit as migrate in – Hensen’s node is the functional equivalent of dorsal lip of the amphibian blastopore.

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The Primitive Streak

First cells to ingress through the primitive streak and into blastocoel are endodermal precursors from the epiblast.

These cells undergo an epithelial-to-mesenchymal transformation, basal lamina beneath them breaks down.

Steak elongates further toward future head region.

Same time secondary hypoblast (endoblast) cells continue to migrate anteriorly from the posterior margin of the blastoderm. Hypoblast directs the movement of the primitive streak.

Figure 12.4 Cell movements of the primitive streak and fate map of the chick embryo

Figure 12.4 Cell movements of the primitive streak and fate map of the chick embryo. (A–C) Dorsal view of the formation and elongation of the primitive streak. The blastoderm is seen at (A) 12–14 hours, (B) 15–17 hours, and (C) 18–20 hours after the egg is laid. (D–F) Formation of notochord and mesodermal somites as the primitive streak regresses, shown at (D) 20–22 hours, (E) 23–25 hours, and (F) the four-somite stage. Fate maps of the chick epiblast are shown for two stages, the definitive primitive streak stage (C) and neurulation (F). In (F), the endoderm has ingressed beneath the epiblast, and convergent extension is seen in the midline. The movements of the mesodermal precursors through the primitive streak at (C) are shown. (Adapted from several sources, especially Spratt 1946; Smith and Schoenwolf 1998; Stern 2005a,b.)

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The Primitive Streak

Henson’s node gives rise to prechordal plate mesoderm (head of embryo forms anterior to Hensen’s node), chordamesoderm (head process and notochord), and anterior somites.

Cell’s that ingress through middle give rise to somites, heart and kidneys.

Cells’s becoming the posterior portion of the streak – lateral plate and extraembryonic mesoderm.

Steak has a continual changing cell population.

Anterior – endoderm, head mesoderm, notochord

Posterior – majority of mesodermal tissue.

Primitive streak defines the

axes of the avian embryo.

Extends from posterior to anterior.

Migrating cells enter thru dorsal side

and move to ventral side.

Separates left and right sides of embryo.

Figure 12.5 Migration of endodermal and mesodermal cells through the primitive streak

Figure 12.5 Migration of endodermal and mesodermal cells through the primitive streak. (A) Stereogram of a gastrulating chick embryo, showing the relationship of the primitive streak, the migrating cells, and the hypoblast and epiblast of the blastoderm. The lower layer becomes a mosaic of hypoblast and endodermal cells; the hypoblast cells eventually sort out to form a layer beneath the endoderm and contribute to the yolk sac. Above each region of the stereogram are micrographs showing the tracks of GFP-labeled cells at that position in the primitive streak. Cells migrating through Hensen’s node travel anteriorly to form the prechordal plate and notochord; those migrating through the next anterior region of the streak travel laterally but converge near the midline to make notochord and somites; those from the middle of the streak form intermediate mesoderm and lateral plate mesoderm (see the fate maps in Figure 12.4). Farther posterior, the cells migrating through the primitive streak make the extraembryonic mesoderm (not shown). (B) This scanning electron micrograph shows epiblast cells passing into the blastocoel and extending their apical ends to become bottle cells. (A after Balinsky 1975, photographs from Yang et al. 2002; B from Solursh and Revel 1978, courtesy of M. Solursh and C. J. Weijer.)

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Figure 12.4 Cell movements of the primitive streak and fate map of the chick embryo (Part 5)

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Chicken embryo, 21 hours.

Credit: © Carolina Biological/Visuals Unlimited

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Primitive streak & groove

Figure 12.4 Cell movements of the primitive streak and fate map of the chick embryo

Figure 12.4 Cell movements of the primitive streak and fate map of the chick embryo. (A–C) Dorsal view of the formation and elongation of the primitive streak. The blastoderm is seen at (A) 12–14 hours, (B) 15–17 hours, and (C) 18–20 hours after the egg is laid. (D–F) Formation of notochord and mesodermal somites as the primitive streak regresses, shown at (D) 20–22 hours, (E) 23–25 hours, and (F) the four-somite stage. Fate maps of the chick epiblast are shown for two stages, the definitive primitive streak stage (C) and neurulation (F). In (F), the endoderm has ingressed beneath the epiblast, and convergent extension is seen in the midline. The movements of the mesodermal precursors through the primitive streak at (C) are shown. (Adapted from several sources, especially Spratt 1946; Smith and Schoenwolf 1998; Stern 2005a,b.)

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Figure 12.4 Cell movements of the primitive streak and fate map of the chick embryo (Part 7)

Somites: Blocks of mesoderm that become subdivided into a sclerotome, myotome, and dermatome.

Sclerotomes form vertebrae and ribs, myotomes form skeletal muscles for the back and appendages, and dermatomes give rise to dermis.

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Extraembryonic Membranes

Protect and nourish the embryo

Terrestrial vertebrates have four extraembryonic membranes

chorion
amnion
allantois
yolk sac
The chorion and amnion enclose the embryo
The chorion surrounds the entire embryo

in birds and reptiles it functions in gas exchange
The amnion encloses the embryo and forms an open volume between the embryo & the amnion called the amniotic cavity

The amniotic cavity fills with amniotic fluid, which envelops the embryo and cushions it
The allantois is an outgrowth of the gut
In reptiles and birds, it stores nitrogenous wastes
The yolk sac encloses the yolk in vertebrates with yolk-rich eggs
In humans, there is no yolk sac, but the yolk aids in formation of red blood cells

Chick Embryo Whole Mounts:

24-hour

33-hour

72-hour

96-hour

Whole mounts of

24-Hour Chick Embryo

Ectodermal derivatives Mesodermal derivatives Extraembryonic regions

Whole mounts of

33-Hour Chick Embryo

Newly hatched chick on day 21.

Credit: © Jerome Wexler/Visuals Unlimited

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Questions Slide 1 of 2 10 questions

1. Ninety-eight percent of an egg shell is calcite, which is calcium carbonate (CaCO3). Calcium is the major component of bone as well (in the form of calcium phosphate, or apatite). The calcium that goes into the shell comes both from the hen’s diet and from her bones. The average weight of a chicken egg shell is 7.26 grams. A truly remarkable hen can lay an egg a day. In a year, a hen can put into her eggshells over 25 times her own skeletal weight in calcium.

Obviously, this must be replaced. The standard feed given to laying hens is 3 to 4% calcium, and one hen receives about a 100 gm of feed a day. Based on the weight of the shell, is this sufficient to replace the calcium lost in a superhen laying every day? Do the calculations to support your answer.

CaCO3 = 100.9 grams / mole

Atomic weights of molecules from the periodic table.

Ca = 40.08

C = 12.01

O = 16.00

Questions Slide 2 of 2

2. Why is the amniote egg considered to be an important evolutionary development?

3. What is the function of the air space in the chick egg?

4. What is the importance of chalazae?

5. Why is initial cleavage of the chick egg meroblastic rather than holoblastic?

6. What is the cavity that underlies the area pellucida? What is its importance relative to the primitive streak?

7. What is the role of the primitive streak and Hensen’s node?

8. Define the term somite.

9. What structure is found between the somites and what is its role?