Introduction and Embryonic Development - Zebrafish (L3) Flashcards
Vertebrates
Phylum of organisms that possess a spinal chord supported by vertebrae. One example of a vertebrate is Danio rerio, the zebrafish, a teleost (bony fish).
Other examples are birds and 4-footed tetrapods (amphibians, reptiles, and mammals, including humans.)
Dissecting microscopes
The Zebrafish embryo’s body and surrounding chorion membrane are almost transparent so that development is easy to observe using dissecting microscopes.
Cartilage
To study development of the skeleton in more detail, we will stain the cartilage of fixed 7-day old embryos with alcian blue and examine the distribution of cartilage along the head and body.
Alcian blue
Stain used to view cartilage of fixed 7-day old Zebrafish embryos.
Fertilization
Occurs when a haploid sperm cell and a haploid egg cell fuse to become a zygote. After fertilization, the development of a zebrafish embryo is divided into 5 broad periods, each of which includes several visually distinct stages of development.
Zygote
The first diploid cell of a new organism, results from fertilization of a haploid sperm and haploid egg.
Chorion
Zebrafish zygotes are surrounded by a thin, transparent membrane called the chorion. The chorion lifts off the surface of a fertilized egg as a block to polyspermy.
Animal pole
The dorsal surface of the zygote.
Vegetal pole
The ventral surface of the zygote. Contains more yolk than the animal pole.
Blastodisc
About 10 minutes after fertilization, the yolk-free cytoplasm of the single-cell zygote begins to move to the animal pole, where it is seen as a transparent bulge, the blastodisc. Subsequent development will occur in the blastodisc.
Cleavage
Occurs about 45 minutes after fertilization. Cell division without an overall change in embryo size.
Meroblastic vs. holoblastic cleavage
Cleavage is restricted to the blastodisc and does not penetrate the yolk (this is meroblastic cleavage).
If the cleavage plane cut through the entire cell including the yolk, it would be holoblastic cleavage.
Blastomeres
The cells formed from cleavage.
Cleavage period
Cleavage occurs synchronously during the cleavage period, with one cell division about every 15 minutes.
Blastula period
From 128 cell stage until the onset of gastrulation, which occurs after 13 divisions.
Blastulas
Ball of cells with irregular spaces in between them. The process of cell division becomes much less organized in blastulas. The cell cycle becomes longer, the organization of cleavage planes becomes indeterminate, and cell division begins at the animal pole and crosses the embryo in a wave.
Yolk syncytial layer
AKA YSL. The blastomeres that sit on the surface of the yolk have remained connected to it throughout cleavage via the cytoplasm. These cells now fuse and release their cytoplasm and nuclei into the cytoplasm of the yolk cell to form the YSL.
The YSL nuclei continue to undergo mitotic divisions for ~3 cycles, but the cytoplasm of the YSL stays together and does not cleave.
YSL is a developmental structure characteristic of teleost fishes. At first, it forms a ring around the edge of the blastodisc, the ring then spreads inward until it forms a continuous border between the embryo and its yolk, eventually covering the entire surface of the yolk.
The YSL is a sort of membrane that separates the embryo proper from the yolk, which contains nutrients deposited by the embryo’s mother.
The correct expression of enzymes in the YSL cytoplasm and transporters at the YSL membrane is vital for the transport of minerals and food stores out of the yolk, until the embryo has developed mouth structures and swimming abilities that permit it to feed.
“High stage”
c 3.3 hpf. The yolk is flattened and the blastodisc is a high mound of cells upon it. Subsequently, the animal-vegetal axis shortens, two stages pass, and the YSL surface begins to done towards the animal pole. This is the dome stage, an obvious sign that epiboly is beginning.
Hours post fertilization
AKA hpf.
Zebrafish development is relatively rapid. At their preferred temperature of 28.5 degrees celsius, the first cell division occurs at 0.75 hours. Mid-blastula occurs at 3 hpf, gastrulation at 5 hpf, first body movements at 18 hpf. Most major organs are formed after 24 hpf and the fish begins to eat within 5 days.
Epiboly
4.33-9 hpf. “Around the ball.” A process of cell migration which begins as the blastodisc things and moves with the YSL to spread completely over the surface of the yolk cell. The yolk cell itself bulges toward the animal pole.
Blastoderm
As epiboly begins, the blastodisc becomes called blastoderm. During the process, embryo stages are often described in terms of blastoderm coverage. If 50% of the distance between the poles along the animal-vegetal axis is covered with blastoderm, then the embryo’s stage is 50% epiboly.
50% epiboly stage
At 50% epiboly stage, epiboly pauses for about 90 minutes as gastrula begins.
Gastrula period
Populations of cells move to the interior of the embryo and three tissue layers and the embryonic axis (anterior-posterior) form.
Gastrulation ends when epiboly is complete at 10 hpf.
Three tissue layers formed during gastrulation
Endoderm, mesoderm, and ectoderm
Germ ring
Cells begin to migrate under the leading edge of the blastoderm during germ ring stage
Embryonic shield
As gastrulation proceeds, the organization of the body plan of the zebrafish begins with a bulge of cells called the embryonic shield that forms at one side of the blastoderm.
Once the embryonic shield forms, epiboly continues at a rate of ~15% p/h while gastrulation continues.
Shield stage
The embryonic shield is visible on one side of the blastoderm.
Epiblast
The upper layer, which sends cells into the hypoblast. Becomes the ectoderm layer and its derived structures.
Hypoblast
The lower layer. Becomes mesoderm and endoderm.
Neural plate
Formed at 90% epiboly, when a part of the ectoderm glattens in the head region near the animal pole of the embryo forming the neural plate. This will fold into a tube becoming the first part of the nervous system.
Segmentation period
10.3-24 hpf. As segmentation progresses, the embryo lengthens, then the tail begins to form. An obvious feature is the development of somites. The embryo begins to move during this period. Organs including the brain, blood, blood vessels, and kidney also form during this period.
Somites
Body segments which form during segmentation. Compartments of mesoderm that will give rise to the muscles and vertebrae.
Notochord
Somites form around the notochord, a long, rod-shaped bundle of mesodermal cells that form the primitive head to tail axis in the embryo.
Brain
In segmentation, becomes clearly visible as a series of 5 lobes in the head.
Spinal chord
Nerves extend posterior from the brain. Dorsal to the notochord, which supports it from underneath, connected by the floor plate.
Heart
At approximately 24 hpf, the fish’s heart begins to beat and to circulate erythroblast.
Erythroblast
Embryonic blood cells which appear clear when the heart begins to beat.
Pharyngula period
24-48 hpf. Pharyngeal arches formed. Embryo continues to lengthen and straighten out, the pectoral fins begin to form, and pigment cells become visible in retina and in patchy stripes along the body.
Will also see color from erythroblasts, which now have a bit of hemoglobin protein for gas transport and exchange.
Pharyngeal arches
Made of cartilage structures in the head, later form the jaw and gills. Form during the pharyngula period.
Melopores
AKA pigment cells. Congregate to form dark brown patches on the skin and embryo’s eyes begin to darken. This happens from around 28 hpf.
Hatching glands
Around the same time as pigment cells congregate, hatching glands anterior to the embryo’s heart secrete a proteolytic enzyme that begins degrading the chorion membrane, and as the embryo’s body movements become increasingly coordinated and strong, it will hatch out of the chorion, approximately from 48-72 hpf.
Gut and liver
During the 3rd day of development, 48-72 hpf, the embryos continue to grow, and complete organ development of endodermal structures like the gut and liver form.
Pectoral fins
Begin to extend in length around 60 hpf.
Swim bladder
Develops on the 4th day of development. Resembles an air bubble located between the embryo trunk and the remaining yolk ball.
The inflation of a swim bladder allows the embryos to rise or fall vertically while swimming. This is necessary for the embryo to hunt for external food sources (as its yolk supplies are rapidly dwindling by day 4, and they begin to eat on day 5).
