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Transcript of Chicken embryology
The early embryological development in chicken reviewed on in whole-mount preparations of the embryonic plate
ORGANS OF THE HEN INVOLVED IN THE FORMATION OF THE EGG:
The chicken embryo develops in the egg, partly within the body of the hen but mainly during the brood period after the lay.
STEPS BEFORE, DURING AND AFTER FERTILIZATION:
Proteins and lipids are synthesized in the liver of the hen before ovulation and stored in layers in the egg yolk.
The mature egg (= oocyte) is released by the ovary during ovulation and is then incorporated in the funnel-shaped opening (infundibulum) of the oviduct.
Fertilization by the sperm of the cock can occur at that location during a short time period (approximately in the first 15 minutes). Therefore, the sperm cells have to pass the whole way through the oviduct from the cloaca up to the infundibulum.
The sperm cells can be stored temporarily in the vagina where selection is taking place.
After penetration of the sperm cell the egg membrane becomes the fertilization membrane, which inhibits penetration by other sperm cells.
THE FERTILIZED EGG
The fertilized oocyte (= zygote) starts immediately with the cleaving process and a small germ layer is formed at the surface of the yolk (blastodisc). The egg is loaded with albumen at the magnum. The two semi permeable shell membranes are formed at the isthmus while the poriferous calcified shell is set off in the uterus. The egg is layed around 24 to 28 hours after fertilization. A light contraction of the egg mass takes place after cooling down and results in the formation of an air chamber between the two aforementioned shell membranes.
THE GERM LAYER IN THE EARLY STAGES FOLLOWING FERTILIZATION
In these early stages, the area opaca (ao) appears as a dark zone at the external border of the germ layer and is in contact with the yolk mass. The central region of the germ layer is localized at the upper part of a hole (the blastocoel = 2). This region is transparent and is therefore called the area pellucida (ap). The area pellucida is composed by two layers, the epiblast (1) at the surface and the hypoblast (3) in the deeper part.
The gastrulation process starts with the formation of a groove (arrow in C) and forms first at the caudal (tail) side the primitive groove, also called the primitive streak (ps). The cells in the central zone divide frequently and migrate laterally in between the epiblast and the hypoblast.
THE GERMINAL DISC IN UPPER VIEW (A), CROSS-SECTION BEFORE (B) AND DURING GASTRULATION (C).
ap = area pellucida, ao = area opaca, ps = primitive streak, 1 = Epiblast (forms the ectoderm), 2 = Blastocoel, 3 = Hypoblast (forms the endoderm), 4 = Subgerminal cavity,
5 = Yolk
18 HOURS AFTER FERTILIZATION
In the central region, the area pellucida (= transparent) can be seen surrounded by the dark area opaca (= untransparent). The primitive streak lies in the middle of the area pellucida as a clear line. At the tail side of the area pellucida it is characterized by a primitive groove surrounded by the two thicker walls of the blastoderm, the primitive walls. In the front, the primitive streak ends in a small pit, the primitive pit or Hensen's node. Just in front of this node one can distinguish the formation of the notochord (chorda). This structure induces the formation of the neural plate and later the neural groove.
STAINED WHOLE-MOUNT PREPARATION 18 HRS AFTER FERTILIZATION
1 = Proamnion, 2 = Area opaca (dark), 3 = Area pellucida (transparent), 4 = Embryonal region, 5 = Neural plate, 6 = Chorda, 7 = Hensen's node, 8 = Primitive streak
20 HRS AFTER FERTILIZATION
The head fold is visible as a half circular elevated structure in the front of the germ layer. The proamnion lies in front of (=pro) this head fold as a clear zone. The amnion is one of the membranes that will protect the embryo in a later stage.
STAINED WHOLE-MOUNT PREPARATION 20 HRS AFTER FERTILIZATION
1 = Proamnion, 2 = Area opaca (dark), 3 = Area pellucida (transparent), 4 = Embryonal region, 5 = Head fold, 6 = Neural groove, 7 = Neural plate, 8 = Chorda, 9 = Hensen's node, 10 = Primitive streak
24 HRS AFTER FERTILIZATION
The somites are formed in the mesoderm at the left and right side of the neural walls. In this stage, they are visible as 4 to 5 segmented paired blocks. Later on, these structures will differentiate in to the vertebrae, the ribs, a part of the skin and the dorsal muscles. Only this head region elevates above the underlying area pellucida. In this preparation, one can see the chorda (notochord) in the region of the differentiating foregut.
STAINED WHOLE-MOUNT PREPARATION 24 HRS AFTER FERTILIZATION
1 Area opaca (dark), 2 Area pelucida (transparent), 3 Neural fold, 4 head folding, 5 Foregut, 6 en ng Neural groove, 7 Somite, 8 Chorda, 9 unsegmented mesoderm, 10 Hensen's node, 11 Primitive streak
33 HRS AFTER FERTILIZATION
The embryo is about 4 mm long and the first flexion of the originally straight embryo starts in the head region and the cranial flexure will be visible a few hours later. At this stage 12 to 13 somites are formed. The eye vesicles are rather large. The forebrain vesicle or prosencephalon will divide, the midbrain vesicle or mesencephalon remains undivided while the hindbrain vesicle or rhombencephalon will form a series of smaller neuromeres. The diamond-shapedsinus rhomboidalis is still present as the only opening of the neural tube and the primitive streak is only rudimentary. The infundibulum (= derived from the diencephalon) appears as a half circular structure at the ventral side of the caudal part of the forebrain. The notochord or chorda dorsalis ends just behind this ventral vesicle.
STAINED WHOLE-MOUNT PREPARATION 33 HRS AFTER FERTILIZATION
1 = Proamnion, 2 = Prosencephalon, 3 = Mesencephalon, 4 = Rhombencephalon, 5 = Somite, 6 = Eye vesicle, 7 = Foregut, 8 = Chorda (translucent), 9 = Heart, 10 = Lateral mesoderm,
11 = Spine, 12 = Sinus rhomboidalis, 13 = Primitive streak, 14 = Blood islands
36 HRS AFTER FERTILIZATION
The heart is a S-shaped tube which protude to the right of the embryo (in upper view). The further development of the heart is now mesodermal. Outside, in the area vasculosa (= forseen of blood vessels) the formation of blood islands continues. The primitive streak can only still be discerned below the sinus rhomboidalis.
STAINED WHOLE-MOUNT PREPARATION 36 HRS AFTER FERTILIZATION
1 = Prosencephalon,
2 = Eye vesicle,
3 = Mesencephalon,
4 =Rhombencephalon, 5 = Heart, 6 = Lateral mesoderm, 7 = Somite, 8 = Spine, 9 = Sinus rhomboidalis,
10 = Primitive streak, 11 = Small blood vessel,
12 = Blood islands
STAINED WHOLE-MOUNT PREPARATION 48 HRS AFTER FERTILIZATION
1 Amnion, 2 Metencephalon, 3 Mesencephalon, 4 Optic cup + lens, 5 Prosencephalon, 6 Otic vesicle, 7 Branchial arches, 8 Atrium , 9 Ventricle, 10 Lateral fold, 11 Lateral mesoderm, 12 Vitelline arteria / vein, 13 Somite, 14 Spine, 15 Tail fold
48 HRS AFTER FERTILIZATION
The position of the embryo with respect to the yolk is strongly achanging. In addition to the head fold of the amnion, also the lateral and caudal amniotic folds begin to form. The outgrowth of the cranial flexure is so strong that the forebrain and hindbrain vesicles become almost located next to each other. The cephalic region of the embryo is twisted in such a manner that the left side comes to lie next to the yolk. A second flexure appears at the transition of the head and the body just behind the heart region. The embryo takes now the shape of a C. The head becomes covered by a double fold. These folds definitely establish the first extra embryonic membrane (=outside of the embryo): the amnion membrane. The vitelline (yolk rich) arteries and veins become connected with the extra embryonic circulatory vessels. A few branchial grooves are already visible.
THE HEN AND
72 HRS AFTER FERTILIZATION
The rotation of the embryo to the left is arrived such behind the region of the heart and only the caudal part of the embryo must twist 90 degrees. The two flexures in the head region are almost completed. The fourth pharyngeal groove develops and the pharyngeal arches are thicker. Due to the cranial flexure, the pharyngeal region (= region of the trachea) is now located at the ventral side of the head. The fore and hind limbs at the level of the 16th to the 20th respectively the 27th to the 32th somite pairs are visible as small buds at an incubation time of about 3 days.
STAINED WHOLE MOUNT PREPARATION 72 HOURS AFTER FERTILIZATION
1 = Auditive (otic) vesicle, 2 = Myelencephalon, 3 = Metencephalon, 4 = Amnion, 5 = Mesencephalon, 6 = Optic vesicle + lens, 7 = Diencephalon, 8 = Epiphyse, 9 = Telencephalon, 10 = Branchial arches, 11 = Heart, 12 = Forelimb (wing) bud, 13 = Vitelline arteria/vein, 14 = Hindlimb (leg) bud, 15 = Tail
The brain divides in to 5 vesicles: telencephalon and diencephalon (both formed by the division of the forebrain vesicle), mesencephalon, metencephalon and myencephalon (both formed by the division of the hindbrain vesicle).
The lens placode (placode=plate) will form the lens vesicle, the optic vesicle will become the optic cup and the auditory placode the auditory pit.
The heart differentiates in to 4 compartiments: the sinus venosus, connected with the veins, the atrium, the U-shaped ventricle and the bulbus cordis. The atrium and ventricle are well distinguishable in the figure.
Authors: Dr. François van Herp, Dr. Sharon Kolk, Dr.Elisabeth Pierson, Drs. Wilbert Janssen
Depts. Molecular Animal Physiology, General Instrumentation and Biology education.
Radboud University Nijmegen ©