1) The germ disk; 2) epiblast; 3) hypoblast; 4) yolk
The second phase of gastrulation in birds is immigration (Fig.18). It consists of complicated movements of cells of the epiblast. The cells start to move from the cranial (head) edge toward the caudal (tail) edge of the embryo. Two streams of moving cells come across one another at the median line on the caudal part of the embryo. Then the streams fuse, turn and move toward the cranial part along the median line of the body, forming thickened cell cord - primitive streak. The anterior broaden part of the primitive streak is termed as the Hensen' node (the primitive knot), which is effectively the equivalent of the dorsal lip of the amphibian blastopore.
A depression that forms within the streak is the primitive groove. The primitive groove is a dip through which the cells of epiblast move deep into and migrate on both sides of the primitive streak. Initially, the cells migrate anteriorly and ventrally forming the foregut endoderm, and displace the hypoblast cells. Gradually more epiblastic cells migrate on both sides of the primitive streak intervening between endoderm and ectoderm. These cells give rise to the middle germ layer - mesoderm.
B
Fig.18. Gastrulation in Birds
A – formation of the primitive streak and the primitive node: 1) the primitive node; 2) the primitive pit; 3) the primitive streak; 4) the primitive groove
B – formation of mesoderm (immigration).
More epiblast cells ingress and form the head mesoderm and the anterior part of the notochord. The notochord is formed from some of the mesodermal cells in a middle layer between the ectoderm and the endoderm along the primitive streak. Following this, the primitive streak regress, moving Hensen’s node posteriorly. As it moves, the more posterior regions of the notochord develop. At its most posterior position, Hensen’s node forms the anal region. Notochord establishes the future axis of the vertebral column.
In the process of gastrulation, mesenchyme, embryonic connective tissue is forming. Mesenchymal cells are mainly mesodermal in origin but may be derived from the other germ layers. So, the cells migrate from their site of origin, surrounding and penetrating developing organs. Mesenchyme consists of loosely arranged stellate cells suspended in a gelatinous matrix. Their processes make contact with those of adjoining cells.
At the end we have three layers, the endoderm which has displaced the hypoblast, the ectoderm, which is what is left of the epiblast, and lots of mesoderm in between. Together the three germ layers and the mesenchyme give rise to all tissues, all organs and organ systems of the developing organism.
Thus, after gastrulation there follows a phase of histo- and organogenesis in which the primordial organs make their appearance. This phase gradually merges into the periods of growth and differentiation - both histological and functional.
At the beginning the formation of the axial primordial organs takes place. The complex of the axial organs includes: notochord, neural tube, mesodermal somites (paraxial mesoderm).
Neurulation (Fig.19) is the development of the neural tube form the neuroectoderm. Nerve tissue develops from embryonic ectoderm that is induced to differentiate in this direction by the underlying notochord. At first, a neural plate forms; then the edges of the plate thicken, forming the neural groove. The edges of the groove grow toward each other and ultimately fuse, forming the neural tube. This structure gives rise to the entire central nerve system. Some cells lateral to the neural groove, making up the neural crest, undergo extensive migrations and give rise to most of the peripheral nervous system, as well as a number of other structures (melanocytes, medulla of the adrenal gland, cells of the diffuse endocrine system. The rest of intra-embryonic ectoderm gives rise to the skin epidermis and its derivatives.
Fig.19
The circumferential continuity of the endoderm is restored with the formation of a tube the definitive gut.
Alongside the neural tube and the notochord, mesoderm forms metamerically arranged somites (Fig.20). More laterally is a strip of intermediate mesoderm (syn.: nephrogonotome, or segment stalk). Pairs of somites and nephrotomes are metamerically segmented portions of the mesoderm. The splanchnotome (or lateral plate mesoderm) is a non-segmented portion of the mesoderm, placed the most laterally.
Fig.20. Differentiation of the germ layers in Birds
ectoderm; 2) nerve tube; 3) somite; 4) nephrogonotome (segment stalk); 5) parietal layer of splanchnotome; 6) coelomic cavity; 7) visceral layer of splanchnotome; 8) splanchnotome (lateral mesoderm); 9) notochord; 10) endoderm; 11) blood vessels in the yolk sac wall.
Further development involves subsequent differentiation of the mesoderm. The somite divides into dermatome, myotome and sclerotome (Fig.21). The dermatome, outer mass of cells, gives rise to the connective tissue layer of the skin (the dermis) and subcutaneous tissues. Cells proliferating from the dorsal part of the inner wall of the somite form the myotome and give rise to somatic muscle. The cells of sclerotome (the ventral part of the inner wall) migrate medially, envelop the notochord and contribute to the axial skeleton and meninges.
The segmented stalks will give rise to the kidneys and gonads.
Fig.21. . Differentiation of the germ layers in Birds
ectoderm; 2) nerve tube; 3) dermatome; 4) myotome; 5) sclerotome; 6) nephrogonotome;
7) notochord; 8) aorta; 9) coelom; 10) amniotic fold; 11) extraembryonic ectoderm;
12) parietal layer of splanchnotome; 13) body fold; 14 ) visceral layer of splanchnotome;
15) endoderm; 12) blood vessels in the yolk sac wall.
The splanchnotome splits onto parietal and visceral layers, which bound coelom cavities. The parietal layer of splanchnotome is in contact with ectoderm (somatopleure), and the visceral layer is in contact with endoderm (splanchnopleure). The lateral plate mesoderm gives rise to serous lining of cavities: pericardium, pleura, peritoneum, as well as cardiac muscle (myoepicardial plate, derived from the visceral layer of splanchnotome).
Mesenchyme cells are pluripotent, i.e. they may differentiate into a wide variety of cells. The mesenchyme gives rise to all types of connective tissue cells, as well as blood cells and blood vessels, hematopoietic organs, smooth muscle cells.
Endoderm gives rise to intestine epithelium and digestive glands (the liver, the pancreas), epithelial lining of the lungs, the urinary bladder etc.
EXTRA-EMBRYONIC (PROVISORY) ORGANS
Extra-embryonic portions of the germ layers form extra-embryonic (provisory) organs.
Extra-embryonic organs provide environment for the embryo development and vital activity. They function during the period of embryogenesis and disappear with the end of the embryonic development, when foetus attains ability for independent existence.
In birds extra-embryonic organs are:
amnion;
serous membrane;
yolk sac;
allantois.
At the beginning the embryo throws itself flat on the yolk surface. With further growth the embryo raises itself over the yolk mass as result of the formation of the body folds. All germ layers (ectoderm, entoderm, visceral and parietal layers of splanchnotome) take participation in the body fold' formation: they are tucked under the embryo (Fig.21). So, the embryo is lifted off the yolk by the development of infoldings unite along the sides of the sides of the embryo. Thus, connection to the yolk is progressively is restricted.
Amniotic folds, forming by extra-embryonic portions of ectoderm and parietal layer of mesoderm (somatopleure), raise first above the cranial edge, and then - above the whole trunk of the embryo. The folds meet dorsal to the embryo (Fig.22). The ectoderm of one fold grows together with the ectoderm of another fold. The parietal layers of both folds fuse also. Thus, the extra-embryonic somatopleure subdivides into two membranes: the amnion, surrounding the amniotic cavity, and the serous membrane (chorion). The wall of the amniotic cavity is formed by the ectoderm, lining the cavity from the inside, and the parietal layer of mesoderm, lying on the outside. Ectodermal amniotic epithelium secrets amniotic fluid, so the amniotic cavity becomes filled with some fluid. The amnion provides a local aquatic environment for the embryo, eliminates gravitational stresses, the formation of adhesions and the danger of desiccation.
Fig.22. Extra-embryonic organs in Birds
amniotic cavity; 2) allantois; 3) yolk sac; 4) body fold; 5) extraembryonic ectoderm; 6) parietal layer of splanchnotome; 7) amnion wall; 8) amniotic folds joint; 9) serous membrane; 10) embryonic ectoderm; 11) notochord; 12) nerve tube; 13) dermatome; 14) myotome; 15) sclerotome; 16) nephrogonotome;