University:
Kentucky State University
Course:
BIO 111 | Principles of Biology
Academic year:

2020
Views:

284

Pages:

34
Author:

Mireya Blevins

Chapter 12 Early Development in Birds Avian cleavage • The fertilization of chick egg occurs in the hen’s oviduct, before the albumin (egg white) and the shell are secreted to cover it. • Blastodisc: is 2-3 mm in diameter disc and is located in the animal pole of the egg. Avian cleavage • Cleavage occurs during the first day of development, while the egg is still inside the hen, during which time the embryo progresses from a zygote to late blastula Avian cleavage • Subgerminal cavity: the space between the blastoderm and the yolk of avian eggs, which is created when the blastoderm cells absorb water from the albumin and secrete the fluid between themselves and the yolk. • The shedding and death of the deep cells in the center of the blastoderm leave behind a 1cell thick layer called area pellucida that forms most of the actual embryo. • The peripheral ring of blastoderm cells that have not shed their deep cells constitutes the area opaca. • Between area pellucida and area opaca is a thin layer of cells called the marginal zone. Gastrulation of the avian embryo The hypoblast • Most of the cells of the area pellucida remain at the surface forming an “upper layer” called the epiblast. • Shortly after the egg is laid, a local thickening of the epiblast, called the Koller’s sickle, is formed at the posterior edge of the area pellucida. • In between the area opaca and Koller’s sickle is a belt like region called the posterior margin zone (PMZ). Gastrulation of the avian embryo The hypoblast • A sheet of cells at the posterior boundary between the area pellucida and marginal zone migrates anteriorly beneath the surface and combines with the hypoblast islands to form the primary hypoblast. • The sheet of cells that grows anteriorly from the Koller’s sickle is called secondary hypoblast or endoblast. Blastocoel like cavity Gastrulation of the avian embryo The hypoblast • The avian embryo comes entirely from the epiblast. • The three germ layers of the embryo proper plus the amnion, chorion, and allantois extraembryonic membranes are formed solely from the epiblast. • The hypoblast cells form portions of the extraembryonic membranes, especially the yolk sac and the stalk linking the yolk mass to the endodermal digestive tube. • The hypoblast cells also provide chemical signals that specify the migration of epiblast cells Gastrulation of the avian embryo The primitive streak • The mammalian and avian gastrulation takes place through the primitive streak. • As the primitive streak forms, a depression called the primitive groove forms within it. • The primitive groove is homologous to the amphibian blastopore, and the primitive streak is homologous to the blastopore lip. Embryo at 12- 14 hrs Embryo at 15- 17 hrs Embryo at 18- 20 hrs Primitive streak Gastrulation of the avian embryo The primitive streak • The anterior end of the primitive streak is a regional thickening of cells called Hensen’s node (Hensen’s knot). • The center of the Hensen’s node (sometimes called primitive pit) contains a funnel- shaped depression through which cells can enter the embryo to form the prechordal plate and notochord. • Hensen’s node is equivalent to the dorsal lip of the amphibian blastopore (the organizer). Hensen’s node Primitive pit Gastrulation of the avian embryo The primitive streak • The primitive streak defines the major body axes of the avian embryo. • 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 Gastrulation of the avian embryo The primitive streak Elongation of the primitive streak • Convergent extension is responsible for the progression of the streak- a doubling in a streak length is accompanied by a concomitant halving of its width. Gastrulation of the avian embryo The primitive streak Elongation of the primitive streak • The elongation of the primitive streak appears to be coextensive with the anterior migration of the secondary hypoblast cells. • The hypoblast directs the movement of the primitive streak. Gastrulation of the avian embryo The primitive streak Formation of endoderm and mesoderm • As soon as the primitive streak has formed, the epiblast cells begin to migrate through it and into the blastocoel. Gastrulation of the avian embryo The primitive streak Formation of endoderm and mesoderm • The first cells to migrate through Hensen’s node are those destined to become the pharyngeal endoderm of the foregut. • The germinal crescent: a region in the anterior portion of the avian and reptilian blastoderm area pellucida containing the hypoblast displaced by migrating endodermal cells. It contains the primordial germ cells (precursors of the germ cells) which later migrate through the blood vessels to the gonads Germinal crescent Gastrulation of the avian embryo The primitive streak Formation of endoderm and mesoderm • The next cells entering through Hensen’s node remain between the endoderm and the epiblast to form the prechordal plate mesoderm. Gastrulation of the avian embryo The primitive streak Formation of endoderm and mesoderm • The next cells passing through the Hensen’s node become the chordamesoderm. • The chordamesoderm has two components: the head process and the notochord. Embryo at 20- 22 hrs Embryo at 23- 25 hrs Embryo at 4 somite stage Gastrulation of the avian embryo The primitive streak Formation of endoderm and mesoderm • Also, the cells that migrate through the Hensen’s node give rise to the medial somite. Gastrulation of the avian embryo The primitive streak Formation of endoderm and mesoderm • The cells that ingress through the middle of the streak give rise to the lateral part of the somite and the intermediate mesoderm which produces the heart and the kidney. Gastrulation of the avian embryo The primitive streak Formation of endoderm and mesoderm • Cells that ingress through the posterior portion of the streak make the lateral plate and extraembryonic mesoderm. Gastrulation of the avian embryo The primitive streak Formation of endoderm and mesoderm • After the ingression of the mesoderm cells, epiblast cells remaining outside of but close to the streak will form medial (dorsal) structures such as the neural plate, while those epiblast cells farther from the streak will become epidermis Gastrulation of the avian embryo The primitive streak Formation of endoderm and mesoderm • The brain components starting from the (Forebrain) forebrain to the level of future ear vesicle originate from the initial neural plate. (Midbrain) • The rest of the nervous system including the posterior hindbrain and all of the spinal Hensen’s node Rhombencephalon (Hindbrain) cord originate from a small region of neural ectoderm just lateral and posterior to the (Ear vesicle) Hensen’s node (sometimes called the caudal lateral epiblast) Head of chick embryo Gastrulation of the avian embryo The primitive streak Formation of endoderm and mesoderm Hensen’s node Gastrulation of the avian embryo Regression of the primitive streak and epiboly of the ectoderm • Primitive streak start to regress, moving the Hensen’s node from near the center of area pellucida to a more posterior position. Gastrulation of the avian embryo Regression of the primitive streak and epiboly of the ectoderm • The regressing streak leaves in its wake the notochord. Gastrulation of the avian embryo Regression of the primitive streak and epiboly of the ectoderm • The notochord starts at the level where the ears and (posterior) hindbrain form and it extends caudally to the tailbud • The notochord induces the formation of the hindbrain and spinal cord. • The anterior parts of the brain are induced by the underlying tissues including the pharyngeal endoderm, prechordal plate and head mesoderm. Gastrulation of the avian embryo Regression of the primitive streak and epiboly of the ectoderm • The enclosure of the yolk by the ectoderm (epiboly) involves the production of new cellular material and the migration of the presumptive ectodermal cells along the underside of the vitelline envelope. Surrounded by the vitelline envelope Gastrulation of the avian embryo Regression of the primitive streak and epiboly of the ectoderm • The elongated filopodia that extend from the marginal cells of area opaca acts as a locomotor apparatus by which the marginal cells pull the other ectodermal cells around the yolk. • The filopodia bind to fibronectin of the chick vitelline envelope and this binding facilitate the ectodermal migration around the vitelline envelope. Axis specification and the avian organizer • While the cells of the posterior portions of the embryo are still part of a primitive streak and entering inside the embryo, cells at the anterior end are already starting to form organs. • Although the formation of chick body axis is accomplished during gastrulation, axis specification begins earlier during the cleavage stage. Axis specification and the avian organizer The role of gravity and the PMZ • The conversion of the radial symmetry in the blastoderm into a bilateral symmetry in body structures appears to be determined by gravity. • When the egg passes through the hen’s reproductive tract, it is rotated for about 20 hrs in the shell gland. Axis specification and the avian organizer The role of gravity and the PMZ • Early on, the ability to initiate a primitive streak is found throughout the marginal zone. • However, once the PMZ has formed, it controls the other regions of the margin. • Not only do the cells of the PMZ initiate the formation of primitive streak and gastrulation, they also prevent other regions of the margin from forming their own primitive streaks. Axis specification and the avian organizer The role of gravity and the PMZ • The PMZ contains cells that act as the equivalent of the amphibian Nieuwkoop center. • The entire marginal zone produces Wnt8c. • The PMZ cells secrete Vg1. • Wnt8c and Vg1 act together to induce expression of Nodal in the future embryonic epiblast next to the Koller’s sickle and the PMZ. Axis specification and the avian organizer The role of gravity and the PMZ • Nodal activity is required to initiate the formation of primitive streak. • However, the secretion of Cerberus- an antagonist of Nodal- by the primary hypoblast cells prevent primitive streak formation. • Once formed, the primitive streak secretes its own Nodal antagonist- the Lefty protein- that prevents the formation of any further primitive streaks. Nodal (Cerberus) Nodal Lefty Nodal Axis specification and the avian organizer Left- right axis formation • As the primitive streak reaches its maximum length, transcription of sonic hedgehog (Shh) gene becomes restricted to the left side of the embryo by the action of activin and its receptor. • Pitx2 protein is active in various organ primordia and specifies which side will be the left, therefore, it is important in directing the asymmetry of embryonic structures. Axis specification and the avian organizer Left- right axis formation Cerberus mRNA Nodal mRNA Pitx2 mRNA

Chapter 12 Early Development in Birds

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