M.Sc.Semester III Paper -409 Developmental Biology and Immunology Topic: Gastrulation Dr.Tabrez Ahmad • The most characteristic event occurring during the third week of gestation is gastrulation. • Gastrulation is a formative process by which the three germ layers, which are precursors of all embryonic tissues, and the axial orientation, are established in embryos. • During gastrulation, the bilaminar embryonic disc is converted into a trilaminar embryonic disc. • Extensive cell shape changes, rearrangement, movement, and alterations in adhesive properties contribute to the process of gastrulation. • The embryo during this stage is called a gastrula. Process of Gastrulation • Gastrulation begins with the formation of the primitive streak on the surface of the epiblast. • Initially, the streak is vaguely defined, but in a 15- to a 16-day embryo, it is clearly visible as a narrow groove with slightly bulging regions on either side. • The cephalic end of the streak, the primitive node, consists of a slightly elevated area surrounding the small primitive pit. • Cells of the epiblast migrate toward the primitive streak. • Upon arrival in the region of the streak, they become fl ask-shaped, detach from the epiblast, and slip beneath it. This inward movement is known as invagination. • Once the cells have invaginated, some displace the hypoblast, creating the embryonic endoderm, and others come to lie between the epiblast and newly created endoderm to form mesoderm. • Cells remaining in the epiblast then form ectoderm. • Thus, the epiblast, through the process of gastrulation, is the source of all of the germ layers, and cells in these layers will give rise to all of the tissues and organs in the embryo. Each of the three germ layers (ectoderm, mesoderm, and endoderm) gives rise to specific tissues and organs: • Embryonic ectoderm gives rise to the epidermis, central and peripheral nervous systems, eyes and internal ears, neural crest cells, and many connective tissues of the head. • Embryonic endoderm is the source of the epithelial linings of the respiratory and alimentary (digestive) tracts, including the glands opening into the gastrointestinal tract, and glandular cells of associated organs such as the liver and pancreas. • Embryonic mesoderm gives rise to all skeletal muscles, blood cells, the lining of blood vessels, all visceral smooth muscular coats, serosal linings of all body cavities, ducts and organs of the reproductive and excretory systems, and most of the cardiovascular system. In the body (trunk or torso), excluding the head and limbs, it is the source of all connective tissues, including cartilage, bones, tendons, ligaments, dermis, and stroma (connective tissue) of internal organs. Factors controlling the process of Gastrulation • Cell migration and specific cation are controlled by fibroblast growth factor 8 (FGF8), which is synthesized by streak cells themselves. • This growth factor controls cell movement by down regulating E-cadherin, a protein that normally binds epiblast cells together. • FGF8 then controls cell specific cation into the mesoderm by regulating Brachyury (T) expression. Consequences Gastrulation results in three important outcomes: 1. The formation of the embryonic tissues called germ layers. The germ layers include the endoderm, ectoderm, and mesoderm. Each germ layer will later differentiate into different tissues and organ systems. 2. The formation of the embryonic gut or archenteron. 3. Gastrulation is the beginning of morphogenesis (development of body form). 4. The appearance of the major body axis. Types of Gastrula Gastrulation of a Coeloblastula A coeloblastula is a hollow ball of cells, one cell thick. Gastrulation in a blastula of this type involves invagination, ingression, or delamination. Invagination involves the blastula folding in on itself, creating a pocket with an opening. These are known as the archenteron and blastopore, and will become parts of the gut. The in-fold becomes the endoderm, while the outer later becomes the ectoderm. The resulting gastrula is known as a coelogastrula, because it remains hollow. This can be seen in the above image. The blastocoel is simply the empty space inside of the blastula. Coeloblastulae can also undergo a type of gastrulation that produces a solid gastrula, known as a stereogastrula. During this form of development, the cells of the blastula divide and migrate into the blastocoel, eventually filling the space. The movement of cells in this form of gastrulation is known as ingression. The cells on the inside will develop as endoderm, while the surface cells will develop as ectoderm. This can be seen in many cnidarians, such as sea anemones and jellyfish. The final form of gastrulation is called delamination, and exists when the cells of a coeloblastula each divide one, creating a layer of cells surrounding the original blastula. This form is seen in only in some groups related to jellyfish. Gastrulation of a Stereoblastula A stereoblastula is a blastula that exists as a solid mass of cells. Gastrulation in stereoblastulae differs from gastrulation in a coeloblastula, because there is no internal space for the cells to divide into. Instead, cells on the surface of the ball divide at a faster rate, until the surface of the ball is covered with a new layer of cells. This layer functions as the ectoderm, while the solid ball in the middle forms the endoderm, as in the previous form of stereogastrula. The archenteron will form later, from inside the solid mass of cells. Gastrulation of a Discoblastula A discoblastula, unlike the other forms of blastulae, does not form a ball of cells around the original cell. Rather, the cells are arranged in a disc at one end of the blastula, and each has access to a yolk reservoir at the other end. Gastrulation in a discoblastula involves the ends of the disc of cells curving, and growing back toward each other. The bottom layer develops as the endoderm, while the top layer further from the yolk develops as the endoderm. This is known as involution. Meaning of Gastrulation: The blastula passes into the stage called gastrula by the process—Gastrulation. This process is extremely important in the ontogenetic process of an animal, because the blue-print of the future organisation is laid down during this phase. During this crucial and dynamic process major presumptive organ-forming areas of the blastula become reorganized in a fashion that allows their ready transformation into the fundamental body plan of a species. Gastrulation is essentially a process of migration of cells from one place to the other in the embryo. Besides movement of cells, considerable nuclear differentiation also takes place. In almost all animals it results in: (i) The establishment and differentiation of three primary germinal layers—ectoderm, mesoderm and endoderm, (ii) The establishment of nuclear differentiation and (iii) The beginning of the control of genetic factors over development. 2. Basic Mechanism in Gastrulation: The process of gastrulation involves following three cellular activities, cell-movement, cell- contact and cell-division. All these mechanisms are carried in a nicely co-ordinated and integrated way. Number of factors is believed to be responsible for this coordination, but it has not been possible to pin point the final answer. It is undeniable that this process is controlled largely by intrinsic factors which are correlated with the external as well as internal conditions. 3. Methods used to Study Gastrulation: The correct observation of incidences during gastrulation was started from the findings of W. Vogt in 1923. Vogt used vital dyes (Janus green and Neutral red) to mark the cells in an early gastrula and noted that cells during gastrulation actually migrate from one place to the other. The vital dye technique of Vogt resulted into the application of several other methods: (i) Visible differences in the cytoplasmic particles were used as natural marker, (ii) Tagging of the cells with carbon particles and (iii) Tagging of the cells with radioactive substances. 4. Morphogenetic Movement of Cells in Gastrulation: During gastrulation, cells from one region of embryo move to another to take up their future fateful position. Two terms, emboly and epiboly which are quite opposite in their meanings, are generally applied to explain the process of movement. Emboly means the throwing in or insertion of cells and epiboly signifies the extending upon. The movement of cells establishes a particular form and involved in organ formation in embryo—so this movement is designated as the morphogenetic movement. Fig. 5.15 shows the movement of cells in gastrulation. Fundamentally, the morphogenetic movement is similar but the details of the process vary greatly. Following types of cells movement occur: Epiboly: It involves the extension along the anteroposterior axis and peripheral divergence. Emboly: The inward movement of cells is classified into different types depending on the behaviour of migrating cells. These are: (i) Invagination: It denotes the infolding of a layer of cells to form a cavity encircled by infolded cells. Generally in the gastrulation of Amphioxus and frog, the wall of the blastoderm is pushed inside the blastocoel. This creates a new cavity called the archenteron which communicates with the exterior by a blastopore. This process of inpushing goes on and the inpushed layer forms the walls of the cavity. The archenteron (or primitive gut) completely obliterates the blastocoel. (ii) Involution: It implies the inward rotation of cells as seen in the gastrulation of amphibian and avian eggs. From one end near the edge of the blastoderm, the cells begin to move inwards to form the inner lining of the blastoderm. (iii) Convergence: It means the movement of cells to a particular region of the gastrula. In amphibian egg, the migration of cells to the external edge of the blastoporal lip is designated as convergence. The same phenomenon of convergence of cells is seen in the formation of primitive streak in chick embryo. (iv) Divergence: This phenomenon is opposite to convergence, when involuted cells diverge to take up their future positions inside the gastrula.