Lecture (3) Embryology PDF

|Page1 Lecture (3) Embryology Types of eggs - Animal eggs contain large amounts of protein, lipid, and glycogen to nourish the embryo. These materials are collectively called yolk. -Yolk proteins are synthesized in the liver in vertebrate...

|Page1 Lecture (3) Embryology Types of eggs - Animal eggs contain large amounts of protein, lipid, and glycogen to nourish the embryo. These materials are collectively called yolk. -Yolk proteins are synthesized in the liver in vertebrates, or in the fat body of insects. Animal – vegetal polarity On the basis of presence or absence and amount of yolk particles, the eggs classified as follows: A- According to the amount of yolk the egg arranged into the following 1-Alecithal type When the yolk particles are entirely lacking as in placentals mammals. 2- Oligolecithal type Here the yolk particles are present but in little amount as in Echinodermata, Amphioxus. 3-Mesolecithal type: The yolk particles are present in Moderate amount as in toad. 4-Macrolecithal type -The yolk particles are in larg amount. |Page1  |Page2 -The yolk is concentrated in the vegetal pole. The size of nucleus and cytoplasm reduce and lie in the form of a small germinal disc at the animal pole called blastodisc. - In birds and reptiles and egg laying mammals, B- According to distribution of yolk 1- Isolecithal type: The yolk particles are distributed evenly throughout the cytoplasm , as in fishes. 2- Centrolecithal type: The yolk particles are restricted to the center of the egg, as in Insects and other Arthropods 3-Telolecithal type: The yolk accumulated at one end or pole of the egg, as in toad and some fishes. Zygote: is a fertilized egg. A fertilized egg becomes an embryo as soon as the first cleavage occurs.The zygot ultimately, produced a diploid multicellular organism by the several repeated and organised mitotic divisions and cellular differentiation. |Page2  |Page3 Cleavage -Cleavage: is the mitotic division of the egg or, is the first step after fertilization. The cells that arise from cleavage are known as blastomeres. -The pole of the egg with the highest concentration of yolk is referred to as the vegetal pole while the opposite is referred to as the animal pole. -Cleavage: is initiated by the appearance of a groove or constriction called cleavage furrow. -The furrow appears first at one point of the egg. For example, the furrow appears at the animal pole. The furrow then deepens and extends downward on both sides. The two ends meet at the vegetal pole. -The furrow then extendeds inwards radially, finally constricting the egg into two blastomeres. Influence of yolk on cleavage, in two ways: 1- It affects the rate of cleavage 2- It determines the pattern of cleavage -Determinate cleavage: -The developmental fate of the cells being set early in the embryo development. - Each blastomere produced by early embryonic cleavage does not have the capacity to develop into a complete embryo, as in most protostomes. Indeterminate cleavge: -Each blastomere produced by early embryonic cleavage can individually develop into a whole organism, as in deuterostomes. |Page3  |Page4 Planes of cleavage There are four main planes of cleavage. They are as follows: 1- Meridional plane The cleavage furrow passes through the center of animals- vegetal axis and bisects the both poles of the egg. 2- Vertical plane The cleavage tend to pass in a direction from the animal pole toward the vegetal pole. It does not pass through the median axis of the egg. 3- Equatoril plane The cleavage furrow bisects the egg at right angles to the main axis and half way between the animal and vegetal poles. 4- Latitudinal plane The cleavage furrow is similar to the equatorial but it courses through the cytoplasm on either side of the equatorial plane. It also called transverse or horizontalplane. Patterns of cleavage A- Complate or holoblastic cleavage In holoblastic, the entire egg divides by each cleavage furrow, observed in isolecithal eggs or in mesolecithal eggs, It may be : 1- Equal holoblastic cleavage, in microlecithal or isolecithal eggs, it produces blastomeres of approximately equal size. 2- Unequal holoblastic cleavage, in telolecithal eggs, it produces blastomeres of unequal size, due to the concentration of the yolk at the vegetal pole. In which, a few but larger blastomeres (macromeres) are formed at the vegetal pole. Smaller but large number of micromeres are formed at the animal pole. |Page4  |Page5 B- Meroblastic or partial cleavage In meroblastic cleavage, only a portion of the egg divides (partial or incomplete cleavage). observed in telolecithal eggs. It may be : 1- Discoidal meroblastic cleavage It occurs in reptiles and birds. Here the nucleus and cytoplasm are placed at the animal pole as a disc called blastodisc, and this disc alone divides, the huge mass of yolk does not participate in cleavage. 2- Superficial meroblastic cleavage In centrolecithal ovum, as in insects, where there is a moderate to large amount of yolk in the center of the egg. The nuclei travel to the peripheral of the egg and the cleavage remains restricted to the peripheral cytoplasmic. It is easier to develop cleavage plane at the periphery of the egg. |Page5