Embryology
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- Date Submitted: 02/02/2015 11:13 AM
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The egg types seen in vertebrate embryology
There are three different egg types in vertebrate embryology; Isolecithal, mesolecithal and telolecithal (Hickman, Roberts, Larson, 2001) As seen below.
Isolecithal eggs are eggs with very little yolk which is evenly distributed within the egg (Hickman, Roberts, Larson, 2001).
Mesolecithal eggs are eggs with a moderate amount of yolk that is concentrated to the vegetal pole. The opposite pole contains largely cytoplasm with very little yolk (Hickman, Roberts, Larson, 2001).
Telolecithal eggs are eggs with an abundance of yolk that is densely concentrated to the vegetal pole (Hickman, Roberts, Larson, 2001).
The primary cleavage patterns in vertebrates
There are two primary cleavage patterns found in vertebrates; holoblastic and meroblastic (Miller, Harley, 1999).
Holoblastic cleavage, as seen on the right, typically occurs In isolectihal and mesolectihal eggs, the entire egg divides by each cleavage furrow (Khanna,2004). There are two main types of meroblastic cleavage; discoidal and superficial. In discoidal cleavage, the cleavage furrows extend through the cytoplasm but do not infiltrate the yolk. The embryo forms a blastodisc on top of the yolk. In superficial cleavage, a polynuclear cell is formed. Due to the yolk being present in the centre of the egg cell, the nuclei migrate to the periphery of the egg, the plasma membrane grows inwards thus separating the nuclei into individual cells (Hickman et al, 2006).
Meroblastic cleavage occurs in telolecithal eggs, in which four major cleavage types can be observed; bilateral holoblastic, radial holoblastic, rotational holoblastic, and spiral holoblastic (Gilbert, 2000).As seen below.
Gastrulation
Gastrulation is a stage of embryological development, which takes place after cleavage, in which a single-layered blastula is reorganized into a gastrula, a trilaminar structure consisting of the three germ layers; an outer ectodermal layer, middle mesodermal...
There are three different egg types in vertebrate embryology; Isolecithal, mesolecithal and telolecithal (Hickman, Roberts, Larson, 2001) As seen below.
Isolecithal eggs are eggs with very little yolk which is evenly distributed within the egg (Hickman, Roberts, Larson, 2001).
Mesolecithal eggs are eggs with a moderate amount of yolk that is concentrated to the vegetal pole. The opposite pole contains largely cytoplasm with very little yolk (Hickman, Roberts, Larson, 2001).
Telolecithal eggs are eggs with an abundance of yolk that is densely concentrated to the vegetal pole (Hickman, Roberts, Larson, 2001).
The primary cleavage patterns in vertebrates
There are two primary cleavage patterns found in vertebrates; holoblastic and meroblastic (Miller, Harley, 1999).
Holoblastic cleavage, as seen on the right, typically occurs In isolectihal and mesolectihal eggs, the entire egg divides by each cleavage furrow (Khanna,2004). There are two main types of meroblastic cleavage; discoidal and superficial. In discoidal cleavage, the cleavage furrows extend through the cytoplasm but do not infiltrate the yolk. The embryo forms a blastodisc on top of the yolk. In superficial cleavage, a polynuclear cell is formed. Due to the yolk being present in the centre of the egg cell, the nuclei migrate to the periphery of the egg, the plasma membrane grows inwards thus separating the nuclei into individual cells (Hickman et al, 2006).
Meroblastic cleavage occurs in telolecithal eggs, in which four major cleavage types can be observed; bilateral holoblastic, radial holoblastic, rotational holoblastic, and spiral holoblastic (Gilbert, 2000).As seen below.
Gastrulation
Gastrulation is a stage of embryological development, which takes place after cleavage, in which a single-layered blastula is reorganized into a gastrula, a trilaminar structure consisting of the three germ layers; an outer ectodermal layer, middle mesodermal...