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[1] [2] Embryonic development begins with a sperm fertilizing an egg cell to become a zygote, which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoel is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the germ layers of the embryo form. [3]
In embryology, cleavage is the division of cells in the early development of the embryo, following fertilization. [1] The zygotes of many species undergo rapid cell cycles with no significant overall growth, producing a cluster of cells the same size as the original zygote.
Coregonus is a diverse genus of fish in the salmon family ().The Coregonus species are known as whitefishes.The genus contains at least 68 described extant taxa, but the true number of species is a matter of debate.
Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass that is distinct from the surrounding blastula. [ 19 ] [ 20 ] [ 21 ] The blastocyst is similar in structure to the blastula but their cells have different fates.
Gastrulation in frogs commences in the marginal zone— the region encircling the blastula's equator where the animal and vegetal hemispheres meet—differing from sea urchins where it begins in the most vegetal part. In contrast to the vegetal blastomeres, the endodermal cells in the marginal zone of frogs are smaller and contain less yolk. [2]
The point of sperm entry defines the dorso-ventral axis - cells opposite the region of sperm entry will eventually form the dorsal portion of the body. [ 1 ] [ 3 ] In the frog Xenopus laevis , the animal pole is heavily pigmented while the vegetal pole remains unpigmented. [ 4 ]
The blastocoel further expands and the inner cell mass becomes positioned on one side of the trophoblast cells forming a mammalian blastula, called a blastocyst. The axis formed by the inner cell mass and the blastocoel is the first axis of symmetry of mammalian embryo and determines its attachment point to the uterus.
The internal cells become the inner cell mass, and the outer cells become the trophoblast. [2] Before cell differentiation takes place there are two transcription factors , Oct-4 and nanog that are uniformly expressed on all of the cells, but both of these transcription factors are turned off in the trophoblast once it has formed.