Morulation and Blastulation

Submitted by:

Dr. Sakshi Verma

Assistant Professor

Zoology Department

Morulation

• Morula is the late cleavage stage before the blastocoel has appeared.
• The balstomeres in a morula tend to assume a spherical shape. Their mutual pressure flatters the surfaces of the blastomeres in contact with each other, but the free surfaces of each blastomere remain spherical.
• The solid mass of blastomeres appears to be like a mulberry.
• On account of this superficial similarity, the embryo in this stage is called a morula (Latin for mulberry).

Morulation in different types of egg

Morulation in Isolecithal eggs

Holoblastic cleavage leads to the formation of true morula as in case of coelenterata.

Morulation in mesolecithal egg

In case of mesolecithal eggs holoblastic unequal cleavage occurs no morula is formed because blastocoel start developing just at the 8 celled stage.

Morulation in different types of egg

In centrolecithal eggs of insects no morula is formed

In reptiles, birds and prototherian with macrolecithal and telolecithal eggs morula is not like ball but it is like planoconvex disc like mass of blastomeres at the top of yolk. It is formed by meroblastic and discoidal cleavage.

In human being and most of the eutherian, microlecithal eggs undergoes holoblastic radial, indeterminate and unequal cleavage to form solid ball of 32 celled stage called morula. It is still surrounded by zona pellucida and is formed of two types of blastomeres, micromere and macromeres.

BLASTULATION

• The process of formation of blastula is called blastulation

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Effects of yolk on blastulation

Isolecithal egg (Amphioxus & Echinoderms)

Outer single layers of blastemeres called blastoderms and inner cells forms blastocoel

Moderate Telolecithal eggs (Amphibians)

Cleavage is faster in the animal pole than vegetal pole. Hence blastomeres of vegetal pole are large in size called macromeres.

Effects of yolk on blastulation

Heavily telolecithal eggs (Reptiles, birds)

Cleavage resulted germinal disc. Later on the cells of central part form multilayers and get separated from yolk but the marginal cells continues with yolk and supply nutrition to central germinal disc.

Epiblast: outer blastomere of germinal disc

Hypoblast: layers formed by the migration of few cells of epiblast above subgerminal cavity

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Blastocoel: Cavity between epiblast and hypoblast

BLASTOCYST

It is a distinctive stage of a mammalian embryo. It is a form of blastula that develops from a berrylike cluster of cells, the morula. A cavity appears in the morula between the cells of the inner cell mass and the enveloping layer. This cavity becomes filled with fluid. The blastocyst differs from the blastula in that it is composed of two already differentiated cell types, the inner cell mass and the enveloping layer referred as Trophoblast.

The inner cell mass leads to the formation of embryo while as does not contribute directly to the formation of the embryo but rather serves to establish a connection with the maternal uterus. It is a precursor of the placenta.

Structure of Blastocyst