Short notes on the development of human foetus upto blastula stage

Short notes on the development of human foetus upto blastula stage

The human egg is very small and has practically very little yolk. Hence it is called microlecithal. Whenever little yolk is present is distributed uni­formly throughout the egg. Hence the egg is called isolecithal or homolecithal. Early development includes three stages mainly – cleavage or seg­mentation, morula stage and blastula stage. At the blastula stage there is implantation.

1. Cleavage:

The human zygote undergoes cleavage during its movement along the fallopian tube. The cleavages in zygote are said to be homoplastic as the cleavage is equal. The first cleavage occurs usually within 30 hours after fertilization. The cleavage furrow is vertical passing through the ani­
mal pole to vegetal pole. The two daughter cells so formed are called blastomeres. The second cleavage is also vertical but at right angles to the first one forming four blastomeres of equal size. Subsequent divisions occur in a rapid but orderly fashion producing a number of blastomeres.

2. Morula:

After repeated cleavages, the embryo takes the form of a solid sphere of cells looking like the fruit of the mulberry hence this stage is called morula (a little mulberry) stage. The morula has 16 to 32 cells and is larger than the zygote. It has two types of cells. In about 4 to 6 days after fertilization the morula reaches the uterus. It is still surrounded by the zona pellucida which prevents its sticking to the uterine wall.

3. Blastula:

The outer layer of cells of morula absorbs nutrition from the uterine wall and is now referred to as trophoblast. The cells of trophoblast are flattened and the fluid absorbed by this trophoblast gets collected in the cavity called the blastocoel. At this stage the trophoblast becomes the blastocyst and this stage of development is known as the blastula.

The inner cell mass present in the blastocyst looks like a small knob at one pole and it is called the embryonal knob. The trophoblast (outer layer of blastomeres) does not take part in the formation of the embryo proper. It helps in the formation of the extraembryonic membranes. The cells of the trophoblast that remain on contact with the embryonal knob are called cells of Rauber.