Blastula: definition, formation and segmentation

The blastula, also known as the blastosphere, is a circular or elliptical structure, formed by 6-32 cells that forms after a zygote undergoes cell division, but before its cells have differentiated.

The development of multicellular organisms begins with a single-celled zygote, which undergoes rapid cell division (CDC) to form the blastula. After segmentation has produced over 100 cells, the embryo is called a blastula. The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled cavity (the blastocele).

Mammals at this stage form the blastocyst, a structure characterized by an internal cell mass distinct from the surrounding blastula. During segmentation, cells divide without mass gain; That is, a large single-celled zygote divides into several smaller cells. Every cell within the blastula is called a blastomere.

Segmentation can occur in two ways:

  • holoblastic segmentation (total)
  • Meroblastic (partial) segmentation.
    The type of segmentation depends on the amount of yolk (deutoplasm) present in the eggs. In humans – and in all other placental mammals – where nourishment is provided by the mother’s body, the eggs have a very small amount of yolk and undergo holoblastic segmentation.

Other species, such as birds, with a lot of yolk in the egg to nourish the embryo during development, undergo meroblastic segmentation.
In mammals, the blastula forms the blastocyst at the next stage of development.

In the blastocyst the cells of the blastula are arranged in two layers:

  • the internal cell mass also known as embryoblast The embryoblast will form the embryo.
  • an outer layer called a trophoblast The trophoblast will contribute to the placenta and nourish the embryo.

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform,

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