Cell Stage

Blastomeres

Early cleavage results in blastomeres of similar size.

Vegetal pole

Animal pole

Cleavage furrow

Early cleavage results in blastomeres of similar size.

Vegetal pole

Animal pole

Gray crescent

Gray crescent

Cleavage furrow

J Blastomeres at the animal pole are smaller, and those at the vegetal pole are larger.

J Blastomeres at the animal pole are smaller, and those at the vegetal pole are larger.

Blastomeres

Blastomeres

The embryo develops on top of the yolk as a disc of cells, called a blastodisc.

Cleavage is incomplete.

The embryo develops on top of the yolk as a disc of cells, called a blastodisc.

Cleavage is incomplete.

Single cell layer Yolk core

Single cell layer Yolk core

Superficial cleavage

Multiple ' nuclei

The nuclei migrate to the periphery, and plasma membranes form between them.

Multiple ' nuclei

The nuclei migrate to the periphery, and plasma membranes form between them.

20.4 Patterns of Cleavage in Four Model Organisms Differences in patterns of early embryonic development reflect differences in the way the egg cytoplasm is organized.

Cleavage in mammals is unique

Several features of mammalian cleavage are very different from those seen in other animal groups. First, the pattern of cleavage in mammals is rotational: the first cell division is parallel to the animal-vegetal axis, yielding two blastomeres. The second cell division occurs at right angles: one blas-tomere divides parallel to the animal-vegetal axis, while the other divides perpendicular to it (Figure 20.5a).

Cleavage in mammals is very slow; cell divisions are 12-24 hours apart, compared with tens of minutes to a few hours in non-mammalian species. Also, the cell divisions of mammalian blastomeres are not in synchrony with each other. Because the blastomeres do not undergo mitosis at the same time, the number of cells in the embryo does not progress in the regular (2, 4, 8, 16, 32, etc.) progression typical of other species.

Another unique feature of the slow mammalian cleavage is that the products of genes expressed at this time play roles in cleavage. In species such as sea urchins and frogs, gene expression does not occur in the blastomeres, and cleavage is directed exclusively by molecules that were present in the egg prior to fertilization.

As in other animals that have complete cleavage, the early cell divisions in a mammalian zygote produce a loosely associated ball of cells. However, at about the 8-cell stage, the behavior of the mammalian blastomeres changes. They change shape to maximize their surface contact with one another, form tight junctions, and become a very compact mass of cells (Figure 20.5b).

At the transition from the 16-cell to the 32-cell stage, the cells separate into two groups. The inner cell mass will become the embryo, while the surrounding cells become an encompassing sac called the trophoblast, which will become part of the placenta. Trophoblast cells secrete fluid, creating a cavity (blastocoel) with the inner cell mass at one end (see Figure 20.5b). At this stage, the mammalian embryo

Parallel Plane of first plane cell division

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment