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20.5 The Mammalian Zygote Becomes a Blastocyst

(a) Mammals have rotational cleavage, in which the plane of Perpendicular the first cleavage is parallel to the animal-vegetal (A, V) axis, plane but the planes of the second cell division (shown in beige) are

/ at right angles to each other. (b) Starting late in the 8-cell stage, the mammalian embryo undergoes compaction of its cells, resulting in a blastocyst—a dense inner cell mass on top of a a hollow blastocoel, completely surrounded by trophoblast cells.

Early 8-cell stage

Zona pellucida

Later 8-cell stage (compaction)

Early 8-cell stage

Later 8-cell stage (compaction)

Zona pellucida

Tightjunctions have formed between the cells.

Tightjunctions have formed between the cells.

16-cell stage

Blastocyst (32-cell stage)

16-cell stage

Blastocyst (32-cell stage)

is called a blastocyst to distinguish it from the blastulas of other animals.

Fertilization in mammals occurs in the upper reaches of the mother's oviduct, and cleavage occurs as the zygote travels down the oviduct to the uterus. When the blastocyst arrives in the uterus, the trophoblast adheres to the endometrium (the uterine wall). This event begins the process of implantation that embeds the embryo in the wall of the uterus (see Figure 20.14). In humans, implantation begins on about the sixth day after fertilization. As the blastocyst moves down the oviduct to the uterus, it must not embed itself in the oviduct wall, or the result will be an ectopic or tubal pregnancy—a very dangerous condition. Early implantation is normally prevented by an external proteinaceous layer called the zona pellucida, which surrounds the egg and remains around the cleaving ball of cells. At about the time the blastocyst reaches the uterus, it hatches from the zona pellucida, and implantation can occur.

Specific blastomeres generate specific tissues and organs

In all animal species, cleavage results in a repackaging of the egg cytoplasm into a large number of small cells surrounding a central cavity. Little cell differentiation occurs during cleavage, and in most nonmammalian species, none of the genome of the embryo is expressed. Nevertheless, cells in different regions of the resulting blastula possess different complements of the nutrients and cytoplasmic determinants that were present in the egg.

The blastocoel prevents cells from different regions of the blastula from interacting, but that will soon change. During the next stage of development, the cells of the blastula will move around and come into new associations with one another, communicate instructions to one another, and begin to differentiate. In many animals, these movements of the blas-tomeres are so regular and well orchestrated that it is possible to label a specific blastomere with a dye and identify the tissues and organs that form from its progeny. Such labeling experiments produce fate maps of the blastula (Figure 20.6).

Animal pole

Ectoderm will form epidermal layer of skin.

The neural ectoderm will form the nervous system.

Animal pole

Ectoderm will form epidermal layer of skin.

Plasma Membrane Map

The neural ectoderm will form the nervous system.

Endoderm will form the lining of the gut, the liver, and the lungs.

Vegetal pole

Mesoderm will form muscle, bone, kidneys, blood, gonads, and connective tissues.

Endoderm will form the lining of the gut, the liver, and the lungs.

Vegetal pole

Mesoderm will form muscle, bone, kidneys, blood, gonads, and connective tissues.

20.6 Fate Map of a Frog Blastula The colors indicate the portions of the blastula that will form the three germ layers, and subsequently the frog's tissues and organs.

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