Chapter 5 The fertilised oocyte, with a diameter of 80 to 120 μm, is one of the largest mammalian cells and has a large amount of cytoplasm relative to the size of its nucleus. For structural development to take place, the zygote must divide. This series of mitotic divisions is referred to as cleavage or segmentation. A distinguishing feature of cleavage over the usual form of mitosis is that daughter cells become progressively smaller with each division, hence the term segmentation. As cleavage proceeds, division of the cytoplasm follows nuclear division and the two daughter cells produced are referred to as blastomeres. The two blastomeres divide repeatedly, producing 4, 8, 16 and 32 cells, and division continues until a spherical mass of cells, termed a morula, is formed. At this stage the first tight junctions between cells in the embryo occur. The first cell divisions tend to occur synchronously in all blastomeres. Asynchronous division subsequently occurs and blastomeres divide independently. Division of the fertilised oocyte is usually regular with the plane of the first division orientated vertically, passing through the main axis of the oocyte from the animal pole at the top to the vegetal pole below. The succeeding division, which is also vertical and passes through the main axis at a right angle to the first division, results in four blastomeres (Fig 5.1A). The third division takes place in the equatorial plane. As a consequence of the planes in which division takes place, eight blastomeres are formed, four in the animal hemisphere and four in the vegetal hemisphere. Figure 5.1 Stages of cleavage from the 2‐cell stage to the early blastula stage in Amphioxus, A, and amphibians, B. The yolk content of the fertilised oocyte determines the pattern of cleavage in individual species; retardation of the completion of cytokinesis correlates with increased yolk content. Accordingly, the relative amount of yolk and its distribution throughout the oocyte has a profound influence on how cleavage proceeds and, subsequently, on germ layer formation. Oocytes with a small amount of evenly distributed yolk are referred to as miolecithal oocytes. When the amount of yolk present displaces the embryo‐forming cytoplasm into a small area at the animal pole, such oocytes are referred to as megalecithal. The term medialecithal is applied to oocytes with a moderate amount of yolk. Based on the abundance and distribution of the yolk, cleavage can be classified in a number of ways. The term total or holoblastic cleavage is used to describe divisions in which the entire oocyte divides and the blastomeres produced are of either equal or unequal size. In miolecithal oocytes, such as those produced by some primitive chordates and placental mammals, blastomeres are of approximately equal size. In medialecithal oocytes, yolk accumulation at the vegetal pole retards mitosis and blastomeres of unequal size are produced. This latter form of cleavage occurs in amphibians. In megalecithal oocytes, mitosis is restricted to the animal pole where the cytoplasm is devoid of yolk. The inert yolk mass at the vegetal pole does not divide. This type of division, referred to as partial or meroblastic cleavage, applies to fish, reptiles and birds. Because the site of cleavage is confined to a disc‐shaped area at the animal pole, this type of cleavage is also known as discoidal. The final stage of cleavage is marked by the formation of a blastula, which consists of a single layer of cells lining a central cavity known as the blastocoele. Cleavage in Amphioxus lanceolatum, a primitive chordate, is holoblastic and the blastomeres produced are of almost equal size. As cleavage proceeds, surface depressions between dividing cells are referred to as cleavage furrows (Fig 5.1). The first cleavage spindle forms near the centre of the oocyte. The second division also produces cells of equal size, but after the third division the four cells at the animal pole are slightly smaller than those at the vegetal pole. As divisions proceed and a morula is formed, differences in cell size become more pronounced, with cells at the animal pole smaller than those at the vegetal pole. At the end of cleavage, the developing Amphioxus embryo is referred to as a blastula (Fig 5.1A). This structure consists of a single layer of cells surrounding a central cavity, the blastocoele.
Cleavage
Cleavage in primitive chordates, amphibians, avian species and mammals
Primitive chordates
Amphibians
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