Early embryonic cell division patterns in vertebrates can be broken into two broad categories, holoblastic cleavage (e.g., most amphibians and mammals) and meroblastic cleavage (e.g., birds, reptiles, and teleost fishes) (Fig.
The discoidal and superficial cleavages are the two main types of meroblastic cleavage.
In the absence of a large concentration of yolk, four major cleavage types can be observed: radial holoblastic, spiral holoblastic, bilateral holoblastic, and rotational holoblastic cleavage.
The pattern of embryonic cleavage is determined by two major parameters ; 1) by the amount and distribution of yolk within the cytoplasm and 2) by the organization of the egg. 1. Role of yolk : The amount and distribution of yolk determines where cleavage can occur and also the relative size of the blastomeres .
2-cell stage (3/4 h): The first cleavage furrow, ending the first zygotic cell cycle, is vertically oriented, as is usual until the 32-cell stage. The furrow arises near the animal pole and progresses rapidly towards the vegetal pole, passing through only the blastodisc and not the yolky region of the egg (Fig. 4A).
As viewed in the microscope, the cell cycle is divided into two basic parts: mitosis and interphase. Mitosis (nuclear division) is the most dramatic stage of the cell cycle, corresponding to the separation of daughter chromosomes and usually ending with cell division (cytokinesis).
During the mitotic (M) phase, the cell divides its copied DNA and cytoplasm to make two new cells. M phase involves two distinct division-related processes: mitosis and cytokinesis.
Radial cleavage is a characteristic feature in the early embryonic development of deuterostomes. It is one of the simplest cleavage patterns in which the successful division planes are at 90° relative to each other. Thus this cleavage results in daughter cells that are located exactly on top of one another.
A blastula produced by radial cleavage can be cut along any meridian to get into two identical halves. Radial cleavage is found in echinoderms. (2) Biradial cleavage: When the first three division planes do not stand at right angles to each other, the cleavage is termed as biradial.
Holoblastic cleavage involves the division of the entire egg into blastomeres, and it mainly occurs in isolecithal cells. The patterns followed by these cells during cleavage are radial, spiral, rotational, and bilateral.
The one cell embryo undergoes a series of cleavage divisions, progressing through 2-cell, 4-cell, 8-cell and 16 cell stages.
Spiral cleavage is a distinctive early developmental program displayed by at least eight major animal groups, including annelids (i.e. segmented worms), molluscs (e.g. snails), nemerteans (i.e. ribbon worms) and platyhelminths (i.e. flatworms) (Hejnol, 2010; Henry, 2014; Lambert, 2010) (Fig. 1A).
Mammals undergo holoblastic rotational cleavage, characterized by a slow rate of division, a unique cleavage orientation, lack of divisional synchrony, and the formation of a blastocyst.
Spiral cleavage is a characteristic of Protostomes, and results in determinant cells (Cell that have a determined embryological fate early on during the development of the embryo). In other words, determinant cells are programmed to become a specific type of cell, early on during the process.
There are several types of cleavage symmetry seen in nature: radial(echinoderms, amphibians), spiral (mollusks, annelids), Bilateral (ascidians,tunicates), Rotational (mammals). The two figures below show examples of holoblastic and meroblastic cleavage symmetries.
Characteristics of Cleavage
Cleavage forms a spherical and multicellular development stage which is known as a blastula. The process of formation of multiple cells is known as blastulation. Cleavage in embryos continues until an average cell size as that of the parent cell is achieved.
Biradial symmetry is found in organisms which show morphological features (internal or external) of both bilateral and radial symmetry. Unlike radially symmetrical organisms which can be divided equally along many planes, biradial organisms can only be cut equally along two planes.
The two main types of symmetry are radial symmetry (in which body parts are arranged around a central axis) and bilateral symmetry (in which organisms can be divided into two near-identical halves along a single plane).
Bilateral and radial symmetries. In bilateral symmetry only a single plane divides the organism into two identically reflected halves (reflective symmetry) (Fig. 1a), whereas a radially symmetric structure can be halved at multiple planes to produce identical parts (n-fold rotational symmetry) (Fig. 1b).
Bilateral cleavage divides the egg bilaterally across both horizontal and vertical planes. This produces a bilateral symmetry across both poles, and we see this cleavage pattern in tunicates, our long ago chordate ancestor. Spiral cleavage is when cleavage occurs in a spiral rotation.
[IV] Bilateral cleavage:
In bilateral cleavage, the blastula can be cut vertically only along one plane to get two identical halves, the right and the left.
In contrast to the radial cleavage found in many other metazoans, in spiral cleavage each micromere is not placed directly above its sister macromere, but is displaced towards the right (or towards the left, depending on the organism) with respect to its sister macromere.
(A) In mitosis, diploid cells replicate chromosomes during S phase and segregate sister chromatids during M phase, so that diploid daughter cells are produced. (B) In meiosis, two chromosome-segregation phases, meiosis I and meiosis II, follow a single round of DNA replication during the premeiotic S phase.
During cytokinesis, the cleavage furrow appears suddenly on the surface of the cell and deepens rapidly (Figure 18-30).
The cell cycle has two major phases: interphase and the mitotic phase (Figure 1). During interphase, the cell grows and DNA is replicated. During the mitotic phase, the replicated DNA and cytoplasmic contents are separated, and the cell divides.