Cleavage is the first phase of embryonic development

What is cleavage? Cleavage is a rapid series of mitotic divisions that occur just after fertilization. There are two critical reasons why cleavage is so important: Generation of a large number of cells that can undergo differentiation and gastrulation to form organs. 2. Increase in the nucleus / cytoplasmic ratio. Eggs need a lot of cytoplasm to support embryogenesis. It is difficult or impossible for one nucleus to support a huge cytoplasm, and oocytes are one of the largest cells that exist. One small nucleus just cannot transcribe enough RNA to meet the needs of the huge cytoplasm. A larger nucleus to cytoplasmic ratio is optimal for cell function. Cell division occurs rapidly after fertilization to correct this problem.

Cleavage differs from normal mitoses in 2 respects Blastomeres do not grow in size between successive cell divisions as they do in most cells. This leads to a rapid increase in the nucleus / cytoplasmic ratio. Cells undergoing cleavage have mainly S and M phases of the cell cycle (little or no G1 or G2). Cleavage occurs very rapidly, and mitosis and cytokinesis in each round of cell division are complete within an hour. Typical somatic cells divide much more slowly (several hours to days) and even the fastest cancer cells divide much slower than occurs in a zygote during cleavage. Cleavage differs in different types of eggs. The presence of large amounts of yolk alters the cleavage pattern, leading to incomplete cleavage that characterizes birds and reptiles. Two areas of interest: How does the process of cleavage differ in different organisms? What mechanisms regulate cleavage?

Eggs are classified by how much yolk is present Isolecithal eggs (iso = equal) have a small amount of yolk that is equally distributed in the cytoplasm (most mammals have isolecithal eggs). Mesolecithal eggs (meso = middle) have a moderate amount of yolk, and the yolk is present mainly in the vegetal hemisphere (amphibians have mesolecithal eggs). Telolecithal eggs (telo = end) have a large amount of yolk that fills the cytoplasm, except for a small area near the animal pole (fish, reptiles, and birds). Centrolecithal eggs have a lot of yolk that is concentrated within the center of the cell (insects and arthropods).

The pattern of cleavage of the zygote depends upon the pattern of yolk distribution Holoblastic cleavage: occurs in isolecithal eggs (mammals, sea urchins). The entire egg is cleaved during each division. Meroblastic cleavage occurs when eggs have a lot of yolk. The egg does not divide completely at each division. Two types: a. Discoidal cleavage is limited to a small disc of cytoplasm at the animal pole. All of the yolk filled cytoplasm fails to cleave (characteristic of telolecithal eggs such as birds). b. Superficial cleavage is limited to a thin surface area of cytoplasm that covers the entire egg. The inside of the egg that is filled with yolk fails to cleave (centrolecithal eggs such as insects).

Typical cleavage patterns of isolecithal, mesolecithal, telolecithal and centrolecithal eggs

Sea urchins have isolecithal eggs and undergo holoblastic cleavage Cleavage plane: this is the plane in which cleavage occurs. It is oriented at right angles to the metaphase plate. In sea urchins, the first cleavage is meridional. Meridional cleavage runs from one pole to another (top to bottom), like the meridian on a globe. The second cleavage is also meridional. Equatorial cleavage encircles the zygote like the equator on the globe. The third cleavage in the sea urchin is equatorial. This creates an animal and vegetal half.

The fourth cleavage is unique The fourth cleavage is unique. Equal cytokinesis occurs in the four blastomeres of the animal pole, giving rise to 8 mesomeres (all the same size). Unequal cytokinesis occurs in the vegetal pole. This causes 4 large macromeres and 4 small micromeres The 5th cleavage is meridional. All mesomeres divide equally as do the macromeres. As cleavage progresses, all blastomeres adhere at the outer surface, but attachment is lost at the inner surface. The blastocoel is a cavity formed due to the unequal adherence of blastomeres.

Amphibians have mesolecithal eggs and undergo holoblastic cleavage Amphibian eggs have a lot of yolk, however, they are still able to undergo holoblastic cleavage. The 1st cleavage is meridional, as is the 2nd. The 3rd cleavage is equatorial. The cleavage is displaced toward the animal pole due to the yolk. This results in 4 small animal blastomeres and 4 large vegetal blastomeres. Morula (morum = mulberry) at the 16 to 32 cell stage the embryo is called a morula because it looks like a mulberry. morula

The blastocoel is displaced to the animal pole in amphibians Blastula = from the 128 cell stage onward the amphibian embryo is a blastula. The outer surface of the amphibian blastula has cells connected by specialized cell junctions. Tight junctions create a seal that isolates the outside of an embryo from the inner layer. Tight junctions polarize the apical and basal surfaces. The basal portions of cells start secreting into the blastocoel. Desmosomes attach the blastomeres together on the outside. Gap junctions connect all surface blastomeres.

Mammalian eggs have rotational cleavage that is holoblastic The mammalian egg is a little slow. It begins to cleave in the oviduct and continues until it implants in the wall of the uterus (1 cleavage / 24 hr). Asynchronous cleavage: mammalian embryos are unusual in that they have asynchronous cleavage. Not all blastomeres divide at the same time. The first cleavage is meridional, and the second cleavage is rotational. The 2 blastomeres divide in different planes (one is equatorial and one is meridional.

Mammalian embryos undergo compaction at the 8 cell stage At first, the blastomeres of mammalian embryos have a loose arrangement, and touch only at the basal surfaces. After compaction, blastomeres adhere tightly, maximizing the area of contact. During compaction, each blastomere undergoes polarization. Tight junctions develop on the outer surface, allowing proteins to specialize. Cells take up fluids from the uterine environment and secrete into the blastocoel. Gap junctions form on the outer cells to aid in intercellular communication.

A blastocoel develops as cleavage proceeds to the 32-64 cell stage After compaction at the 8-16 cell stage, there are 2 types of blastomeres. Outside blastomeres are tightly joined and number about 9-14. They surround 2-7 inside blastomeres that are loosely joined. Cavitation: the outside blastomeres start to take up fluid from the uterus and pump it into the center, creating the blastocoel. The blastocyst is the hallmark of early embryonic development in mammals. Inner cell mass: this gives rise to the embryo, and develops from the inside blastomeres Trophoblast: a structure consisting of outside blastomeres, this contributes to forming the placenta.

Embryonic stem cells can be cultured from the inner cell mass Cells in the inner cell mass are undifferentiated, they multiply indefinitely, and are known as embryonic stem cells. Stem cells are totipotent = they have the potential to form any tissue. These cells are of great scientific and medical importance. They can be removed from the embryo, genes can be introduced into the cells, and then they can be placed back in the blastocyst. This is how one constructs transgenic or “knock out” mice. The embryonic stem cells are also used to grow certain types of tissue in culture. Theoretically, it should be possible to grow structures such as ears, muscles, nerves, and skin for transplantation to sick individuals. Interestingly, if you inject adult, differentiated cells back into the environment of the morula or blastula, they become undifferentiated, and they can redifferentiate to form many parts of the body.

Early development and cleavage in humans How do twins develop?

Development of monozygotic or identical twins Monozygotic twins develop from one zygote by splitting at various stages of development (from the 2 cell to the blastocyst stage). The stage of splitting effects the overall structure of the embryo and extraembryonic membranes. What are conjoined twins and how do they arise? Where do fraternal twins come from? Sextuplets?

Conjoined twins are identical twins who develop from a single fertilized ovum. are always the same sex and race. are more often female than male, at a ratio of 3:1. occur once in 40,000 births but only once in 200,000 live births. may be caused by any number of factors, being influenced by genetic and environmental conditions.

Birds, reptiles, and fishes have telolecithal eggs that completely support embryogenesis; they undergo meroblastic cleavage In contrast to holoblastic cleavage, where the entire zygote divides into blastomeres, meroblastic cleavage leaves a large portion of the zygote uncleaved. There are 2 types of meroblastic cleavage, discoidal and superficial. Discoidal: In birds and reptiles, the 1st cleavage is meridional. It starts at the animal pole but does not progress far. The 2nd and 3rd cleavages are also meridional. The 4th cleavage is equatorial, and it creates a layer of small cells on top of the huge uncleaved area below (yolk). Blastoderm: when cleavage has progressed such that there are many blastomeres in the animal pole, it is a blastoderm. Chicken eggs have a blastoderm of about 60,000 cells when the egg is laid.

The next step in development of telolecithal eggs is formation of the upper and lower blastoderm. Epiblast: (epi = upon) this is the upper layer and it forms the embryo proper. Hypoblast: (hypo = under) this is the bottom layer that will form the extraembryonic endoderm that surrounds the yolk. What is the counterpart in mammals? Blastocoel: lies between the 2 layers. Subgerminal space: lies between the hypoblast and yolk.

Insects have centrolecithal eggs and undergo superficial cleavage Periplasm: insect eggs have a superficial area of cytoplasm that is free from yolk. It surrounds the entire egg, and cleavage occurs here. Endoplasm: the yolk-rich cytoplasm in the center of the egg. This area does not undergo cleavage. Cleavage is a misnomer in insects because cell division is delayed until after many rounds of mitosis have been completed.

In Drosophila, nuclei start to undergo mitosis deep within the yolk In Drosophila, nuclei start to undergo mitosis deep within the yolk. No cell division occurs, and the nuclei slowly migrate out toward to periphery. A few nuclei are first observed in the periplasm at the 9 cell division stage. They quickly become enclosed by a plasma membrane and become pole cells (primordial germ cells). Preblastoderm stage: (cycles 10 to 13) Most of the nuclei are present in the periplasm but no cytokinesis has occurred. Still one big multinucleated cell! Cellular blastoderm: At about cycle 14, cytokinesis occurs simultaneously all over the egg. Each nuclei is surrounded by a plasma membrane to become a cell. This corresponds to the blastoderm stage of other embryos.

The cell cytoplasm is divided during cytokinesis Mitosis is followed by cytokinesis, when the cytoplasm divides equally. A contractile ring forms beneath the plasma membrane. It contains a band of actin and myosin filaments. It always forms in the same place that was occupied by the metaphase plate. As the actin and myosin filaments slide by one another, the ring contracts and pinches the 2 cells apart.

Immuno staining of the cortex shows myosin Cytokinesis is caused by subcortical network of actin and myosin filaments. These filaments slide over one another as in muscle, and this causes contraction and a cleavage furrow to form on the cell surface. In holoblastic cleavage, the furrow squeezes around the periphery, like a belt tightening, to pinch the cell in two. In meroblastic cleavage, the furrow starts at the animal pole and progresses into the egg like a knife. It stops when it reaches the vegetal portion. Anti myosin antibodies

The mitotic spindle determines the orientation of the cleavage plane Blastomeres can cleave either equatorially or meridionally. Cytokinesis usually directly follows mitosis, except for superficial cleavage. Cytokinesis invariably occurs in a plane perpendicular to the axis of the mitotic spindle. Thus, the spindle orientation controls the orientation of the contractile ring The proximity between the egg cortex and the mitotic spindle is also important for furrow formation. In eggs where the the outer cortex is displaced from the spindle (birds and insects), by large amounts of yolk, the spindle never activates the cleavage furrow. How does a blastomere know to divide meridionally or equatorially?

Mitotic spindles are oriented with their axis parallel to the longest available cell dimension Mitotic spindles work to keep the cell round in shape. Experiment: It is possible to control how tightly blastomeres adhere by changing the concentration of calcium. High calcium concentrations cause more cell – cell attachment. Low calcium causes minimal attachment. The effect is likely mediated by adhesion molecules such as cadherin. When blastomeres adhere they have a longer axis, and the mitotic spindle is almost always oriented parallel to this axis. As the cell becomes more spherical in low calcium medium, the mitotic spindle orientation starts to become random.

How does a cell know when it should divide? The cyclic activity of a protein dimer controls the activity of the cell cycle Cyclin dependent kinase 1 (cdk1) is an enzyme that is always present in cells. It can phosphorylate other proteins when it is activated. Cyclins are a family of proteins that are produced in cyclic fashion during the cell cycle. Cyclin B is destroyed shortly after metaphase, but accumulates slowly thereafter. M phase promoting factor (MPF): when there is sufficient cyclin B, it combines with cdk1. Additional regulatory changes occur such as phosphorylation of threonine and dephosphorylation of tyrosine. The active kinase phosphorylates specific cell proteins that control mitosis (spindle, nuclear lamins, and chromosomes). The actual targets of M phase promoting factor are an area of intense research interest.

Timing of cleavage divisions Normal eukaryotic cells divide slowly, once every several hours or days. The cell cycle has G1 and G2 periods. During G1 the cell synthesizes RNA and other components for cell growth. Cleavage consists of very rapid successive mitoses. Since the egg has stored large amounts of RNA and other material, it does not need G1 or G2. However, as the number of cells increases, the nucleus / cytoplasmic ratio also increases. The rate of cell division slows because the cell now needs to synthesize its own RNA and grow between divisions. Thus, G1 and G2 are restored = midblastula transition.

Midblastula transition is prominent in Drosophila Nuclei in a Drosophila embryo undergo mitosis every 9 minutes during the early stage of development !!! The 1st 10 mitoses are rapid and synchronous, and only S and M phases exist. After 10 mitoses, the cell cycle increases a little as RNA must be synthesized before each division. Midblastula transition: After 13 mitoses, the rate slows further, mitoses are asynchronous, and G1 and G2 reappear. Other animals, such as mammals and sea urchins, synthesize RNA throughout cleavage and they have no midblastula transition.

How does a blastomere control how fast it divides? M-phase promoting factor is the critical activity for initiation of mitosis. During the first 7 mitoses in Drosophila, cyclin B and cdk1 (components of MPF) are constantly present. During cycles 8-9, cyclin begins to be degraded after each mitosis. String gene: Activates MPF. This gene is constitutively active during the first 13 cycles of mitosis. This is because it is translated from large stores of maternal mRNA. As the nuclear / cytoplasmic ratio increases, more string protein is needed to activate MPF in all of the additional nuclei. Because string protein synthesis occurs during G1 and G2, the subsequent mitoses are retarded in each cycle until normal levels accumulate within the cell.

What does the product of the string gene do? The string protein acts as a phosphatase to remove a phosphate from tyrosine on cdk1. This is important for activation of cdk1 and allows MPF activity to initiate mitosis. Similar proteins are important in human cells.