Amitosis

Last updated January 30, 2025

Amitosis, also known as karyostenosis, direct cell division, or binary fission , is a form of asexual cell division primarily observed in bacteria and other prokaryotes. This process is distinct from other cell division mechanisms such as mitosis and meiosis, mainly because it bypasses the complexities associated with the mitotic apparatus, such as spindle formation. Additionally, amitosis does not involve the condensation of chromatin into distinct chromosomes before the cell divides, thereby simplifying the process of cellular replication.

Several instances of cell division previously thought to be "non-mitotic", such as the division of some unicellular eukaryotes, may actually occur by "closed mitosis",^[1] which differs from open or semi-closed mitotic processes. These processes involve mitotic chromosomes and are classified based on the condition of the nuclear envelope. Amitosis can also affect the distribution of human lactic acid dehydrogenase isoenzymes, which are present in almost all body tissues. An example of amitosis is spermatogenesis. During amitosis, the cell membrane does not divide.

Cells containing two or more nuclei are called binucleated and multinucleated cells, respectively, which can also result from the fusion of cells. Although amitosis differs fundamentally from mitosis without cytokinesis, some similarities exist between amitosis and cell fusion. Amitosis can result in nearly haploid nuclei, which is not possible through mitosis or cell fusion.^[2]

Discovery

Amitosis was first described in 1880 by Walther Flemming, who also described mitosis and other forms of cell division.^[3] Initially it was common for biologists to think of cells having the ability to divide both mitotically and amitotically.^[4]

Process

Amitosis is the division of cells in the interphase state, typically achieved by a simple constriction into two sometimes unequal halves without any regular segregation of genetic material.^[5] This process results in the random distribution of parental chromosomes in the daughter cells, in contrast to mitosis, which involves the precise distribution of chromosomes. Amitosis does not involve the maximal condensation of chromatin into chromosomes, a molecular event observable by light microscopy when sister chromatids align along the metaphase plate.

While amitosis has been reported in ciliates, its role in mammalian cell proliferation remains unconfirmed. The discovery of copy number variations (CNVs) in mammalian cells within an organ ^[6] has challenged the assumption that every cell in an organism must inherit an exact copy of the parental genome to be functional. Instead of CNVs stemming from errors in mitosis, such variations could have arisen from amitosis and may even be beneficial to the cells. Additionally, ciliates possess a mechanism for adjusting the copy numbers of individual genes during amitosis of the macronucleus.^[7]

Mechanism

Additional reports of non-mitotic proliferation and insights into its underlying mechanisms have emerged from extensive work with polyploid cells. Multiple copies of the genome in a cell population may play a role in the cell's adaptation to the environment.^[8]

Polyploid cells are frequently "reduced" to diploid cells by amitosis.^[9] Naturally occurring polyploid placental cells have been observed to produce nuclei with diploid or near-diploid complements of DNA. These nuclei, derived from polyploid placental cells, receive one or more copies of a microscopically identifiable region of chromatin. This amitotic process can result in representative transmission of chromatin. In rat polyploid trophoblasts, the nuclear envelope of the giant nucleus is involved in this subdivision.^[10] Polyploid cells may also be key to the survival processes underlying chemotherapy resistance in certain cells.

Following the treatment of cultured cells with mitosis-inhibiting chemicals, similar to those used in some chemotherapeutic protocols, a small population of induced polyploid cells survives. Eventually, this population gives rise to "normal" diploid cells by forming polyploid chromatin bouquets that return to an interphase state before separating into several secondary nuclei.^[11] The controlled autophagic degradation of DNA and the production of nuclear envelope-limited sheets^[12] accompany the process.^[13] Since this process of depolyploidization involves mitotic chromosomes, it conforms to the broad definition of amitosis.

The scientific literature affirms the involvement of amitosis in cell proliferation and explores multiple amitotic mechanisms capable of producing "progeny nuclei" without "mitotic chromosomes." One form of amitosis involves fissioning, where a nucleus splits in two without involving chromosomes. This has been reported in placental tissues and cells grown from such tissues in rats,^[14] as well as in human and mouse trophoblasts.^[15]^[2] Amitosis by fissioning has also been reported in mammalian liver cells^[16] and human adrenal cells.^[17] Chen and Wan^[18] reported amitosis in rat liver and presented a mechanism for a four-stage amitotic process whereby chromatin threads are reproduced and equally distributed to daughter cells as the nucleus splits in two. In macronuclear amitosis of Tetrahymena, γ-tubulin-mediated MT assembly was required.^[19]

There are multiple reports of amitosis occurring when nuclei bud out through the plasma membrane of a polyploid cell. This process has been observed in amniotic cells transformed by a virus^[20] and in mouse embryo fibroblast lines exposed to carcinogens.^[21] A similar process called extrusion has been described for mink trophoblasts, a tissue in which fissioning is also observed.^[22] Asymmetric cell division has also been described in polyploid giant cancer cells and low eukaryotic cells and is reported to occur by the amitotic processes of splitting, budding, or burst-like mechanisms.^[23]

Examples

An example of amitosis particularly suited to the formation of multiple differentiated nuclei in a reasonably short period of time has been shown to occur during the differentiation of fluid-enclosing hemispheres called domes from adherent Ishikawa endometrial monolayer cells during an approximately 20-hour period.^[24]^[25] During the initial stages of differentiation, particularly within the first 6 hours, aggregates of nuclei from monolayer syncytia undergo a unique process where they become enveloped in mitochondrial membranes. These resulting structures, known as mitonucleons, experience an elevation due to the formation of vacuoles around them. This phenomenon indicates a distinct cellular organization and differentiation process, highlighting the complex interactions between cellular structures during development.^[26] In other systems, such changes accompany apoptosis, but not in differentiating Ishikawa cells, where the processes appear to accompany changes in DNA essential for the newly created, differentiated dome cells. Finally, the chromatin filaments emerging from these processes form a mass from which dozens of dome nuclei are amitotically generated over approximately 3 hours with the apparent involvement of nuclear envelope-limited sheets.^[12]

In development

Examination of fetal guts during development (5 to 7 weeks), colonic adenomas, and adenocarcinomas has revealed nuclei that appear as hollow bells encased in tubular syncytia. These structures can either divide symmetrically by an amitotic nuclear fission process, forming new "bells", or undergo fission asymmetrically, resulting in one of seven other nuclear morphotypes, five of which appear to be specific to development since they are rarely observed in adult organisms.^[27]

The current body of literature suggests that amitosis may be involved in cellular development in humans,^[8] likely during the fetal and embryonic phases of development when the majority of these cells are produced.

When the intestinal stem cells (ISCs) in fruit flies' guts are seriously reduced, they use amitosis to repair the damage. Cells in another part of the gut, called enterocytes, reduce the number of chromosomes without going through the normal division process. This helps replace the lost ISCs, keeping the gut functioning properly.^[28]

Related Research Articles

The cell nucleus is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei, and a few others including osteoclasts have many. The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm; and the nuclear matrix, a network within the nucleus that adds mechanical support.

The cell cycle, or cell-division cycle, is the sequential series of events that take place in a cell that causes it to divide into two daughter cells. These events include the growth of the cell, duplication of its DNA and some of its organelles, and subsequently the partitioning of its cytoplasm, chromosomes and other components into two daughter cells in a process called cell division.

Mitosis is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis is an equational division which gives rise to genetically identical cells in which the total number of chromosomes is maintained. Mitosis is preceded by the S phase of interphase and is followed by telophase and cytokinesis, which divide the cytoplasm, organelles, and cell membrane of one cell into two new cells containing roughly equal shares of these cellular components. The different stages of mitosis altogether define the mitotic phase of a cell cycle—the division of the mother cell into two daughter cells genetically identical to each other.

The nuclear pore complex (NPC), is a large protein complex giving rise to the nuclear pore. Nuclear pores are found in the nuclear envelope that surrounds the cell nucleus in eukaryotic cells. The nuclear envelope is studded by a great number of nuclear pores that give access to various molecules, to and from the nucleoplasm and the cytoplasm. Small molecules can diffuse easily but other larger molecules need to be transported across.

Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there are two distinct types of cell division: a vegetative division (mitosis), producing daughter cells genetically identical to the parent cell, and a cell division that produces haploid gametes for sexual reproduction (meiosis), reducing the number of chromosomes from two of each type in the diploid parent cell to one of each type in the daughter cells. Mitosis is a part of the cell cycle, in which, replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained. In general, mitosis is preceded by the S stage of interphase and is followed by telophase and cytokinesis; which divides the cytoplasm, organelles, and cell membrane of one cell into two new cells containing roughly equal shares of these cellular components. The different stages of mitosis all together define the M phase of an animal cell cycle—the division of the mother cell into two genetically identical daughter cells.

Prophase is the first stage of cell division in both mitosis and meiosis. Beginning after interphase, DNA has already been replicated when the cell enters prophase. The main occurrences in prophase are the condensation of the chromatin reticulum and the disappearance of the nucleolus.

<span class="mw-page-title-main">Spindle apparatus</span> Feature of biological cell structure

In cell biology, the spindle apparatus is the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells. It is referred to as the mitotic spindle during mitosis, a process that produces genetically identical daughter cells, or the meiotic spindle during meiosis, a process that produces gametes with half the number of chromosomes of the parent cell.

Telophase is the final stage in both meiosis and mitosis in a eukaryotic cell. During telophase, the effects of prophase and prometaphase are reversed. As chromosomes reach the cell poles, a nuclear envelope is re-assembled around each set of chromatids, the nucleoli reappear, and chromosomes begin to decondense back into the expanded chromatin that is present during interphase. The mitotic spindle is disassembled and remaining spindle microtubules are depolymerized. Telophase accounts for approximately 2% of the cell cycle's duration.

The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the embryoblast which subsequently forms the embryo, and an outer layer of trophoblast cells called the trophectoderm. This layer surrounds the inner cell mass and a fluid-filled cavity or lumen known as the blastocoel. In the late blastocyst, the trophectoderm is known as the trophoblast. The trophoblast gives rise to the chorion and amnion, the two fetal membranes that surround the embryo. The placenta derives from the embryonic chorion and the underlying uterine tissue of the mother. The corresponding structure in non-mammalian animals is an undifferentiated ball of cells called the blastula.

Cell growth refers to an increase in the total mass of a cell, including both cytoplasmic, nuclear and organelle volume. Cell growth occurs when the overall rate of cellular biosynthesis is greater than the overall rate of cellular degradation.

<span class="mw-page-title-main">Karyogamy</span> Fusion of the nuclei of two haploid eukaryotic cells

Karyogamy is the final step in the process of fusing together two haploid eukaryotic cells, and refers specifically to the fusion of the two nuclei. Before karyogamy, each haploid cell has one complete copy of the organism's genome. In order for karyogamy to occur, the cell membrane and cytoplasm of each cell must fuse with the other in a process known as plasmogamy. Once within the joined cell membrane, the nuclei are referred to as pronuclei. Once the cell membranes, cytoplasm, and pronuclei fuse, the resulting single cell is diploid, containing two copies of the genome. This diploid cell, called a zygote or zygospore can then enter meiosis, or continue to divide by mitosis. Mammalian fertilization uses a comparable process to combine haploid sperm and egg cells (gametes) to create a diploid fertilized egg.

The nuclear lamina is a dense fibrillar network inside the nucleus of eukaryote cells. It is composed of intermediate filaments and membrane associated proteins. Besides providing mechanical support, the nuclear lamina regulates important cellular events such as DNA replication and cell division. Additionally, it participates in chromatin organization and it anchors the nuclear pore complexes embedded in the nuclear envelope.

HMGN proteins are members of the broader class of high mobility group (HMG) chromosomal proteins that are involved in regulation of transcription, replication, recombination, and DNA repair.

Endoreduplication is replication of the nuclear genome in the absence of mitosis, which leads to elevated nuclear gene content and polyploidy. Endoreduplication can be understood simply as a variant form of the mitotic cell cycle (G1-S-G2-M) in which mitosis is circumvented entirely, due to modulation of cyclin-dependent kinase (CDK) activity. Examples of endoreduplication characterised in arthropod, mammalian, and plant species suggest that it is a universal developmental mechanism responsible for the differentiation and morphogenesis of cell types that fulfill an array of biological functions. While endoreduplication is often limited to specific cell types in animals, it is considerably more widespread in plants, such that polyploidy can be detected in the majority of plant tissues. Polyploidy and aneuploidy are common phenomena in cancer cells. Given that oncogenesis and endoreduplication likely involve subversion of common cell cycle regulatory mechanisms, a thorough understanding of endoreduplication may provide important insights for cancer biology.

<span class="mw-page-title-main">Nuclear dimorphism</span>

Nuclear dimorphism is a term referred to the special characteristic of having two different kinds of nuclei in a cell. There are many differences between the types of nuclei. This feature is observed in protozoan ciliates, like Tetrahymena, and some foraminifera. Ciliates contain two nucleus types: a macronucleus that is primarily used to control metabolism, and a micronucleus which performs reproductive functions and generates the macronucleus. The compositions of the nuclear pore complexes help determine the properties of the macronucleus and micronucleus. Nuclear dimorphism is subject to complex epigenetic controls. Nuclear dimorphism is continuously being studied to understand exactly how the mechanism works and how it is beneficial to cells. Learning about nuclear dimorphism is beneficial to understanding old eukaryotic mechanisms that have been preserved within these unicellular organisms but did not evolve into multicellular eukaryotes.

<i>Jaagsiekte sheep retrovirus</i> Species of virus

Jaagsiekte sheep retrovirus (JSRV) is a betaretrovirus which is the causative agent of a contagious lung cancer in sheep, called ovine pulmonary adenocarcinoma.

Ran also known as GTP-binding nuclear protein Ran is a protein that in humans is encoded by the RAN gene. Ran is a small 25 kDa protein that is involved in transport into and out of the cell nucleus during interphase and also involved in mitosis. It is a member of the Ras superfamily.

Lamina-associated polypeptide 2 (LAP2), isoforms beta/gamma is a protein that in humans is encoded by the TMPO gene. LAP2 is an inner nuclear membrane (INM) protein.

HERV-R_7q21.2 provirus ancestral envelope (Env) polyprotein is a protein that in humans is encoded by the ERV3 gene.

Chilodonella uncinata is a single-celled organism of the ciliate class of alveolates. As a ciliate, C. uncinata has cilia covering its body and a dual nuclear structure, the micronucleus and macronucleus. Unlike some other ciliates, C. uncinata contains millions of minichromosomes in its macronucleus while its micronucleus is estimated to contain 3 chromosomes. Childonella uncinata is the causative agent of Chilodonelloza, a disease that affects the gills and skin of fresh water fish, and may act as a facultative of mosquito larva.

References

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