Cell Physio II

Oocyte

An immature egg (before completion of maturation divisions, i.e. meiosis).

Follicle

in the context of the gametes, refers to the somatic cells surrounding the oocyte as it grows. These may be numerous and arranged in complex ways, as in mammals, but in most animals takes the form of a single-layered epithelium made of (what else?) follicle cells. Ovulation is release of the oocyte from the follicle and is not synonymous with oocyte maturation, though these usually coincide.

Follicle cell

specialized, nurturing somatic cells that surround developing germ cells or functional units in various organs, playing critical roles in hormone production, structural support, and maturation.

Vitellogensis

The process of oocyte growth; specifically, of yolk accumulation.

Egg

mature oocyte (after completion of meiosis), ovum, female gamete. In some animals (e.g. sea urchins, cnidarians) fertilization occurs at this stage.

Zygote

fertilized oocyte or egg, in effect; but strictly, the zygote is the diploid cell resulting from the fusion of gametes, that is, it isn’t a zygote until oocyte meiosis is complete and pronuclei have gotten together.

Pronucleus

(pl. pronuclei) — haploid nucleus of male (sperm) or female (egg) gamete which will fuse with its counterpart to form the diploid nucleus of the zygote in the process of fertilization

Centrosome

also known as the microtubule organizing center (MTOC), is the organelle responsible for organizing the aster during cell division. In most animals (except for the mouse) both of the centrosomes in the zygote are derived from the basal body (centriole) of the sperm flagellum.

Microtubule-organizing center

specialized eukaryotic cellular structure that nucleates, anchors, and organizes microtubules, typically focusing on their minus ends

basal body

an organelle that forms the base of a flagellum or cilium.

Meiosis

also known as "maturation division" — is a type of cell division, which leads to a reduction in the number of chromosomes from diploid (2n) to haploid (n), and the formation of gametes. Meiosis comprises two separate divisions (meiosis I and meiosis II), which result in the formation of an egg and polar bodies from oocytes, or four sperm from a spermatocyte.

Spore

incidentally, if the haploid products of meiosis keep dividing as a haploid cell lineage, as in fungi and many other non-animal eukaryotes, they are called spores, not gametes.

Polar body

a product of oocyte maturation, a tiny sister-cell of the primary or secondary oocyte, which contains the DNA discarded in the process of meiosis, and little of anything else. In many species two polar bodies are formed, in others the first polar body divides, so that a total of three polar bodies may be observed. Polar bodies mark the animal pole of the egg by definition.

Animal pole

Is the pole of the egg where polar bodies are produced. In eggs that have unequal distribution of yolk in the cytoplasm, the animal pole is less yolky than the opposite vegetal pole, and is more "animated", meaning that cell divisions occur there at a faster rate than at the more "vegetative" vegetal pole.

Germinal vesicle (GV)

The greatly enlarged nucleus of a primary oocyte. It is not clear why this term persists now that it is obvious the GV is just a great big nucleus. Maybe it's because developmental biologists like to say

GVBD

(that is, "germinal vesicle breakdown") instead of "NEBD" (for "nuclear envelope breakdown").

Nucleolus

it's that thing in the GV that looks like a marble. Essentially all cell nuclei include a nucleolus, which is a superstructure devoted to ribosomal RNA synthesis and assembly. The size of the nucleolus is proportional to the rate of ribosome synthesis. This accounts for the exceptional size in oocytes.

Homologues

in diploid organisms which reproduce sexually, most cells contain a paternally- and maternally-derived copy of each chromosome. These homologous chromosomes likely differ at numerous loci and may even be structurally distinct (i.e., as a consequence of major chromosome rearrangements).

Bivalent

the physical entity consisting of paired homologous chromosomes, condensed and coupled by a chiasma (see below). It is the bivalents that are sorted on the spindle during meiosis I.

Chiasma

a point at which paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands.

Recombination

when used in the context of meiosis, this term refers to events in which homologous chromosomes exchange a non-negligible stretch of sequence. More precisely, recombination is the process in which such an exchange is likely to take place. During meiosis, recombination initiates with a number of…

DSBs

ouble-strand breaks; these initiate a DNA-repair-like process in which a broken end invades a homologous chromosome (or a replicated sister, but this has no interesting consequence), perhaps initiating…

Crossing-over

It is at the sites where strands from two chromatids trade places that actual genetic exchange might take place. Note that most DSBs are resolved without crossover. Hundreds of DSBs form in early meiotic prophase; usually there are one or two that become crossovers. Morphologically visible crossovers that survive meiotic prophase and persist into metaphase I are called chiasmata (singular chiasma).

Synapsis

process of pairing homologous chromosomes; facilitated by assembly of the synaptonemal complex which is a superstructure connecting two homologous chromosomes. Although the synaptonemal complex is often described as a mediator of recombination, it appears more likely that recombination initiates the process which culminates in the paired assembly. In this view, the purpose of synapsis may be to make it less likely that things get scrambled during the aftermath of DSB, recombinational repair, and crossover resolution.

Cohesion

in meiosis, this common English word refers to the protein glue that holds sister chromatids together through meiosis I. It so happens that in the aftermath of a crossover event, sister chromatid cohesion, mediated by proteins called cohesins, is the main thing holding homologues together in a bivalent as they align on the spindle.

Nondisjunction

what's supposed to happen is that homologous chromosomes pair, recombine, and come apart at meiosis I, and then sister chromatids separate in meiosis II. This, when it all works, is called disjunction. Hence, when it does not, one has non-disjunction. Non-disjunction results in aneuploidy, which is surprisingly troublesome. While single-chromosome aneuploidies may not be cell-lethal, such a condition is rarely tolerated by the whole organism. (Known exceptions in humans are the trisomy of chromosome 21, the smallest one; this is the basis for Down Syndrome.)

Maturation Promoting Factor (MPF)

he cytoplasmic activity capable of causing an oocyte to continue meiosis, through maturation (that is, stopping at the usual point to await fertilization). MPF turns out to be identical to the cytoplasmic activity that operates during mitosis - and is therefore now referred to as "mitosis promoting factor" in textbooks - and biochemically it is a protein kinase (that is, an enzyme that phosphorylates - adds phosphates to - proteins) consisting of…

Protein kinase

an enzyme that modifies other proteins by chemically adding phosphate groups to them (phosphorylation), acting as a molecular switch to regulate cell signaling, metabolism, division, and apoptosis

Phosphorylates

Phosphorylation is the crucial biochemical process of adding a phosphate group to an organic molecule, such as a protein, sugar, or lipid, typically mediated by enzymes called kinases.

Cyclin

some protein that some people found that rose and fell in abundance coincident with mitosis in clam and sea urchin embryos. It turned out to be a required component of MPF! There are actually a lot of cyclins now, but the ones they were looking at were eventually called cyclin B and are now called M cyclins. They, like other cyclins, have no enzymatic activity themselves, they bind to and activate…

Cyclin Depedant Kinase (CDK)

this is the actual enzyme. There are several of these too, but the one that matters for mitosis is Cdk1, which you will also see referred to as cdc2. Cyclin plus Cdk makes an active protein kinase: to some extent, the specificity is determined by the cyclin component, which may function as a kind of targeting subunit. At any rate, the Cyclin B/Cdk1 complex is MPF.

Anaphase Promoting Complex (APC)

composed of a dozen or so proteins that collaborate to tag specific cell cycle regulatory proteins by covalent addition of ubiquitin. Ubiquitin-tagged proteins are promptly degraded by the proteasome, a major non-specific protease. APC's principle targets at the metaphase-anaphase transition are the M cyclins and securin. Destruction of M cyclin reduces Cdk activity, whereas securin is an inhibitor of the protease, separase. Once relieved from inhibition, separase degrades the cohesins that keep sister chromatids together. Cohesin destruction permits sister chromatid separation along the spindle in anaphase.

Contracting Ring

it may be a ring, and it might be contractile! The equatorial band of actin filaments and associated proteins, including myosin II, that assembles in the cortex after anaphase onset is somehow responsible for the motive power that constricts the cleavage furrow during cytokinesis.

Rho

a plasma membrane-associated small GTPase that is widely used by cells to regulate assembly of contractile proteins (i.e., actin and myosin and all their friends). Like many other related small GTPases used for intracellular signaling, Rho is a) "active" when bound to GTP; b) "inactive" when bound to GDP, and c) a lousy GTPase on its own. Accessory regulators accelerate GTP hydrolysis (GAPs: GTPase activating proteins) and nucleotide exchange (GEFs: Guanine nucleotide exchange factors). Hence, GTPase "activating" proteins inactivate the GTPase by activating its GTPase activity. Rho is switch-like; GAPs turn switch off; GEFs turn switch on

Abscission

the last act of cytokinesis, once the contractile ring is finished, is to sever the last little canal. This is done by the collection, then synchronous fusion, of secretory granules at the midbody.