List of abbreviations		 Vocabulary
of micros-
copic
anatomy
specialist terms
explained in
English +
German

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Editor:
Dr. med.
H. Jastrow
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Overview Chromosomes (Chromosomae):
Pages with explanations are linked to the text below the images if available! (Labelling is in German)
chromosomes in
metaphase (rat)		 chromosomes
Claudius cell (rat)		 chromosom
detail (rat)		 chromosomes + multi-
lamellar body (rat)		 chromosoms of a seba-
ceous gland cell (rat)		 Kinetochor +
spindle fibres (rat)
chromosomes
hypophsis (rat) 		 detail: pairing in
metaphase		 skin (human)
anaphase		 detail 1 detail 2 detail 3 human skin
prometaphase
Tonsilla pha-
ryngea (human)		 detail: formation of chro-
mosomes in prophase		 Tonsilla pharyngea
Prophase 2 (Mensch)		 detail 1: chromosome
formation		 detail 2 detail 3 Kinetochor liver
cell (rat)
Chromosomes (Terminologia histologica: Chromosomae) are the transport form of genetic information during cell division, i.e. mitosis or meiosis. They are formed by condensation and helical conformation of desoxyribonucleic acid (DNA) which carries the genetic infomation. DNA consists of 2 hlically arranged chains of molecules of desoxyribose, a sugar molecule interconnected by phosphate groups. One of the four nucleotids  (Adenin [A], Cytosin [C], Guanin [G] or Thymin [T]) is connected to one carbon nucleus of each sugar molecule. The basic nucleotids of one chain tightly connect to those of the other chain forming the base pairs. Hereby A in chain 1 is always coupled to T of chain 2 and C of chain 1 is tighted to G of chain 2 and vice versa. The length of the longest human DNA chains comprised of ~250.000.000 base pairs, is about 10 cm, the diameter only 2 nanometers (nm; = 0.0000002 cm). Each chromosome has only one linear DNA molecule. For a reasonable spatial arrangement the DNA is wired around histone proteins which are also components of chromosomes. There are 5 major types of histon proteins (H1, H2A, H2B, H3 and H4). Histone proteins  2-4 arrange to disk-like octamers, i.e. units of 8 linked proteins consiting of 2 molecules of H2A, H2B, H3 and H4 each. Since these proteins are rich in basic amino acids the negatively charged phosphate groups of the DNA bind to them. Hereby the DNA winds around the histone octamers in a manner that about 146 base pairs are wired around each octamer. This unit of histone octamer and attached base pairs is called nucleosome  and has a diameter of ~11 nm. Thus the DNA appears like a chain of pearls whereby the spheres are formed by the nucleosomes interconnected by thin sections of the DNA stripe. The chains of pearls are called chromatin fibrils. The formation of the nucleosomes is supposed to be managed by 2 additional proteins in presence of DNA: nucleoplasmin and N1 protein, that are not inculded into the octamer and thus called non-histone proteins. The histone protein 1 H1 which is not included in nucleosome formation attaches to the DNA opposite to the nucleosomes and links with neighbouring H1 units to form a framework causing a larger helical arragngement of nucleosomes. Further scaffold proteins attach to the H1 e.g., the enzyme topoisomerase II. The resulting tubes of arranged chromatin fibrils have diameters of about 30 nm. The 700 nm wide chromosome sections are formed by further folding of the tubes and winding around each other. They are densely packed in form of the chromatids. Both chromatids of a chromosome are linked to each other at theCentromer (= primary incisure). Specific sequences of nucleic acids present in the centromer called CEN sequence. The centromer-binding factor, a complex of 3 proteins, binds to this sequence of the DNA. It on the other hand has binding sites for microtubules that attach here during mitosis. These microtubules are also called spindle fibres and attach to opposite sites of the centromer at the daughter chromatids to cause their separation. A kinetochor consists of disk-like stacks of protein layers (inner, central and outer) in the centromer region of a chromosome. A diffuse corona attaches to the outer lamella which contains the microtubule-binding protein (CLIP-170) like the outer lamella and the microtubular motor proteins dynein and CENP-E. The kinetochor binds to the plus end of microtubules.

The number of chromosomes of organisms is species specific and constant. With exception of the ova and spermatocytes all other somatic cells of humans have, with exception of few multiploid cells, a double set of chromosomes, i.e. they are diploid comprised of 46 chromosomes, 44 autosomes and 2 gonosomes. Autosomes are all chromosomes bearing the genetic information for all non-sex specific protiens of the body. One of the 2 chromosomes in each pair is from the mother, the other from the father. These corresponding chromosomes that carry the same genes are called homologous chromosomes. The gonosomes which are also called heterosomes or sey chromosomes carry a large number of genes with the information of sex-specific proteins and of proteins required for the regulation and formation of primary and secondary sex organs. There are 2 different gonosomes, the X-chromosome and the Y-chromosome. While all cells in females have a pair of X-chromosomes, cells of males show 1 X- and 1 Y-chromosome. In females only the inforation of one X-chromosome is used whereas the second X-chromosome is inactive and condensed as a X-chromatin or heterochromatic body (Barr body). Such a Barr body is detectable in cells of the oropharyngeal mucosa with the light- or electron microscope as a lump of heterochromatine which has a diameter of 1 - 2 µm and which is attached to the inner membrane of the cell nucleus.
The female reproductive cells (ova) are haploid, i.e. posses only a half set of chromosomes in which all autosomes are only present once plus one gonosome which always is an X-chromosome (number of chromosomes: 22 + X). Male reproductive cells, i.e. spermatocytes are also haploid with the difference that their gonosome may either be an X- or an Y-chromosome. By fertilisation the ovum gets the chromosomes of the spermatozoon and the resulted fertilised ovum has a full (diploid) set of chromosomes (either femal: 44 + XX or male: 44 + XY). The vast majority of cells is euploid, i.e. diploid with complete set of 46 chromosomes in the cell nucleus. In some organs, however, certain cells may have multiple sets of chromosomes. This means they are polyploid. An example of polyploid cells are hepatocytes. It is assumed that this is caused by the extremely high metabolic activity. Usually these cells are tetraploid, i.e. have a 4x set of chromosomes. Occasionally, hexaploid (6x) or octaploid (8x) cells are encountered. In contrast to that tumor cells may be triploid (3x) or pentaploid (5x) or other irregular sets of chromosomes.
The shape and the size as well as the bands of chromosomes in light microscopic staining are constant allowing a classification using the size and length of chromosome branches to the centromer. In a karyogram all chromosomes are arranged according to their size using a micrograph of usually a lymphocyte in light microsopic stain after administration of a drug which stops mitosis in metaphase when chromosomes are best visible. The best stain for visualisation of the bands of chromosomes is orcein. The visible bands are caused by the differences of chromatin fibil density in the regions of the chromosomes. Further the relation of A-T to G-C pairs has an influence on stain. The typical bands with size and location of the centromere allow to create karyograms which are important for genetic investigations to detect abnormalities e.g., trisomy (one chromosome is present 3x) or monosomy (present only once). The micrographed chromosomes have 2 long processes, the chromatids which are connected at the centromer as mentioned above. According to the position of the centromer we can distinguish: metacentrisc chromosomes (centromer in the centre: chromosomes 1,2,3,11,19,20,X), submetacentric chromosomes (centromer not exactly in the centre: chromosomes 6,7,8,12), acrocentric chromosomes (centromer located close to the end: chromosomes 13,14,15,21,22,Y), subacrocentric chromosomes (centromer located closer to the end than to the centre: chromosomes 4,5,9,10,16,17,18). Since even in metacentric chromosomes the centre is not absolutely exactly in the middle it is possible to always distinguish 2 longer and 2shorter arms of each chromosome. The longer ones are labelled with q and the shorter ones with p. Further there are secondary laces in the short arms of the acrocentric chromosomes 13, 14, 15, 21 and 22. These locations are the nucleolus-organisator centres which are responsible for the formation of nucleoli. The ends of the short arms of these chromosomes are called satellits behind the incisure.
Chromosomes are electron-dense structures in a transmission electron microscope which can only be seen in prometaphase, metaphase and anaphase of the mitosis. They are 2 to 10 µm in length and both chromatids are about 0.5 µm in thickness. A clear determination which chromosome is regarded is not possible without special procedures.

--> Euchromatine, heterochromatine, synaptonemal complex, cell nucleus, mitosis
--> Electron microscopic atlas Overview
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