A lymph node or lymph gland is an ovoid or kidney-shaped organ of the lymphatic system, of the adaptive immune system, present throughout the body. They are linked by the lymphatic vessels as a part of the circulatory system. Lymph nodes are major sites of B and T lymphocytes, other white blood cells. Lymph nodes are important for the proper functioning of the immune system, acting as filters for foreign particles and cancer cells. Lymph nodes do not have a detoxification function, dealt with by the liver and kidneys. In the lymphatic system the lymph node is a secondary lymphoid organ. A lymph node is enclosed in a fibrous capsule and is made up of an outer cortex and an inner medulla. Lymph nodes have clinical significance, they become inflamed or enlarged in various diseases which may range from trivial throat infections, to life-threatening cancers. The condition of the lymph nodes is important in cancer staging, which decides the treatment to be used, determines the prognosis; when swollen, inflamed or enlarged, lymph nodes can be hard, tender.
Lymph nodes are oval shaped and range in size from a few millimeters to about 1 -- 2 cm long. Each lymph node is surrounded by a fibrous capsule, which extends inside the lymph node to form trabeculae; the substance of the lymph node is divided into the inner medulla. The cortex is continuous around the medulla except where the medulla comes into direct contact with the hilum. Thin reticular fibers of reticular connective tissue, elastin form a supporting meshwork called a reticulin inside the node. B cells are found in the outer cortex where they are clustered together as follicular B cells in lymphoid follicles and the T cells are in the paracortex; the lymph node is divided into compartments called lymph nodules each consisting of a cortical region of combined follicle B cells, a paracortical region of T cells, a basal part of the nodule in the medulla. The number and composition of follicles can change when challenged by an antigen, when they develop a germinal center. Elsewhere in the node, there are only occasional leukocytes.
As part of the reticular network there are follicular dendritic cells in the B cell follicle and fibroblastic reticular cells in the T cell cortex. The reticular network not only provides the structural support, but the surface for adhesion of the dendritic cells and lymphocytes, it allows exchange of material with blood through the high endothelial venules and provides the growth and regulatory factors necessary for activation and maturation of immune cells. Lymph enters the convex side of the lymph node through multiple afferent lymphatic vessels, flows through spaces called sinuses. A lymph sinus which includes the subcapsular sinus, is a channel within the node, lined by endothelial cells along with fibroblastic reticular cells and this allows for the smooth flow of lymph through them; the endothelium of the subcapsular sinus is continuous with that of the afferent lymph vessel and with that of the similar sinuses flanking the trabeculae and within the cortex. All of these sinuses drain the filtered lymphatic fluid into the medullary sinuses, from where the lymph flows into the efferent lymph vessels to exit the node at the hilum on the concave side.
These vessels are smaller and don't allow the passage of the macrophages so that they remain contained to function within the lymph node. In the course of the lymph, lymphocytes may be activated as part of the adaptive immune response; the lymph node capsule is composed of dense irregular connective tissue with some plain collagenous fibers, from its internal surface are given off a number of membranous processes or trabeculae. They pass inward, radiating toward the center of the node, for about one-third or one-fourth of the space between the circumference and the center of the node. In some animals they are sufficiently well-marked to divide the peripheral or cortical portion of the node into a number of compartments, but in humans this arrangement is not obvious; the larger trabeculae springing from the capsule break up into finer bands, these interlace to form a mesh-work in the central or medullary portion of the node. In these trabecular spaces formed by the interlacing trabeculae is contained the proper lymph node substance or lymphoid tissue.
The node pulp does not, however fill the spaces, but leaves, between its outer margin and the enclosing trabeculae, a channel or space of uniform width throughout. This is termed the subcapsular sinus. Running across it are a number of finer trabeculae of reticular connective tissue, the fibers of which are, for the most part, covered by ramifying cells; the subcapsular sinus is the space between the capsule and the cortex which allows the free movement of lymphatic fluid and so contains few lymphocytes. It is continuous with the similar lymph sinuses; the lymph node contains lymphoid tissue, i.e. a meshwork or fibers called reticulum with white blood cells enmeshed in it. The regions where there are few cells within the meshwork are known as lymph sinus, it is lined by reticular cells and fixed macrophages. The subcapsular sinus has clinical importance as it is the most location where the earliest manifestations of a metastatic carcinoma in a lymph node would be found; the cortex of the lymph node is the outer portion of the node, underneath the capsule and the subcapsular sinus.
It has a deeper part known as the paracortex. The subcapsular sinus drains to the trabecul sinuses, the lymph flows into the medullary sinuses; the outer cortex consists of the B c
The lymphatic system is part of the vascular system and an important part of the immune system, comprising a large network of lymphatic vessels that carry a clear fluid called lymph directionally towards the heart. The lymphatic system was first described in the seventeenth century independently by Olaus Rudbeck and Thomas Bartholin. Unlike the circulatory system, the lymphatic system is not a closed system; the human circulatory system processes an average of 20 litres of blood per day through capillary filtration, which removes plasma while leaving the blood cells. 17 litres of the filtered plasma is reabsorbed directly into the blood vessels, while the remaining three litres remain in the interstitial fluid. One of the main functions of the lymph system is to provide an accessory return route to the blood for the surplus three litres; the other main function is that of defense in the immune system. Lymph is similar to blood plasma: it contains lymphocytes, it contains waste products and cellular debris together with bacteria and proteins.
Associated organs composed of lymphoid tissue are the sites of lymphocyte production. Lymphocytes are concentrated in the lymph nodes; the spleen and the thymus are lymphoid organs of the immune system. The tonsils are lymphoid organs that are associated with the digestive system. Lymphoid tissues contain lymphocytes, contain other types of cells for support; the system includes all the structures dedicated to the circulation and production of lymphocytes, which includes the bone marrow, the lymphoid tissue associated with the digestive system. The blood does not come into direct contact with the parenchymal cells and tissues in the body, but constituents of the blood first exit the microvascular exchange blood vessels to become interstitial fluid, which comes into contact with the parenchymal cells of the body. Lymph is the fluid, formed when interstitial fluid enters the initial lymphatic vessels of the lymphatic system; the lymph is moved along the lymphatic vessel network by either intrinsic contractions of the lymphatic passages or by extrinsic compression of the lymphatic vessels via external tissue forces, or by lymph hearts in some animals.
The organization of lymph nodes and drainage follows the organization of the body into external and internal regions. The lymph vessels empty into the lymphatic ducts, which drain into one of the two subclavian veins, near their junction with the internal jugular veins; the lymphatic system consists of lymphatic organs, a conducting network of lymphatic vessels, the circulating lymph. The primary or central lymphoid organs generate lymphocytes from immature progenitor cells; the thymus and the bone marrow constitute the primary lymphoid organs involved in the production and early clonal selection of lymphocyte tissues. Bone marrow is responsible for both the creation of T cells and the production and maturation of B cells. From the bone marrow, B cells join the circulatory system and travel to secondary lymphoid organs in search of pathogens. T cells, on the other hand, travel from the bone marrow to the thymus. Mature T cells join B cells in search of pathogens; the other 95 % of T cells begin a process of a form of programmed cell death.
Secondary or peripheral lymphoid organs, which include lymph nodes and the spleen, maintain mature naive lymphocytes and initiate an adaptive immune response. The peripheral lymphoid organs are the sites of lymphocyte activation by antigens. Activation leads to clonal affinity maturation. Mature lymphocytes recirculate between the blood and the peripheral lymphoid organs until they encounter their specific antigen. Secondary lymphoid tissue provides the environment for the foreign or altered native molecules to interact with the lymphocytes, it is exemplified by the lymph nodes, the lymphoid follicles in tonsils, Peyer's patches, adenoids, etc. that are associated with the mucosa-associated lymphoid tissue. In the gastrointestinal wall the appendix has mucosa resembling that of the colon, but here it is infiltrated with lymphocytes. Tertiary lymphoid organs are abnormal lymph node–like structures that form in peripheral tissues at sites of chronic inflammation, such as chronic infection, transplanted organs undergoing graft rejection, some cancers, autoimmune and autoimmune-related diseases.
TLOs are regulated differently from the normal process whereby lymphoid tissues are formed during ontogeny, being dependent on cytokines and hematopoietic cells, but still drain interstitial fluid and transport lymphocytes in response to the same chemical messengers and gradients. TLOs contains far fewer lymphocytes, assumes an immune role only when challenged with antigens that result in inflammation, it achieves this by importing the lymphocytes from lymph. The thymus is a primary lymphoid organ and the site of maturation for T cells, the lymphocytes of the adaptive immune system; the thymus increases in size from birth in response to postnatal antigen stimulation to puberty and regresses thereafter. The loss or lack of the thymus results in severe immunodeficiency and subsequent high susceptibility to infection. In most species, the thymus consists of lobules divided by septa which are made up of epithelium and is therefore an epithelial organ. T cells mature from
Neutrophils are the most abundant type of granulocytes and the most abundant type of white blood cells in most mammals. They form an essential part of the innate immune system, their functions vary in different animals. They are formed from stem cells in the bone marrow and differentiated into subpopulations of neutrophil-killers and neutrophil-cagers, they are short-lived and motile, or mobile, as they can enter parts of tissue where other cells/molecules cannot. Neutrophils may be banded neutrophils, they form part of the polymorphonuclear cells family together with eosinophils. The name neutrophil derives from staining characteristics on hematoxylin and eosin histological or cytological preparations. Whereas basophilic white blood cells stain dark blue and eosinophilic white blood cells stain bright red, neutrophils stain a neutral pink. Neutrophils contain a nucleus divided into 2–5 lobes. Neutrophils are a type of phagocyte and are found in the bloodstream. During the beginning phase of inflammation as a result of bacterial infection, environmental exposure, some cancers, neutrophils are one of the first-responders of inflammatory cells to migrate towards the site of inflammation.
They migrate through the blood vessels through interstitial tissue, following chemical signals such as Interleukin-8, C5a, fMLP, Leukotriene B4 and H2O2 in a process called chemotaxis. They are the predominant cells in pus, accounting for its whitish/yellowish appearance. Neutrophils are recruited to the site of injury within minutes following trauma and are the hallmark of acute inflammation; when adhered to a surface, neutrophil granulocytes have an average diameter of 12–15 micrometers in peripheral blood smears. In suspension, human neutrophils have an average diameter of 8.85 µm. With the eosinophil and the basophil, they form the class of polymorphonuclear cells, named for the nucleus' multilobulated shape; the nucleus has the separate lobes connected by chromatin. The nucleolus disappears as the neutrophil matures, something that happens in only a few other types of nucleated cells. In the cytoplasm, the Golgi apparatus is small and ribosomes are sparse, the rough endoplasmic reticulum is absent.
The cytoplasm contains about 200 granules, of which a third are azurophilic. Neutrophils will show increasing segmentation. A normal neutrophil should have 3–5 segments. Hypersegmentation occurs in some disorders, most notably vitamin B12 deficiency; this is noted in a manual review of the blood smear and is positive when most or all of the neutrophils have 5 or more segments. Neutrophils are the most abundant white blood cells in humans; the stated normal range for human blood counts varies between laboratories, but a neutrophil count of 2.5–7.5 x 109/L is a standard normal range. People of African and Middle Eastern descent may have lower counts. A report may divide neutrophils into segmented bands; when circulating in the bloodstream and inactivated, neutrophils are spherical. Once activated, they change shape and become more amorphous or amoeba-like and can extend pseudopods as they hunt for antigens. Neutrophils have a preference to engulf refined carbohydrates over bacteria. In 1973 Sanchez et al. found that the neutrophil phagocytic capacity to engulf bacteria is affected when simple sugars are digested, that fasting strengthens the neutrophils' phagocytic capacity to engulf bacteria.
However, the digestion of normal starches has no effect. It was concluded that the function, not the number, of phagocytes in engulfing bacteria was altered by the ingestion of sugars. In 2007 researchers at the Whitehead Institute of Biomedical Research found that given a selection of sugars, neutrophils engulf some types of sugar preferentially; the average lifespan of inactivated human neutrophils in the circulation has been reported by different approaches to be between 5 and 90 hours. Upon activation, they marginate and undergo selectin-dependent capture followed by integrin-dependent adhesion in most cases, after which they migrate into tissues, where they survive for 1–2 days. Neutrophils are much more numerous than the longer-lived monocyte/macrophage phagocytes. A pathogen is to first encounter a neutrophil; some experts hypothesize. The short lifetime of neutrophils minimizes propagation of those pathogens that parasitize phagocytes because the more time such parasites spend outside a host cell, the more they will be destroyed by some component of the body's defenses.
Because neutrophil antimicrobial products can damage host tissues, their short life limits damage to the host during inflammation. Neutrophils will be removed after phagocytosis of pathogens by macrophages. PECAM-1 and phosphatidylserine on the cell surface are involved in this process. Neutrophils undergo a process called chemotaxis via amoeboid movement, which allows them to migrate toward sites of infection or inflammation. Cell surface receptors allow neutrophils to detect chemical gr
Histopathology refers to the microscopic examination of tissue in order to study the manifestations of disease. In clinical medicine, histopathology refers to the examination of a biopsy or surgical specimen by a pathologist, after the specimen has been processed and histological sections have been placed onto glass slides. In contrast, cytopathology examines free cells or tissue micro-fragments. Histopathological examination of tissues starts with biopsy, or autopsy; the tissue is removed from the body or plant, then...often following expert dissection in the fresh state...placed in a fixative which stabilizes the tissues to prevent decay. The most common fixative is formalin; the tissue is prepared for viewing under a microscope using either chemical fixation or frozen section. If a large sample is provided e.g. from a surgical procedure a pathologist looks at the tissue sample and selects the part most to yield a useful and accurate diagnosis - this part is removed for examination in a process known as grossing or cut up.
Larger samples are cut to situate their anatomical structures in the cassette. Certain specimens can undergo agar pre-embedding to assure correct tissue orientation in cassette & in the block & on the diagnostic microscopy slide; this is placed into a plastic cassette for most of the rest of the process. In addition to formalin, other chemical fixatives have been used. But, with the advent of immunohistochemistry staining and diagnostic molecular pathology testing on these specimen samples, formalin has become the standard chemical fixative in human diagnostic histopathology. Fixation times for small specimens are shorter, standards exist in human diagnostic histopathology. Water is removed from the sample in successive stages by the use of increasing concentrations of alcohol. Xylene is used in the last dehydration phase instead of alcohol - this is because the wax used in the next stage is soluble in xylene where it is not in alcohol allowing wax to permeate the specimen; this process is automated and done overnight.
The wax infiltrated specimen is transferred to an individual specimen embedding container. Molten wax is introduced around the specimen in the container and cooled to solidification so as to embed it in the wax block; this process is needed to provide a properly oriented sample sturdy enough for obtaining a thin microtome section for the slide. Once the wax embedded block is finished, sections will be cut from it and placed to float on a waterbath surface which spreads the section out; this is done by hand and is a skilled job with the lab personnel making choices about which parts of the specimen microtome wax ribbon to place on slides. A number of slides will be prepared from different levels throughout the block. After this the thin section mounted slide is stained and a protective cover slip is mounted on it. For common stains, an automatic process is used; the second method of histology processing is called frozen section processing. This is a technical scientific method performed by a trained histoscientist In this method, the tissue is frozen and sliced thinly using a microtome mounted in a below-freezing refrigeration device called the cryostat.
The thin frozen sections are mounted on a glass slide, fixed & in liquid fixative, stained using the similar staining techniques as traditional wax embedded sections. The advantages of this method is rapid processing time, less equipment requirement, less need for ventilation in the laboratory; the disadvantage is the poor quality of the final slide. It is used in intra-operative pathology for determinations that might help in choosing the next step in surgery during that surgical session; this can be done to slides processed by frozen section slides. To see the tissue under a microscope, the sections are stained with one or more pigments; the aim of staining is to reveal cellular components. The most used stain in histopathology is a combination of hematoxylin and eosin. Hematoxylin is used to stain nuclei blue, while eosin stains cytoplasm and the extracellular connective tissue matrix pink. There are hundreds of various other techniques. Other compounds used to color tissue sections include safranin, Oil Red O, congo red, silver salts and artificial dyes.
Histochemistry refers to the science of using chemical reactions between laboratory chemicals and components within tissue. A performed histochemical technique is the Perls' Prussian blue reaction, used to demonstrate iron deposits in diseases like Hemochromatosis. Antibodies have been used to stain particular proteins and carbohydrates. Called immunohistochemistry, this technique has increased the ability to identify categories of cells under a microscope. Other advanced techniques include in situ hybridization to identify specific DNA or RNA molecules; these antibody staining methods require the use of frozen section histology. These procedures above are carried out in the laboratory under scrutiny and precision by a trained specialist Medical laboratory scientist (Hist
Endothelium refers to cells that line the interior surface of blood vessels and lymphatic vessels, forming an interface between circulating blood or lymph in the lumen and the rest of the vessel wall. It is a thin layer of single-layered, squamous cells called endothelial cells. Endothelial cells in direct contact with blood are called vascular endothelial cells, whereas those in direct contact with lymph are known as lymphatic endothelial cells. Vascular endothelial cells line the entire circulatory system, from the heart to the smallest capillaries; these cells have unique functions in vascular biology. These functions include fluid filtration, such as in the glomerulus of the kidney, blood vessel tone, neutrophil recruitment, hormone trafficking. Endothelium of the interior surfaces of the heart chambers is called endocardium. Endothelium is mesodermal in origin. Both blood and lymphatic capillaries are composed of a single layer of endothelial cells called a monolayer. In straight sections of a blood vessel, vascular endothelial cells align and elongate in the direction of fluid flow.
The foundational model of anatomy makes a distinction between endothelial cells and epithelial cells on the basis of which tissues they develop from, states that the presence of vimentin rather than keratin filaments separate these from epithelial cells. Many considered the endothelium a specialized epithelial tissue. Endothelial cells are involved in many aspects of vascular biology, including: Barrier function - the endothelium acts as a semi-selective barrier between the vessel lumen and surrounding tissue, controlling the passage of materials and the transit of white blood cells into and out of the bloodstream. Excessive or prolonged increases in permeability of the endothelial monolayer, as in cases of chronic inflammation, may lead to tissue edema/swelling. Blood clotting; the endothelium provides a non-thrombogenic surface because it contains, for example, heparan sulfate which acts as a cofactor for activating antithrombin, a protease that inactivates several factors in the coagulation cascade.
Inflammation Formation of new blood vessels Vasoconstriction and vasodilation, hence the control of blood pressure Repair of damaged or diseased organs via an injection of blood vessel cells Angiopoietin-2 works with VEGF to facilitate cell proliferation and migration of endothelial cells Endothelial dysfunction, or the loss of proper endothelial function, is a hallmark for vascular diseases, is regarded as a key early event in the development of atherosclerosis. Impaired endothelial function, causing hypertension and thrombosis, is seen in patients with coronary artery disease, diabetes mellitus, hypercholesterolemia, as well as in smokers. Endothelial dysfunction has been shown to be predictive of future adverse cardiovascular events, is present in inflammatory disease such as rheumatoid arthritis and systemic lupus erythematosus. One of the main mechanisms of endothelial dysfunction is the diminishing of nitric oxide due to high levels of asymmetric dimethylarginine, which interfere with the normal L-arginine-stimulated nitric oxide synthesis and so leads to hypertension.
The most prevailing mechanism of endothelial dysfunction is an increase in reactive oxygen species, which can impair nitric oxide production and activity via several mechanisms. The signalling protein ERK5 is essential for maintaining normal endothelial cell function. A further consequence of damage to the endothelium is the release of pathological quantities of von Willebrand factor, which promote platelet aggregation and adhesion to the subendothelium, thus the formation of fatal thrombi. Anatomy photo: Circulatory/vessels/capillaries1/capillaries3 - Comparative Organology at University of California, Davis, "Capillaries, non-fenestrated" Histology image: 21402ooa – Histology Learning System at Boston University Endothelium Journal of Endothelial Cell Research, Informa Healthcare Endothelium and inflammation Platelet Activation, University of Washington
Sclerotherapy is a procedure used to treat blood vessels or blood vessel malformations and those of the lymphatic system. A medicine is injected into the vessels, it is used for young adults with vascular or lymphatic malformations. In adults, sclerotherapy is used to treat spider veins, smaller varicose veins and hydroceles. Sclerotherapy is one method for the treatment of spider veins varicose veins, venous malformations. In ultrasound-guided sclerotherapy, ultrasound is used to visualize the underlying vein so the physician can deliver and monitor the injection. Sclerotherapy takes place under ultrasound guidance after venous abnormalities have been diagnosed with duplex ultrasound. Sclerotherapy under ultrasound guidance and using microfoam sclerosants has been shown to be effective in controlling reflux from the sapheno-femoral and sapheno-popliteal junctions. However, some authors believe that sclerotherapy is not suitable for veins with reflux from the greater or lesser saphenous junction, or for veins with axial reflux.
Sclerotherapy has been used in the treatment of spider veins and varicose veins for over 150 years. Like varicose vein surgery, sclerotherapy techniques have evolved during that time. Modern techniques including ultrasonographic guidance and foam sclerotherapy are the latest developments in this evolution; the first reported attempt at sclerotherapy was by D Zollikofer in Switzerland, 1682 who injected an acid into a vein to induce thrombus formation. Both Debout and Cassaignaic reported success in treating varicose veins by injecting perchlorate of iron in 1853. Desgranges in 1854 cured 16 cases of varicose veins by injecting tannin into the veins; this was 12 years after the probable advent of great saphenous vein stripping in 1844 by Madelung. However, due to high rates of side-effects with the drugs used at the time, sclerotherapy had been abandoned by 1894. With the improvements in surgical techniques and anaesthetics over that time, stripping became the treatment of choice. Work continued on alternative sclerosants in the early 20th century.
During that time carbolic acid and perchlorate of mercury were tried and whilst these showed some effect in obliterating varicose veins, side-effects caused them to be abandoned. Prof. Sicard and other French doctors developed the use of sodium carbonate and sodium salicylate during and after the First World War. Quinine was used with some effect during the early 20th century. At the time of Coppleson's book in 1929, he was advocating the use of sodium salicylate or quinine as the best choices of sclerosant. Further work on improving the technique and development of safer more effective sclerosants continued through the 1940s and 1950s. Of particular importance was the development of sodium tetradecyl sulfate in 1946, a product still used to this day. George Fegan in the 1960s reported treating over 13,000 patients with sclerotherapy advancing the technique by focussing on fibrosis of the vein rather than thrombosis, concentrating on controlling significant points of reflux, emphasizing the importance of compression of the treated leg.
The procedure became. However it was poorly understood or accepted in England or the United States, a situation that continues to this day amongst some sections of the medical community; the next major development in the evolution of sclerotherapy was the advent of duplex ultrasonography in the 1980s and its incorporation into the practise of sclerotherapy that decade. Knight was an early advocate of this new procedure and presented it at several conferences in Europe and the United States. Thibault's article was the first on this topic to be published in a peer-reviewed journal; the work of Cabrera and Monfreaux in utilising foam sclerotherapy along with Tessari's "3-way tap method" of foam production further revolutionised the treatment of larger varicose veins with sclerotherapy. This has now been further modified by Whiteley and Patel to use 3 non-silicone syringes for more long lasting foam. Injecting the unwanted veins with a sclerosing solution causes the target vein to shrink, dissolve over a period of weeks as the body absorbs the treated vein.
Sclerotherapy is a non-invasive procedure taking only about 10 minutes to perform. The downtime is minimal, in comparison to an invasive varicose vein surgery. Sclerotherapy is the "gold standard" and is preferred over laser for eliminating large spider veins and smaller varicose leg veins. Unlike a laser, the sclerosing solution additionally closes the "feeder veins" under the skin that are causing the spider veins to form, thereby making a recurrence of the spider veins in the treated area less likely. Multiple injections of dilute sclerosant are injected into the abnormal surface veins of the involved leg; the patient's leg is compressed with either stockings or bandages that they wear for two weeks after treatment. Patients are encouraged to walk during that time, it is common practice for the patient to require at least two treatment sessions separated by several weeks to improve the appearance of their leg veins. Sclerotherapy can be performed using microfoam sclerosants under ultrasound guidance to treat larger varicose veins, including the great and small saphenous veins.
After a map of the patient's varicose veins is created using ultrasound, these veins are injected whilst real-time monitoring of the injections is undertaken using ultrasound. The sclerosant c
Cytogenetics is a branch of genetics, concerned with how the chromosomes relate to cell behaviour to their behaviour during mitosis and meiosis. Techniques used include karyotyping, analysis of G-banded chromosomes, other cytogenetic banding techniques, as well as molecular cytogenetics such as fluorescent in situ hybridization and comparative genomic hybridization. Chromosomes were first observed in plant cells by Karl Wilhelm von Nägeli in 1842, their behavior in animal cells was described by Walther Flemming, the discoverer of mitosis, in 1882. The name was coined by another German anatomist, von Waldeyer in 1888; the next stage took place after the development of genetics in the early 20th century, when it was appreciated that the set of chromosomes was the carrier of the genes. Levitsky seems to have been the first to define the karyotype as the phenotypic appearance of the somatic chromosomes, in contrast to their genic contents. Investigation into the human karyotype took many years to settle the most basic question: how many chromosomes does a normal diploid human cell contain?
In 1912, Hans von Winiwarter reported 47 chromosomes in spermatogonia and 48 in oogonia, concluding an XX/XO sex determination mechanism. Painter in 1922 was not certain whether the diploid number of man was 46 or 48, at first favoring 46, he revised his opinion from 46 to 48, he insisted on man having an XX/XY system. Considering their techniques, these results were quite remarkable. In science books, the number of human chromosomes remained at 48 for over thirty years. New techniques were needed to correct this error. Joe Hin Tjio working in Albert Levan's lab was responsible for finding the approach: Using cells in culture Pre-treating cells in a hypotonic solution, which swells them and spreads the chromosomes Arresting mitosis in metaphase by a solution of colchicine Squashing the preparation on the slide forcing the chromosomes into a single plane Cutting up a photomicrograph and arranging the result into an indisputable karyogram, it took until 1956 until it became accepted that the karyotype of man included only 46 chromosomes.
The great apes have 48 chromosomes. Human chromosome 2 was formed by a merger of ancestral chromosomes. Barbara McClintock began her career as a maize cytogeneticist. In 1931, McClintock and Harriet Creighton demonstrated that cytological recombination of marked chromosomes correlated with recombination of genetic traits. McClintock, while at the Carnegie Institution, continued previous studies on the mechanisms of chromosome breakage and fusion flare in maize, she identified a particular chromosome breakage event that always occurred at the same locus on maize chromosome 9, which she named the "Ds" or "dissociation" locus. McClintock continued her career in cytogenetics studying the mechanics and inheritance of broken and ring chromosomes of maize. During her cytogenetic work, McClintock discovered transposons, a find which led to her Nobel Prize in 1983. In the 1930s, Dobzhansky and his coworkers collected Drosophila pseudoobscura and D. persimilis from wild populations in California and neighboring states.
Using Painter's technique they studied the polytene chromosomes and discovered that the wild populations were polymorphic for chromosomal inversions. All the flies look alike whatever inversions they carry: this is an example of a cryptic polymorphism. Evidence accumulated to show that natural selection was responsible. Using a method invented by L'Héritier and Teissier, Dobzhansky bred populations in population cages, which enabled feeding and sampling whilst preventing escape; this had the benefit of eliminating migration as a possible explanation of the results. Stocks containing inversions at a known initial frequency can be maintained in controlled conditions, it was found that the various chromosome types do not fluctuate at random, as they would if selectively neutral, but adjust to certain frequencies at which they become stabilised. By the time Dobzhansky published the third edition of his book in 1951 he was persuaded that the chromosome morphs were being maintained in the population by the selective advantage of the heterozygotes, as with most polymorphisms.
The lily is a favored organism for the cytological examination of meiosis since the chromosomes are large and each morphological stage of meiosis can be identified microscopically. Hotta et al. presented evidence for a common pattern of DNA nicking and repair synthesis in male meiotic cells of lilies and rodents during the zygotene–pachytene stages of meiosis when crossing over was presumed to occur. The presence of a common pattern between organisms as phylogenetically distant as lily and mouse led the authors to conclude that the organization for meiotic crossing-over in at least higher eukaryotes is universal in distribution. Following the advent of procedures which allowed easy enumeration of chromosomes, discoveries were made related to aberrant chromosomes or chromosome number. In some congenital disorders, such as Down syndrome, cytogenetics revealed the nature of the chromosomal defect: a "simple" trisomy. Abnormalities arising from nondisjunction events can cause cells with aneuploidy in one of the parents or in the fetus.
In 1959, Lejeune discovered patients with Down syndrome had an extra copy of chromosome 21. Down syndrome is referred to as trisomy 21. Other numerical abnormalities discovered include sex chromosome abnormalities. A female with only one X chromosome has Turner syndrome, whereas an additional X chromosome in a male, resulting in 47 total chromosomes, has Klinefelter syndrome. Many other sex chromosome combinations are compatible with live bir