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Primitive erythroblasts in the circulating blood of the chick embryo continue to divide while synthesizing hemoglobin (Hb). Hb measurements on successive generations of erythroblasts show that there is a progressive increase in the Hb content of both interphase and metaphase cells. Furthermore, for any given embryo the Hb content of metaphase cells is always significantly greater than that of interphase cells. The distribution of Hb values for metaphase cells suggests that there are six Hb classes corresponding to the number of cell cycles in the proliferative phase. The location of erythroblasts in the cell cycle was determined by combining Feulgen cytophotometry with thymidine radioautography on the same cells. Measurements of the Hb content for erythroblasts in different compartments of the cell cycle (G1, S, G2, and M) show a progressive increase through the cycle. Thus, the amount of Hb per cell is a function of the number of cell divisions since the initiation of Hb synthesis and, to a lesser degree, the stage of the cell cycle. Earlier generations of erythroblasts synthesize Hb at a faster rate than the terminal generation. Several models have been proposed to explain these findings.

Parenchymal cells from adult rat liver have been established in primary monolayer culture. Donor animals are subjected to a partial hepatectomy and, 4 days later, cells are prepared by collagenase perfusion of the regenerated liver. The hepatic parenchymal cells, separated from nonparenchymal material and suspended in serum-free medium, are placed in plastic tissue culture dishes, where they form a monolayer within 24 h. The monolayer cells exhibit minimal mitotic activity and demonstrate several major metabolic functions characteristic of liver in vivo; these include albumin synthesis and secretion, gluconeogenesis from 3-carbon precursors, responsiveness to insulin and glucagon, glycogen synthesis, and activity of two microsomal enzymes. These functions are present in the monolayer cells for several days at activities similar to those observed in the liver in vivo. The findings indicate that hepatic parenchymal cells in this monolayer system are viable and behave in many respects like normal adult rat liver.

Mitotic chromosomes of rat kangaroo cells were irradiated with a green argon laser microbeam without prior dye sensitization. Deoxyribonucleic acid-negative chromosome paling was observed. The irradiated cells were isolated and cloned into viable populations.

HeLa cells were found to enter and complete mitosis in the presence of puromycin or actinomycin D. Daughter cells formed in the presence of puromycin contained completed nuclear membranes. Daughter cells resumed protein synthesis shortly after completing mitosis however, this resumption of protein synthesis could be prevented by actinomycin D. Data presented here indicate that all cellular constituents necessary for successful initiation and completion of mitosis are produced prior to the onset of mitosis. They also indicate that new messenger RNA synthesis is required for resumption of protein synthesis after mitosis, and that the inhibition of protein synthesis observed in mitotic mammalian cells 18 a result of messer ger RNA depletion. © 1967.

HeLa cells were found to enter and complete mitosis in the presence of puromycin or actinomycin D. Daughter cells formed in the presence of puromycin contained completed nuclear membranes. Daughter cells resumed protein synthesis shortly after completing mitosis however, this resumption of protein synthesis could be prevented by actinomycin D. Data presented here indicate that all cellular constituents necessary for successful initiation and completion of mitosis are produced prior to the onset of mitosis. They also indicate that new messenger RNA synthesis is required for resumption of protein synthesis after mitosis, and that the inhibition of protein synthesis observed in mitotic mammalian cells 18 a result of messer ger RNA depletion. © 1967.

Rabbit antisera raised against antigens solubilized from the membranes of a transplantable Balb/c adenocarcinoma precipitated an antigen that was not expressed by the normal resting mammary gland or any other normal mouse tissue. The same antisera were used to show the presence of this antigen in other mammary adenocarcinomas, although not in tumours of different histological types. The antigen was, however, transiently expressed by mammary gland cells during the hyperplasia that both precedes and accompanies lactation. A connection is sought between the existence of this antigen and the membrane structural changes hat take place during mitosis. [ABSTRACT FROM AUTHOR]

DNA synthesis, mitosis and ploidy of dividing cells were studied during 30 h after wounding around wounds inoculated with Agrobacterium tumefaciens, around sterile wounds and in control stems of Vicia faba. DNA synthesis was examined by counting nuclei labelled with 3H‐thymidine in slides in which mitoses had been counted and analyzed before autoradiography. The main results were that around inoculated wounds, DNA synthesis and the number of mitoses showed a peak between 12 and 22.5 h. Both types of wounding induced mitoses, many of them polyploid (DNA content higher than 4C), both in the pith and the cortex, whereas in the control stems only diploid mitoses, mostly in the stelar area, were seen. The first polyploid (8C) mitoses around the inoculated wounds took place at 12 h and at 15.5 h 32C mitoses were seen; around the sterile wounds the first 8C divisions occurred at 26 h. The frequency of polyploid mitoses and their degree of ploidy continued to be considerably higher around the crown gall than around the control wounds. When a cell with a higher than 4C content is induced to divide, the 12 chromosomes, as a rule, consist of four, eight, 16 or 32 chromatids, instead of the normal two. The early division of highly polyploid cells around the inoculated wounds is obviously caused by growth factors which are known to be produced by the bacteria. It appears possible that this ability to synthesize excessive amounts of growth factors is subsequently transferred to the host cells through bacterial DNA. [ABSTRACT FROM AUTHOR]

Excised primary roots of Phaseolus vulgaris L. were treated with canavanine and the effect on arginine utilization was studied. Arginine utilization was observed to be depressed to a low level by canavanine (1.8 × 10-4 M) in rapidly dividing and growing tissues. In addition, canavanine inhibited at ginine utilization to a lesser degree in root sections composed of mature and non-dividing tissues. This demonstrated that canavanine inhibition is not limited to those tissues where active DNA synthesis occurs. Canavanine was observed to inhibit the onset of mitosis in primary roots as evidenced by a significant reduction of the frequency of mitotic figures. In addition, this amino acid was found to affect the course of mitosis once it was initiated by altering the relative frequency of mitotic stages, decreasing the percentage of prophase figures and increasing the percentage observed in the telophase. An influence on the rate of spiralization and despiralization of the chromosomes is suggested. The effects on mitosis are probably related to growth-inhibitory and toxic effects of canavanine. The possible function of canavine as an allelopathic substance is hypothesized. [ABSTRACT FROM AUTHOR]

Keratinocytes from normal human skin and psoriasis lesions were propagated in vitro. Several substances known to increase intracellular levels of cyclic AMP were added to the in vitro system. these caused a decrease in mitosis by blocking cells in the G2 portion of the cell cycle. For both normal and psoriatic cells, the mitotic response was qualitatively and quantitatively similar. The results support other studies in which it has not been possible to demonstrate significant qualitative differences between normal and psoriatic keratinocytes in vitro. [ABSTRACT FROM AUTHOR]

There are numerous ways of assessing the activity of the sebaceous glands in rodents. For example, gland size and the incidence of mitoses have been extensively used. Gland size is extremely tedious to measure and, since it depends on both the transit time of the sebaceous cells and their production, is not proportional to sebaceous output. Whereas the incidence of mitoses is a true measure of activity in a holocrine structure, it is only one factor in secretion, which also depends on intracellular synthesis. For these reasons, direct measurement of sebaceous secretion is desirable.

Mitotic activity in sebaceous glands is mainly focal, occurring mostly in buds or aggregates of undifferentiated cells from the walls of the excretory ducts and from the periphery of the acini. As these buds grow into sebaceous units, differentiation begins in their centers. Replacement thus takes place from a succession of cell clusters in the acini. Sebaceous acini, therefore are transient or unstable structures, which undergo variation in shape and volume according to the amount and distribution of mitotic activity. Therefore, in both sebaceous glands and epidermis, the main kinetic units are cell clusters which originate one after another from groupings of stem cells which are simultaneously entering mitosis.

Mitosis and cytokinesis in Gonyostomum semen (Ehrenberg) Diesing have been investigated with the light microscope. During prophase nucleoli disappear and the chromatid structure of the chromosomes becomes apparent. Separation of chromatids at anaphase is accompanied by progressive fusion of the progeny chromosomes. This process continues into telophase by which stage the progency nuclei consist of dense masses of chromatin with occasional chromosomes extending from their equatorial surfaces. By the end of telophase, nucleoli are reforming and the interphase nuclear morphology is reestablished. Mitosis is followed by cytokinesis, which is a relatively lengthy phase. In early cytokinesis the 2 interphase nuclei are present and thee is no indication of the forthcoming division of the cytoplasm. Later in cytokinesis a membrane is formed between the 2 nuclei. Final separation of the progeny individuals is accomplished by vigorous movements of swimming cells or, in the case of palmelloid cells, by the deposition of a mucilaginous layer. [ABSTRACT FROM AUTHOR]

The cell cycle has been determined in eight in-patients with psoriasis. The cell cycle time for the germinative cell compartment has been found to be 91 h, and the turnover time for the epidermal compartment, beneath the keratin layer to be 120 h (5 days). The duration of the S phase (DNA synthesis) has been found to be 10 h, and that for mitosis 30 min. If it is accepted that the turnover time for the keratin layer is 2 days, then the turnover time for the epidermis in psoriasis is 7 days, and not 3-4 days as previously described. Psoriasis is characterized by a number of changes in the epidermal cell. There is an increase in the total number of cells per unit area of skin, in the number of germinative cells, in the size of the cells (Fry & McMinn, 1968), and in the number of mitotic figures. In addition, the turnover time of the epidermis has been stated to be shorter in psoriasis. Weinstein & Van Scott (1965) reported that, for the epidermal compartment beneath the keratin layer, the turnover time was 2 days, compared to 13 days in normal skin. These observations have not been satisfactorily explained or investigated, but they imply an alteration in the cell cycle of the epidermal cell in psoriasis compared to normal. The cell cycle is the behaviour of a cell during its lifetime and has been defined by Howard &amp Pelc (1953) as the interval between completion of mitosis and the subsequent completion of mitosis in one or both daughter cells (Fig. 1). There are four recognized components of the cell cycle and it would seem imperative to know how these are altered in psoriasis, as this would undoubtedly help in our understanding of the biological fault. The only previous study of the cell cycle in psoriasis has been that of Weinstein & Frost (1968). The results obtained in that particular study do not correlate with the turnover time for psoriatic epidermis postulated by Halprin (1972). We therefore felt it necessary to determine the cell cycle in psoriasis, using more sophisticated techniques than those used by Weinstein & Frost. [ABSTRACT FROM AUTHOR]

The labelling indices of the epithelium of the lesions of oral lichen planus (LP) have been estimated, using in vitro labelling with tritiated thymidine. The relationship between this epithelial labelling index and the underlying inflammatory cell density was also examined. The total labelling activity in the LP lesion was comparable with that of normal mucosa but was redistributed as follows. The labelling index of the basal epithelial layer was increased above that of normal controls and the labelling in the suprabasal layer was decreased. Thus 50% of mitoses occur basally in involved atrophic LP epithelium. The implications of this redistribution of mitotic activity are discussed. The labelling index of epithelium was reduced where the inflammatory cell density was exceptionally high, but no simple inverse relationship between labelling index and inflammatory cell density was apparent. This supports the concept that the primary event in lichen planus seems to occur in the basal epithelial layer followed by an inflammatory cell response in the underlying connective tissue. Subsequent secondary damage to the epithelium, including inhibition of mitosis, may be produced by this inflammatory infiltrate only if it is very dense. [ABSTRACT FROM AUTHOR]

On the ventral side of the African fruit bat web 12 × 50 mm areas of epidermis were removed by adhesive tape strippings. The regenerative reaction close to the wound was studied on both sides of the web by the Colcemid method and by counting cells. Waves of increased proliferation occurred followed by increase in cell number. The reaction was local and limited. It was definitely strongest on the dorsal side opposite the centre of the wound. The results are discussed in relation to the many theories that have been proposed to explain the increase in cell proliferation around a wound and the consequent re-establishment and maintenance of the steady state condition found in normal epidermis. The theory which most easily explains the results on the time course, the degree and the localization of the alterations of cell proliferation and cell number found here after Sellotape stripping is the chalone theory of growth control. [ABSTRACT FROM AUTHOR]

A pronounced 24-hour fluctuation occurs in the mitotically active tissues of the rat maxillary incisor. High and low periods of activity were seen during the daytime as well as at night. A consistent increase in the number of mitotic figures occurred approximately four hours subsequent to a change in the light-dark regimen. [ABSTRACT FROM AUTHOR]

The effect of insulin in vivo on the proliferation and cellularity of epididymal adipose tissue of growing rats was investigated. Following the intraperitoneal administration of small amounts of insulin (500 μU/ rat, twice a day), which did not influence the blood sugar level and food intake, it was found that repeated administration of insulin for 48-72 h leads in adipose tissue to an enhanced incorporation of C-2-thymidine into DNA and to an increase of the total amount of DNA and RNA in the fat body. The enhanced DNA synthesis in adipose tissue of insulin-treated rats was marked in nuclear DNA and absent in mitochondrial DNA. After fractionation of adipose tissue by collagenase an enhanced DNA synthesis was found in the fraction of adipose and stromavascular cells. Morphological examination of adipose tissue of insulin-treated rats revealed cells in the phase of mitotic division and an increased ratio of fat cells of smaller size. Calculation of the number of cells in the epididymal fat body revealed that, after administration of insulin, the number of adipose and stromavascular cells increased. [ABSTRACT FROM AUTHOR]

Mouse peripheral blood lymphocytes were cultured with phytohaemagglutinin or Concanavalin A and at various times antiserum to each mitogen was added. Stimulation could be completely blocked for 5-6 hr and partially inhibited for at least 48 hr, and probably longer. The block produced by antiserum was specific and dependent on the relative concentrations of mitogen and antiserum. The gradual release of activation from the requirement for mitogen was not due to the production of a soluble non-specific mediator since the stimulatory activity of culture super-natants of fresh lymphocytes could be completely blocked in turn by antiserum to the original mitogen. Other experiments made the operation of a locally concentrated mediator unlikely. These data support a direct proliferative mechanism of lymphocyte transformation to these mitogens. [ABSTRACT FROM AUTHOR]

The influence of activated carbon and aseptic conditions has been studied on the growth of the primary root of wheat seedlings in order to ascertain whether or not the growth effect of activated carbon is connected with the occurrence of epiphytic bacteria. Growth was measured as mitotic activity, rate of cell elongation and duration of cell elongation. The surface infection of the septic roots probably consisted of common airborn and waterborn bacteria. Aseptic conditions increased the rate of cell elongation by ca 70 % but had no effect on the meristem activity. Activated carbon increased mitoses in the meristem and slightly augmented the duration of cell elongation but had no effect on the rate of elongation. The effects of sepsis and carbon were independent and appeared additative. Activated carbon removed inhibitors produced by the root tip itself but not those formed by the bacteria. In these experiments neither group of inhibitors seemed to contain IAA-like substances. [ABSTRACT FROM AUTHOR]

The open carboxymethylcellulose (CMC) haemolytic plaque technique was used to study the antibody-forming capacity of daughter ceils arising from single plaque-forming ceils (PFC) after in vitro mitosis. Spleen cells from mice immunized 2–5 days previously with sheep red blood cells (SRBC) were used. The number of in vitro mitoses was increased by giving mice colcemid 2 hours before killing. The experimental protocol involved allowing cells to form haemolytic plaques in a liquid medium; selecting the largest available PFC; transferring them to wash droplets to await colcemid-escape and completion of mitosis; separating the two daughter ceils from each other by micromanipulation; transfer of each daughter cell pair to a CMC plaque-revealing monolayer; and continuous assessment of plaque formation rate by daughter cell pair members. The whole procedure was carried out with the micromanipulation set up at 37°. Altogether, seventeen experiments on IgM (direct)-PFC and two on IgG (enhanced)-PFC were performed. Of eighty-seven daughter cell pairs transferred, eighty-six behaved symmetrically with both daughter ceils forming antibody (seventy-four cases) or not forming antibody (twelve cases). Arguments are presented to suggest that this failure rate of 14 per cent is due to technical factors inherent in the micromanipulation steps. In the great majority of cases, daughter cells were symmetrical in size and plaque formation rate. Plaques from daughter ceils were always of similar morphology, and when mixed sheep and goat RBC monolayers were used to reveal plaques, both cells of a pair always behaved similarly in that either both formed a clear plaque or both formed a turbid plaque. The results are discussed in the light of the hypothesis of Tannenberg and Malaviya (1968) for asymmetric divisions amongst PFC, which they do not favour. It is stressed, however, that the possibility of asymmetric divisions amongst antibody- forming cell precursors is not eliminated. [ABSTRACT FROM AUTHOR]

Human peripheral lymphocytes from all donors tested were mitotically activated when cultured in the presence of X-irradiated cells from continuously cultivated lymphoid cell lines (LCL). X-irradiated cells from LCL originating from normal individuals were effective as well as those of Burkitt's tumour origin and cells from LCL reported to be free of herpes-like virus were as effective as those reported to contain virus. The activation was of much greater intensity than the normal mixed lymphocyte reaction but occurred over a similar culture period. The reaction occurred only under conditions permitting direct cell contact between the two populations of cells. Most stimulation was obtained with viable X-irradiated LCL cells. Disrupted LCL cells were ineffective. [ABSTRACT FROM AUTHOR]

Albino guinea-pigs were actively sensitized with complete Freund's adjuvant. Tuberculin skin reactivity and tuberculin-induced in vitro mitotic responsiveness of circulating lymphocytes were determined sequentially in each animal after sensitization. In nine of ten animals studied, both skin reactivity and in vitro mitotic response to tuberculin appeared 3 weeks after sensitization. [ABSTRACT FROM AUTHOR]

Mean rates of mitosis, determined over short intervals of time by blocking the cell cycle at metaphase, fall immediately after X-irradiation in most parts of the root meristem of Zea mays. The fall continues for several hours and is then replaced by a rise until normal rates are reached. In the quiescent centre, however, the mean mitotic cycle shortens to one-third of its previous duration within an hour of irradiation. The rate of mitosis then fails to nearly its previous level before rising again at the same time as the acceleration of the cycle in the other regions. Finally quiescence sets in again as the other regions return to greater mitotic activity. The observations suggest (1) that the reaction of the quiescent centre is not simply due to the relief of some inhibition imposed by the surrounding cells, (2) that something other than the different DNA levels of the various regions is in part responsible for the differential response, and (3) the resistance of the quiescent centre to irradiation is a real phenomenon requiring explanation. [ABSTRACT FROM AUTHOR]

Mitosis in the nucleus of Cyanophora paradoxa is shown to be generally typical of other eukaryotic cells. Basal bodies and the flagella apparatus appear to replicate before nuclear division. At preprophase, microtubules approach the nucleus from the periphery of the cell to form the usual extranuclear spindle by prophase; at this stage the nucleolus begins its breakdown, which is soon complete. The spindle poles, clearly defined by the orientation of microtubules, never contained any obvious structural component, and basal bodies were never associated with the spindle. By prometaphase, breakdown of the nuclear envelope is accompanied by invasion of the nucleus by the extranuclear microtubules. During anaphase, two plates of chromosomes separate inside the spindle containing both continuous and chromosomal microtubules. Remnants of the nuclear envelope often coat the chromosomes. Cleavage, initiated by early anaphase, is achieved by an asymmetric, longitudinal constriction of the cell membrane, which passes between the replicated flagellar apparatuses. The plane of cleavage was always oriented at right angles to the spindle axis, and did not involve any detectable microfibrillar elements. Telophase nuclei reform as normal. These observations indicate that the host cell of the symbiotic pair cannot be considered as a dinoflagellate as had been previously suggested. Division of the cyanelles is independent of nuclear division in the host, which therefore contains variable numbers of these symbionts. 'Wall' material is secreted into the furrow of the cleaving cyanelles. Furthermore, a very characteristic organelle was discovered in many cells invariably appressed to the cyanelles. Its dense granular contents were contained within a single unit membrane, and it was also frequently associated with endoplasmic reticulum. [ABSTRACT FROM AUTHOR]

The life cycle of Plasmodiophora brassicae was studied in stained preparations of Brassica napus and B. rapa callus tissue cultures and clubbed roots. Resting spores germinated in situ in callus cells to produce naked protoplasts which underwent plasmogamy to form multinucleate primary plasmodia. Mitosis then occurred and was followed by cleavage of the plasmodia into zoosporangia containing uninucleate zoospores. The zoospores were released and fused in pairs (without karyogamy) and then penetrated living callus cells where growth and mitosis took place to give multinucleate secondary vegetative plasmodia. The vegetative plasmodial nuclei fused in pairs and then underwent meiosis immediately. This was followed by cleavage of the plasmodial cytoplasm to yield uninucleate, haploid resting spores. P. brassicae had a haploid chromosome number of five in both 2-year-old callus cells and in recently harvested club-root tissues. [ABSTRACT FROM AUTHOR]

Roots of Allium sativum and Zea mays were given 150 rads and 1800 rads of X-rays respectively. Cells of the quiescent centre respond at once by coming into mitosis although most are at G1 before irradiation. There is a stimulation of DNA synthesis in the quiescent centre, but not elsewhere, immediately after irradiation. Some cells, however, undergo mitosis without having synthesized DNA and this results in prophase and metaphase nuclei without the normal 4C complement of DNA. [ABSTRACT FROM AUTHOR]

Secondary cultures of embryonic rat cells (17-19 day of gestation) require for growth the presence of serum in the culture medium; a growth stimulating fraction has been isolated from fetal calf serum. It will be shown that cells incubated in medium without this active fraction are stopped in the G1 phase of the cell cycle about 3 hours prior to the beginning of DNA synthesis. This has been confirmed by incorporation of methyl-[³H]thymidine, labelling index, mitosis index and cytophotometry. Cells already triggered at this critical point come into mitosis in spite of the fact that, the stimulating factor is omitted from the culture medium. Resting cells, preincubated in inactive medium for 15 to 40 hours can be stimulated by adding the active component to the medium; they resume DNA synthesis after a lag phase of about 20 hours and come into mitosis another 10 hours later. Embryonic rat cells after spontaneous transformation into a permanent cell line do not require the presence of this stimulating factor for at least 3 cell generations. For secondary cultures of embryonic mouse and hamster cells the same active serum fraction is essential for growth. Embryonic mouse cells after spontaneous transformation into a permanent cell line are as independent from this stimulating activity as permanent rat cells; the same holds for KB and HeLa cells. [ABSTRACT FROM AUTHOR]

In HeLa S3 cells grown in suspension and synchronized with thymidine, while nuclear DNA is replicated during the S-phase, cytoplasmic DNA is synthesized with two peaks in the S- and G2- phases: the first has its maximum 1–2 h before that of nuclear DNA ; the second takes place after that of nuclear DNA, approximately at the 9th hour from the removal of synchronizing thymidine from the culture medium. Newly synthesized nuclear and G2-phase-cytoplasmic DNAs in CsCl gradients have buoyant density of 1.698 and 1.688 g/cm3, respectively. The buoyant density of S-phase-cytoplasmic DNA is 1.701 g/cm3, probably corresponding to the closed circular duplex form of mitochondrial DNA. The kinetics of synthesis of nuclear DNA is exponential, differing from that of S- and G2-phase-cytoplasmic DNAs which show an asymptotic shape of [3H]thymidine labeling. Ethidium bromide inhibits the synthesis in vivo of both S and G2 peaks of cytoplasmic DNA, but does not appreciably influence the labeling of nuclear DNA. [ABSTRACT FROM AUTHOR]

Nuclei isolated from various points in the mitotic cycle were examined for RNA synthesis m the presence of Mg2+ or Mn2+ + (NH4)2SO4. A peak of activity 2.5--3 h after mitosis was only slightly increased by Mn2+ + (NH4)2SO44) over that in the presence of Mg2+, and inhibited some 50% by low levels of α-amanitin in the presence of either Mg2+ or Mn2+ + (NH4)2SO4. A G2 peak of activity 2.5--3 h before mitosis was 100% increased by Mn2++ (NH4)2SO4 over that observed in the presence of Mg2+, the increase being completely sensitive to low levels of α-amanitin, whereas the Mg2+ activity remained insensitive. Nucleoli isolated some 2.5--3 h before mitosis exhibited RNA synthesis which was unstimulated by Mn2+ (NH4)2SO4 and insensitive to α-amanitin. Similarities in RNA synthesis in isolated nucleoli and isolated (G2 nuclei in the presence of Mg2+ with respect to pH optima and actinomycin-D sensitivity were observed. It is concluded that there is a high basal level of α-amanitin-insensitive RNA synthesis activity which is nucleolar and therefore probably ribosomal, rising to a peak some 2.5--3 h before mitosis, and a major peak of α-amanitin-sensitive nucleoplasmic RNA synthesis activity some 2.5--3 h after mitosis, thereafter dropping to a low level. [ABSTRACT FROM AUTHOR]

Epidermal mitotic activity, thickness, and cellularity were measured in a blind study of serial biopsies after several different application schedules of vitamin A acid to hairless mice. A single dorsal application increased the mitotic index by two hours (P < 0.005), the thickness of the basal and prickle layers by fifty hours (P < 0.005), and the number of cells per unit length of epidermis by seventy-four hours (P < 0.0125) as compared to vehicle controls and the no-treatment controls. The rapidity of mitotic response, which was not seen with vitamin A alcohol, along with other evidence, suggests an initial stimulation of a cell population in G2. [ABSTRACT FROM AUTHOR]

The effects of human autopsy skin and liver extracts were studied on mitotic incidence in cultures of human epidermal cells. Whereas skin extracts inhibited mitosis significantly, liver extract had no effect. Neither skin nor liver extract had a significant inhibitory effect on mitosis of monkey kidney cells in vitro. These results demonstrate that the mitotic inhibitory effects of the skin extracts are tissue specific. The evidence also indicates that skin extracts act directly on the epidermal cells, a point which has not previously been demonstrated. Thus, human skin contains tissue-specific mitotic inhibitory factors similar to those described in non-human tissues. [ABSTRACT FROM AUTHOR]

Aqueous extracts of newborn rat epidermis inhibit mitosis of cultivated keratinocytes obtained from guinea pigs and humans. The mitotic activity of cultivated human keratinocytes and fibroblasts obtained from the uninvolved skin of subjects with psoriasis is similar to the mitotic activity of these cells obtained from involved psoriatic skin. This observation is consistent with the model of repression of mitotic activity in normal skin and a derepression of mitotic activity in psoriatic epidermis. Extracts of liver also inhibit mitosis of guinea pig keratinocytes. Cultivated keratinocytes do not exhibit the same specificity of inhibition by epidermal extracts that has been reported for surviving fragments of mouse ear. [ABSTRACT FROM AUTHOR]

The sequence of morphological changes associated with the migration of the epidermal cell from the germinative layer to the cornified layer, cessation of mitosis, formation of tonotilaments, appearance of keratohyalin granules, loss of the nucleus and development of keratin fibers, must coincide with a series of molecular changes resulting from alterations in the cellular profile of catalytic activities. Undoubtedly, this chemical differentiation is genetically programmed in time and in substance, although environmental influences may modulate the activities of specific metabolic systems.

Using split epidermal sheets treated with colchicine, a quantitative study was carried out on hairless mouse epidermal melanocytes and those in mitosis, during 14 daily exposures to UV. It was demonstrated that the mitotic figures were first noted after 3 days of irradiation and their incidence varied with the duration of UV treatment; the number of cells in arrested mitosis during the 3 hour period was 0.2-0.4, 0.6-0.7 and 1.0-1.3 per cent of dopa-positive melanocytes, respectively, after 3, 5 and 7 to 14 days of exposure. Comparison of those values and the rate of population increase of melanocytes indicates that the numerical increase in melanocytes during the first 5 days is attributable largely to a mechanism other than the division of melanocytes. In addition, it is likely that the population increase between the 5th to 14th days is due, to a considerable extent, to a similar process. In regard to the mechanism accounting for the population increase in melanocytes, the following processes or possibilities, other than the division of melanocytes are discussed: the activation of pigment formation in amelanogenic melanocytes and the migration of dermal melanocytes into the epidermis. [ABSTRACT FROM AUTHOR]

Statistical analysis of clustering of mitotic figures in adult human epidermis shows that this clustering is highly significant, in excess of what can be explained by chance. To explain this, some cause for the clustering can be looked for. The results are consistent with the existence of a mitosis stimulating factor that is balanced by a diurnally varying anti-mitotic factor or factors -- perhaps the adrenal steroid chalone complex of present theory. Such a mitosis stimulating factor would have the characteristics of an epidermal antichalone. [ABSTRACT FROM AUTHOR]

The structural organization of the stratum corneum in several scaling disorders of the skin has been studied in alkali swollen frozen sections with the light microscope, and in tape stripped specimens with the scanning electron microscope. In psoriasis, lichenification, and lamellar ichthyosis, which are characterized by high epidermal mitotic activity, the stratum corneum lacks the neatly stacked cellular arrangement typical of normal stratum corneum. In direct contrast the stratum corneum of patients with ichthyosis vulgaris appears nearly normal in this respect. Of the scaling disorders studied, ichthyosis vulgaris is also unique in having normal, or even subnormal epidermal mitotic activity. This suggests that the columned stacking pattern of the normal stratum corneum is incompatible with a high mitotic activity. [ABSTRACT FROM AUTHOR]

In order to initiate studies to determine whether the restriction of mitosis to the basal layer in normal mammalian epidermis is a function of intrinsic changes in the cell or of environmental factors, a procedure which facilitates the isolation and separation of epidermal cells into suspensions of predominantly basal cells and of spinous and granular cells, respectively, was established. The comparatively high viability and good morphological preservation of the cells obtained by this method allowed us (1) to observe and describe morphological characteristics of keratinocytes in various stages of differentiation using phase contrast microscopy and (2) to study their respective proliferative capabilities in vitro using cell culture and metabolic tracer techniques. Over 200 successful primary cultures of basal cells have been performed to date but 56 attempts with suspensions composed of spinous and granular cells were unsuccessful. The incorporation of thymidine-3H, a specific precursor of DNA, into acid precipitable form after 8 hours incubation was nearly 100 fold greater in basal cell suspensions than in suspension of more differentiated cells. These observations suggest that the factors responsible for the absence of mitosis in cells from the upper epidermal layers are related to irreversible changes in the cell itself rather than to in situ environmental influence. Other evidence to support this tentative view is discussed. [ABSTRACT FROM AUTHOR]

An embryonic induction is said to operate when one population of cells acts upon another population to change the behavior of the second group in a developmentally significant way. In summary, it is evident that the responses to induction are of such dissimilar nature that they can hardly be caused by single mechanism. Determination almost certainly is a nuclear phenomenon: mitotic stimulation may be due to some peculiar type of nutritional interaction: morphogenetic interaction may involve complex of molecules at the interface of the two tissue as well as function of specific organelles within epithelial cells.

gIncrease in the size of thalli of C. scutata was achieved by division and growth of the marginal cells. During mitosis, the nuclear envelope dis-integrated and the daughter nuclei remained widely separated after telophase. The plane of cytokinesis in the marginal cells was either along a radius of the discoidal thalli or parallel to their circumference. For radial divisions, this alga utilized a phragmoplast typical of higher plants, and a modified phragmoplast when dividing circumferentially. The phylogenetic implications of these findings are discussed. [ABSTRACT FROM AUTHOR]

Mitosis, cytokinesis, and cellular organization during interphase are described. Interphase cells possess a single microbody-like organelle which occurs between, and is appressed to,the chloroplast and nucleus. The microbody-like organelle divides during mitosis and the division of the chloroplast. Anaphase is unusual in that chromosome-to-spindle pole distance remains constant even through the 2 groups of chromosomes become widely separated. When anaphase is half completed, a vacuole forms in the interzonal region and appears to be involved in further separation of the chromosomes. Vacuolar development is also involved with events of early interphase. The cytology of K. flaccidum is compared to those of Ulothrix fimbriata and Stigeoclonium helveticum. The comparison does not support the present classification of the 3 species, and indicates the value of comparative fine structural studies in the classification of ulotrichalean algae. [ABSTRACT FROM AUTHOR]