#### TL;DR How many cells are in your body? A simple question de...
Robert Hooke began the scientific study of cells, known as cell bio...
Yeasts are single-celled, eukaryotic - organisms whose cells have ...
> Since the cell number and cell size of various organs or systems,...
***Cell number might vary depending on gender, body height, body ma...
A ***gross estimation*** of the total human cell number was obtaine...
General estimations based on mean cell volume or weight have not pr...
Results for the total number of cells in the human as given by the ...
A more precise method than the gross estimation would be to sum the...
Their final estimate for the total number of human cells was:3.7... 2013 http://informahealthcare.com/ahb ISSN: 0301-4460 (print), 1464-5033 (electronic) Ann Hum Biol, 2013; 40(6): 463–471 ! 2013 Informa UK Ltd. DOI: 10.3109/03014460.2013.807878 RESEARCH PAPER An estimation of the number of cells in the human body Eva Bianconi 1 , Allison Piovesan 1 , Federica Facchin 1 , Alina Beraudi 2 , Raffaella Casadei 3 , Flavia Frabetti 1 , Lorenza Vitale 1 , Maria Chiara Pelleri 1 , Simone Tassani 4 , Francesco Piva 5 , Soledad Perez-Amodio 6 , Pierluigi Strippoli 1 , and Silvia Canaider 1 1 Department of Experimental, Diagnostic and Specialty Medicine, University of Bologna, Bologna, Italy, 2 Medical Technology Lab, Prometeo Lab (RIT), Istituto Ortopedico Rizzoli, Bologna, Italy, 3 Department for Life Quality Studies, University of Bologna, Rimini, Italy, 4 Institute of Communication and Computer Systems, Athens, Greece, 5 Department of Specialized Clinical Sciences and Odontostomatology, School of Medicine, Polytechnic University of Marche, Ancona, Italy, and 6 Biomaterials for Regenerative Therapies Group, Institute for Bioengineering of Catalunya (IBEC), Barcelona, Spain Abstract Background: All living organisms are made of individual and identifiable cells, whose number, together with their size and type, ultimately defines the structure and functions of an organism. While the total cell number of lower organisms is often known, it has not yet been defined in higher organisms. In particular, the reported total cell number of a human being ranges between 10 12 and 10 16 and it is widely mentioned without a proper reference. Aim: To study and discuss the theoretical issue of the total number of cells that compose the standard human adult organism. Subjects and methods: A systematic calculation of the total cell number of the whole human body and of the single organs was carried out using bibliographical and/or mathematical approaches. Results: A current estimation of human total cell number calculated for a variety of organs and cell types is presented. These partial data correspond to a total number of 3.72 ! 10 13 . Conclusions: Knowing the total cell number of the human body as well as of individual organs is important from a cultural, biological, medical and comparative modelling point of view. The presented cell count could be a starting point for a common effort to complete the total calculation. Keywords Cell size, human cell number, organ, total cell count, theoretical issue History Received 26 September 2012 Revised 19 March 2013 Accepted 9 May 2013 Published online 5 July 2013 Introduction Since Schleidens and Schwann’s first formulations of cell theory (Mazzarello, 1999), it has been known that all living organisms are made of individual and identifiable cells, whose size, number and type ultimately define the structure, functions and size of an organism. Cell size appears to be regulated by the amount of DNA (ploidy): for example, the diploid yeast is larger than the haploid one (Su & O’Farrell, 1998); cells in a tetraploid salamander are twice the size of those in a diploid salaman- der, although the corresponding organs in the two animals have the same size, because the organisms of the tetraploid salamander contain half as many cells as those of the diploid (Conlon & Raff, 1999). Nevertheless, the issue of what kind of relationship or signalling there is between cell size and DNA content is still unresolved. It has been hypothesized that there is a maximal protein-production rate from a genome because there is an upper limit to gene transcription and translation rates. Since proteins have a limited lifetime, there is a maximal amount of protein-associated cell mass that can be supported by a genome (Gomer, 2001). In size-control mechanisms, even cell communication has a role in defining specific organs, as reported by Depaepe et al. (2005). Cell number, on the other hand, appears to be under strict genetic and developmental control in lower organisms: for example, bacteria and yeasts are composed of single cells, while the adult Caenorhabditis elegans worm consists of exactly 959 (male) or 1031 (ermaphrodite) somatic cells (Alberts et al., 2002). In contrast, how cell number is determined in higher organisms is not understood, although it is presumably regulated by wider mechanisms of homeostasis such as proliferation, differentiation and cell death. According to this, Notch signalling can prolong precursor cell division and can maintain stem cells in a self-renewing pattern of division, thereby indirectly increasing the number of differ- entiated cells that are ultimately produced (Conlon & Raff, 1999). There are currently several examples of secreted factors that regulate tissue size, by both increasing and limiting proliferation. Examples are myostatin, which regu- lates skeletal muscle mass; leptin, which regulates the amount of adipose tissue; growth hormone and insulin-like growth Correspondence: Pierluigi Strippoli, De partment of Experimental, Diagnostic and Specialty Medicine, University of Bologna, Bologna, Italy. Tel: +39 051 2094100. Fax: +39 051 20 94110 . E-mail: pierluigi.strippoli@unibo.it factors, which regulate total body mass (Roisin-Bouffay & Gomer, 2004), and Hh-Gli signalling factors, which play a central role in controlling precursor cell numbers in the brain and other organs (Stecca & Ruiz i Altaba, 2009). Some factors, such as insulin-signalling pathway, are central in controlling cell number and size from worms to humans, supporting the idea that key functions of the pathway have been powerfully conserved through evolution (Moore, 2003). However, it has also been suggested that there may be fundamental differences in the mechanisms by which mam- mals and insects control their body size. This is the case of c-myc, indicated as a crucial signal mediator: in mice its reduction resulted in multi-organ hypoplasia, while Drosophila c-myc mutants were found to be smaller because of hypotrophy (Trumpp et al., 2001). In complex organisms, composed of many billions of cells, it is expected that control of cell number does not reach precision at the level of single cells. However, at least a defined order of magnitude in body cell number should be conserved among humans, on the basis of several consider- ations. First, although there are obvious differences in body height and mass among adult individuals, these variations are not greater than one order of magnitude and they may be due to an increase in cell size along with or in absence of an increase in cell number, such as in obesity (Avram et al., 2007; Salans et al., 1973). In addition, analysis of cell number at single organ or tissue level shows a considerable degree of conservation among human organisms, i.e. blood cells count. Biological experimental data point to an ‘‘organ-size check- point’’ that generates organs of reproducible shape and size in metazoans by regulating cell division, cell growth and apoptosis, involving genes regulating patterning or controlling cell adhesion and cell polarity (Leevers & McNeill, 2005). Finally, consistency of basic molecular and cellular mechan- isms for the development of an initial single cell (zygote) into a complete organism with a reproducible anatomy and physiology and conservation of these mechanisms through evolution, suggest that a fine control of cell number is exerted at genetic level in a similar manner for all individuals of a certain species. In fact, there are clear physical constraints on upper and lower limits of organism and organ size, such as bone or heart mechanics and surface-to-volume ratio, and biological mechanisms aimed at controlling body and tissue cell number (Gomer, 2001; Hafen & Stocker, 2003). The aim of this work is to discuss the theoretical issue of the total number of cells that compose the standard human adult organism. First, we noticed that these data were typically mentioned in the literature without citing a reference; second, we observed wide ranges among data reported by different sources, ranging from 10 12 –10 20 . We followed different lines of research in order to: undertake a systematic survey of available information about the number of cells of a human organism known to date; assess this number using rough estimations for the body as a whole; provide a framework and accurate count of specific organs or cell types. In addition, we discuss the relevance of this topic with regard to its applications in biology and medicine, the complexity of deriving the total cell number and, finally, we launch an international open debate aimed at a final and documented solution of the problem. Materials and methods Different approaches were used to retrieve all data available to date on the total human cell number for a human being or for a specific system/organ or cell type. Moreover, we obtained useful data from the literature in order to calculate the total cell number of specif ic system/organ or cell types not yet known. Since the cell number and cell size of various organs or systems, as well as the size of the organ or system itself, may vary according to several parameters such as age, sex, weight, pathology or evolutionary adaptations, we searched for a single reference for the ‘‘average man’’ or ‘‘standard human being’’. Since this does not exist, for the purposes of this work we chose a 30-year old young adult, weighing 70 kg, 1.72 m tall and with a body area of 1.85 m 2 (Irving, 2007). When retrieved data were different from this ‘‘average man’’ this was indicated in the specific section. Bibliographical search In order to f ind primary literature articles with information about the cell number of a human organism, we systematically searched the PubMed database (http://www.ncbi.nlm.nih.gov/ pubmed/). Searching for the MeSH term ‘‘Cell Count’’, corresponding to ‘‘cell number’’, led to non-specific results (more than 168 000 items found). The PubMed search was therefore adjusted by using MeSH sub-headings and by restricting results to Homo sapiens. The search was performed using the expression: (‘‘Cell Count’’[Mesh] OR ‘‘cell number’’) AND (‘‘Body Weights and Measures’’[Mesh] OR ‘‘Body Size’’[Mesh] OR ‘‘Body Constitution’’[Mesh] OR ‘‘Body Composition’’[Mesh]) AND ‘‘Humans’’[Mesh] AND (1809[PDAT]: 2012/01/31[PDAT]). In addition, a search including the explicit expression ‘‘cell number’’ along with the terms ‘‘body’’ or ‘‘organism’’, limited to humans but not including the MeSH sub-headings used above, was performed in the title/abstract fields of PubMed. In this case the PubMed query was: (Human[Title/Abstract] OR ‘‘Humans’’[Mesh]) AND (body[Title/Abstract] OR organism[Title/Abstract]) AND ‘‘cell number’’[Title/Abstract] AND (1809[PDAT]: 2012/01/31[PDAT]). Finally, a further search, not including restriction to the presence of ‘‘body’’ or ‘‘organism’’ terms and directed to the presence of the expression ‘‘cell number’’ in the item title was performed by the query: ‘‘Cell number’’[TI] AND human AND (1809[PDAT]: 2012/01/31[PDAT]). At the same time, we performed a specific search of the PubMed database with the purpose of finding primary literature articles with information on how to calculate the total cell number of a specific system/organ or cell type not yet known. The publication date of the articles searched ranged from 1809 to January 2012. Moreover, we reviewed printed versions of available texts covering Biology, Genetics, Histology, Anatomy, Physiology and other potentially useful available printed sources. In addition, we searched for this information in the main repositories of electronic versions of 464 E. Bianconi et al. Ann Hum Biol, 2013; 40(6): 463–471 books, NCBI Books (http://www.ncbi.nlm.nih.gov/sites/ entrez?db ¼ books) as well as of Google Books (http:// books.google.com/books), in their versions available up to January 2012. The NCBI Books collection includes many reference textbooks widely used in the biomedical field. It was queried using a general expression formulated according to the NCBI Entrez query language: (‘‘Human body’’ OR ‘‘Human organ- ism’’) AND (Cells OR ‘‘Cell number’’) or a specific request. The Google Books collection includes books on any subject whose text has been freely made totally or partially available. It was searched by using the expression: (‘‘Human body’’ OR ‘‘Human organism’’) (Cells OR ‘‘Cell number’’) or a specific request. The internet search engine Google was searched using the expression: (‘‘Human body’’ OR ‘‘Human organism’’) AND (Cells OR ‘‘Cell number’’) or a specific request. Morphological estimation It was possible to calculate cell volumes using an ultramicro- scopic cell or an organ picture by the ‘‘solid revolution method’’. First, we filtered the pictures in which the scale bar was reported, the cell/organ was well magnified and its edge was cleaned. We plotted the Cartesian axes on the cell/organ picture so that the axes origin was on the cell edge, the cell lay on the positive abscissa and this axis split the cell into two identical parts. We then traced bars with equal base size and intersecting cell/organ edge in the middle of their top side (Figure 1); the bars were built so that the sum of their areas was, as much as possible, equal to the area of the cell/organ contained in the first quadrant of the Cartesian axes. An approximation of the cell/organ volume was obtained by the following formula: V approx ¼ X n i¼1 ! # f ðx i Þ 2 # Dx In order to attain a better approximation of the cell/organ volume, we chose to sample the function f(x), that represents the cell/organ edge, in the middle of each interval Dx. Moreover we sub-divided the abscissa axis (the ‘‘a’’ segment in Figure 1) laid upon the shape into n equal parts, each with length . Finally, we re-scaled the cell/organ volume in cubic micrometres. Mathematical calculations Most of the time, data obtained by previous described methods required mathematical elaborations to find the total cell number of a human being or the specific system/organ or a cell type searched. Every time more than one datum was given, the mean value and the corresponding errors (standard deviation, SD) were reported alongside the value together with the corresponding reference that reports them. Furthermore, when cell size was necessary to perform the calculation, we declared it in the organ-specific results section. Moreover, a gross estimation of the total human cell number was obtained by dividing the mass or volume of a reference adult human body by the mass or volume, respectively, of an average human cell. Results Estimation of the human total cell number in the literature We optimized the PubMed search in order to obtain useful information about the composition of the human body in terms of cell number. The search strategies in PubMed database described in the ‘‘Bibliographical search’’ section led us to retr ieve 3407 articles. However, only two articles addressing the specific issue of the global cell number in the human body were found (see ‘‘Original article’’ in Appendix A, Table A1; online only). Searches in printed books, in online NCBI and Google Books gave useful estimations of total cell number in the human body (see ‘‘Printed book’’ and ‘‘Online book’’ in Appendix A, Table A1; online only). None of these values were justified by primary literature data and no citation source was available. Many websites retrieved via Google search reported estimations for the human whole body cell number. Due to criteria used by the Google search engine, we consulted the first 100 results, which led to estimations ranging from 5 ! 10 12 to 7 ! 10 16 , with a single site reporting 2 ! 10 20 . Excluding the results without an available primary source, the estimation of the total human cell number, supported by a bibliographical reference, ranges from 10 12 –10 16 , with a modal value of 10 13 (see Figure 2 and Table A1 in Appendix A; online only). Gross estimation of the human total cell number by mass and by volume Although high variability in the size and weight of cells from different normal tissues makes it difficult to choose a reference for human cell weight and volume average values, we assumed that a global compensation among body regions exists. Figure 1. Schematic representation of the cell image position in the Cartesian axes. It was possible to calculate the cell volume by the ‘‘solid revolution method’’ approximated by the summation formula, using an ultramicroscope cell picture. In particular, Cartesian axes have to be drawn on the cell image so that the x-axis is a symmetrical axis of the cell, the upper half of the cell (represented by the curve line in the figure) lies in the first quadrant and the cell edge intersects the origin of the axes. Bars of identical width have to be drawn in the cell area included between the cell edge and the x-axis, intersecting the cell edge in the middle of their top side. In this manner the sum of the areas of all the bars will approximate the area of this cell portion, while the cell volume is obtained by measuring each bar height and inserting it in the summation formula (see details in the ‘‘Morphological estimation’’ section). DOI: 10.3109/03014460.2013.807878 Human cell number 465 Therefore, since the mean weight of a mammalian cell has been estimated to be 1 ng (Makarieva et al., 2008), for a standard body weight of 70 kg (Irving, 2007), there would be 7 ! 10 13 cells. The mean volume of a mammalian cell has been estimated to be 4 ! 10 &9 cm 3 (Alberts et al., 2002) (i.e. 4 pL). The human body mean volume has been calculated by Nagao et al. (1995) by two different physical methods: the obtained values were 63.83 and 66.61 L. Considering the average value, the resulting cell number is 1.63 ! 10 13 cells. Moreover, con- sidering the body volume indicated by Irving (2007), equal to 60 ! 10 3 cm 3 , the total human cell number resulted to be 1.50 ! 10 13 (Figure 2). However, if a single, 90 fL volume, blood red cell were to be considered (Tønnesen et al., 1986), this would translate into a larger number of total cells, 7.24 ! 10 14 . On the other hand, the estimation of a mean volume of 6 pL for each endothelial cell and of a total number of 35 ! 10 12 human endothelial cells (Genest et al., 1983) would result in an over-estimated body volume of 210 L. Estimation of total cell number of specific systems, organs or cell types We present here results on the cell number of specific systems, organs or cell types obtained to date with our research. Some of the values regarding the cell number of whole human organs or cell type were obtained directly from an in- depth bibliographical search. Other data on total cell number of whole human organs or cell type needed integration of some or all methods described above. We report below these methods and results together for each of them (see Appendix B, Table B1; online only). Articular cartilages: Femoral condylar, humeral head and trochlear surface of talus cartilage total cell number Articular cartilage cell number was calculated for some main joints: the femoral condylar cartilage, the shoulder (humeral head) and the ankle (trochlear surface of talus). Femoral, humeral head and talus cartilage volumes have been reported at 2503 ' 568 mm 3 (lateral femur cartilage), 2770 ' 536 mm 3 (medial femur cartilage) (Baysal et al., 2004), 4200 mm 3 ' 1120 (Vanwanseele et al., 2004) and 3320 ' 550 mm 3 (Millington et al., 2007), respectively. Femoral, humeral head and talus cartilage cell densities have been reported at 14 100 ' 3200 cells/mm 3 , 14 600 cells/ mm 3 and 12 150 cells/mm 3 (Stockwell, 1971), respectively. Based on these data, the calculated cell numbers of two femoral condyles, two humeral heads and two talus cartilages were 1.49 ' 0.46 ! 10 8 , 1.23 ' 0.35 ! 10 8 , 8.06 ' 1.56 ! 10 7 , respectively. Biliary system: Gallbladder and biliary ducts epithelium total cell number The biliary system is composed of the gallbladder and biliary ducts (Borley, 2005a). We calculated the gallbladder internal surface (47.03 cm 2 ), by gallbladder inter nal volume (Irving, 2007) and size (Borley, 2005a) assuming that it has a prolate spheroidal shape. By dividing the estimated surface by the single cell basal surface (29.20 ' 4.21 mm 2 ), attained by mor phological estimation (as described in the Methods section) from an epithelium microimage (Wolf & Scarbrough, 2012), we obtained the total number of gallbladder epithelium cells to be 1.61 ' 0.23 ! 10 8 . Besides the columnar epithelium, gallbladder width is composed of a sub-epithelial stroma and complex smooth fibromuscolar layers. Sub-epithelial stroma and smooth fibromuscolar layers volumes were calculated by morphological estimation (Bergman et al., 2004). We calculated the interstitial Cajal-like cells (4.94 ' 0.05 ! 10 5 ) and other stromal gallbladder cells (8.48 ' 0.09 ! 10 6 ) starting from data reported by Hinescu et al. (2007). We then calculated the total myocytes cell number (considering the maximum length and the minimum diameter of each cell (Portincasa et al., 2004)) by dividing the smooth fibromuscolar layer volume (2.80 ' 0.46 ! 10 12 mm 3 ) by the single myocyte cell volume (1770 ' 350 mm 3 ), obtain- ing 1.58 ' 0.40 ! 10 9 cells. As for to the biliary ducts, we were able to estimate the total epithelial cell number of the extra hepatic ducts (7.03 ' 5.30 ! 10 7 ) by dividing the total surface of the ducts (Castelain et al., 1993; Khalil et al., 2005) by the cylindr ical epithelial cell basal surface. Bones: Osteocytes total number Since the total volume of bone tissue (BV) was not directly measurable, it was necessary to compute it star ting from total skeleton weight (TW), extrapolated from the literature (Trotter, 1954; Seale, 1959). The amount of cortical and trabecular tissue was also investigated (77% and 23%, respectively) (Malluche & Faugere, 1986). Based on Figure 2. Human total cell number comparison. *Total cell number calculated for the whole organ; **total cell number calculated only for some cell types of the organ. 466 E. Bianconi et al. Ann Hum Biol, 2013; 40(6): 463–471 micro-CT analysis, it was possible to obtain the average porosity (bone volume over total volume fraction, BV/TV) of both types of tissue and the correlation between bone porosity and ash density (Tassani et al., 2011), defined as the ratio between the bone weight and total geometrical volume of the specimen, including empty spaces of porosity. BV was computed as follows: Ash Density ¼ m ! BV=TV þ b; T s V ¼ TW ! Ash Density; and B s V ¼ BV=TV ! T s V where m is the slope of the linear regression (trabecular 1.04 g/cm 3 , cortical 1.15 g/cm 3 ) and b is the intercept (0.03 g/ cm 3 for both trabecular and cortical bone), R 2 ¼ 0.97 for trabecular bone and R 2 ¼ 0.91 for cortical bone (Tassani et al., 2011). BV/TV is measured as the average value for the trabecular (13 ' 5%) and cortical (89 ' 11%) bone. Knowing the average ash density (trabecular 0.16 ' 0.05 g/cm 3 , cor- tical 1.06 ' 0.14 g/cm 3 ) of the two types of tissue and their dry TW which is 3939 ' 471 g (906 ' 108 g trabecular and 3033 ' 363 g cortical) average from literature (Seale, 1959; Trotter, 1954), it is possible to compute total skel- etal volume (T s V, trabecular 5618.04 ' 1931.49 cm 3 , cor- tical 2916.85 ' 513.27 cm 3 ) and total bone skeletal volume (B s V, trabecular 722.56 ' 377.99 cm 3 , cortical 2602.17 ' 553.50 cm 3 ). The osteocyte (OCY) number estimated in a man less than 50 years old was expressed as a number for mm 2 of bone surface (trabecular 98.97 ' 1.24 cells/mm 2 cortical 56.34 ' 1.69 cells/mm 2 ) (Torres-Lagares et al., 2010). In order to compute a volumetric analysis assuming bi-dimen- sional cells distribution, a uniform distribution was hypothe- sized. Therefore, linear density was first obtained (trabecular 9.95 ' 0.09 cells/mm, cortical 7.51 ' 0.16 cells/mm) and finally the number of OCY for mm 3 was computed (trabecular 984.59 ' 21.37 cells/mm 3 , cortical 422.89 ' 21.97 cells/ mm 3 ). Consequently the OCY total numbers for whole bone tissue is 7.11 ' 3.72 ! 10 8 , trabecular and 1.10 ' 0.24 ! 10 9 , cortical. Bone marrow total cell number The number of bone marrow nucleated cells was 1.11 ! 10 10 (mean of 10 subjects, SD ¼ 5.25, count in rib sections) or 1.04 ! 10 10 (mean of the same 10 subjects, SD ¼ 3.36, count in crest aspirates) per body kilograms (Harrison, 1962). Using the average of the two values, we have an estimate of 7.53 ' 2.18 ! 10 11 cells for the reference weight of 70 kg (Irving, 2007). Liver total cell number Since hepatic volume is 1470 cm 3 (Irving, 2007), hepatocyte volume is 4900 mm 3 (Prothero, 1982) and the parenchimal cell percentage of the total liver is 80% (Borley, 2005b), we estimated the total hepatocyte cell number at 2.41 ! 10 11 . Total stellate cells (2.41 ! 10 10 ) were calculated as 1/10 of hepatocyte cells (Geerts, 2001), while total Kupffer cells (9.63 ! 10 10 ) were obtained as 4-times the total stellate cells (Dong et al., 2007). Therefore, the total cell number of the liver is 3.61 ! 10 11 . Nervous system: Glial cells total number Since the total glial cells in the nervous system are 10–50 times the neurons (Kandel et al., 2000; Standring et al., 2005) (for details, see Table B1 in Appendix B; online only), their number is estimated as 3.00 ' 0.66 ! 10 12 . Pancreas: Islet total cell number The total number of islet cells (2.95 ' 0.78 ! 10 9 ) was calculated by multiplying the cell number per islet 1.56 ' 0.02 ! 10 3 (Pisania et al., 2010) and the mean number of islets in the human pancreas equal to 1.89 ' 0.5 ! 10 6 (Meier et al., 2008). Skin: Epidermal and dermal total cell number We found the density of corneocytes, epidermal nucleate cells, Langerhans cells and melanocytes in Hoath and Leahy (2003). We obtained the relative total cell number by dividing the standard human body surface (1.85 m 2 ) (Irving, 2007) by the specific density, to obtain an overall number of 1.76 ' 0.44 ! 10 11 cells. Epidermal Merkel cells resulted 3.62 ! 10 9 as 0.20–5.00% compared to the total epidermal cells (Boulais & Misery, 2007). By dividing the human body surface by the specific densities of fibroblasts (Randolph & Simon, 1998) and mast cells (Grimbaldeston et al., 2000) we obtained an overall of 1.85 ' 0.26 ! 10 12 dermal cells. Therefore, the total dermal and epidermal cell number were found to be 2.03 ' 0.30 ! 10 12 (for details, see Appendix B, Table B1; online only). Small intestine: Jejunum and ileum enterocytes total number To calculate the surface of the jejunum and the ileum of the small intestine (0.35 m 2 ), we considered them as two cylin- ders. Their lengths and diameters are 2.00 m and 2.50 cm (jejunum) and 3.00 m and 2.00 cm (ileum) (Borley, 2005c). Then, considering the valvulae conniventes, the total surface (1.04 m 2 ) increases 3-fold (Teodori 1987). By an exact graphyc reproduction (Cattaneo & Baratta, 1989), we estimated 1000 ' 3.40 cells per villus. Knowing that there are 13 ' 1 villi/mm 2 , the total number of cells covering them is estimated at 1.35 ! 10 10 . The total number of cells covering the cryptae is 3.24 ' 0.14 ! 10 9 if we consider that they represent 12/50 of the cryptae covering each villus (Weiss & Greep, 1981; Cattaneo & Baratta, 1989) and that the estimated number of cryptae and villi is roughly the same (Cattaneo & Baratta, 1989). Therefore, the total enterocyte cell number was calculated to be 1.67 ' 0.71 ! 10 10 . Supradrenal gland total cell number Normal mean adrenal gland volumes have been reported to be 5.70 (SD ¼ 4.90 and 1.90 for the right and left gland, respectively) in a study that considered a group of 52 men with an average age of 48.4 years (Geraghty et al., 2004). Cortex and medulla zones represent 90% and 10%, respect- ively, of the total gland volume. Reticularis, fasciculata and glomer ularis zones represent 7%, 78% and 15%, respectively, of the cortex total volume (Martini 1994). Based on these data, we calculated the volumes of the reticularis (0.36 cm 3 ), DOI: 10.3109/03014460.2013.807878 Human cell number 467 fasciculata (4.00 cm 3 ), glomerularis (0.77 cm 3 ) and medulla (0.57 cm 3 ) zones. Cell volumes of the fasciculata, glomerularis and medulla zones were 1200, 870 and 970 mm 3 , respectively (Bocian-Sobkowska et al., 1997). These data referred to new- born humans, but cell size does not change in this organ during lifetime (Staton et al., 2004). We calculated the radius (6.25 mm) of the reticularis zone cells, described as round shaped (Borley, 2005d), by a morphological analysis of a microimage (Hui et al., 2009) and then the volume (1023 mm 3 ). Finally, by dividing the single zone into the specific cell type volumes, we obtained the mean of the total cell number of the single zones, reported in Table B1. The total cells of the two supradrenal glands are 1.66 ' 0.16 ! 10 10 (for details, see Appendix B, Table B1; online only). Vessels: Total endothelial cell number The endothelium is composed of roughly 50–70 mm long and 10–30 mm wide cells, which cover the internal layer of the vessel (Fe ´ le ´ tou, 2011). To calculate the total endothelial cell number, we used two different approaches to estimate the vessel’s internal surface and we then divided it for a single cell area. For systemic capillaries, superior and inferior vena cava, we considered their length as 8.00 ! 10 9 cm (Loe & Edwards, 2004), 7.00 cm (Testut & Latarjet, 1959; Johnson et al., 2005), and 23.50 cm (Testut & Latarjet, 1959), respectively, and their average diameter as 7.50 ! 10 & 4 (Loe & Edwards, 2004), 3.00 cm (Ganong, 2006; Germann & Stanfield, 2006), respectively. For the other vessels, we estimated their length by using the blood volume occupying the vessels, their diameter and assuming vessels were cylindrical (Ganong, 2006; Germann & Stanfield, 2006; Guyton & Hall, 2002; Mountcastle, 1980; Rhoodes & Planzer, 2004; Testut & Latarjet, 1959). The calculations led to a total number of 2.54 ' 1.05 ! 10 12 endothelial cells. Current estimation of human total cell number Our current estimation of human total cell number was calculated only on a variety of organs and cell types, as listed in Appendix B, Table B1 (online only). These partial data correspond to a total number of 3.72 ' 0.81 ! 10 13 (Figure 2). This number invalidates previous references, at least those indicating 10 12 as the human total cell number and creates the starting point for a complete work that considers all the components of the human body. Discussion The estimation of a reference cell count for the human whole body was revealed to be not a trivial task. The results of our bibliographical research show considerable variation in terms of order of magnitude, which is not justifiable by inter- individual differences (see Appendix A, Table A1; online only). Furthermore, we noted a consistent lack of citations of the original literature providing the pertinent data in printed books, online available books and websites. This is not totally unexpected, following our difficulties in retrieving articles discussing this specific issue by systematic searches in the PubMed database. General estimations based on mean cell volume or weight have not proved to be reliable, due to the high variability in cell size, volume and weight among different cell types and, in turn, due to high variability in cellularity for each cell type in the human organism. The most reliable way to determine the human number of cells seems to be to sum the cell counts for individual organs. This also presents the advantage of providing valuable reference data for the study of each part of the human organism. Data about static, kinetic and pathological cell counts for organs and systems have been produced in the context of a wide variety of fields in biology and medicine. We have shown here that in many cases a precise count may be obtained from the literature. However, several problems have emerged while conducting this analysis. First, a major difficulty was estimation of the number of stromal and accessory cells in tissues and organs for which, instead, an accurate count of parenchymal cells has been performed. Secondly, it was difficult to obtain data for diffuse systems, e.g. vessels or nerves, both in their different section and in their global dimensions. In fact, a systematic survey of the whole biomedical literature is not a trivial exercise, because the pertinent data may have often have been disclosed in the context of a particular study. Finally, different sources of variability in cell number may hamper estimation of a general reference number. Some organ or tissue cellularity varies in function of sex, age or evolutionary adaptation, not only because of a pathological condition. For instance, erythrocyte cell count differs in males and females as well as during pregnancy and in populations adapted to high-altitudes. However, this appears to be a minor problem because it is expected that these variations do not reach an order of magnitude and a mean estimation remains a reasonable end-point for this research. As reported in our current partial estimation of total cell number of the ‘‘average man’’ (Irving, 2007) we have an SD equal to 0.81 ! 10 13 , which is less than one order of magnitude. We believe that knowledge of the cellularity of organs and total body would be not only be culturally important but it may also have biological and medical relevance, as demon- strated by some key applications described below. A quantitative model of development should explain how it is possible to sustain a proliferation rate able to lead to a whole human organism starting from a single cell. The wide fields of cell growth and stem cell proliferation are potentially included in this problem. Modelling of whole organs or systems needs cell number data in order to produce afford- able physiological views from the cells to the whole body. For example, a recent report of ion transport by pulmon- ary epithelia (Hollenhorst et al., 2011) cited quantitative available data about the alveolar surface and cell number (Crapo et al., 1982). The determination of an organ or a tissue cell number is relevant in medicine for diagnostic and prognostic proced- ures. In fact, cellularity of biopsies leads to assessment of specific pathologic states due to cell number deregulation. For example, estimation of total number of hepatocytes in cir rhotic patients gave a mean value of 1.72 ! 10 11 (Imamura et al., 1991) in comparison with the mean value of 2.40 ! 10 11 reported by us for a healthy organ. In the case of blood cell counts, accessibility of tissue and standardization of method- ology has led to quantitative and qualitative characterization 468 E. Bianconi et al. Ann Hum Biol, 2013; 40(6): 463–471 of cell types and sub-types. In the same way, it would be important to know the cell number of other tissues and organs, along with the mechanisms regulating it, which could be crucial in understanding the biology and kinetics of cancer (Albanes & Winick, 1988), as well as the development of many other human diseases. For instance, dysregulation of organ size control has been implicated in diabetes and hypertrophy (Yang & Xu, 2011) and trisomy 21, where the relatively greater volume of the cell population of heart, skeletal muscle, liver and brain is only a partial compensation for the smaller cell number and not strong enough to produce normal organ mass (Landing & Shankle, 1995). Moreover, estimation of cellularity, organ and cell size of diseased organs would benefit modern strategies of human biological and disease research. In fact, the size of some organs can decrease as a result of malnutrition and disease during gestation or critical growth periods. For example, it has been proposed that the number of adult nephrons may be determined during renal development in utero and may be related to foetal malnutrition (Barker, 1995; Lackland, 2005). Finally, from an evolutionary point of view, data about cell number in different species could allow a measure of organism and organ complexity valuable in order to classify and understand inter-species variability at phenotypic level (Herculano-Houzel, 2011; Kothari et al., 1978). The formal description of cell types and number of different organisms could be valuable, in conjunction with other measures of complexity such as gene number and function (Szathmary et al., 2001), in order to develop better indices quantitatively related to the ‘‘complexity’’ of an organism (Grizzi & Chiriva-Internati, 2005). Since we believe that the total human cell number may be obtained by summing all data related to the single organs and that the systematic work on each human section is complex and requires a meticulous work, we believe that this issue is an ideal candidate for a collaborative effort. The problems outlined here could stimulate attainment and publication of useful data worldwide, in order to provide a systematic organ-by-organ view of human organism cellular- ity and a final detailed estimation of the total cell number in a standard human adult. Ideally, the last step could be preparation of a forum paper and an on-line database resource summarizing the complete picture by integrating data from different expert contributors. With this aim, we release our current estimations (Figure 2) so that they are available for correction, integration and completion from any interested researcher. These final results could be, in future develop- ments, integrated into or related to formal description of anatomical entities provided by ongoing attempts to formalize anatomical data through the creation of ontologies (Baldock & Burger, 2005; Hayamizu et al., 2005). Conclusions We have shown the importance and the usefulness of searching for a reference number of the cells present in a human body, providing hypotheses for a solution of the problem. Our current estimation of 3.72 ! 10 13 cells for a variety of organs and cell types is higher than some estimations found in the literature. We believe that our initial reference table for cell number in the human body, when completed, possibly with a common effort, will have many useful applications in all biomedical fields needing quantita- tive measurement in order to build structural, functional, pathological and comparative models of human organs and of the whole body. Acknowledgements The authors wish to thank Paolo Comeglio and Lucia Mancini for helping with the revision of the manuscript. Declaration of interest The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper. 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The corrected version is shown in this issue. DOI: 10.3109/03014460.2013.807878 Human cell number 471 A more precise method than the gross estimation would be to sum the cell counts for individual organs. This method has another advantage which is to provide valuable reference data for the study of each part of the human organism. The determination of the number of cells in the human body is determined with higher accuracy using this method. Robert Hooke began the scientific study of cells, known as cell biology in the 17th century. In 1839 Matthias Schleiden and Theodor Schwann introduced the idea that cells are the fundamental unit of life - cell theory. Cell theory states that living organisms are made up of cells, that they are the basic structural unit of all organisms, and that all cells come from pre-existing cells. ***Cells are the basic unit of structure in all organisms and also the basic unit of reproduction.*** !["cell theory"](https://i.imgur.com/2RY1CD2.png) *Robert Hooke's microscope* General estimations based on mean cell volume or weight have not proved to be reliable because of a high variability in cell size, volume and weight among different cell types. A ***gross estimation*** of the total human cell number was obtained by dividing the mass/volume of a reference adult human body by the mass/volume of an average human cell. The final result was for the total number of cell in the human body was \sim 10^{13} cells. Their final estimate for the total number of human cells was: 3.72 \pm 􏰃 0.81\times􏰁 10^{13}  Results for the total number of cells in the human as given by the different methods discussed in this paper. #### TL;DR How many cells are in your body? A simple question deserves a simple answer. Unfortunately things are not that simple. The reported total cell number of a human being ranges between $10^{12}$ and $10^{16}$ and it is widely mentioned without a proper reference. This paper discusses the theoretical issue of the total number of cells that compose the standard human adult organism and provides a systematic calculation of the total cell number of the whole human body and of the single organs. The most reliable way to determine the human number of cells seems to be to sum the cell counts for individual organs. The authors found a total number of $3.72 \times􏰁 10^{13}$ Yeasts are single-celled, eukaryotic - organisms whose cells have a nucleus enclosed within membranes - microorganisms classified as members of the fungus kingdom !["yeast"](https://i.imgur.com/XJJaAnU.png) ***Cell number might vary depending on gender, body height, body mass etc... *** There are differences but these variations are not greater than one order of magnitude. > Since the cell number and cell size of various organs or systems, as well as the size of the organ or system itself, may vary according to several parameters such as age, sex, weight, pathology or evolutionary adaptations, we searched for a single reference for the ***‘‘average man’’ or ‘‘standard human being’’.*** Since this does not exist, for the purposes of this work ***we chose a 30-year old young adult, weighing 70kg, 1.72m tall and with a body area of 1.85 $m^2$*** A ***gross estimation*** of the total human cell number was obtained by dividing the mass/volume of a reference adult human body by the mass/volume of an average human cell. The final result was $\sim 10^{13}$ cells.