List of human cell types

The list of human cell types provides an enumeration and description of the various specialized cells found within the human body, highlighting their distinct functions, characteristics, and contributions to overall physiological processes. Cells may be classified^[1] by their physiological function, histology (microscopic anatomy),^[2] lineage, or gene expression.

Total number of cells

The adult human body is estimated to contain about 30 to 40 trillion (4×10¹³) human cells,^[3]^[4]^[5] with the number varying between 20 and 100 trillion depending on factors such as sex, age, and weight. Additionally, there are approximately an equal number of bacterial cells. The exact count of human cells has not yet been empirically measured in its entirety and is estimated using different approaches based on smaller samples of empirical observation.^[6]^[7]^[8]^[3]^[9]^[10]^[4]^{[excessive citations]} It is generally assumed that these cells share features with each other and thus may be organized as belonging to a smaller number of types.^[11]^[12]

Classification

As a definition of "cell type" is yet to be agreed, it is not possible yet to arrive at a precise number of human cell types.^[13] There is, for example, significant variation in these cell types depending on the specific surface proteins they possess.

An extensive listing of human cell types was published by Vickaryous and Hall in 2006, collecting 411 different types of human cells, including 145 types of neurons.^[12]

The Human Cell Atlas project, which started in 2016, had as one of its goals to "catalog all cell types (for example, immune cells or brain cells) and sub-types in the human body".^[14] By 2018, the Human Cell Atlas description based the project on the assumption that "our characterization of the hundreds of types and subtypes of cells in the human body is limited", but the word hundreds was removed in later versions.^[15]^[14]

On 2021, Stephen Quake guessed that the upper limit of the number of human cell types would be around 6000, based on a reasoning that "if biologists had discovered only 5% of cell types in the human body, then the upper limit of cell types to discover is somewhere around 6000 (i.e., 300/0.05)."^[11]

Other different efforts have used different numbers. A count of cells in the human body published in 2023 divided the cells in about 400 types to perform the calculation.^[7]

The main cellular components of the human body by count^[3]
	Cell type	% cell count
	Red blood cells (Erythrocytes)	84.0
	Platelets	4.9
	Bone marrow cells	2.5
	Vascular endothelial cells	2.1
	Lymphocytes	1.5
	Hepatocytes	0.8
	Neurons and glia	0.6
	Bronchial endothelial cells	0.5
	Epidermal cells	0.5
	Respiratory interstitial cells	0.5
	Adipocytes (fat cells)	0.2
	Dermal fibroblasts	0.1
	Muscle cells	0.001
	Other cells	2.0

In 1996, scientists revealed a 'map' of 16,000 human genes.^[16] This led to estimates that humans likely had around 100,000 genes^[17] (or regions that code for human proteins). However, actual sequencing did not start before around 1999, and it was not until 2003^[18] that the first complete draft of a human genome revealed that there were roughly 20,000-25,000 protein-coding genes, as most DNA does not code for any protein. It is difficult to say that there have not been similar mistakes when estimating how many cells humans have as there are still substantial gaps in understanding human cells.^{[citation needed]}

Efforts to characterize all human cell types

Several efforts have been made to make a list or a map of all human cells.^[19]^[20]^[21] One of the largest and most recent is the HuBMAP (Human BioMolecular Atlas Program).^[22]

The HuBMAP project has organized 1551 different samples in 17 collections, each dedicated to a different system. However, this project still only mapped about 31 of the human body's 70 organs. Their datasets and visualisations place great emphasis on biomarkers and location in the body, but less on cell development and how cells can change over time. Usually specific surface proteins are used to identify cells, and based on this they are put into different categories.

Another major effort to make an overview of these proteins that allows us to observe cell types is the Human Protein Atlas.^[23]

A similar project, the Human Brain Project has also attempted to map the human brain, although much of the publicly accessible model does not have cellular resolution.^[24]^[25]

Standards and naming conventions

So far not all cells which can be found in the human body have been documented. There is no good way to make the experiment where one checks if all cell types identified so far could be taken from and measured in a single donor, proving that the cell types are universal to all humans. This is partly due to a lack of standards, as scientists are still not entirely sure what is needed to measure, in order to capture every cell type which can be found.^{[citation needed]}

Some attempts have been made – and some are still in progress– for creating standards for identifying cells consistently.^[26]^[27]^[28] The Cell Ontology provides arguably the most comprehensive metadata standard to date, cataloging over 2500 cell classes and being used actively by the Human Cell Atlas community.^[29]

There is still no standard which is used industry wide, nor any definitions which have been accepted by the wider scientific community, often making it difficult to say whether some collected and observed cells are really one or multiple types of cells. This lack of standards makes it difficult to estimate how many cell types and how many cells of each type can be found in the human body, as well as difficult to predict which young cells one would need to develop with mature cells. The list in this article also contains inconsistencies due to multiple sources using different conventions.^{[citation needed]}