Although estimates are necessarily approximate, the adult human body is thought to have somewhere in the region of 1013 to 1014 cells (a recent calculation is provided in Bianconi E et al. [2013]; PMID 23829164). In addition to our body cells, we each host our own personal microbiome, a diverse mixture of microbial organisms and viruses with a total of more than 10 times as many cells as in our bodies! The bulk of these cells are located in our intestines, and many of the bacterial cells are beneficial: some ferment complex indigestible carbohydrates, for example, and others synthesize various vita mins upon which we depend, including folic acid, vitamin K, and biotin.
While the average diameter of eukaryotic cells is ~10–30 μm, some specialized cells can grow much larger. Mammalian egg cells are ~100 μm in diameter but other eggs that store nutrients required for development can be much larger, such as an ostrich egg. Some cells are very long. Human muscle-fiber cells can extend as long as 30 cm, and individual human neurons can reach up to 1 meter in length.
Complex animals have many highly-specialized cells, and histology textbooks rec ognize over 200 different cell types in adult humans. Histology is a comparatively crude way of classifying cells, however, relying heavily on differences in cell size, morphology, and ability to take up certain stains. These limitations, plus the recognition that some of our cells are difficult to access and study, means that cell diversity has been grossly underestimated.
More recent molecular and functional studies indicate that human cell diversity is orders of magnitude greater than suggested by histological classification. Neurons, for example, are now known to be extremely diverse. Recent estimates suggest that there may be >10,000 types of human neuron and they are linked to each other by astonishingly complex connections (some individual neurons can be connected to 100,000 other neurons). Certain types of somatic DNA rearrangements are frequent during neurogenesis and may contribute to neuron diversity.
B and T lymphocytes display a special type of diversity. As they mature, they undergo cell-specific DNA rearrangements, so that individual B cells from a single person can produce different immunoglobulins, and similarly individual T cells can exhibit different T-cell receptors.
Variability in cell life span and turn over
The average life span of a human cell is thought to be of the order of 7–10 years, but life spans vary enormously according to cell type (Table 1). Some cells are very long-lived. At the other extreme are cells that live for only a few days or weeks, being replaced by new cells generated ultimately from stem cells. The cells with the shortest life spans are those that are continually subjected to external challenges and stresses and/or high workloads.
Table1. EXAMPLES OF LIFE SPAN/TURNOVER OF HUMAN CELLS
Different methods have been used to assess cell turnover (the rate at which cells die off and are replaced by new cells). Easily accessible blood and epithelial cells have been readily studied, but a more general approach involves administering labeled nucleotides and following the incorporation of the label into DNA as cells divide in animal models, such as mice (the approach is not used in humans for safety reasons, and short-lived mice are never going to be good models in this case). It has, however, been possible to measure the turnover of our cells using a retrospective carbon-dating procedure in humans (atmospheric 14C was introduced into the food chain at the time of extensive nuclear bomb testing from the late 1950s to the time of the test ban treaty in 1963; see the legend to Table 1).
Germ cells are specialized for reproductive functions
In multicellular organisms, development and growth are separated from reproductive functions. During growth and development there is a need for cell division to create increasing numbers of cells or to replace defective or worn-out cells by new cells. That is done by a standard type of cell division known as mitosis, in which a cell divides to give two daughter cells that are usually identical to each other and to the parent cell, and we will illustrate how the DNA replicates and is distributed equally between the daughter cells later in this chapter. However, sometimes during development and differentiation, cell division is asymmetric and results in two daughter cells with different properties (see Box 1).
Box1. ASYMMETRIC CELL DIVISIONS
Figure 2.12 Male and female germ-line development and gametogenesis. (A) Diploid primordial germ cells migrate to the embryonic gonad (the male testis or the female ovary) and enter rounds of mitosis that establish spermatogonia (in males) and oogonia (in females). (B) These undergo further mitotic divisions, growth, and differentiation to produce diploid primary spermatocytes and diploid primary oocytes, which can enter meiosis. (C) Meiosis I. After DNA duplication, the cells become tetraploid but then divide to produce two diploid cells. In male gametogenesis, the cell division is symmetrical, generating identical, diploid secondary spermatocytes. In female meiosis I, by contrast, the division is asymmetric; the secondary oocyte is much larger than the first polar body, which is discarded. (D) Meiosis II. The diploid secondary spermatocyte and secondary oocyte divide without prior DNA synthesis to give haploid cell products. In male gametogenesis, this division is again symmetrical, producing two haploid spermatids from each secondary spermatocyte. In female meiosis II, the egg produced is much larger than the second (also discarded) polar body. (E) Maturation produces four spermatozoa and a single egg.
A specialized population of germ cells is set aside to carry out reproductive functions; in evolutionary terms, the remaining somatic cells provide a vessel to carry these reproductive cells in order to achieve reproduction. In plants and primitive animals, ordinary somatic cells can give rise to germ cells throughout the life of the organism. However, in most of the animals that we understand in detail—insects, nematodes, and vertebrates, the germ cells are set aside very early in development as a dedicated germ line and represent the sole source of gametes.
The germ cells are the only cells in the body capable of meiosis, the specialized cell divisions that give rise to mature sperm and egg cells (allowing the genetic material to be transmitted from one generation to the next). In mammals, germ-line cells derive from primordial germ cells that are induced in the early embryo.
