C

cell structure and function The cell is the basic
structural and functional unit of all living organ-
isms. About 100 trillion cells make up one of the
most complex of such organisms, the human.

Types of Cells in the Human Body

There are three basic types of cells in the body:
stem cells, germ cells, and somatic cells.
The foundation of life: stem cells Stem cells are
the primal, undifferentiated cells that give rise to
all other cells. They are primarily abundant and
functional during early embryonic development
(embryonic stem cells). These are the cells of the
blastocyst, the earliest form of a new life, and at
this stage are totipotent: They have the ability to
become any other kind of cell. Genes instruct
dividing stem cells how to differentiate or form
specific kinds of cells that then develop into vari-
ous organs and body structures.

UMBILICAL CORD BLOOD STEM CELLS

The BLOODthat remains in the UMBILICAL CORD

and PLACENTAat birth is an abundant source of

multipotent BLOOD STEM CELLS. Cord blood trans-

plantation is an emerging treatment for LEUKEMIA

and other cancers as well as SICKLE CELL DISEASE

and other blood disorders. Many people now opt

to collect and store or donate the cord blood of

their newborns after birth.

As the body takes shape stem cells become
increasingly diffuse and specialized, transitioning
to pluripotent (able to become cells of distinct
body systems such as cardiovascular or gastroin-
testinal) and finally multipotent (able to become
cells of specific kinds, such as BLOODor BONE). The
most versatile stem cell that remains when devel-
opment is complete is the blood stem cell, which

has the ability to differentiate into various types of

blood cells throughout life. Other adult stem cells

(also called somatic stem cells to distinguish them

from embryonic stem cells) exist in most body tis-

sues though are interspersed among other cells.

Their role remains unclear though they appear

responsible for large-scale regeneration of tissue

such as can occur in the LIVER.

The cells of reproduction: germ cellsGerm cells,

also called gametes, are the cells of reproduction:

the OVAor eggs (female) and the spermatozoa or

SPERM (male). Gametes are haploid cells; each

GAMETEcontains one-half the complement of chro-

mosomes. When two gametes merge in CONCEP-

TION, the resulting ZYGOTE acquires the full

complement of genetic material.

The cells of the functioning body: somatic cells

All cells that are not stem cells or germ cells are

somatic cells. Somatic cells make up more than 99

percent of the cells in the adult body. They are

diploid cells; each somatic cell contains the full

complement of chromosomes. Somatic cells make

up the organs and structures of the body. They are

the body’s primary working units, responsible for

carrying out the myriad functions of METABOLISM

that support life. Though similar in structure and

function, somatic cells are broadly diverse in their

activities and specializations.

Cell Structure

Most cells have standard, key structural compo-

nents in common. These include

• PLASMAmembrane, the cell’s outer wall made
  up of a protein layer and a lipid (fatty) layer,
  that separates the cell’s contents from its exter-
  nal environment yet permits interaction
  between the cell and the external environment

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