HUMAN BIOLOGY

n Like all animals, humans begin life as a single cell from

which tissues and organs soon begin to develop.

n Links to Organ systems 4.8, How eggs and sperm form

16.2, 16.4, Fertilization 16.6

after fertilization, the zygote soon

becomes a ball of cells

Section 16.6 described how fertilization produces a zygote,

the first cell of a new individual. Within a day or two after

fertilization, cell divisions convert the zygote to a ball of

cells (Figure 17.1). This process is

called cleavage. It occurs while the

zygote is traveling down the ovi-

duct toward the uterus. By the time

it reaches the uterus, the zygote is

a cluster of sixteen cells called a

morula (moe-roo-lah), from a Latin

word for mulberry.

Each new cell that forms during

cleavage is called a blastomere.

Each one ends up with a differ-

ent portion of the egg’s cytoplasm.

Which bit of cytoplasm a blastomere

receives helps determine the devel-

opmental fate of cells that arise from

it later on. For example, its fate may

be to become the forerunner of some

kind of nervous tissue or perhaps

of epithelium.

Three primary tissues are the first to form

After cleavage comes gastrulation (gas-tru-lay-shun), a

process that rearranges the morula’s cells. It lays out the

basic organization for the body as cells are arranged into

three primary tissues, the germ layers (Table 17.1). The

outer one is called ectoderm, the middle one mesoderm,

and the inner one endoderm. Body tissues and organs will

develop from groups of cells in each layer.

Next, cells become specialized

In the germ layers, different genetic instructions begin

to operate in different groups of cells. This is the start of

cell differentiation. In simple terms, this process makes

cells “specialists” for a particular function. For example,

you may remember from Chapter 9 how various subsets of

T cells, each with a different function, differentiate in

the thymus.

There are about 200 types of differentiated cells in the

body of an adult. In each type, the genetic instructions

needed for its specialized function operate. For instance,

as your eyes developed, certain cells turned on genes for a

transparent protein called crystallin. These differentiated

cells then formed the lens of each eye. They are the only

body cells that make crystallin.

Although each type of differentiated cell has its own

particular genetic marching orders, a differentiated cell

still has all the genes that parents pass to an embryo.

Efforts to “reverse engineer” adult cells back into stem

cells, as described in the introduction to Chapter 4, rely on

this fact. It also is why scientists have been able to clone

adult animals—that is, to create a genetic copy—from some

types of differentiated cells. Chapter 21 discusses some

examples of cloning.

17.1 Overview of Early Human Development

F i g u r e 17.1 Cleavage in a zygote creates a ball of cells. In

the diagram of the first cleavage, on the left, the small spheres

are the polar bodies described in Section 16.2. They contain

unneeded material and will disintegrate. Cleavage produces a

morula by the fourth day after fertilization. (© Cengage Learning)

zygote after

first cleavage

morula

polar

bodies

Germ Layer Body Parts in an Adult

Ectoderm Nervous system and sense organs

Pituitary gland

Outer layer of skin (epidermis) and its

associated structures, such as hair

Mesoderm Cartilage, bone, muscle, and various

connective tissues

Cardio vascular system and blood

Lymphatic system

Urinary system

Reproductive system

Outer layers of the digestive tube and of structures

that develop from it, including parts of the

respiratory system

Endoderm Lining of the digestive tube and of structures

that develop from it, such as the lining of the

respiratory airways

Table 17.1 The Three Germ Layers and Tissues

and Organs That Form from Them

blastomere Cell that forms
during cleavage of a zygote.

cell differentiation Process
by which newly formed cells
become specialized for a
certain function.

cleavage Rounds of cell
division that transform a
zygote into a ball of cells.

gastrulation Process of
early development that pro-
duces the three germ layers.

germ layers The three pri-
mary tissues that form as an
early embryo develops. The
outer tissue is ectoderm,
the middle one is meso-
derm, and the inner one is
endoderm.

330 Chapter 17

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