HUMAN BIOLOGY

Development anD aging 331

What are the first steps in embryonic

development?

• Cleavage of the zygote into a ball of cells occurs shortly after
  fertilization.


• Gastrulation forms the three germ layers.


• Cell differentiation specializes cells for their final roles in the
  body. Tissues and organs form during morphogenesis.


• Each stage of early embryonic development builds on structures
  that were formed during the preceding stage.

taKe-Home message

Growth,

tissue

specialization

Organ

formation

Fertilization Cleavage Gastrulation

B A sperm and an

egg fuse at their

plasma membrane.

Then the nucleus of

one fuses with the

nucleus of the other

to form the zygote.

D Cell divisions,

migrations, and

rearrangements

produce two or three

primary tissues, the

start of specialized

tissues and organs.

E Subpopulations

of cells are sculpted

into specialized

organs and tissues

in spatial patterns

at prescribed times.

top view

F Organs

increase in size

and gradually

assume their

specialized

functions.

Gamete

formation

A Eggs form and

mature in female

reproductive organs.

Sperm form and

mature in male

reproductive organs.

C Cell divisions

carve up different

regions of egg

cytoplasm for

daughter cells.

F i g u r e 17. 2 An early embryo begins

to develop soon after fertilization.

This flow chart shows the stages

from fertilization to about 6 weeks.

For clarity, membranes surrounding

the embryo are not shown, and

several stages are shown in cross

section. (© Cengage Learning)

morphogenesis Process

by which specific tissues

and organs form.

A process called morphogenesis forms organs

Morphogenesis (“the beginning of form”) is the process
by which body tissues and organs form. Several factors
influence these changes. They include cell division in
certain areas and the growth and movement of cells and
tissues from one place to another.
For example, most of the bones of your face descended
from cells that migrated from the back of your head when
you were an early embryo. This migration takes place
in stages:

In a similar way, neurons in the

center of the developing brain creep

along parts of glial cells or axons of

other neurons until they reach their

final destination. Morphogenesis

also requires sheets of tissue to fold and certain cells to die

on cue. You will read about some other examples of these

events in Section 17.4.

Figure 17.2 summarizes the stages of development we

have been discussing. It is important to remember that by

the end of each stage, the embryo is more complex than it

was before. Normal development requires that each stage

be completed before the next one begins.

© Cengage Learning

A cell adheres to one point

and protrudes forward.

The protruding portion of

the cell attaches to a new

point.

The trailing portion of the

cell releases its grip.

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