Pregnancy Lasts from
Fertilization to Delivery
LEARNING OBJECTIVES
1.
Explain
the major events in embryonic and fetal
development.
2.
Describe
the differences between fetal circula-
tion and normal human circulation.
ro m th e tim e fe r tiliz a tio n o cc u r s to d e liv e r y
o f a baby, a ty p ic a l p r e g n a n c y la sts a b o u t 38
w e e k s. H o w ev e r, th e d u e d a te is c a lc u la te d
fro m th e first d a y o f th e m o th e r ’s la st m e n -
str u a l p e r io d — a d o c u m e n ta b le tim e o f th e c y c le — w h ic h
o c c u r r e d 2 w e e k s p rio r to fe r tiliz a tio n . D u r in g th is tim e o f
d e v e lo p m e n t, th e fe r tiliz e d e g g d iv id e s a n d g ro w s fro m 1
c e ll to a p p r o x im a te ly 1 tr illio n c e lls . T h e c e lls d iffe r e n ti-
a te in to tis s u e s a n d o rg a n s as th e em b ry o d e v e lo p s a h u -
m a n sh a p e a n d a p p e a r a n c e .
M o st o f th e c h a n g e s occu r w ith in th e
embryonic peri-
od,
w h ich la sts from fe r tiliz a tio n th r o u g h 8 w ee k s. A fte r 8
w e e k s, th e em b ryo is co n sid er ed to b e a fetu s. B y th e early
p art (8 to 12 w e e k s) o f th e
fetal period,
th e organ s h ave fo rm ed
an d d iffe r e n tia te d . G ro w th is th e m ajor ev e n t d u rin g th e re -
m a in d e r o f th e fe ta l p eriod ; n o n ew c h a n g e s in sh a p e or or-
g a n s occur, b u t th e le n g th o f th e fe tu s in c r e a se s 12-fold , w ith
sig n ifica n tly g r e a te r c h a n g e s in w e ig h t. B y ab ou t 33 w ee k s,
th e fe tu s is
viable
(can live o u tsid e th e m o th e r ), so th e p h y si-
o lo g ic a l e v e n ts o f lab or ca n b e g in an d th e b a b y ca n b e born .
L e t ’s ta k e a c lo se r lo o k at th e e v e n ts o f em b r y o n ic d e -
v e lo p m e n t.
During the Embryonic Phase,
the Embryo Changes from a Single
Cell to a Differentiated Organism
T h e first s te p in d e v e lo p m e n t is
fe r tiliz a tio n , w h ic h o ccu rs in th e
u te r in e
tu b e s
( F ig u r e
1 6 . 1 2 ).
T h e fe r tiliz e d e g g is n o w c a lle d a
z y g o t e a n d c o n ta in s 4 6 c h r o m o -
so m e s: 23 fro m th e o v u m a n d 23
fro m th e sp er m .
W e e k 1
o f d e v e l o p m e n t .
In th e first tw o d a y s, th e
z y g o te im m e d ia te ly u n d e r g o e s ra p id m ito tic c e ll d iv isio n s
3.
Identify
the hormones associated with preg-
nancy and their functions.
o f th e z y g o te c a lle d c le a v a g e , fo r m in g n u m e r o u s c e lls
th a t r e s e m b le th e o r ig in a l c e ll. (R e fe r to C h a p te r 3 for
a d isc u ssio n o f m ito s is.) T h e p r o g r e ssiv e ly s m a lle r c e lls
p r o d u c e d b y c le a v a g e a re c a lle d
b la s t o m e r e s . T h r o u g h su c c e ssiv e
d iv isio n s w ith in th e first w e e k , it
b e c o m e s a m o r u la . T h e m o r u la is
su r r o u n d e d b y th e z o n a p e llu c id a ,
a c le a r g ly c o p r o te in layer. B y th e
en d o f th e fo u r th day, th e n u m b e r
o f c e lls in th e m o r u la in c r e a s e s as it c o n tin u e s to m o v e
th r o u g h th e u te r in e tu b e to w a rd th e u te r in e cavity.
W h e n th e m o r u la e n te r s th e u te r in e c a v ity on d a y 4
or 5, g ly c o g e n -r ic h s e c r e tio n fro m th e g la n d s o f th e e n -
d o m e tr iu m p e n e tr a te th e m o r u la
a n d r e o r g a n iz e th e c e lls a ro u n d
a la r g e , flu id -fille d ca v ity c a lle d a
b la s t o c y s t c a v ity . T h e d e v e lo p in g
m a ss is n o w c a lle d a b la s t o c y s t .
T h o u g h it h a s h u n d r e d s o f c e lls ,
th e b la sto c y st is a b o u t th e sa m e
siz e as th e o r ig in a l z y g o te . T h e
b la sto c y st r e m a in s fr e e w ith in th e
u te r in e ca v ity for 1 -2 d ays b e fo r e
it a tta c h e s to th e u te r in e w a ll.
T h e b la sto cy st b e g in to re a r r a n g e in to tw o d istin c t
stru ctu r es: th e in n e r c e ll m a s s is lo c a te d in te r n a lly an d
e v e n tu a lly d ev elo p s in to th e em b ry o w h ile th e t r o p h o b la s t
(
TROF-o-blast
) is an o u te r su p e rficia l la y er o f c e lls th a t
fo rm s th e w a ll o f th e b la sto m e r e . It w ill u ltim a te ly d ev elo p
in to th e fe ta l p o r tio n o f th e p la c e n ta , th e site o f e x c h a n g e o f
n u tr ie n ts a n d w a ste s b e tw e e n th e m o th e r an d fe tu s.
H o w ev er, th e b la sto c y st is n o t sa fe u n til it n e s tle s in to
th e u te r in e lin in g , a b o u t d a y 6 a fte r fe r tiliz a tio n , d u r in g
th e se c o n d w e e k , in a p r o c e ss c a lle d im p l a n t a t i o n (se e
F ig u re 1 6 .1 2 ). A s th e b la sto c y st im p la n ts , th e in n e r c e ll
m a ss is o r ie n te d to w a rd th e e n d o m e tr iu m .
z y g o t e
(ZI-g-ot) The
single cell resulting
from the union of male
and female gametes;
the fertilized ovum.
m o r u la
(MOR-u-la)
A solid sphere of cells
produced by succes-
sive cleavages of a
fertilized ovum about 4
days after fertilization.
b la s t o c y s t
(BLAS-
to-sist) In the develop-
ment of an embryo, a
hollow ball of cells that
consists of a blastocyst
cavity (the internal
cavity), a trophoblast
(outer cells), and an
inner cell mass.
488 CHAPTER 16
The Reproductive Systems
previous page 523 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online next page 525 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online Home Toggle text on/off