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CHAPTER
Elaine N. Marieb
Katja Hoehn
28
PART A
Human
Anatomy
& Physiology
SEVENTH EDITION
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Pregnancy
and Human
Development
From Egg to Embryo



Pregnancy – events that occur from
fertilization until the infant is born
Conceptus – the developing offspring
Gestation period – from the last
menstrual period until birth
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From Egg to Embryo



Preembryo – conceptus from
fertilization until it is two weeks old
Embryo – conceptus during the third
through the eighth week
Fetus – conceptus from the ninth
week through birth
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Relative Size of Human Conceptus
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Figure 28.1
Accomplishing Fertilization

The oocyte is viable for 12 to 24 hours

Sperm is viable 24 to 72 hours

For fertilization to occur, coitus must occur no
more than:


Three days before ovulation

24 hours after ovulation
Fertilization – when a sperm fuses with an egg
to form a zygote
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Sperm Transport and Capacitation


Fates of ejaculated sperm:

Leakage

Low pH of vagina

Failure to reach cervix

Destroyed by phagocytosis/lost in cavity

Failure to reach tubes
Sperm must undergo capacitation before they can
penetrate the oocyte
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 28.2a
Completion of Meiosis II and Fertilization


Upon entry of sperm, the secondary
oocyte:

Completes meiosis II

Casts out the second polar body
Fertilization – when the pronuclei come
together
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Events Immediately Following Sperm Penetration
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Figure 28.3
Cleavage and Implantation

Cleavage – rapid mitosis of the
zygote following fertilization.
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Preembryonic Development



The first cleavage produces two
daughter cells called blastomeres
Morula – the 16 or more cell stage
(72 hours old)
By the fourth or fifth day the
preembryo consists of 100 or so cells
(blastocyst)
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Preembryonic Development

Blastocyst – a fluid-filled hollow sphere
composed of:
1. A single flattened layer of cells called
trophoblasts
2. An inner cell mass

Trophoblasts take part in placenta formation

The inner cell mass becomes the embryonic disc
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Cleavage: From Zygote to Blastocyst
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Figure 28.4
Implantation

6-7 day

trophoblast implants into the
endometrium – high levels of
estrogen and progesterone

2 layers implant in endometrium
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Implantation

blastocyst is covered over by
endometrial cells

completed by the fourteenth day
after ovulation
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Implantation of the Blastocyst
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Figure 28.5a
Implantation of the Blastocyst
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Figure 28.5b
Implantation



corpus luteum -secrete estrogen and progesterone
Chorion – developed from trophoblasts after
implantation, continues this hormonal stimulus
Between the second and third month, the placenta:

Assumes the role of progesterone and estrogen
production

Is providing nutrients and removing wastes
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Hormonal Changes During Pregnancy
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Figure 28.6
Placentation

Formation of the placenta from:

Embryonic trophoblastic
tissues

Maternal endometrial tissues

These form the chorion
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Placentation

The placenta is fully formed
and functional by the end of
the third month
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Placentation

Embryonic placental barriers
allow for passage of nutrients but
the blood does not intermix

The placenta also secretes other
hormones – human placental
lactogen, human chorionic
thyrotropin, and relaxin
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Placentation
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Figure 28.7a–c
Placentation
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Figure 28.7d
Placentation
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Figure 28.7f
Germ Layers

Inner cell mass produces 4
membranes during the first 2-3
weeks of development

Amnion, yolk sac, allantois,
chorion
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Embryonic Membranes

Amnion – a transparent membrane filled with
amniotic fluid

Provides a buoyant environment that
protects the embryo

Helps maintain a constant homeostatic
temperature

Amniotic fluid comes from maternal blood,
and later, fetal urine
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Embryonic Membranes

Yolk sac – a sac on the ventral surface of
the embryo

Forms part of the digestive tube

Produces earliest blood cells and
vessels

Is the source of primordial germ cells
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Embryonic Membranes


Allantois – a small outpocketing at the caudal
end of the yolk sac

Structural base for the umbilical cord

Becomes part of the urinary bladder
Chorion – helps form the placenta

Encloses the embryonic body and all other
membranes
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Gastrulation



During the 3rd week, the two-layered
embryonic disc becomes a three-layered
embryo – gastrulation
Primitive streak – raised dorsal groove
that establishes the longitudinal axis of the
embryo
The primary germ layers are ectoderm,
mesoderm, and endoderm
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Gastrulation

Notochord – rod of
mesodermal cells that
serves as axial support
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Primary Germ Layers



Ectoderm – forms structures of the
nervous system and skin epidermis
Endoderm – forms epithelial linings
of the digestive, respiratory, and
urogenital systems
Mesoderm – forms all other tissues
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings