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(1)
(2)

Gestation

(or pregnancy)

is the carrying of

developing young within the female reproductive tract

Human pregnancy averages:

280 days or

40 weeks or

9 months

(3)

The embryo and placenta take

shape during the first month of

pregnancy

Human development begins with

fertilization

in the oviduct creating a

zygote

Cleavage continues producing a solid ball of cells called a

morula

which then becomes a hollow ball of cells called a

blastocyst

whose

inner cell mass becomes the embryo.

The outer cell layer of the blastocyst called the

trophoblast or

blastoderm

will become implanted in the uterine wall & form part of

the placenta

Gastrulation

occurs and organs develop from the three embryonic

(4)

Figure 27.15ab

Trophoblast

Cavity

Uterine

cavity

Blastocyst

Cleavage starts

Fertilization

of mature

egg by

sperm

Oviduct

Secondary

oocyte

Ovulation

Ovary

Blastocyst

(implanted)

Endometrium

Uterus

Uterine

cavity

(5)

4 extraembryonic membranes

develop

amnion

surrounds the embryo and

forms a fluid-filled amniotic cavity that protects the embryo.

yolk sac

in reptiles, stores yolk, and

in humans, source of blood cells and initial circulation

allantois

contributes to the umbilical cord,

forms part of the urinary bladder, and in reptiles, stores embryonic waste.

chorion

contributes to the placenta and

(6)

Figure 27.15e

Embryo:

Endoderm

Mesoderm

Ectoderm

Chorionic

villi

Amnion

Chorion

(7)

Figure 27.15f

Placenta

Amnion

Amniotic

cavity

Mother’s

blood

vessels

Allantois

Yolk

sac

Chorionic

villi

(8)

Placenta – contains both

embryonic & maternal parts

Formed from chorion and trophoblast

mediates exchange of nutrients, gases, and the products of excretion

between the embryo and the mother.

consists of

chorionic villi

closely associated with the blood vessels of

(9)

Hormonal changes during

pregnancy

1

st

10 Weeks

HCG or human chorionic gonadotropin –

Secreted by developing embryo about

8-10 days after fertilization

thought to be cause of morning

sickness

detected in urine during pregnancy

tests

Progesterone-

secreted by corpus luteum

After 10 weeks

HCG

production stops (eliminating

morning sickness)

Progesterone

(10)

Figure 27.16-0

TRIMESTER 1

TRIMESTER 2

TRIMESTER 3

Gill pouches (primitive

gill-like structures) Placenta

Limb buds Tail Umbilical cord Amnion Umbilical cord Placenta

Most radical changes for mother and embryo; embryo particularly susceptible to radiation, drugs, alcohol.

The fetus continues to grow; its eyes can open, teeth form, and bones begin to harden. The placenta begins to secrete progesterone and stops secreting hCG. The corpus luteum degenerates.

The fetus grows rapidly and gains strength. The mother’s abdominal organs become squeezed, causing frequent urination, digestive troubles, and backaches. Babies born prematurely—as early as 24 weeks—may survive, but they require special medical care.

Timeline of Human Development Conception

Week 5: Highly organized multicellular embryo about 7 mm long, with notochord and coelom. Brain and spinal cord taking shape. Gill pouches will develop into parts of throat and ear.

Week 8: All major structures present in rudimentary form. The embryo is about 4 cm long. Fetus can move its limbs and head and make facial expressions.

Week 14: Fetus is about 6 cm long. Features have been refined, and the fetus now appears more human.

Week 20: The fetus is about 19 cm long, weighs about 0.5 kg, and has eyebrows, eyelashes, fingernails, and toenails and is covered with fine hair. It may be quite active and “kick.” The mother’s abdomen is markedly enlarged. With limited space, the fetus bends into the fetal position.

Week 40 (newborn): The circulatory system and

(11)

The induction of labor involves

positive feedback

Oxytocin and prostaglandins cause uterine

contractions

that in turn stimulate the release of

more

oxytocin

and prostaglandins.

The result is a steady increase in contraction

intensity

climaxing in forceful muscle contractions that

(12)

Figure 27.17a

from

ovaries

Induces oxytocin

receptors on uterus

Oxytocin

from fetus and

mother’s pituitary

Stimulates uterus

to contract

Stimulates

(13)

Childbirth is induced by hormones

and other chemical signals

The series of events that expel an infant from the uterus is called

labor

.

Hormonal changes induce birth.

Estrogen makes the uterus more sensitive to oxytocin.

Oxytocin acts with prostaglandins to initiate labor.

The cervix dilates to about 10 cm.

(14)

Labor occurs in three stages:

Latent

- dilation of the cervix,

Active

- expulsion, delivery of the

infant, and

Figure

Figure 27.15ab Trophoblast Cavity UterinecavityBlastocystCleavage startsFertilizationof matureegg bysperm Oviduct Secondary oocyte Ovulation Ovary Blastocyst (implanted)Endometrium Uterus Uterinecavity
Figure 27.15e Embryo: Endoderm Mesoderm Ectoderm Chorionic villi AmnionChorion Allantois Yolk sac
Figure 27.15f Placenta Amnion Amniotic cavity Mother’s  blood
Figure 27.17a from ovaries Induces oxytocin receptors on uterus Oxytocin

References

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