Chapter 18

Development

Fertilization, Cleavage, and

Implantation

• Sperm and egg unite: 6-24 hours after

intercourse, sperm survives days in female

reprod. tract

• One sperm fertilizes one egg

– Occurs in upper oviduct

– Acrosome: contains digestive enzymes

– Sperm digests thru follicle cells and zona pellucida

– Sperm membrane fuses with egg membrane

– Zona pellucida made impermeable to other sperm

Fertilization

How many sperm normally fertilize a

single egg?

A. 1

B. 2

C. 4

D. More than a million

• Cleavage involves: cell division without cell

growth, up to about day four

– Function is to restore cell size to normal (i.e.

smaller than the egg cell)

– Produces a “morula” – a solid ball of cells

Early Development

Blastocyst

• Inner cell mass cells form embryo

proper

– Source of embryonic stem cells

• Outer (trophoblast) cells form placenta

– Secrete hCG (human chorionic

gonadotropin) after implantation

– Extends life of corpus luteum

– Basis of pregnancy test

Implantation

Implantation

The cells of the blastocyst that produce

the embryo proper are called:

A. Zona cells

B. Trophoblast cells

C. Inner cell mass

D. Amnion

Gastrulation

• Cells migrate to produce a three layered embryo

• Ectoderm: outermost layer

• Forms nervous system, skin, etc.

• Mesoderm: middle layer

• Muscle, bones, blood, gonads, etc.

• Endoderm: innermost layer

• Viscera, gut, etc.

Neurulation Forms Neural Tube

• The neural tube forms the CNS (brain &

spinal cord)

• Inductive interactions between tissues

– Chemical signals influence development of

nearby tissue

– Ex: Neural tube induced by notochord

Somites: form bone & muscle

The neural tube develops into:

A. The digestive tube

B. Reproductive tract

C. Brain and spinal cord

Extraembryonic Membranes and

Placenta

• Extra-embryonic membranes

– Amnion: lines amniotic cavity, filled with

amniotic fluid

– Chorion: makes human chorionic

gonadotropin (hCG)

Placenta

• Functions

– Nutrient exchange with mother

• Umbilical cord connects fetus to placenta

• Filters nutrients, waste, and provides

antibodies for fetus

• No mixing of mother and fetal circulations

– Endocrine organ: produces hormones

• hCG

• Estrogen and Progesterone

The following are functions of the

placenta

A. Nutrient exchange with the mother

B. Produces hormones required for

pregnancy

During the first stage of labor, the cervix opens (dilates) and thins out (effaces) to allow

the baby to move into the birth canal. In figures A and B, the cervix is tightly closed. In

figure C, the cervix is 60 percent effaced and 1 to 2 cm dilated. In figure D, the cervix is

90 percent effaced and 4 to 5 cm dilated. The cervix must be 100 percent effaced and 10

centimeters dilated before a vaginal delivery.

Factors That Contribute to Labor

• Estrogen: Progesterone ratio

– High P promotes uterine quiescence (early

pregnancy)

– High E promotes uterine contractility (late

pregnancy)

• Oxytocin promotes uterine contractions

via neuroendocrine reflex

Lactation

• During Pregnancy:

– High E and P during pregnancy promote

breast development but inhibit milk prodn.

• After Childbirth:

– E & P levels plummet

– Prolactin promotes milk synthesis

– Oxytocin induces milk ejection via

neuroendocrine reflex

Environmental Disruptions During the

Embryonic Period Cause Major Birth

Defects

Lower limbs

Spina bifida

Spina bifida baby