Chapter 24: Development

Chapter 24.1: Embryonic Period

First Week of Development

- Fertilization- Can occur during a 3-day window

- Early Embryonic Development- Rapid cell division- Development of inner cell mass and

trophoblast- Blastocyst implants into uterine wall

Frontal plane

1. Fertilization(occurs within uterine tube 12–24 hours after ovulation)

2. Cleavage(first cleavage completed about 30 hours after fertilization) 3. Morula

(3–4 days after fertilization)

4. Blastocyst (4 1/2–5 days after fertilization)

Uterine cavity

5. Implantation (occurs about 6 days after fertilization)

UterusEndometrium

Myometrium

OvulationOvary

Frontal section through uterus, uterine tube, and ovary

Second Week of Development

- Trophoblast penetrates uterine wall and continues to grow

- Human chorionic gonadotropin (hCG) sustains progesterone and estrogen secretion

- Inner cell mass develops into amniotic cavity and yolk sac

- Extraembryonic mesoderm begins placenta formation

Amniotic cavityFormation of yolk sac

Extraembryonic mesoderm

Third Week of Development

- Gastrulation: formation of three primary germ layers- Ectoderm- Mesoderm- Endoderm

- Mesodermal cells develop into notochord- Neural tube becomes brain and spinal cord

Third Week of Development

Umbilical vein

Umbilical arteries

Maternal blood

Amniotic cavity

Embryo

Yolk sac

Fourth-Eighth Weeks- All major organs and body systems begin to

develop- Embryo triples in size and becomes three-

dimensional (embryonic folding)

20-day embryo 24-day embryo

32-day embryo 44-day embryo 52-day embryo

Chapter 24.2-Fetal Period

Fetal Development

- Very few new structures appear and rate of body growth is very fast

- Fetus is less vulnerable to drugs, radiation, and microbes

10-week fetus 13-week fetus 26-week fetus