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Ch28: Human Development. Explain the stages of development starting with fertilization and ending with the neonatal period. Discuss the major events of the first, second, and third trimesters of development.

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Ch28 human development l.jpg

Ch28: Human Development

Explain the stages of development starting with fertilization and ending with the neonatal period.

Discuss the major events of the first, second, and third trimesters of development.

Review the changes occurring in some organs as the infant goes from life in utero to neonate.

week 10

Prenatal development l.jpg

Embryonic development

fertilization - 8 weeks

Fetal development

9 weeks - birth

Prenatal Development

time period from fertilization to birth = gestation

Postnatal Development

Oocyte at ovulation l.jpg
Oocyte at Ovulation

  • 2º oocyte arrested in Metaphase II

  • Completion of Meiosis only if fertilization occurs

  • Follicular cells of corona radiata protect ovum as it breaks through ovary wall

Fig 28-1

Fertilization l.jpg

Taking place in ??

Viability of gametes:

  • Oocyte 12-24 h

  • Sperm 12-48 h

    Acrosome contains hyaluronidase  acrosomal reaction breaks down intercellular cement between adjacent follicle cells

    Single sperm fuses with oocyte

    amphimixis - fusion of sperm and oocyte pronuclei

The first trimester weeks 1 12 fetus size 3 in weight 14 g l.jpg
The first Trimesterweeks 1-12; fetus size ~ 3 in.; weight ~ 14 g





Basic organ plan and tissues laid out –

most susceptible to damage or disorganization at this time

Cleavage l.jpg

Early division of zygote into multiple cells without increase in size, partitions contents



solid ball of cells



with blastocoele cavity

Implantation embedding of blastocyst into uterine lining begins at day 7 l.jpg
Implantation - increase in size, partitions contentsembedding of blastocyst into uterine liningbegins at day 7

Blastocyst - with blastocoele cavity

Trophoblast - outer layer of cells

Inner cell mass - will form embryo

Trophoblast forms syncytial trophoblast- erodes into endometrium

Cellular trophoblast - carries nutrients to inner cell mass

Lacunae and primary villi formed by trophoblast

All of these form placental tissues

Fig 28-3

Day 10 l.jpg
Day 10 increase in size, partitions contents

Embryo completely embedded in endometrium

Amnion and yolksac visible

Blastodisc formation (2 cell layers)

  • Epiblast

  • Hypoblast

Gastrulation 3 germ layers formed l.jpg
Gastrulation: 3 Germ Layers Formed increase in size, partitions contents

day 12:

Ectoderm (forms from epiblast)

Nervous system


Endoderm (forms from hypoblast)

Mucosae (eg: GI-tract

Associated glands


Everything else

Formation of extra embryonic membranes l.jpg
Formation of Extra-embryonic Membranes increase in size, partitions contents

visible after day 10:

Amnion – Protection of embryo/fetus

Yolk sac –

Early site of blood cell formation

Placentation l.jpg
Placentation increase in size, partitions contents

Fig 28-5

Development of placenta from edges of blastocyst

Placenta = organ that forms from the chorion and the endometrium and allow the embryo/fetus to exchange nutrients and waste.

Chorionic villi provide surface area forexchange

Nutrient and gas exchange happens without actual blood exchange

Umbilical cord - contains two umbilical arteries and one umbilical vein

Fig 28-6

Embryogenesis l.jpg
Embryogenesis increase in size, partitions contents

Following gastrulation, formation of viable embryo

Head fold and tail fold develop

Critical period organogenes

Teratogens, Teratology = ?

Rubella and syphilis


FAS and smoking

Second and third trimester l.jpg
Second and Third Trimester increase in size, partitions contents

  • After the end of 8 weeks: Fetal development

  • Week 12: all organ systems laid out

  • Most teratogens not lethal anymore – but produce major defects

  • 3rd trimester mostly for size increase and maturity.

Fig. 28-7

Labor and delivery l.jpg
Labor and Delivery increase in size, partitions contents

22 weeks of gestation absolute minimum (normal?)

Parturition: Expulsion of fetus and placenta due to fetal oxytocin 

Stages of labor

1. dilation stage - cervix stretches

2. expulsion stage - fetus delivered

3. placental stage - placenta expelled

The End

Fig. 28-9/10