Ch. 3 Overview of Human Embryology and Development

Embryology

Study of individual origin and development.

Prenatal period

Time from conception to birth.

Embryonic period

First 8 weeks post-fertilization.

Fetal period

Remaining 30 weeks of development.

Zygote

Fertilized oocyte that develops into an embryo.

Blastomeres

Daughter cells formed from the zygote.

Morula

Cluster of 12-16 blastomeres.

Blastocyst

Fluid-filled structure with about 60 cells.

Cleavage

Cell division without growth.

Bilaminar embryonic disc

Inner cell mass divided into two sheets.

Epiblast

Upper layer of the bilaminar embryonic disc.

Hypoblast

Lower layer of the bilaminar embryonic disc.

Amniotic sac

- Formed by an extension of epiblast

- Outer membrane: forms amnion

- Inner membrane: forms the amniotic sac cavity filled with amniotic fluid

Yolk sac

Forms digestive tube; extension of hypoblast.

Primitive streak

Site where epiblast cells migrate inward.

Gastrulation

Process forming three germ layers.

Endoderm

First migrating cells replace hypoblast.

Mesoderm

Cells between epiblast and endoderm. Next group of migrating cells.

Ectoderm

Surface cells left at surface that do not migrate inward, formed from epiblast cells.

Neurulation

Ectoderm forms brain and spinal cord.

Neural tube

Hollow tube forming from the neural plate, cranial part of the neural tube becomes the brain

Somites

First body segments forming vertebrae, ribs, and axial muscles. Paraxial mesoderm

Ossification centers

Sites where bone formation begins.

Surfactant

Substance that prevents lung collapse.

Neural plate

ectoderm thickens

Neural groove

ectoderm folds inward

Neural crest

Forms sensory nerve cells, ganglia, and melanocytes

Intermediate mesoderm

begins as a continuous strip of tissue just lateral to the paraxial mesoderm (urogenital system)

End of second month

All organs have appeared and the placenta is fully functioning, embryonic development complete

Fetal development

Beginning of third month, head growth begins to slow and body increases in length, ossification centers appear in bones, sex can be determined

5-7 months

Mother begins to feel fetal movement, lungs lack surfactant

8-9 months

Fetus usually rotates so head is pointed down toward cervix