Survey of Embryonic Development Exercise 44 Survey of Embryonic Development

Objectives: Fertilization, zygote, morula, blastocyst, gastrula, fetus, chorion, chorionic villi, placenta, amnion, yolk sac, umbilical cord Embryonic structures and functions

Fertilization Fusion of two haploid (1n) gametes, occurs in uterine tube usually within a day after ovulation Sperm Delivers paternal chromosomes to fertilization site Egg Provides all the cellular organelles, nourishment, genetic programming for development AND maternal chromosomes ~2000x the volume of the sperm cell Secondary oocyte has been suspended in Metaphase II until now Fig. 29-1

Zygote Product of fertilization (union of gametes) Contains 46 chromosomes (normal # for a somatic cell) Fig. 29-1

Prenatal Development **Atlas Diagrams 1st trimester Embryological & early fetal development Beginnings of organ systems appear 2nd trimester Organs and systems nearly complete 3rd trimester Rapid fetal growth Most organ systems functional

Embryonic Development: MORULA Day 3-4 of development/cleavage (cell divisions) Solid multicellular (> 4-cell stage) ball Morula = “mulberry” Reaches uterus on day 4 Fig. 29-2

Embryonic Development: BLASTOCYST Over next 2 days Hollow ball with inner cavity Outer cellular layer (“trophoblast”, provides nutrients) Inner layer of cells (clustered at one end, will form the embryo) Implants in uterine wall Fig. 29-2

Embryonic Development: GASTRULA Day 12 Third layer forms—cells on surface migrate to central line: “primitive streak” Migrating cells move between the 2 layers Now have 3 embryonic layers: Ectoderm Mesoderm Endoderm Fig. 29-4

Embryonic Germ Layers Ectoderm Mesoderm Endoderm Neural tissue Connective tissues Epithelia & glands Muscle tissue Fig. 29-4

Embryo to Fetus Fetus: Start of month 3 to delivery Fig. 29-5

Extra-embryonic membranes Yolk sac Endoderm & mesoderm Amnion Ectoderm & mesoderm Allantois Chorion Mesoderm & trophoblast

Yolk sac Endoderm + mesoderm Pouch Important site of blood cell formation Fig. 29-5

Amnion Ectoderm + mesodermal cells form this two layered structure Enlarges throughout development Amniotic cavity contains amniotic fluid Surrounds, cushions developing embryo/fetus Fig. 29-5

Chorion Mesoderm and trophoblast cells form this vascular layer Provides transport of nutrients to the growing embryo/fetus (multicellular now, so diffusion alone won’t suffice) Chorionic villi: branches in contact with maternal tissues Embryonic blood vessels in each Supplies embryonic heart (beating by week 3) Gases and nutrients diffuse between embryonic and maternal circulations—no mixing of actual blood (separated by trophoblast)

Chorion Fig. 29-6

Placenta Appearance of blood vessels in the chorion = 1st step in creation of a functional placenta Temporary structure in uterine wall that allows the diffusion between fetal and maternal circulatory systems Fetal side Maternal side Fig. 29-6

Umbilical cord Connection between fetus and placenta Contains the allantois, placental blood vessels, & yolk stalk of the yolk sac Fig. 29-6

Fig. 29-8

Fig. 29-10

Video: The Miracle of Life