Embryology

Mitosis

After an egg is fertilized, it need to increase in cell number so it can become a multi-cellular organism (as humans obviously are). So it has to do this through mitosis.

Another word for mitosis is cleavage.

Stages in Embryonic Development

  1. Basically through cleavage, after it becomes a solid ball of (many) cells, which will at this point be called a morula

    • morula = solid ball of cells

    • result of early cleavages

    • takes about 96 hours

  2. Day 6 of the development, the morula “blasts” out into a hollow ball of cells called a blastula.

    • on top of the blastula is an inner cell mass where the embryonic stem cells are located

    • lots of telomerase, making the cells “immortal”

  3. Once the blastula reaches several hundred cells, the embryo then indents in itself becoming 3 layers

    • endoderm = inner layer

    • mesoderm = middle layer

    • ectoderm = outer layer

  4. On weeks 7-8 differentiation occurs, which is the changing of stem cells into 210 different types of cells, tissues, and organs

    • endoderm ➡ liver, pancreas, lining of organs

    • mesoderm ➡ muscles, skeleton, circulatory system

    • ectoderm ➡ skin, nervous system, hair, nails

In chart form:

sperm + egg

⬇

zygote

⬇

morula

⬇

blastula

⬇

gastrula

⬇ ⬇ ⬇

endoderm mesoderm ectoderm

⬇

embryo

⬇

fetus

Stem Cells

Stem cells are undifferentiated cells that have the ability to become specialized cells

  • totipotent

  1. adult

    • some adult tissues contain stem cells, such as the brain, bone marrow, blood, vessels, muscle, skin, and liver

    • these cells give rise to “like” cells (liver ➡ liver)

    • hoped to be used to treat Parkinson’s, diabetes, etc.

  2. embryonic

    • grown from in-vitro fertilized eggs (controversial)

    • taken from 4-6 day old blastula

    • toti/pluripotent ➡ can become (potentially) any type of cell, being able to truly differentiate

    • trade-off: abundance of telomerase may lead to cancer, which also destroys an embryo

Site of Development

External: Development of the embryo/fetus outside of mother’s body

  • In water

    • eggs of many fish and amphibians are fertilized and develop externally in an aquatic environment

    • the food source for the developing embryo is the yolk stored in the egg

  • On land

    • eggs of birds, reptiles, and some mammals develop externally after being fertilized internally

    • the food source is the yolk

Internal: Development of embryo inside mother’s body

  • Placental Mammals (humans, whales)

    • after internal fertilization, the embryo develops internally in a uterus

    • the food for the developing embryo comes from the mother/placenta

    • placenta = combination of maternal and embryonic tissues

      • materials exchanged by diffusion

      • blood supplies don’t mix

    • umbilical cord = attaches embryo to placenta, contains blood vessels

  • Marsupials (kangaroo, opossum*)

    • internal fertilization and development occur WITHOUT nourishment from the mother (embryo born at a pre-mature stage)

    • the food source is the yolk

    • the embryo migrates and development is completed externally in a pouch containing mammary glands

      * non-placental mammal

Structures in the Developing Embryo (egg)

shell - provides protection

amnion - surrounds the embryo, blood vessels penetrate yolk sac to transport food to the developing embryo

yolk - food source for developing embryo

allantois - respiratory surface, also is a storage site for the nitrogenous waste, uric acid

chorion - the outer membrane surrounding the other embryonic membranes

air space - cushions embryo

albumin - protein reserve/egg white