Embryology

• gametes form from contact between sperm and eggs
• during fertilization only the nucleus of the sperm enters and fuses with the egg
• the mitochondria of the sperm does not enter the egg but gets left behind, it is the reason why we only have mitochondria from the mom and not the dad
• the acrosomal sac is full of enzymes that digest the surface of the egg, and is located on the head of the sperm; it allows the sperm to enter the egg
• sperm and eggs are produced from gonads
• gonads produce gametes (eggs or sperms)
• a zygote forms when the egg and sperm fuse during fertilization
• zygotes divide by mitosis to form embryos
• cleavage is a type of mitosis and happens after fertilization; during cleavage one cell becomes 2 cells and then 4 cells, and then finally becomes 8 cells
• an embryo has three layers: ectoderm, mesoderm, endoderm
• after cleavage, gastrulation takes place, or the formation of the germ layers
• during the blastula stage, the inner layer of cells will become the endoderm, while the outer layer of cells will become the ectoderm
• in the blastocyst stage, the inner mass of cells is called an embryoblast, which will eventually develop into the fetus
• the middle layer of cells that are formed from the inside outward will become the mesoderm
• adult stem cells only have limited potential to differentiate
• the trophoblast forms around the outside of the embryoblast and it provides nutrients to the developing embryo
• adult stem cells are multipotent, meaning they can only differentiate into certain types of tissues
• embryonic stem cells are pluripotent, meaning they can differentiate into any kind of tissue
• embryonic stem cells can form any type of cell in the body
• invagination is the process whereby the cells fold in on themselves to create the gut cavity
• stem cells can be found in bone marrow, blood vessels, brain tissue, skin, liver, pancreas, skeletal muscle, testes, ovaries, and umbilical cord blood
• gastrulation occurs when the three primary germ layers (ectoderm, mesoderm, and endoderm) begin to separate and migrate during development
• stem cell therapy involves using stem cells to repair or replace damaged tissues
• after gastrulation is organogenesis, which is when the body organs begin to form, cells interact, and differentiate
• after organogenesis, is growth where the organs increase in side and the adult body form is attained
• the animal development order

  gamete formation, fertilization (zygote), cleavage, gastrulation, organogenesis, and growth
• neurulation is the process of the neural tube forming in the embryo and the formation of the neurons and brain
• parthenogenesis is the production of an egg without fertilization by a female parent; an unfertilized egg can become a complete organism without the need of sperm
• parthenogenesis is seen in frogs, salamanders, desert lizards, some fish and arthropods
• cleavage


  the process of nucleus and cell division in the zygote
• there are four main types of cleavage: radial holoblastic, spiral holoblastic, discoidal meroblastic, and rotational holoblastic
• an animal-vegetal axis is visible on the embryo before cleavage begins
• vegetal pole


  formed by the presence of yolk at only one end; does not undergo division and fertilization
• animal pole


  the region containing mostly cytoplasm; lots of enzymes are near the cytoplasm and this is where cell division occurs; undergoes division and fertilization
• the cleavage depends on whether the axis is vegetal or animal pole
• merula is a ball of cells that is the first stage of the embryo
• Spiral cleavage


  the first two planes of cleavage are vertical and produce blastomere of equal size. The third cleavage is horizontal and the blastomere are unequal. Found in mollusk (snails, clams, etc) and annelids (worms).
• radial cleavage

  the first two cleavage are horizontal and produce equal blastomere. the third plane of cleavage are also horizontal but unequal. Found in starfish and frogs.
• rotational cleavage 


  after the second cleavage, one pair of blastomere comes to lie right angles to the others (found in humans and mice)
• discoidal cleavage


  the cleavage is restricted to a small disc at one end of the embryo (found in birds)