Fertilization and Development Flashcards
polyspermy
When an egg gets fertilized by more than one sperm. It usually results in unsuccessful fertilization
What is the first step of fertilization
An exocytosis of digestive enzymes from the acrosome of the sperm
What is the acrosomal reaction
Exocytosis of enzymes from the acrosome occurs. The enzymes digest the jelly of the outer membrane, while actin filaments grow from the acrosomal process. The sperm binds to the egg cell membrane receptors
What is the fast block to polyspermy?
Contact and fusion of the egg and sperm occur. The egg’s membrane voltage changes (depolarization) as Na+ ion channels open and Na+ ions flood the cell membrane. This prevents any further sperm cells from fusing with the cell membrane.
What is the slow block to polyspermy?
Cortical Reaction
Ca++ ions are released from the endoplasmic reticulum. The Ca++ ions help cortical granules (special egg organelle) fuse with the plasma membrane. Sperm binding receptors are removed from the plasma membrane and a fertilization envelope forms. The forming of the fertilization envelope is termed the slow block to polyspermy
progesterone
A hormone release from the oocyte that helps with sperm motility among other functions.
zona pelludica
Extracellular egg matrix. Includes the receptors for sperm.
cleavage
Once fertilization occurs, a rapid period of mitosis, S, and M phases are carried out. The cytoplasm divides into smaller units called blastomeres.
Blastomere
The smaller units that result from rapid cell division upon fertilization
Morula
a solid ball of cells resulting from rapid cell division from fertilization.
blastocoel
A fluid filled cavity within the morula that begins to hollow out the morula.
Blastula
a hollow ball of cells (about 128 cells or 7 cleavages)
Two Poles:
- Vegetal Pole: High in Yolk
- Animal Pole: Low in Yolk
Yolk distribution is the most important driving factor that influences the cleavage pattern in many animals.
How does yolk distribution affect cleavage?
The heavy yolk in birds and other animals sinks to the bottom of the cell and there is less of the cytoplasm available for cleavage. Those with high yolk content end up with smaller replications and those like humans with very little yolk end up with more equal cleavages.
gastrula
We see a shape change
- Forms from a blastula.
- A new central cavity called the archenteron is formed that will become the primitive gut.
- The opening of the archenteron is called the blastopore. This forms the mouth in protosomes and the anus in deuterostomes.
-Three germ layers in a dome shape with a hole at the bottom (blastopore):
Ectoderm
Mesoderm
Endoderm
Deuterostomes
chordates and echinoderms
Protosomes
Mollusks, Arthropods, and annelids
What is the order of fertilized cell division groups?
Zygote -> Morula - > Blastula -> Gastrula
Ectoderm
- Outermost layer - Most sensitive to radiation
- Skin
- Nails
- Hair
- Sweat glands
- Tooth enamel
- Cornea and lens of eye
- Adrenal medulla
Mesoderm
- Middle layer
- Bone
- Muscle
- Blood
- Connective Tissues
- Notochord
- Excretory, lymphatic systems
- dermis of skin
- adrenal cortex
Endoderm
- Innermost layer
- Epithelial linings of respiratory, excretory, and digestive tracts, urinary bladder, and lining of reproductive systems and urethra
- Glands such as the thymus, thyroid, and parathyroid
- Liver and pancreas
Organogenesis
We see a big shape change
- Condensing of the mesoderm gives rise to the notochord. This is a rod of stiffened tissue that gives structure to all chordates
- Neural plate forms from the ectoderm and rolls into the neural tube which becomes the CNS (brain and spinal chord)
- Somites are formed, they represent blocks of mesoderm that give rise to vertebrae and muscle. Somites are arranged along the notochord
Amniotes
Bird, reptiles, and mammals
All contain a fluid like sac called the amnion
Extraembryonic membranes
Think everything surrounding the embryo, amniotic fluid, part of the placenta, etc
- Located outside the embryo:
- Yolk Sac: Encloses the yolk, provides needed nutrients
- Amnion: Absorbs shocks, keeps embryo from drying out
- Chorion: Gas exchange, forms part of the placenta
- Allantois: Disposes of nitrogen waste and gas exchange. In humans it does not store wastes, but is involved in O2 and nutrient transport.
- Visibly found in most shelled eggs
Yolk Sac
Encloses the yolk, provides needed nutrients
Amnion
Absorbs shocks, keeps embryo from drying out
Chorion
Gas exchange, forms part of the placenta
Allantois
Disposes of nitrogen waste and gas exchange. In humans it does not store wastes, but is involved in O2 and nutrient transport.
Totipotent
- Capable of developing into all different cell types
- A zygote is Totipotent plus the next three cell divisions to a Morula
Pluripotent
- Capable of developing into many different cell types, but NOT extraembryonic tissues.
- Stem Cells
- Blastocyst inner mass cells are pluripotent
Unipotent
Normal organ cells that create clones of themselves
Implantation
- Occurs in the Endometrium and occurs before the end of the first week
- It’s the blastocyst that adheres to the endometrium. (blastocyst is the mammalian version of the blastula)
blastocyst
Mammalian version of the blastula
Human yolk sac
Plays a role in developing the digestive tract. Does not provide nourishment like egg laying animals
Umbilical cord
- Connects the human embryo to parts of the yolk sac, allantois, and amnion
- Well endowed with blood vessels.
Chorion development
Develops around the embryo and secretes chorion gonadotropin hormone that helps maintain the corpus leutum
Corpus Leutum
Secretes progesterone during the first three months of pregnancy (trimester I)
After the first three months the placenta takes over to produce progesterone and estrogen (steroid hormones)
Trimester 1
- Most growth and differentiation occurs here
- Most sensitive to radiation and drug toxicity
- Main period for organogenesis
- After eight weeks the embryo is called a fetus
- Outer layer of the blastocyst called the trophoblast grows within the endometrium to form the placenta
Trimester 2
- All major organs formed
- Pregnancy is obvious
- Corpus luteum deteriorates as levels of hCG decrease.
- The placenta takes over the role of making progesterone and estrogen
Trimester 3
- Further development
- Fetus can generally survive at 8 months
viviparous
young are born alive after nourishment in the uterus from the placenta
oviparous
Young hatch from eggs laid outside the mother
trophoblast
The outer layer of the blastocyst that attaches to the endometrium and helps form the placenta
Fraternal vs identical twins
Fraternal: Two eggs drop and are fertilized
Identical: The cell divides after fertilization
Summary of human development
zygote -> Morula -> blastocyst -> embryo -> fetus - > baby
Homeotic gene
A gene that regulates the placement and spatial organization of body parts in animals, plants, and even fungi
These control the development and the fate of many cell groups
A homeotic mutation in an insect could result in a leg being placed where an antenna should be.
