Animals Development I: Fertilization and Cleavage Flashcards
Stages of Animal Development
- Fertilization
- Cleavage
- Gastrulation (MORPHOGENESIS)
- Organogenesis (MORPHOGENESIS)
Morphogenesis
biological process that results in an organism’s shape + body organization
Fertilization
process in which a single haploid sperm fuses with a single haploid egg to form a zygote
- sperm + egg cells possess specific features that make this process possible
- to ensure that offspring has 2N chromosomes. only ONE sperm can fuse with each egg
Fertilization: Egg Features
- Yolk
- Jelly layer/ Zona Pellucida (ZP)
- Vitelline Envelope (VE)
- Cortical Granules
Fertilization: Sperm Features
- Sperm structure
- Bindin proteins
- Acrosome
Egg: Yolk
nutrients to support growth of the developing zygote
Egg: Zona Pellucida (ZP)
composed of glycoproteins (proteins with sugars stuck to them) that release species-specific chemoattractants that guide sperm to the egg
- mammals: zona pellucida
- placental mammals: a layer of follicular cells surrounding the ZP
Egg: Vitelline Envelope (VE)
the ZP is separated from the egg by a second membrane outside the cell’s PM
Egg: Cortical Granules
underneath the egg’s plasma membrane are these: vesicles containing enzymes that will degrade the proteins that hold the vitelline envelope around the PM when fertilization occurs
Sperm: Sperm Structure
consists of a head containing sightly packed DNA, a flagellar tail for swimming, and many mitochondria to power movement
Sperm: Bindin Proteins
bindin is a protein found in the PM of the sperm which are species-specific and recognize + bind to egg PM receptors
Sperm: Acrosome
organelle in the head which contains digestive enzymes to degrade the ZP to allow sperm to reach the egg PM
Polyspermy
fusion of more than one sperm with an egg; genetically incompatible with life + results in zygote death
- two mechanisms prevent this: “flask” and “slow” block in fertilization
Steps of Fertilization
- sperm is attracted to + connects with the ZP
- The interactions between receptors on the sperm cell and glycoproteins on the egg cell initiate the ACROSOME REACTION. Digestive enzymes are released from the acrosome in the sperm and destroy the ZP to create a pathway for the sperm to reach the egg
- Sperm reaches egg PM, and the sperm PM BINDIN PROTEINS contact the bindin receptors on the egg PM, allowing the sperm and egg to fuse. The proteins and receptors are species-specific, meaning it is difficult for different species to fertilize one another
- Sperm + egg PM fusion initiates ELECTRICAL DEPOLARIZATION of the entire egg, temporarily preventing other sperm from fusing with the egg PM (“fast block”)
- The membrane depolarization initiates a wave of CALCIUM released across the PM
- The calcium initiates the CORTICAL REACTION in the egg, where the granules fuse with the gg PM, releasing digestive enzymes that degrade the egg membrane bindin receptors
- Bindin receptor destruction causes the vitelline layer to lift away from the egg PM, creating the FERTILIZATION ENVELOPE, a barrier that prevents additional sperm from reaching the egg (“slow block”)
- These events culminate in egg activation, causing the egg to recognize that fertilization occurred and results in development initiation
“Fast Block”
Sperm + egg PM fusion initiates ELECTRICAL DEPOLARIZATION of the entire egg, temporarily preventing other sperm from fusing with the egg PM (“fast block”)
Bindin Receptors
have two jobs:
1. sperm docking site
2. hold the vitelline layer against the PM
“Slow Block”
Bindin receptor destruction causes the vitelline layer to lift away from the egg PM, creating the FERTILIZATION ENVELOPE, a barrier that prevents additional sperm from reaching the egg (“slow block”
Cleavage
series of rapid cell divisions following fertilization
- divisions occur every 10 minutes
- cells divide without growing between divisions, so the large single-celled zygote divides into smaller cells called BLASTOMERES
Blastula
what the embryo is after cleavage has produced 100+ blastomeres
- a spherical layer of blastomeres considered to be the first embryonic tissue (BLASTODERM)
- blastoderm surrounds a fluid/yolk-filled cavity called the BLASTOCOEL, which is essential for gastrulation
Unique Mammal Cleavage
the later stages of cleavage are different in mammals; the blastocyst and trophoblast
- inner cell mass forms the embryo + trophoblast forms embryonic portion of placenta
Blastocyst
mammalian blastula
- has inner cell mass and outer cell layer called the trophoblast
Cell Specialization Factors in Cleavage
cells get instructions from two different types of information:
- intrinsic (internal/”lineage”)
- extrinsic (external/”positional)
Intrinsic
information inherited from the mother cell, as a result of cell division
- EX: cell inherits molecules in its cytoplasm that tells it that it belongs in the neural lineage of the body (CYTOPLASMIC DETERMINANTS)
Extrinsic
information received from the cell’s surrounding environment/neighboring cells
- EX: cell gets chemical signals from a neighbor to become a type of photoreceptor (INDUCTION)
