What are the 3 phases of fertilization?
Phase 1: Penetration of the corona radiata Phase 2: Sperm binding (SED1 protein) and penetration of the zona pellucida Phase 3: Fusion of sperm and oocyte cell membranes
What is the acrosome reaction?
Lysis by enzymes Fusion of the outer acrosomal membrane and sperm cell membrane, resulting in the release of acrosomal enzymes
Sequence of events in human fertilization
Acrosome intact (on sperm head) Acrosome vesiculates Sperm binds zona pellucida Sperm digests & moves through zona pellucida followed by sperm-egg biomembrane fusion Sperm nucleus enters egg cytoplasm
Fast component and Slow component in zona pellucida
What is the fast component?
Fast component: change in resting potential of oocyte plasma membrane prevents further binding of sperm
What is the slow component?
Slow component: release of cortical granules (lysosomes) containing enzymes that destroy sperm receptors (ZP3)
Penetrating the zona pellucida requires what?
Acrosomal enzymes, specifically acrosin
Zona Pellucida Proteins and Fertilization
1. Receptor for ZP3 on sperm head binds to ZP3 in egg zona pellucida 2. ZP3-Receptor binding leads to clustering of receptors to side of sperm head 3. Resulting acrosome reaction releases acrosin which digests hole through zona pellucida 4. ZP2 receptor binds to ZP2 keeping sperm attached to zona pellucida *Note: Experimental removal of carbohydrate residues from ZP3 prevents sperm-egg binding and fertilization
Where are the ZP3 receptors?
On the zona pellucida
What is required for the sperm to be able to fuse with the egg?
Immunoglobin superfamily protein Izumo A mutation in Izumo creates the inability for the sperm to fuse to the egg and undergo this acrosome reaction
Sperm head and tail enter oocyte Second meiotic division with production of second polar body Unequal contribution of cytoplasm, equal contribution of genetic material
(Fraternal/non-identical) Develop when two separate eggs are fertilized and implant in the uterus Result from the fertilization of 2 different eggs with 2 different sperms
(identical) Result from the fertilization of one egg and one sperm When a single fertilized egg splits into two genetically identical parts Share the same DNA set, but can actually look very different, but can also be identical twins
Polar (Half-identical) Twins
Polar body twins occur when the oocyte splits before fertilization and the polar body is also fertilized by a different sperm Twins share half their genes in common (from the mother) and the other half different (from the two sperm). They share some features of identical twins and some features of fraternal twins and are so-called half-identical twins DNA-based zygosity tests are currently not available to determine if twins are polar-body twins
Cells within one individual have different genetic makeup Some cells have normal number of chromosomes, others do not (one cell may have 46 chromosomes and another may be lacking one and only have 45)
Mitotic non-disjunction early in cellular division
Gonadal or tissue mosaicism
-Mosaic Down Syndrome -Segmental Neurofibromatosis (NF1) -McCune-Albright Syndrome: GNAS1 mutations
Mosaicism vs Chimerism
-Mosaic: genetically different cells, same zygote (e.g. Blaschko's lines) -Chimera: genetically different cells, different zygotes *Different color skin over body
-Multiple cell lines (genotypes) with different set of chromosomes in a single individual -Fraternal twins that "fuse" to form a single individual -Individual with bilateral "assymetry" penis/testicle and ovary/uterine tube; mix of Xy and XX (IVF) -40 cases of no genetic "relationship" between mother and offspring
X chromosome inactivation
-Early in embryogenesis in mammals, one X chromosome is functionally inactivated through a process called X chromosome inactivation (lyonization) -Because this inactivation occurs randomly, ALL NORMAL FEMALES HAVE ROUGHLY EQUAL POPULATIONS OF TWO GENETICALLY DIFFERENT CELL TYPES AND ARE THEREFORE A TYPE OF MOSAIC Cat: Paternal X=black Maternal X=orange -The tortoiseshell pattern of fine patches of black and orange reflects the pattern of X chromosome inactivation in the hair follicles
The process of nuclear fusion, whereby 2 haploid cells fuse into a single diploid cell or zygote takes about _____ hours
Fusion of pronuclei
Return to diploid chromosome number Mitotic division of the zygote begins Embryogenesis begins
A series of mitotic divisions of the zygote. It is asymmetrical (daughter cells are unequal in size) and asynchronous (only one cell will divide at a time) Oocyte + fertilization--> zygote-->2 cell blastula-->4 cell blastula-->morula-->blastocyst
Consists of cells called blastomeres Oocyte + fertilization--> zygote-->blastula-->morula-->blastocyst
What is a morula?
A cluster of blastomeres (16-32 blastomeres) Gets more and more compact
Make up a blastula/blastocyst Are totipotent up to the 8-cell stage (i.e. each blastomere can form a complete embryo by itself).
Refers to a stem cell that can differentiate into every cell within the organism, including extraembryonic tissues
Is a group of cells that is small enough to fit into Roosevelt's eye on the face of a US dime
Involves fluid secreted within the morula that forms the blastocyst cavity, thus forming the blastocyst 1. The inner cell mass of the blastocyst is called the embryoblast (becomes the embryo). The embryoblast cells are pluripotent (can turn into ectoderm, mesoderm and endoderm) 2. The outer cell mass of the blastocyst is called the trophoblast (becomes the fetal portion of the placenta) Zona pellucida begins to dissipate
The outer cell mass of the blastocyst is called the trophoblast (becomes the fetal portion of the placenta)