Fertilization and Development Flashcards
Why Study development
- Understand and treat congenital abnormalities
- Discover what drugs are safe during pregnancy by testing them on other animals
- Understand the working of the mature body in a new light
Development
- Go from 1 cell→ multicellular body
- Develop a body plan
- Involves: signaling, mitosis, differentiation, apoptosis, migration—all coordinated among the cells
Differentiation
• As the cells divide, different transcriptional programs are turned on via signals from the environment
o Whatever’s on the left side will be in one daughter cell and whatever’s on the right side will be in another daughter cell
o Draw OUT DIFFERENTIATION
o Each division on the left side is asymmetric: produces one cell that retains its undifferentiated capacity as a stem cell
• Can form into whatever cell
o Each division on the right side is asymmetric: the other cell from each mitotic division acquires new traits through the transcription of genes
• Cannot form into whatever cell
Phases of Division
Phases of division
Common to all organisms
fertilization, cleavage, gastrulation, neurulation, organogenesis
unique to a few: metamorphosis
fertilization
• Gametes meet, generating a 1 celled, diploid organism called a zygote
cleavage
• Cell division
gastrulation
• First migration of cells to form the gut
organogenesis
• Development of the organs
metamorphosis
• Transition from larval stage to adult
steps of fertilization
acrosomal reaction, cortical reaction
acrosomal reaction
- Enzymes stored in the sperm acrosome digest a hole in the jelly coat of the egg
- Beneath the jelly coat, the sperm binding proteins bind sperm, trigger fusion of plasma membranes
- Sperm releases its nucleus into the egg
cortical reaction
• Sperm binding to receptor triggers intracellular signaling events that cause release of Ca2+ from ER
• Ca2+ causes vesicles within the egg to be released
• Enzymes within
Pull zona pellucida away from egg, hardening to protective “fertilization envelope”
Cleave off sperm receptors (including any attached sperm)
roles of Ca2+
• Triggers exocytosis of cortical vesicles, hardening of the zona
Slow block to polyspermy
• Triggers egg activation
Huge increases in rate of cellular respiration and protein syntheisis
DNA synthesis begins
cleavage
- Cell division without cell growth
- Resulting ball of cells stays roughly the same size, but increasing cell number
- Cells alternate between M and S phase, no G1 or G2
- Resulting ball of cells =blastula (each cells is a blastomere)
- Ball forms a central cavity called a blastocoel, fills with water
yolks of different species
- Some egg laying organisms have large yolk sacs, so cell division is pushed toward one side (animal pole), yolk sac side is vegetal pole
- Mammals, and some others have a less pronounced yolk sac and cell division is more well distributed
what is a yolk
• Food/nutrient sac
cleavage in mammals
- Mammal embryos go through compaction, where they condense the ball of cells
- Then expanf the blastocoel space
- The embryo in this stageis called a morula
- Blastula is then called a blastocyst
- First differentiation event creates inner cell mass and trophoblat layer
cessation of cleavage
- Cleavage appears to stop when the ratio of nucleus to cytoplasm in each blastomere is right
- Fertilized egg: very small nuc-cytoplasm ratio
- Each cleavage decreases size of cytoplasm
cleavage in mammals
• Cleavage occurs in the oviduct (fallopian tube) as the egg descends from the ovary to the uterus
implantation in mammals
- Implantation occurs when the trophoblast adheres to the wall of the uterus
- Uterine wall envelopes the mebryoprocess o
- Process of implantation takes about 1 week
- Trophoblast begins to proliferate and invade –becomes the placenta
ovary
- The human ovary is both an endocrine gland and a reproductive organ
- Two functions are intricately tied
- Egg develops within a follicle
- Follicle produces hormones
implantation and differentiation
- Implantation and invasion of the uterine wall by the trophoblast continues
- Further differentiation events occur in the inner cell mass
- 2 cell types: epiblast and hypoblast= bilaminar disc
- both begin to proliferate and migrate
formation of “germ layers’
- Germ layers are the 3 precurosors to tissues that form in the embryo
- From these, all the tissues of the body form
- Endoderm
- Mesoderm
- Ectoderm
- All 3 are derived from the epiblast
- Hypoblast goes on to become “extra-embryonic” tissue
gastrulation
• Process of cell migration and dramatic reorganization
• Paves the way for tissue differentiation
• After gastrulation 3 cell layers (germ layers)
o Endoderm (inner most) becomes the lining of organs & lining of CV system
o Mesoderm(middle) becomes skeleton, muscle, CV system, bottom of skin
o Ectoderm (outermost) becomes top layer of skin, nervous system, and germ cels
notocord formation
• In the mesoderm, the notochord begins to form
organogenesis
- Starts with neurulation—formation of the nervous system ((chordates))
- Doral (posterioir) mesoderm cells pinch off to become a rod called notochord (becomes the vertebral discs)
- Then ectoderm cells above this site change shape and curve inward (neural plate)
- Neural plate invaginated and pinches off, becoming neural tube
two sets of cells develop around the neural tube
neural crest cells and somites
neural crest cells
develop from neural tube (ectoderm) migrate around the body becoming nerves
somites
develop from notochord (mesoderm) become vertebrae) (humans) also contribute to body segmentation in other vertebrates
- In some organisms somites contribute to segmented body plans
- In other organisms somites contribute to the formation of repetitive structures
spinal bifida
- Error in neural tube closure
* Most common debilitating birth defect in US
organogenesis pII
- The embryo folds in on its self during this process, to make a central cavity the length of the embryo—this becomes the gut
- The embryo stays connected to the yolk sac through a duct of the gut, this later becomes the umbilical cord (and after birth, the belly button)
important factors in organogenesis
• Cytoskeleton
o Actin and microtubules make these programmed cell migrations possible
o Cells express adhesion molecules that help them adhere tot the ECM
o ECM plays an active role in guiding them in the riht direction
• Apoptosis
o Cells die in order to create a tissue pattern (tadpole tail, human hands)
o Cells compete for survival (only neurons with the most connections live)
how do cells know what to become
- Long before the cells differentiatie and begin to look different, their fate is decided
- Factors within the cell itself (autonomous specification)
- Factors from outside the cell, usually signaling (uconditional specification)
- Combination of factors creates a unique internal/external ienvironment in which each cell may make unique changes in terms of gene expression
determination
cells are committed to a fate
differentiation
cells go through the genetic and morphological changes to become specialized
the theory of fate mapping
undifferentiated cells in the blastula all appear the same However cells occupying a particular location ALWAYS deveop into a subset of specialized cells in the adult
morphogens
- Cytoplasmic factors that convey positional information
- Are often autonomous regulators, but can be sectred, but can be secreted to work in a conditional manner
- Its not just presence/absence of morphogens but concentration
cell fate and location
- Cells distinguish direction by stationary cilia, act like anyennas
- From this info, cells candecide how to migrate, etc
- Determines R and L patterns
kartagener’s syndrome
• Defect in stationary cilia lead to incorrect placement of organs, without being able to detect the flow across the cell surface, cilia can’t correctly identify location
birth defects
- By understanding the precise order and timing of events, we can better understand at what critical points in development things go wrong
- i.e. spinal bifida occurs around d28 of human gestation (lack of adequate folic acid)
- i.e. cleft palate: fusion of the structures that will form the upper jaw 5-7 weeks gestation (maternal ingestion of certain compounds)
