BIOL 216 ch2 development

fertilization

formation of a diploid zygote from a haploid egg and sperm

features of fertilization

• recognition at a distance

• contact recognition and binding

• fusion of egg and sperm

• blocks to polyspermy

• activation of egg and first cleavage

recognition at a distance in sea urchin

• resact released from jelly layer into surrounding seawater
• sperm recognize and bind resact, swimming in direction of higher resact concentration

resact

soluble glycoprotein derived from jelly layer of sea urchin egg

chemotaxis

migration of cells towards a soluble concentration gradient of a stimulant

contact recognition in sea urchin

facilitated by a carbohydrate molecule in egg jelly layer binding to a receptor on sperm plasma membrane

sea urchin fertilization

• sperm head contacts jelly coat
• acrosomal reaction

acrosomal reaction

acrosomal vesicle fuses with inner plasma membrane and releases digestive enzymes out of cell to penetrate jelly coat

acrosomal process

• pointy projection forming at the tip of sperm head due to polymerization of actin monomers to form an actin filament
• surface proteins can bind to receptors on egg membrane

bindin

protein molecule on acrosomal process that binds to vitelline layer of egg

vitelline layer

layer below jelly coat that contains species-specific sperm receptors

fast block to polyspermy

rapid depolarization of fused sperm-egg plasma membrane after binding sperm with egg

• prevents additional sperm from fusing immediately after binding
• caused by increase in Na+ concentration inside the cell

slow block to polyspermy (cortical reaction)

intracellular calcium release causes cortical granules to fuse with inner plasma membrane of egg

• enzymes released clip receptors and lift vitelline layer from membrane
• vitelline layer hardens to form fertilization envelope
• fused sperm nucleus enters egg cell

cortical granules

specialized vesicles containing enzymes in unfertilized egg cells which fuse with inner membrane upon fertilization

proteinases and glycosidases

enzymes in cortical granules which separate vitelline layer from plasma membrane

mucopolysaccharides

long molecules that form the fertilization envelope and create an osmotic gradient to expand envelope from inner membrane

peroxidases

crosslinks macromolecules in vitelline layer to harden fertilization envelope

hyalin protein

coats outer surface of egg to form hyaline layer

timeline of sea urchin fertilization

1. binding of sperm to egg
2. acrosomal reaction
3. depolarization (fast block to polyspermy
4. increased intracellular calcium levels
5. cortical reaction
6. formation of fertilization envelope complete
7. increased intracellular pH
8. increased protein synthesis
9. fusion of egg and sperm nuclei complete
10. onset of DNA synthesis
11. first cell division

fertilization in mammals

• internal fertilization
• glycoprotein coat and seminal proteins are removed from surface of acrosome by substances in uterus/fallopian tubes (capacitation)

capacitation process

glycoprotein coat and seminal proteins are removed from acrosome surface by substances in uterus and fallopian tubes

• increases sperm metabolism and motility
• necessary for binding sperm and egg
• triggered by basic bicarbonate ions
• 5-6 hours

zona pellucida

ECM surrounding plasma membrane in egg cell

• promotes species-specific fertilization with receptors that bind sperm cells
• initiates acrosomal reaction

microvilli in egg plasma membrane

facilitate binding and fusion between sperm and egg cells

• located outside of egg plasma membranes

acrosomal reaction in mammals

• sperm contact zp
• acrosomal enzymes begin to dissolve zp
• actin filament comes into contact with zp
• calcium influx
• cortical granules inside oocyte fuse to outer membrane

3 proteins of zona pellucida

• ZP1
• ZP2
• ZP3 (bind with sperm plasma membrane receptors)

cortical reaction in mice

• released contentts from cortical granules remove carbohydrate from ZP3 preventing additional binding to sperm
• partly cleave ZP2 to harden zona pellucida

cleavage

first stage of early embryonic development after fertilization

• characterized by a series of mitotic cell divisions with not significant growth
• cell cluster the same size as original zygote
• skips G1 and G2 of cell cycle
• little to no protein synthesis

blastomeres

smaller cells produced from cleavage

vegetal pole

region where yolk is concentrated

animal pole

region with low yolk concentration

blastula

hollow ball of cells with a fluid filled cavity

• 5-7 cleavage divisions
• more than 100 cells from cleavage

blastocoel

fluid filled cavity of blastula

cleavage furrow

indentation on surface of developing embryo

• first 2 are parallel to line connecting animal and vegetal poles
• third is perpendicular (equator)

location of sperm entry

always animal pole

• critical in setting up a dorsal/ventral axis
• triggers rotation of outer cell cortex and exposing gray crescent
• entry point establishes location of gastrulation initiation

gray crescent

light colored band of exposed nonpigmented cytoplasm along the edge of vegetal pole

• contains cytoplasmic determinants needed for normal development of blastomeres
• forms at 2-cell stage
• marks future dorsal side
• uncommon in mammals

cytoplasmic determinants

substances in maternal egg that affect gene expression and early development

• eg. mRNAs encoding transcription factors
• contained within gray crescent

why smaller blastomeres form at animal pole

higher concentration of yolk at vegetal pole push mitotic apparatus and cleavage furrow away from vegetal pole

holoblastic

cleavage furrow passes entirely through the egg

• present in mammals, frogs, echinoderms

meroblastic

cleavage furrow does not pass through the yolk portion of embryo

• higher concentration of yolk
• bundle of cells in animal pole and not in vegetal pole
• present in birds, fish, reptiles

first transcription of zygote genes

occurs when blastula contains more than 4000 cells

• previous activities initiated and controlled by mechanisms already present in egg cell

cleavage in sea urchin vs. frog

sea urchin: evenly distributed yolk platelets throughout the cell results in cells of similar size at 8-cell stage
frog: yolk concentration in vegetal pole result in smaller cells in animal pole at 8-cell stage

blastocoel in sea urchin vs. frog

sea urchin: generally present throughout the entire blastula
frog: present in animal hemisphere of blastula

gastrulation

reorganization of embryo from a spherical blastula into a multi-layered organism

ectoderm

outer layer of embryo

• gives rise to epidermis of skin and derivatives
• nervous and sensory systems
• pituitary gland, adrenal medulla
• jaws and teeth
• germ cells

mesoderm

middle layer of embryo

• gives rise to skeletal and muscular systems
• circular and lymphatic systems
• excretory and reproductive systems (except germ cells)
• dermis of skin
• adrenal cortex

endoderm

inner layer of embryo

• gives rise to epithelial lining of digestive tract and associated organs
• epithelial lining of respiratory, excretory, reproductive tracts and ducts
• thymus, thyroid, parathyroid glands

movement of mesoderm mesenchymal cells during sea urchin gastrulation

migrate from vegetal pole toward blastocoel

• will eventually secrete calcium carbonate to form internal skeleton of sea urchin

mesenchymal stem cells

multipotent cells that can differentiate into a variety of cell types

• osteoblasts, chondrocytes (skeletal system)

movement of endoderm during sea urchin gastrulation

invagination of vegetal plate (blastopore)

• formation of future digestive tube

filopodia

cytoplasmic projections that facilitate cell attachment and migration

• extend from mesodermal cells above invaginated endoderm
• attach to ectodermal cells at animal pole and contract to extend archenteron

archenteron

future digestive tube formed by endodermal cells

gastrula stage in sea urchin

archenteron fuses with blastocoel wall and forms digestive tube with mouth (animal) and anus (vegetal)

• 3 germ layers

3 germ layers of gastrula stage in sea urchin

ectoderm: outer layer of cells
mesoderm: collection of cells within that will form future skeleton
endoderm: digestive tube

blastopore in sea urchin

opening of archenteron at vegetal pole

• becomes future anus

frog gastrulation

initiated by blastopore formation in this organism

• cells roll over the dorsal lip and move through blastopore into blastocoel cavity

• animal pole cells spread over outer surface of embryo

blastopore in frog

crease that forms on the dorsal side of late blastula

• becomes future mouth

dorsal lip

inward fold of vegetal pole cells of the blastopore

archenteron formation in frog gastrulation

• blastopore extends via cell invagination of mesoderm cells until it encircles the embryo
• ectoderm spreads over outer surface
• endoderm and mesoderm expand internally and blastocoel cavity shrinks

spiral cleavage

mouth develops from blastopore

• meroblastic cleavage
• determinate cleavage

radial cleavage

anus develops from blastopore

• holoblastic cleavage

• indeterminate cleavage

• deuterostomes

chick gastrulation

organism’s embryo at gastrulation consists of epiblast and hypoblast

epiblast in chick

upper layer containing cells that form the embryo in chick gastrulation

hypoblast in chick

lower layer containing cells that form part of the sac that surrounds the yolk in chick gastrulation

primitive streak

thickening of epiblast midline due to a concentration of migrating cells inward

• found along anterior/posterior axis of avian and mammalian embryos
• begins formation at the beginning of human gastrulation

start of gastrulation in humans

stage that blastocyst implants into endometrium of uterus

movement of cells in blastocyst

formation of inner cell mass and trophoblast layers

inner cell mass

inner group of cells in blastocyst that develop into the embryo

• used for embryonic stem cells

trophoblast

outer epithelium of blastocyst

• secretes enzymes that break down endometrium to facilitate implantation
• forms fetal portion of placenta

implantation of blastocyst

formation of epiblast, hypoblast, and trophoblast layers

• 7 days after fertilization
• formation of epiblast and hypoblast layers

epiblast in human

upper layer of cells that give rise to 3 primary germ layers

• give rise to extraembryonic mesoderm of visceral yolk sac, allantois, amnion

hypoblast in human

lower layer of cells that contributes to extra embryonic membranes (eg. yolk sac)

chorion formation

extraembryonic membrane forms between fetus and mother

• fetal part of placenta
• gives rise to chorionic villi
• begins gastrulation

chorionic villi

allow transfer of nutrients from maternal blood to fetal blood

gastrulation in humans

production of a 3-layered embryo with 4 extraembryonic membranes

extraembryonic membranes of human gastrula

• amnion (amniotic sac)
• chorion
• yolk sac
• allantois

allantois

sac-like structure involved in nutrition and excretion for human embryo

• webbed with blood vessels, collects liquid waste, exchanges gases

amnion

membrane in human embryo that makes the amniotic sac

• provides protection and cushioning

extraembryonic mesoderm

becomes umbilical cord

placenta

temporary organ that connects developing fetus to uterine wall and allows nutrient uptake, gas exchange, thermo-regulation, protects against infection, produces hormones

• begins developing upon implantation of blastocyst
• 3 layers

yolk sac

membrane outside embryo connected to the embryo's midgut through a tube in umbilical opening

• helps with circulation

3 layers of placenta

amnion: innermost layer surrounding fetus
allantois: middle layer
chorion: outermost layer that contacts endometrium

stages of human embryogenesis

1. fertilization
2. cleavage (blastomeres)
3. compaction
4. differentiation (ICM and trophoblast)
5. cavitation
6. hatch from zp
7. implantation
8. differentiation (epiblast, hypoblast, trophoblast)
9. bilaminar disc formation (before gastrulation)
10. mesoderm formation (primitive streak)
11. formation of extraembryonic membranes
12. amniotic sac enlargement

changes involved in gastrulation

reorganization of embryo into 3 germ layers

• cell motility
• cell shape
• cell adhesion

invagination

epithelial sheet bends inward

• forms longer cells

ingression

individual cells leave an epithelial sheet and become freely migrating mesenchyme cells

involution

epithelial sheet rolls inward to form an underlying layer

epiboly

a sheet of cells spreads out by thinning

intercalation

rows of cells combine between one another to create a longer row of cells that is thinner

convergent extension

rows of cells combine but in a specific direction

• major changes in cell shape and number of rows

neurulation

process of neural tube formation

• begins with appearance of neural plate (flat)
• neural plate invaginates to form neural groove with neural folds on each side (folded)
• neural folds gradually approach each other at midline and fuse into a tube (tunnel)

neural tube

precursor to brain and spinal cord

primary neurulation

neural plate creases inward until edges fuse

secondary neurulation

neural tube formation by hollowing out the interior of a solid precursor

formation of neural plate

thickening of ectoderm induced by signals from mesoderm and other surrounding cells

• located at animal pole

somites

blocks of cells formed from mesoderm that give rise to vertebrae

notochord

a flexible, rod like structure in mesoderm layer that extends along anterior/posterior axis

• disappears before birth
• parts become the inner portion of vertebral disks in adults

cadherins

transmembrane proteins that play an important role in cell adhesion

E-cadherin in neurulation

cadherin expressed primarily in ectoderm and neural plate

cadherin 68 in neurulation

cadherin expressed primarily in neural folds

N-cadherin in neurulation

cadherin expressed primarily in invaginated portions of neural plate