BIOL 216 - Topic 2

Fertilization

formation of a diploid zygote from a haploid egg and sperm

Key features of fertilization

recognition at a distance, contact recognition and binding, egg and sperm fusion, blocks to polyspermy, egg activation

Recognition at a distance in sea urchins

sperm recognize and bind resact (species specific) and swim in the direction of higher resact concentration

Resact

released from the jelly layer into the surrounding seawater

Chemotaxis

the migration of cells towards a soluble concentration gradient of a stimulant

Contact recognition in sea urchins

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

Sea urchin fertilization process

sperm contacts the jelly coat, acrosomal reaction occurs

Acrosomal reaction-the acrosomal vesicle fuses with the plasma membrane causing the extracellular release of digestive enzymes that penetrate the jelly coat

The acrosomal process forms due to

the polymerization of actin monomers to form an actin filament

Proteins on the surface of the acrosomal process bind to

receptors on the egg membrane

Bindin

protein molecule on the acrosomal membrane that binds with species specificity to the vitelline layer of the egg

Fast block to polyspermy

fusion of sperm and egg membranes triggers depolarization of the membrane 1-3 seconds after sperm binds to egg, preventing additional sperm from fusing (the depolarization only lasts for about 1 minute)

Depolarization

Na+ channels open, Na+ enters, inside becomes more positive = depolarizes

Blocks to polyspermy

fast transient block (membrane depolarization), slow block (cortical reaction, permanent)

Membrane depolarization

caused by change in Na+ concentration, unfertilized sea urchin eggs have a net negative charge inside that becomes more positive post fertilization, when the sea urchin sperm encounter an egg with a positive charge the sperm-egg fusion is blocked

Cortical reaction

triggered by intercellular calcium release which causes cortical granules (vesicles) to fuse with the membrane, enzymes are released from granules clip receptors lifting the vitelline layer which then hardens to form the fertilization envelope, the sperm nucleus enters

What triggers cortical granule fusion

increase in intracellular calcium levels

Function of cortical granule fusion

fusion with the egg plasma membrane results in the release of proteinases, glycosidases, mucopolysaccharides, peroxidases and hyalin proteins into the extracellular space

Proteinases and glycosidases

separate vitelline layer from plasma membrane

Mucopolysaccharides

osmotic gradient

Peroxidases

crosslinks macromolecules of the vitelline membrane

Hyalin protein

modifies the extracellular matrix of the egg to help black sperm entry, coats outer surface of the egg

Early events of egg activation

increase in cell metabolism

Late events of egg activation

initiation of protein and DNA synthesis in preparation of first cleavage

Timeline for the fertilization of sea urchin eggs

1 second

binding of sperm to egg

2 seconds

acrosomal reaction: plasma membrane depolarization (fast block)

10 seconds

increased intracellular calcium level

20 seconds

cortical reaction (slow block)

1 minute

formation of fertilization membrane complete

2 minutes

increased intracellular pH

5 minutes

increased protein synthesis

20 minutes

fusion of egg and sperm nuclei complete

40 minutes

onset of DNA synthesis

90 minutes

first cell division

Capacitation

process by which the glycoprotein coat and the seminal proteins are removed from the surface of the sperms acrosome by substances secreted by the uterus or fallopian tubes

Aspects of capacitation

increases sperm metabolism and motility, necessary for future sperm and egg binding, triggered by bicarbonate ions (HCO3-) in the vagina, requires about 5-6 hours in humans

Zona pellucida

extracellular matrix of the egg

Function of microvilli in the egg plasma membrane

aid in the facilitation of fusion

Events of mammalian fertilization

sperm comes into contact with the oocyte's zona pellucida and the acrosomal reaction occurs, acrosomal enzymes begin to dissolve the zona pellucida, actin filament comes into contact with the zona pellucida, calcium influx occurs, causing cortical granules inside the oocyte to then fuse to the outer membrane

The three proteins that compose the zona pellucida

ZP1, ZP2, and ZP3

ZP3

place where the sperm plasma receptors bind

Cortical reaction in a mouse

prevents additional sperm from entering the egg, the released cortical granules both removes carbohydrates from ZP3 (so it cannot bind to the sperm plasma membrane anymore) and partly cleaves ZP2 (hardening the zona pellucida)

What does sperm binding and fusion of membranes do for fertilization in mammals

triggers enzymes release from cortical granules

What does enzyme release from cortical granules result in

the cortical reaction and slow block to polyspermy, preventing other sperm from fertilizing the same egg

What occurs after the cortical reaction in mammalian fertilization

the oocyte undergoes its second meiotic division, producing the haploid ovum and releasing a polar body after which the sperm and egg genetic material fuse

How long does it take for the first cell division in sea urchins to occur

90 minutes

How long does it take for the first cell division to occur in humans 12-36 hours

Cleavage

the first stage of early embryonic development after fertilization characterized by a series of mitotic cell divisions, the divisions are rapid (essentially skip G1 and G2 of cell cycle resulting in little to no protein synthesis)

Definition of cleavage

cell division with no significant growth, producing a cluster of cells that is the same size as the original zygote

Result of cleavage

embryo is partitioned into smaller cells called blastomeres

Vegetal pole

region where yolk (nutrients, distribution of which influences the cleavage pattern) is concentrated

Animal pole

opposite to vegetal pole, sperm always enter at this spot

Blastula

produced after 5-7 cleavage divisions, consists of the blastocoel and blastoderm, composed of thousands of cells, term comes into play ones the cleavage has produced over 100 cells

Cleavage furrow

indentation on the surface of the developing embryo

How many cleavage furrows are used in the formation of the blastula

three, the first two are parallel to the line connecting the animal and vegetal pole (longitudinal), the third is perpendicular to that axis (equilatorial)

How does sperm entry affect the axis

critical cue in the setting up of them, triggers a rotation of the outer cell cortex, exposing the grey crescent, the entry point establishes the location of gastrulation initiation

Gray crescent

exposed nonpigmented cytoplasm, light colored band visible in xenopus, has cytoplasmic determinants needed for the normal development of blastomeres, marks the future dorsal side

Ventral

underside

Dorsal

backside

Irregularity of third division

equatorial 8 celled division produces smaller blastomeres at the animal side

Holoblastic

cleavage furrow passes entirely through the egg (frogs, mammals, echinoderms)

Meroblastic

incomplete penetration of the cleavage furrow, so much yolk that cleavage furrow does not pass through the yolk portion of the embryo, cell division occurs in a small area (birds, fish, reptiles)

Blastocoel

fluid filled cavity within the blastula

Blastoderm

spherical layer of cells

What is required in frog development for transcription of zygote genes to occur

the blastula contains 4,000 cells

Gastrulation

cell movements result in a massive reorganization of the embryo from a simple spherical ball of cells, the blastula, into a multi-layered organism

Ectoderm

outer layer of embryo

Parts formed in the ectoderm

epidermis of skin and its derivatives (including sweat glands, hair follicles), nervous and sensory systems, pituitary gland, adrenal medulla, jaws and teeth, germ cells

Mesoderm

middle layer of embryo

Parts formed in the mesoderm

skeletal, muscular, circulatory, lymphatic, excretory, and reproductive (except germ cells) systems, dermis of skin, adrenal cortex

Endoderm

inner layer of embryo

Parts formed in the endoderm

epithelial lining of digestive tract and associated organs (liver, pancreas), epithelial lining of respiratory, excretory, and reproducture tracts and ducts, thymus, thyroid, and parathyroid glands

Sea urchin gastrulation process

1. mesodermal mesenchymal cells migrate from the vegetal pole toward the blastocoel, will eventually secrete calcium carbonate to form the internal skeleton, 2. Vegetal plate invaginates, 3. Mesenchymal cells continue to migrate, 4. Archenteron is formed by endodermal cells, 5. Some mesenchymal cells extend filopodia, 6. Filapodia contract and extend archenteron, 7. Archenteron fuses with blastocoel wall, forming the digestive tube with mouth and anus, after which three germ layers are present

Mesenchymal stem cells

multipotent cells that can differentiate into a variety of cell types including osteoblasts and chondrocytes

Osteoblasts

bone cells

Chondrocytes

cartilage cells

Archenteron

future digestive tube

Filopodia

cellular extensions that facilitate cell attachment and migration

Frog gastrulation

1. Sheets of cells rollover the dorsal lip and move inward, 2. Animal pole cells spread over the outer surface of the embryo, 3. Blastopore extends via the process of cell invagination until it encircles the embryo, becoming an opening into the archenteron, 4. Ectoderm spreads over the outer surface, 5. Endoderm and mesoderm expand internally due to cell involution, 6. Archenteron begins to form, 7. Ectoderm, mesoderm, and endoderm are formed, 8. Blastopore is plugged by yolk (becomes mouth in protostomes (earthworm), becomes anus in deuterostomes (sea urchin))

What initiates frog gastrulation

blastopore formation

Blastopore

crease that forms on the dorsal side of the late blastula

Dorsal lip

the involuted region of the blastopore

Chick gastrulation

What does the chick embryo consist of at gastrulation

the epiblast and hypoblast

Chick Epiblast

upper layer, all of the cells that form the chick embryo originate here

Hypoblast

lower layer, cells form the part of the sac that surrounds the yolk

Primitive streak

thickening found at the midline due to a concentration of migrating cells, found along the A-P axis of avian and mammalian embryos

What is the ultimate outcome of gastrulation

the three cell layers

Trophoblast

the outer epithelium of the blastocyst, eventually forms the fetal portion of the placenta

Inner cell mass (ICM)

group of cells that develop into the embryo

Steps of human development

1. Blastocyst reaches uterus, 2. Blastocyst implants (7 days after fertilization), 3. Extraembryonic membranes start to form (10-11 days) and gastrulation begins (13 days), 4. Gastrulation has produced a three layered embryo with four extraembryonic membranes

Things that occur during the second step of human development

trophoblast secretes enzymes that breakdown the lining of the uterus (endometrium) to facilitate implantation, ICM forms apiblast and hypoblast

Human epiblast

gives rise to the three primary germ layers and to the extraembryonic mesoderm of the visceral yolk sac, and allantois, and the amnion, hypoblast contributes to extra embryonic membranes such as yolk sac

Chorion

outermost layer of the placenta, comes into contact with the endometrium, extraembryonic membrane between fetus and mother, is the fetal part of the placenta and gives rise to chorionic villi

Chorionic villi

allow transfer of nutrients from maternal blood to fetal blood

Amnion

innermost placental layer surrounding the fetus, membrane that makes the amniotic sac; protection/cushion

Allantois

the middle layer of the placenta, sac-like structure, involved in nutrition and excretion, webbed with blood vessels, collects liquid waste from the embryo, and exchanges gases used by the embryo

The four extraembryonic membranes

amnion, yolk sac, allantois, chorion

Extraembryonic mesoderm

primordial umbilical cord