Fertilization, Cleavage, and Gastrulation

Fertilization

Process where two gametes fuse together to create a new individual with genetic potentials derived from both parents

Parthenogenesis

Embryonic development without a sperm

Process of fertilization

1. Contact and recognition between sperm and egg

2. Regulation of sperm entry into the egg

3. Fusion of the genetic material of sperm and egg

4. Activation of egg metabolism to start development

Sperm activation or acrosomal reaction

Fusion of acrosomal vesicle with the sperm plasma membrane leads to the exocytosis of the acrosomal vesicle and release of the protein-digesting enzymes that digest path to egg surface

occurs when sperms reach zona pellucida

Cumulus

Layer surrounding egg made up of ovarian follicular cells in mammals

Capacitation

mammalian sperms are activated by the contents of the female reproductive tract

Contact of gametes

Species specific recognition step

binding of sperm to the extracellular envelope of the egg, passage of sperm through extracellular envelope, and fusion of sperm and egg cell membranes

How do sperms have contact to eggs?

Through binding of specific proteins on egg and sperm surfaces.

What happens once two gametes connect?

Membrane of sperm and egg fuse, which results in the injection of sperm nucleus into the egg

Polyspermy

the entrance of multiple sperm

Fast block to polyspermy

changes in electrical potential of the egg membrane once the egg fuses with sperm. Only lasts 1 minute. Influx of sodium ions into the egg

Slow block to polyspermy

Formation of fertilization envelope. Once sperm enter cortical granules fuse with egg plasma membrane and release contents between cell membrane and vitelline membrane

Contents of cortical granules

Proteases

Mucopolysaccharides

Peroxidase enzymes

Hyalin

Proteases

enzymes that digest the vitelline posts that connect vitelline membrane to plasma membrane, clips off the bindin receptors

Mucopolysaccharides

Produce an osmotic gradient that causes water to rush into the base between the cell membrane and the vitelline membrane, causing the vitelline envelope to expand and become the fertilization envelope

Peroxidase enzymes

hardens fertilization membrane by crosslinking tyrosine residues on adjacent proteins

Hyalin

Forms a coating around the egg

Fusion of genetic material

Two haploid pronuclei fuse to form the diploid zygote nucleus

How do the pronuclei reach each other?

Migrate towards each other via microtubular processes radiated from the centrosomes

Activation of egg metabolism

Influx of free ions into the egg and releases from intracellular storage changes ion concentration and pH levels

What step activates a series of metabolic reactions and development of the embryo?

Change in pH level in egg

What is one significant reaction that follows fertilization?

Burst of a protein synthesis

What are maternal effects?

Early embryonic development controlled by mRNAs stored in the oocyte rather than the embryos own genome

What are the proposed mechanisms of maternal effects?

Masked maternal message hypothesis

Poly (A) tail hypothesis

Translational efficiency hypothesis

(Uncapped messages, Sequestered messages)

Masked maternal message hypothesis

Proteins bound to 3’UTR physically block stored mRNAs from ribosomes, and ionic changes or phosphorylation releases them at fertilization or maturation

Poly A tail hypothesis

Stored mRNAs are kept silent with short poly A tails and the AAUAAA signal and CPE in the 3’UTR let them be repolyadenylated and translated after maturation or fertilization

translational efficiency hypothesis

Low pH in unfertilized oocyte suppresses protein synthesis and the H+ release at fertilization raises cytoplasmic pH which activates the translational machinery and triggers a burst of translation

Uncapped messages hypothesis

Stored mRNAs have a nonmethylated cap and cant be translated until a burst of methylation at fertilization completes the cap

Sequestered message hypothesis

mRNA is physically kept away from ribosomes

Cleavage

A process that the zygote, after fertilization, begins the production of a multicellular multicellular organism

What is involved in cleavage?

short cell cycles where cell division and mitosis repeat without any cell growth periods, mass of the embryo does not increase

End result of cleavage

Blastula, hollow sphere of cells

What are cleavage stage cells called

Blastomeres

What caused the transition from fertilization to cleavage in an embryo

Activation of mitosis promoting factor (MPF)

What are the subunits of MPF

large cyclin B and small cyclin dependent kinase

What happens after MPF is used up?

Cell has to synthesize the components, adding growth stages, know as MBT

Mid blastula transition

Stage where MPF is used up and cell cycle slows as embryo shifts to using its own genome

The two factors that determine pattern of embryonic cleavage

The amount and distribution of yolk within the cytoplasm

Position of asters of the mitotic apparatus that determines the plane of cleavage

Holoblastic

Complete cleavage - radial, spiral, bilateral, rotational

Meroblastic

Incomplete cleavage - bilateral, discoidal, superficial

Amphibian cleavage

Holoblastic and radially symmetrical, contains large amount of yok at vegetal hemisphere so faster cleavage in animal hemisphere, results in different sizes of cells

Bird cleavage

Meroblastic and discoidal, because large amount of yok cleavage can only occur in the blastodisc at animal pole, forms blastoderm

Blastoderm

Sheet of cells the blastodisc becomes

Subgerminal cavity

Space between the blastoderm and the yolk

Area pellucida

One cell thick center left after deep cells are shed

Area opaca

Peripheral ring of cells that are not shed

Epiblast

Upper layer

Hypoblast

Lower layer

Blastocoel

Space between the epiblast and hypoblast

Cleavage of Mammal

rotational holoblastic

Describe the first mammalian cleavage

Normal meridional division

Describe the second mammalian cleavage

Ome blastomere divides meridionally and the other equatorially

Features that distinguish mammalian cleavage

Slow divisions

rotational orientation

marked asynchrony

What does asynchrony mean in mammalian cleavage

Blastomeres do not divide at the same time

Compaction

After the 3rd cleavage, 8-cell blastomeres huddle together into a compact ball

What do the internal cells become after compaction

Inner cell mass (embryo, yolk sac, allantois, amnion)

What do the external cells become after compaction

Trophoblast, forms the chorion

Gastrulation

integrated cell and tissue movements that rearrange the blastula cells and establish the multilayered body plan

What happens to cells during gastrulation?

They get new positions and new neighbors

Three germ layers

Ectoderm

Mesoderm

Endoderm

When are the germ layers first produced?

During gastrulation

Invagination

Infolding of a cell sheet into the embryo

Involution

Inturning of a cell sheet over the basal surface of an outer layer

Ingression

Migration of individual cells into the embryo

Delamination

Splitting of one cell sheet into two sheets

Epiboly

Expansion of one cell sheet over other cells

Where does frog gastrulation begin?

At the future dorsal side of the embryo

What is the blastopore and where does it form?

A slit-like opening at the marginal zone near the blastula’s equator

What movement forms the blastopore?

Involution of future endodermal cells

What drives involution of the marginal zone cells in frogs?

A change in cell affinity

What do animal cells doe during frog gastrulation?

They undergo epiboly and converge at the blastopore

What happens when migrating marginal cells reach the dorsal lip in amphibians?

They turn inward and travel along the inner surface of the outer cell sheet

What do cells that involute through the dorsal lip of amphibians become?

Pharyngeal cells of the foregut, head mesoderm, and notochord

What do the lateral and ventral lips contribute in amphibians?

Additional mesodermal and endodermal precursor cells.

Are the cells of the blastopore lip fixed in amphibians?

No, they are constantly changing

What do the mesodermal cells migrate on in amphibians?

Fibronectin secreted by cells of the blastocoel roof

Which two movements occur together in frog gastrulation?

Involution and epiboly

Which germ layer gives rise to the avian embryo?

Epiblast

What does the hypoblast form in avian embryos?

Parts of external membranes, especially the yolk sac and the stalk linking the yolk to the digestive tube

What marks the start of avian gastrulation?

Formation of the primitive streak

Where does the primitive streak first appear?

As a thickening of the epiblast at the posterior region

What does the primitive streak mark?

The anterior-posterior axis

what causes the thickening that forms the primitive streak?

Ingression of mesodermal cells from the epiblast and migration of lateral posterior epiblast cells towards the center

What is the primitive groove in avian embryos?

A depression in the primitive streak that acts as the blastopore

What is Hensen’s node?

A regional thickening at the anterior end of the streak (primitive knot)

Hensen’s node is equivalent to what in amphibians?

The dorsal lip of the blastopore.

What do cells through Hensen’s node form?

Foregut, head mesoderm, and notochord.

What do cells through the lateral streak give rise to in avian embryos?

Most endodermal and mesodermal tissues.

Which cells migrate through the streak first in avian embryos?

Those destined to become the foregut.

Name the two streams from the lateral streak

A deep stream and a second stream spreading between the hypoblast and epiblast.

What does the deep stream do and form?

Joins the hypoblast along the midline, displaces it laterally, and forms all the endodermal organs.

What does the second stream form?

Mesodermal parts of the embryo and the extraembryonic membrane.

When does the regression phase begin in an avian embryo?

When the streak reaches about 60-75% of the area pellucida length.

What happens during regression in an avian embryo?

Hensen’s node moves from anterior to posterior while cells continue to ingress.

What does the node form at its most posterior position in an avian embryo?

The anal region.

What is the epiblast made of after regression?

Entirely presumptive ectodermal cells.

How do ectodermal cells surround the yolk?

By epiboly, migrating off the blastodisc.

How is mammalian gastrulation related to birds’?

It’s similar.

Where does gastrulation occur in mammals?

In the bilaminar germ disc (epiblast + hypoblast) formed by the inner cell mass.

What does gastrulation set up?

Interactions between newly positioned tissues, leading to further commitment and differentiation.