BIOSC 041 Reproduction & Development Reference: Lecture; Chapters 46 & 47

Processes and Stages of Development

v The processes of development

v Influence of cytoplasmic segregation and induction in cell determination

v Stages of development 1.  Cytoplasmic determinants (egg and sperm) 2.  Fertilization 3.  Cleavage: repackaging the cytoplasm 4.  Gastrulation: producing the body plan 5.  Neurulation: initiating the nervous system 6.  Extraembryonic membranes

The Processes of Development

v Development § A series of progressive changes in shape, form, function § Begins with fertilization of egg with sperm à zygote § Continues throughout an organism’s life cycle

v Development consists of § Growth § Determination § Differentiation § Morphogenesis

The Processes of Development

v Growth § Occurs by cell division and/or expansion §  Starts with zygote

§ Repeated mitotic cell divisions increase cell # § May or may not increase actual size of embryo

The Processes of Development

v The fertilized egg is totipotent §  it can give rise to all other cell types of the organism

v Determination § Commitment of a cell to a particular fate

§  Loss of totipotency §  Influenced by the extracellular environment and the cell

itself acting on the its own genome

v Differentiation § Follows determination § Results from differential gene expression § Cells become structurally and functionally specialized

The Processes of Development

v Morphogenesis § The shaping of the multicellular body and its organs § Results from pattern formation

§ Organization of differentiated tissues into specific structures (e.g., limbs, organs)

§  Influenced by signal transduction pathways that control: § Cell movement within developing embryo (animals) § Apoptosis (programmed cell death)

§ Ultimately under genetic control

Cell Determination: Cytoplasmic Segregation & Induction

v  Chemical signals control cell differentiation

v  Two signal mechanisms are known

1.   Cytoplasmic segregation

§  A factor (signal) is unequally distributed in the zygote and ends up in some daughter cells but not others

§  These cytoplasmic determinants are often transcription factors (proteins that bind DNA to control gene expression)

2.   Induction

§  A factor (signal) is produced and secreted by some cells to induce other cells to differentiate

Cell Determination: Cytoplasmic Segregation

v Polarity occurs early in development § Establishes major body axes

§  Anterior–posterior axis (head – tail)

§  Dorsal–ventral axis (back – belly)

§ The earliest step in differentiation

v Materials are distributed unequally in egg and zygote

§ Dividing cells receive unequal amounts

§ Generates polarity in early embryo

Cytoplasmic Segregation: Asymmetry in the Early Embryo (Part 1)

Cytoplasmic Segregation: Asymmetry in the Early Embryo (Part 2)

Cell Determination: Induction

Example: development of the vertebrate eye

1.  Developing forebrain bulges out at both sides & forms optic vesicles

2.  Optic vesicles signal surface tissue à induce it to form lens placodes

3.  Lens placodes differentiate into lenses

4.  Developing lens induces surface tissue to develop into the cornea

The Stages of Animal Development

1.   Cytoplasmic determinants: Eggs and Sperm are not created equal

2.   Fertilization: the onset of development

3.   Cleavage: Repackaging the Cytoplasm

4.   Gastrulation: Producing the Body Plan

5.   Neurulation: Initiating the Nervous System

6.   Extraembryonic Membranes

1. Sperm & Egg differ in size

v Eggs contain cytoplasmic determinants § Regulate expression of genes affecting developmental fate

of cells §  Strong influence on early development § Cytoplasmic factors set anterior/posterior, and dorsal/

ventral axes

2. Fertilization

v  Union of gametes to form zygote (usually diploid)

v  Contact of (only 1!) sperm with egg’s surface §  Initiates metabolic reactions within egg that trigger onset

of embryonic development

1.   Acrosomal reaction: 1-3 seconds after contact §  Sperm releases hydrolytic enzymes that digest material

surrounding the egg §  Depolarization of egg cell membrane = fast block to

polyspermy

The acrosomal reaction (Fig 47.3)

Sperm nucleus

Sperm plasma membrane

Hydrolytic enzymes

Cortical granule

Cortical granule membrane

EGG CYTOPLASM

Basal body (centriole)

Sperm head

Acrosomal process

Actin

Acrosome Jelly coat (Cumulus)

Egg plasma membrane

Vitelline layer (zona pelucida)

Fused plasma membranes

Perivitelline space

Fertilization envelope

Cortical reaction. Fusion of the gamete membranes triggers an increase of Ca2+ in the egg’s cytosol, causing cortical granules in the egg to fuse with the plasma membrane and discharge their contents. This leads to swelling of the perivitelline space, hardening of the vitelline layer, and clipping of sperm-binding receptors. The resulting fertilization envelope is the slow block to polyspermy.

5 Contact and fusion of sperm and egg membranes. A hole is made in the vitelline layer, allowing contact and fusion of the gamete plasma membranes. The membrane becomes depolarized, resulting in the fast block to polyspermy.

3 Acrosomal reaction. Hydrolytic enzymes released from the acrosome make a hole in the jelly coat, while growing actin filaments form the acrosomal process. This structure protrudes from the sperm head and penetrates the jelly coat, binding to receptors in the egg cell membrane that extend through the vitelline layer.

2 Contact. The sperm cell contacts the egg’s jelly coat, triggering exocytosis from the sperm’s acrosome.

1

Sperm-binding receptors

Entry of sperm nucleus. 4

2. Fertilization

2.   Cortical reaction: rise in Ca2+ stimulates cortical granules to release contents into perivitelline space

2. Fertilization

v These changes cause the formation of a fertilization envelope § Functions as a slow block to polyspermy

Types of Fertilization

1.  Internal fertilization § Male transfers sperm via a penis or other organ into

female’s body

2.  External fertilization § “Spawning”- fishes, amphibians, reptiles

Salmon spawning video: http://www.youtube.com/watch?v=SOllUTBYZtE

3. Cleavage

v Rapid DNA replication and mitosis §  Little actual growth §  Little gene expression

v Formation of blastula §  Large zygote à morula à blastula with blastocoel §  Blastomeres = Individual cells of blastula

v Influenced by: §  Yolk (impedes cleavage furrow- cleavage often incomplete/superficial) §  Maternal cytoplasmic determinants §  Mitotic spindle orientation determines cleavage plane

§  Radial pattern à frogs, sea urchins §  Spiral pattern à molluscs (snail shells) §  Rotational pattern à (mammals = us)

The Mammalian Zygote Becomes a Blastocyst (Part 1)

Cleavage in the Mammalian Embryo

Day 1-2

Day 2-3

Day 3-4

Day 5

Day 6-7

Cleavage in Xenopus (frog): Fate Maps

v Specific blastomeres generate specific tissues & organs § Determined by unequal complements of nutrients and

cytoplasmic determinants in the egg

4. Gastrulation: Producing the Body Plan (Review)

v Gastrulation § Blastula transforms into embryo with three germ layers,

and defined body axes

v Three germ layers form:

§ Endoderm à inner germ layer §  Gives rise to digestive, circulatory, and respiratory tracts

§ Mesoderm à middle germ layer §  Bone, muscle, liver, heart, and blood vessels

§ Ectoderm à outer germ layer §  Epidermis, nervous system

Gastrulation can look very different between groups

Gastrulation in Sea Urchins

Gastrulation in the Frog Embryo

*Primary embryonic organizer

The dorsal lip is very important to development

Gastrulation in the Chick Embryo

5. Organogenesis and Neurulation

v Gastrulation produces an embryo with three germ layers

v Organogenesis occurs next- formation of organs and organ systems

v Neurulation occurs early in organogenesis- begins the formation of the nervous system in vertebrates

Neurulation in the Frog Embryo (Part 1)

Neurulation in the Frog Embryo (Part 2)

6. Extraembryonic Membranes

v Extraembryonic membranes § Originate from germ layers of embryo § Function in nutrition, gas exchange, waste removal

v In the chicken, § Yolk sac forms first

§ Extension of the endodermal tissue of the hypoblast § Constricts at the top to create a tube continuous with

embryo gut § Yolk is digested by the endodermal cells of the yolk sac § Nutrients transported through blood vessels lining outer

surface of yolk sac

Chick Embryo Extraembryonic Membranes

Human Embryo Development

The Mammalian Placenta

Summary of Development/Reproduction

Useful video resources from Khan Academy

Stages of development (video, ~10 min): https://www.khanacademy.org/partner-content/crash-course1/crash-course-biology/v/crash-course-biology-115 Genes (such as Hox) that regulate development (video, ~10 min): https://www.khanacademy.org/partner-content/crash-course1/crash-course-biology/v/crash-course-biology-116