BIO 308 - Dr. Daley - Life Cycles

Embryogenesis - stages between fertilization and hatching

Circle of Life

• Cleavage - series of rapid mitotic divisions of the zygote - get smaller cells - blastomeres

– Cluster of cells called the blastula

• Gastrulation - division slows and dramatic cell movements occur

– germ cell layers form - ectoderm, mesoderm and endoderm

Circle of Life

• Organogenesis - three cells layers interacting with one another to form tissues and organs

• Many organs are made or cells derived from more than one germ layer - skin (ectoderm and mesoderm)

• Many cells undergo long migrations - e.g. precursors of blood cells, pigment cells and gametes

Circle of Life

Germ cells - precursors of gametes and gametes themselves arise from specialized portion of the egg cytoplasm - other cells called somatic cells

Development of gametes - gametogenesis - not usually completed until organism is physically mature

Adult organism eventually undergoes senescence and dies

Developmental History of a Frog

Frog Life Cycle

Gametogenesis and fertilization - seasonal events - depends on other plants and insects - needs moderate temperature

Photoperiod and temp - tells pituitary gland of a female it is spring - a hormone stimulates estrogen production - this stimulates liver to produce yoke protein - via blood to enlarging eggs in ovary

Amplexus and Xenopus Eggs

• Amplexus

Frog Life Cycle - Egg Poles

Vegetal pole of egg - bottom half contains more yoke cells divide more slowly

Animal pole - upper half of egg - divides more rapidly

Progesterone - stimulates eggs to resume meiotic divisions - were “frozen” in Metaphase I

Sperm also occur on a seasonal basis

Frog Life Cycle - Fertilization

Allows egg to complete second meiotic division - egg now as a haploid pronucleus

Egg and sperm pronuclei fuse - zygote (diploid)

Cytoplasm moves to new locations

Activates molecules necessary to begin cleavage & development

Frog Life Cycle - Cleavage

During cleavage - volume of egg stays the same - tens of thousands of cells

Morula and then blastula stages

Animal hemisphere divides faster - get blastocoel

Cleavage & Blastula

Frog Life Cycle - Gastrulation

Begins about 180° from sperm entry point

See dimple called blastopore

Cells migrate through the pore - to animal pole become dorsal mesoderm

Next more cells enter - pore expands - these cells become - lateral and ventral mesoderm

Cells remaining on outside - ectoderm - expands ventrally to surround embryo

Large yolky cells remain at the vegetal (until surrounded by ectoderm) - become endoderm

Frog Life Cycle - Organogenesis

Begins when notochord (mesoderm) tells dorsal ectoderm - not skin - rather form a tube to become the nervous system

Neurula stage - get neural tube - covered by future epidermal cells

Neural crest cells - between epidermal cells and neural tube - almost 4th germ layer

Give rise to pigment cells, peripheral neurons, and cartilage of the face

Neural tube now induce further organogenesis in neighboring cells

Gastrulation

Frog Life Cycle - Organogenesis

Mesodermal tissue adjacent to notochord - becomes segmented into somites

Precursors of back muscle, cartilage of vertebrae & ribs and dermis

Mouth and anus form and embryo elongates - tadpole like

Neuron connections are made - gills form - ready to hatch

Neurulation

Prehatching & Mature Tadpole

Frog Life Cycle - Metamorphosis

Initiated by hormones from the tadpole’s thyroid

In anurans (frogs and toads) almost all systems change

Tail disappears as legs differentiate

Cartilaginous skull become bony

Horny teeth disappear - new jaw shape and fly-catching tongue muscle develops

Gills regress & lungs expand

As metamorphosis end - first germ cells begin

In Rana pipiens - egg development last 3 years

Metamorphosis in Rana

Developmental Patterns - Metazoa

Three major evolutionary paths

Diploblasts - ectoderm and endoderm - no true mesoderm

Cnidarian - coral, jellyfish, sea anemones and hydras

Ctenophores - comb jellies

Developmental Patterns - Metazoa

The bilateral - bilateral symmetry & 3 germ layers

Protostomes - (mouth first) - molluscs, arthropods & worm phyla - mouth is formed first near - at or near the opening to the gut

Anus forms later - coelom from previously solid mesoderm

Two major braches - ecdysozoa - animals that molt - mostly arthropods

Lophotrochozoa - all have spiral cleavage - annelids, molluscs and flatworms

Developmental Patterns - Metazoa

Deutrostomes - ( mouth second) chordates and echinoderms

Body cavities - coeloms - from mesodermal pouches extending from the gut

Amniote Egg

Major evolutionary advance - originated about 255 MYO in amphibian ancestors of reptiles

Now vertebrates could roam the land

Amphibians must return to the water to lay their eggs

Four sacs - Yolk sac - nutritive proteins, amnion - fluid bathing the embryo, allantois, metabolic wastes collect here and chorion - interacts with outside environment - encased in a shell