Chapter 47 Animal Development

Embryonic development/fertilization Preformation: until 18th century; miniature

infant in sperm or egg Epigenesis: proposed by Aristotle; animal

emerges/develops from an unformed egg

Fertilization in a Sea Urchin

At fertilization/conception in a sea urchin:1. Contact: sperm contacts egg’s jelly coat2. Acrosomal reaction: hydrolytic enzymes in acrosome

make a hole in the jelly; actin filaments lengthen from the sperm head - the acrosomal process

3. Growth of acrosomal process: process attaches to receptors on the vitelline layer; enzymes digest a hole into the layer

4. Fusion of membranes of sperm and egg5. Fast block to polyspermy: membrane depolarization

prevents multiple fertilizations; causes the cortical reaction

6. Cortical reaction: release of Ca2+ causes the cortical granules to release enzymes that separates the vitelline from the plasma membrane. The vitelline layer becomes the fertilization envelope, further preventing more sperm from entering the fertilized egg.

Slow block to polyspermy: the fertilization envelope Egg activation: the rise in Ca2+ increases metabolic activity and

protein synthesis

Fertilization in mammals

Fertilization in Mammals:1. Sperm migrates past follicle cells and binds to

receptors in the zona pellucida of the egg.2. The acrosomal reaction: release of hydrolytic

enzymes that break through the zona.3. Sperm reaches the plasma membrane and bind

to receptors on the egg membrane.4. The membranes fuse and sperm contents

spill into the egg.5. The cortical reaction: harden the zona

pelucida (block of polyspermy).

The Fertilized Egg & Cleavage Cleavage: succession of rapid cell divisions. Blastomeres: resultant cells

of cleavage by mitosis No G1 or G2 phase in mitosis The blastomeres willcontain cytoplasm fromthe original, large cell Therefore, the blastomeres will end up with different cytoplasmic contents. This is very important as it sets the stage for major developmental events.

Most animals have eggs and zygotes with definite polarity:

Yolk: nutrients stored in the egg at the vegetal pole.

The lowest concentrationof yolk is at theanimal pole.

The animal pole in many animals becomesthe anterior (head) part of the body.

In frog eggs, the animalpole is dark grey in colordue to melanin granulesthe yolk is yellow.

At fertilization, the greycortex slides over to reveal a non-pigmentedregion called the “greycrescent,” which will become the dorsal side of the embryo.

Cleavage in a frog embryo

1 cell 2 cell 4 cell 8 cell Morula: 16-64 cell stage; solid ball of cells Blastula: at least 128 cells; hollow ball stage

of development Blastocoel: fluid-filled cavity in

late morula and blastula

Cleavage Meroblastic cleavage: in eggs of birds, reptiles,

fishes, and insects; yolk-rich egg with most cell division taking place in the small disc at the animal pole of egg.

Holoblastic cleavage: in eggs of urchins and frogs where there is little yolk.

Gastrulation in sea urchins Gastrulation:

rearrangement of cellls of the blastula to form a triploblastic gastrula1. Mesenchyme cells migrate

into the blastocoel2. Vegetal pole invaginates

(blastopore becomes ?)3. Endoderm Archenteron Digestive tube4. Archenteron fuses with

the ectoderm 5. Gastrulation is complete

Gastrulation in frogs1. Blastocoel off-center;

wall is more than one-cell thick

2. Dorsal lip of theblastopore formson side of blastula.Cells of endo/mesodermmove inward (“involution”).Ecotoderm spreads overthe surface of embryo.

3. The three germ layerscontinue to move inward,filling in the space ofthe blastocoel.The lip becomes circular.

4. Blastopore filled = yolk plug

Organogenesis: formation of organsGerm Layer Organs and Tissues in

the AdultEctoderm Skin, glands, nails, epithelial lining of

mouth and rectum, sense receptors in epidermis, cornea and lens of eye, nervous system, adrenal medulla, tooth enamel, epithelium of pineal and pituitary gland

Endoderm Epithelial lining of digestive tract (except mouth and rectum), epithelial lining of respiratory system; liver, pancreas, thyroid, parathyroids, thymus; lining of urethra, urinary bladder, and reproductive system.

Mesoderm Notochord, skeletal system, muscular system, circulatory and lymphatic system, excretory system, reproductive system (except germ cells); dermis of skin, lining of body cavity, adrenal cortex.

Organogenesis starts with folds, splits, and clustering of cells

The organs that first develop in all frog and chordate embryos are the– Neural tube: forms from the ectoderm just

above the archenteron; becomes the brain and spinal chord

– Notochord: forms from the dorsal mesoderm

– Somites: forms from the mesoderm just lateral to the notochord; arranged on both sides along the notochord; becomes the vertebrae of backbone and skeletal muscles.

Amniote Development Two solutions to reproducing on land:

1. Shelled egg (amniotic egg)2. Uterus of placental animals Both provide a watery/fluid environment

that surrounds the embryo The fluid and embryo are surrounded by a

membrane called the amnion.

Amniotes

Avian Development1. Meroblastic

“incomplete” cleavage:cleavage atop a largeyolk mass. A blastodiscwith two layers (epiblastand hypoblast) is formed.

2. Gastrulation: Some cellsof the epiblast migrateinto the interior by the primitive streak. Somecells move laterally to formthe mesoderm, and othersmove downward to formthe endoderm.

3. Early organogenesis:Archenteron forms whenendoderm pinches upward. The embryowill remain attached tothe yolk by the yolk stalkwhich formed from the hypoblast.The neural tube, somites,and notochord form thesame way as in the frog.

Extraembryonic membranes in a chick1. Yolk sac: membrane over

the yolk; blood vessels in sac will deliver nutrients to embryo.

2. Amnion: membrane around the embryo.

3. Chorion: cushions the embryo against mechanical shock.

4. Allantois: disposal sac for uric acid. It will expand, pushing the chorion against the vitelline membrane. It will also serve as a respiratory organ for the embryo, delivering oxygen to the embryo.

Mammalian Development1. 7 Days: Blastocyst

containing inner mass ofcells reaches uterus. Theouter layer of cells is called the trophoblast.It will form the fetal portion of the placenta.

2. Trophoblast secretes enzymes that help itpenetrate the endometrium.Trophoblast thickens andproduces fingerlike projections into the maternal tissue.-Epiblast embryo-Hypoblast yolk sac

Mammalian Development3. Extraembryonic membranes

develop:-Trophoblast Chorion-Epiblast Amnion and

placenta4. Gastrulation: cells from the

epiblast move inward through the primitive streak(as seen in the chick). The three germ layers areformed and are surroundedby mesodermal extra-embryonic membranes.

Late in the second week of human gestation, the embryo has two cell layers, an

epiblast and a hypoblast.

The following website has some images of human embryo development. http://www.med.unc.edu/embryo_images/unit-welcome/welcome_htms/contents.htm

Epiblast cells invaginate at the primitive streak. They will form the mesoderm cells.

The Cellular and Molecular Basis for Morphogenesis

Reorganization of the cytoskeleton will change a cell’s shape. Example: Formation of the neural tube is do to microtubule elongation and microfilament contraction.

Movement of cells via lamellipodia (flat sheets of cells moving) or filopodia (spikes of cells moving). Example: Gastrulation – invagination of cells by filopodia.

Convergent extension is when cells merge together to become narrower (convergent) and longer (extension).

-Example: Archenteron elongation-ECM fibers may direct cell movement-Cell adhesion molecules (CAMs): located on cell surfaces bind to CAMs on other cells.-Cadherins: cell-to-cell adhesion molecules; require Calcium to function; very important in the formation of blastula.

The Fate of Cells Depend on:1.Cytoplasmic determinants2.Cell-cell induction

- chemical signals - cell-surface interactions affect gene expression

Fatemaps Fatemaps: cell lineage is determined

-Embryologist W. Vogt found that you can determine which parts of the embryo will be derived from regions of the blastula.

Cytoplasmic Determinants Location melanin and yolk in frog eggs

determine the animal and vegetal poles, which in turn determine the anterior-posterior axis.

Entry of sperm into egg triggers formation of grey crescent, which determines the dorsal-ventral axis.

Axis of cleavage: -1st cleavage in frog embryo is equal.-Uneven cleavage mayproduce abnormalembryo.

Induction: cells influence on another cell’s fate

Hans Spemann and Hilde Mangold (1920s) discovered that the dorsal lip of the blastopore signals a series of inductions that result in the formation of the neural tube and other organs.

Dorsal lip = primary organizer

Pattern Formation in Vertebrate Limbs Pattern formation: the development of an

animal’s spatial organization; the arrangement of organs and tissue.

Positional information: molecules that tell cells its own location and what it will become.

Pattern formation of limbs determined by two areas:1. AER (apical ectodermal ridge): a limb-bud

organizer; made up of mesoderm and very thick layer of ectoderm; it is responsible

the distal-proximal (outward) growth of limbs).

-AER release FGF (fibroblast growth factors), which are protein signals for

distal-proximal limb growth.

2. ZPA (zone of polarizing activity): located at the posterior side of limb bud. It is necessary for the posterior-anterior growth of the limb.- ZPA secretes a protein, sonic hedghog.

Fertilization in a Sea Urchin

Fertilization in mammals

Blastula

Gastrula Ectoderm Endoderm

Mesoderm

Epidermis associated withstructure (nails,hair, skin, etc.)

Brain andnervous sys.

Embryonic gut

Inner liningof digestive

tract

Glands(including

pancreas/liver)

Inner liningof respiratory

tract

Notochord

SomitesMesenchyme(migratory cells)

Dermis(inner layer)

Circulatory System

Bones &Cartilage

Gonads

Muscle

ExcretorySystem

Outer coveringof organs