u Chapter 47 Animal Development

Embryonic development/fertilization u Preformation: until 18th century; miniature infant in sperm or egg u Epigenesis: proposed by Aristotle; animal emerges/develops from an unformed egg

Fertilization in a Sea Urchin

uAt fertilization/conception in a sea urchin: 1. Contact: sperm contacts egg’s jelly coat 2. 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 egg 5. 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. uSlow block to polyspermy: the fertilization envelope uEgg 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 u Cleavage: succession of rapid cell divisions. u Blastomeres: resultant cells of cleavage by mitosis  No G1 or G2 phase in mitosis  The blastomeres will contain cytoplasm from the 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: u Yolk: nutrients stored in the egg at the vegetal pole. u The lowest concentration of yolk is at the animal pole. u The animal pole in many animals becomes the anterior (head) part of the body. u In frog eggs, the animal pole is dark grey in color due to melanin granules the yolk is yellow. u At fertilization, the grey cortex slides over to reveal a non-pigmented region called the “grey crescent,” which will become the dorsal side of the embryo.

Cleavage in a frog embryo u 1 cell  2 cell  4 cell  8 cell  u Morula: cell stage; solid ball of cells u Blastula: at least 128 cells; hollow ball stage of development u Blastocoel: fluid-filled cavity in late morula and blastula

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

Gastrulation in sea urchins u Gastrulation: rearrangement of cellls of the blastula to form a triploblastic gastrula 1. Mesenchyme cells migrate into the blastocoel 2. Vegetal pole invaginates (blastopore becomes ?) 3. Endoderm  Archenteron  Digestive tube 4. Archenteron fuses with the ectoderm 5. Gastrulation is complete

Gastrulation in frogs 1.Blastocoel off-center; wall is more than one-cell thick 2.Dorsal lip of the blastopore forms on side of blastula. Cells of endo/mesoderm move inward (“involution”). Ecotoderm spreads over the surface of embryo. 3.The three germ layers continue to move inward, filling in the space of the blastocoel. The lip becomes circular. 4. Blastopore filled = yolk plug

Organogenesis: formation of organs Germ LayerOrgans and Tissues in the Adult Ectoderm 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 u 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 u 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 Development 1. Meroblastic “incomplete” cleavage: cleavage atop a large yolk mass. A blastodisc with two layers (epiblast and hypoblast) is formed. 2. Gastrulation: Some cells of the epiblast migrate into the interior by the primitive streak. Some cells move laterally to form the mesoderm, and others move downward to form the endoderm.

3.Early organogenesis: Archenteron forms when endoderm pinches upward. The embryo will remain attached to the yolk by the yolk stalk which formed from the hypoblast. The neural tube, somites, and notochord form the same way as in the frog.

Extraembryonic membranes in a chick 1. 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 Development 1.7 Days: Blastocyst containing inner mass of cells reaches uterus. The outer layer of cells is called the trophoblast. It will form the fetal portion of the placenta. 2.Trophoblast secretes enzymes that help it penetrate the endometrium. Trophoblast thickens and produces fingerlike projections into the maternal tissue. -Epiblast  embryo -Hypoblast  yolk sac

Mammalian Development 3.Extraembryonic membranes develop: -Trophoblast  Chorion -Epiblast  Amnion and placenta 4.Gastrulation: cells from the epiblast move inward through the primitive streak (as seen in the chick). The three germ layers are formed and are surrounded by 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.

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

The Cellular and Molecular Basis for Morphogenesis u Reorganization of the cytoskeleton will change a cell’s shape. Example: Formation of the neural tube is do to microtubule elongation and microfilament contraction.

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

uConvergent 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 determinants 2.Cell-cell induction - chemical signals - cell-surface interactions affect gene expression

Fatemaps u 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 u Location melanin and yolk in frog eggs determine the animal and vegetal poles, which in turn determine the anterior-posterior axis. u Entry of sperm into egg triggers formation of grey crescent, which determines the dorsal- ventral axis. u Axis of cleavage: -1 st cleavage in frog embryo is equal. -Uneven cleavage may produce abnormal embryo.

Induction: cells influence on another cell’s fate u 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 u Pattern formation: the development of an animal’s spatial organization; the arrangement of organs and tissue. u Positional information: molecules that tell cells its own location and what it will become. u 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 EctodermEndoderm Mesoderm Epidermis associated with structure (nails, hair, skin, etc.) Brain and nervous sys. Embryonic gut Inner lining of digestive tract Glands (including pancreas/liver) Inner lining of respiratory tract Notochord Somites Mesenchyme (migratory cells) Dermis (inner layer) Circulatory System Bones & Cartilage Gonads Muscle Excretory System Outer covering of organs