Animal Development Chapter 47

The Miracle of Life Human embryo

Fertilization Function  to bring the haploid nuclei of sperm and egg together to form a diploid zygote Contact of the sperm with the egg’s surface  Initiates metabolic reactions within the egg that trigger the onset of embryonic development Studied in Sea Urchins Very few minor differences in mammlian system Made up of three reactions  Acrosomal  Cortical  Activation of the egg

The Acrosomal Reaction The acrosomal reaction  Is triggered when the sperm meets the egg  Releases hydrolytic enzymes that digest material surrounding the egg  Sperm membrane adhere to receptor proteins on the egg plasma membrane  Egg and sperm fuse with in seconds

Fusion of egg and sperm Causes ion channels to open and sodium ions to flow into the egg Depolarizes the egg cell membrane Prevents other sperm from entering  sets up a fast block to polyspermy Polyspermy  multiple sperm entering the egg resulting in an aberrant # of Chromosmes

The Acrosomal and Cortical Reactions

The acrosomal reaction 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 Egg plasma membrane Vitelline layer Fused plasma membranes Perivitelline space Fertilization envelope Cortical reaction. Fusion of the gamete membranes triggers an increase of Ca 2+ 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 Figure 47.3

The Cortical Reaction Fusion of egg and sperm plasma membranes triggers a signal-transduction pathway. Ca 2+ from the eggs ER is released into the cytosol and propagates as a wave across the fertilized egg  IP 3 and DAG are produced.  IP 3 opens ligand-gated channels in the ER and the Ca 2+ released stimulates the opening of other channels High concentrations of Ca 2+ cause cortical granules to fuse with the plasma membrane and release their contents into the perivitelline space. The vitelline layer hardens into the fertilization envelope: a component of the slow block to polyspermy. The plasma membrane returns to normal and the fast block to polyspermy no longer functions.

Activation of the Egg High concentrations of Ca 2+ in the egg stimulates an increase in the rates of cellular respiration and proteins synthesis. In the meantime, back at the sperm nucleus...  The sperm nucleus swells and merges with the egg nucleus  diploid nucleus of the zygote.  DNA synthesis begins and the first cell division occurs.

Fertilization in Mammals Capacitation, a function of the female reproductive system, enhances sperm function. A capacitated sperm migrates through a layer of follicle cells before it reaches the zona pellucida. The envelopes of both the egg and sperm nuclei disperse.  The chromosomes from the two gametes share a common spindle apparatus during the first mitotic division of the zygote

Building the Body of the Animal Cleavage Gastrulation Organogenesis

Cleavage Cleavage follows fertilization. The zygote is partitioned into blastomeres. Each blastomere contains different regions of the undivided cytoplasm and thus different cytoplasmic determinants.

Gastrulation Rearranges the cells of a blastula into a three- layered embryo, called a gastrula, that has a primitive gut The three layers produced by gastrulation  Are called embryonic germ layers The ectoderm  Forms the outer layer of the gastrula The endoderm  Lines the embryonic digestive tract The mesoderm  Partly fills the space between the endoderm and ectoderm

Figure ECTODERMMESODERMENDODERM Epidermis of skin and its derivatives (including sweat glands, hair follicles) Epithelial lining of mouth and rectum Sense receptors in epidermis Cornea and lens of eye Nervous system Adrenal medulla Tooth enamel Epithelium or pineal and pituitary glands Notochord Skeletal system Muscular system Muscular layer of stomach, intestine, etc. Excretory system Circulatory and lymphatic systems Reproductive system (except germ cells) Dermis of skin Lining of body cavity Adrenal cortex Epithelial lining of digestive tract Epithelial lining of respiratory system Lining of urethra, urinary bladder, and reproductive system Liver Pancreas Thymus Thyroid and parathyroid glands

Organogenesis Various regions of the three embryonic germ layers  develop into the rudiments of organs during the process of organogenesis Notochord  Stiff dorsal rod characteristic of all chordates  develops from the mesoderm Neural Plate  forms from the ectoderm. Curves inwardrolling into the Neural Tube  Becomes the CNS

Birds & Reptiles  Amniotes The four extraembryonic membranes are the yolk sac, amnion, chorion, and allantois. Cells of the yolk sac digest yolk providing nutrients to the embryo. The amnion encloses the embryo in a fluid- filled amniotic sac which protects the embryo from drying out. The chorion cushions the embryo against mechanical shocks. The allantois functions as a disposal sac for uric acid.

The four extraembryonic membranes are the yolk sac, amnion, chorion, and allantois.  Cells of the yolk sac digest yolk providing nutrients to the embryo.  The amnion encloses the embryo in a fluid- filled amniotic sac which protects the embryo from drying out.  The chorion cushions the embryo against mechanical shocks.  The allantois functions as a disposal sac for uric acid. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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Mammalian Development Blastocyst  mammalian version of the baastula Inner cell mass  develops into the embryo  embryonic stem cells Trophoblast  fetal portion of the placenta