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Chapter 47 Animal Development BIOL 223 Embryonic Development • Embryonic development • determined by the zygote’s genome • and molecules in the egg called cytoplasmic determinants • Cell differen2a2on • specializaBon of cells in structure and funcBon • Morphogenesis • process by which an animal takes shape Stages of embryonic development • Three stages of development • Cleavage PLAY • cell division creates a hollow ball of cells • called a blastula • Gastrula2on • cells are rearranged into a three-‐ layered gastrula • Organogenesis • the three layers interact and move to give rise to organs • Due to posiBonal informaBon 1 FerBlizaBon • FerBlizaBon • brings the haploid nuclei of sperm and egg together • forming a diploid zygote • The sperm’s contact with the egg’s surface • iniBates metabolic reacBons in the egg • that trigger the onset of embryonic development The Acrosomal ReacBon • Acrosome • Structure at the Bp of the sperm • releases hydrolyBc enzymes • that digest material surrounding the egg • acrosomal reac2on • Gamete contact and/or fusion • depolarizes the egg cell membrane • sets up a fast block to polyspermy • Not seen in mammals • Charge inside of egg changes from negaBve to posiBve The CorBcal ReacBon • cor2cal reac2on • induces a rise in Ca2+ • that sBmulates cor2cal granules • to release their contents outside the egg • causes formaBon of a fer2liza2on envelope • that funcBons as a slow block to polyspermy 2 Fig. 47-3-5 Sperm plasma membrane Sperm nucleus Fertilization envelope Acrosomal process Basal body (centriole) Sperm head Actin filament Cortical Fused granule plasma membranes Perivitelline Hydrolytic enzymes space Acrosome Jelly coat Vitelline layer Sperm-binding receptors Egg plasma membrane EGG CYTOPLASM AcBvaBon of the Egg • sharp rise in Ca2+ in the egg’s cytosol • increases the rates of cellular respiraBon and protein synthesis by the egg cell • egg is now acBvated • sperm nucleus merges with the egg nucleus • and cell division begins Cleavage • cleavage • a period of rapid cell division without growth • parBBons the cytoplasm of one large cell • • Morula • Blastula • • into many smaller cells called blastomeres Solid ball of cells (~4-‐32 cells) ball of cells with a fluid-‐filled cavity called a blastocoel (a) Fertilized egg (b) Four-cell stage (c) Early blastula (d) Later blastula 3 Cleavage • The eggs and zygotes of many animals • have a definite polarity • except mammals • Polarity defined by distribuBon of yolk (stored nutrients) • vegetal pole has more yolk • animal pole has less yolk • three body axes • established by the egg’s polarity • and by a corBcal rotaBon following binding of the sperm • CorBcal rotaBon • exposes a gray crescent opposite to the point of sperm entry Fig. 47-7 Dorsal Right Anterior Posterior Left Ventral (a) The three axes of the fully developed embryo Animal pole Animal hemisphere Vegetal hemisphere Pigmented cortex Point of sperm nucleus entry First cleavage Future dorsal side Gray crescent Vegetal pole (b) Establishing the axes Cleavage Fig. 47-8-6 • 0.25 mm 0.25 mm • Cleavage planes usually follow a paVern that is relaBve • to the zygote’s animal and vegetal poles Zygote 2-cell stage forming Animal pole 4-cell stage forming 8-cell stage Blastocoel Vegetal pole Blastula (cross section) 4 Cleavage • Cell division is slowed by yolk • Holoblas2c cleavage • complete division of the egg • occurs in species whose eggs have liVle or moderate amounts of yolk • sea urchins and frogs • As well as mammals • Meroblas2c cleavage • incomplete division of the egg • occurs in species with yolk-‐rich eggs • repBles and birds GastrulaBon • Gastrula • Three layered embryo with a primiBve gut • Called germ layers • Gastrula2on • Process of making a gastrula • rearranges the cells of a blastula • Into the three layers • embryonic germ layers • ectoderm forms the outer layer • endoderm lines the digesBve tract • mesoderm partly fills the space between the endoderm and ectoderm Frog GastrulaBon • GastrulaBon in the frog (holoblasBc cleavage) • frog blastula is many cell layers thick • Cells of the dorsal lip • originate in the gray crescent • and invaginate to create the archenteron • Cells conBnue to move from the embryo surface • into the embryo by involu2on • • • • These cells become the endoderm and mesoderm The blastopore encircles a yolk plug when gastrulaBon is completed The surface of the embryo is now ectoderm • innermost layer is endoderm • middle layer is mesoderm 5 SURFACE VIEW CROSS SECTION Animal pole Blastocoel Dorsal lip of blastopore Dorsal lip of blastopore Blastopore Early gastrula Vegetal pole Blastocoel shrinking Archenteron Ectoderm Mesoderm Blastocoel remnant Endoderm Archenteron Key Blastopore Future ectoderm Future mesoderm Future endoderm Late gastrula Yolk plug Blastopore Chick GastrulaBon • GastrulaBon in the chick (meroblasBc cleavage) • The embryo forms from a blastoderm • upper layer of the blastoderm (epiblast) • • And sits on top of a large yolk mass moves toward the midline of the blastoderm • and then into the embryo toward the yolk • primi2ve streak • movement of different epiblast cells • • Thickening midline gives rise to the endoderm, mesoderm, and ectoderm Fig. 47-11 Dorsal Fertilized egg Anterior Left Primitive streak Embryo Right Yolk Posterior Ventral Primitive streak Epiblast Future ectoderm Blastocoel Migrating cells (mesoderm) Endoderm Hypoblast YOLK 6 Organogenesis (NeurulaBon) • organogenesis • various regions of the germ layers • • develop into rudimentary organs Early in vertebrate organogenesis • the notochord forms from mesoderm • and the neural plate forms from ectoderm neural plate soon curves inward • • • forming the neural tube • will become the central nervous system Neural crest cells • develop along the neural tube of vertebrates • and form various parts of the embryo (nerves, parts of teeth, skull bones, and so on) • Mesoderm lateral to the notochord forms blocks called somites • Lateral to the somites, the mesoderm splits to form the coelom Fig. 47-12 Eye Neural folds Somites Tail bud Neural plate Neural fold SEM 1 mm 1 mm Notochord Neural crest cells Coelom Somite Neural tube Neural Neural fold plate Neural crest cells Notochord Ectoderm Archenteron (digestive cavity) Outer layer of ectoderm Mesoderm Endoderm Neural crest cells Archenteron (c) Somites (a) Neural plate formation Neural tube (b) Neural tube formation DerivaBves of Embryonic Germ Layers ECTODERM Epidermis of skin and its derivatives (including sweat glands, hair follicles) Epithelial lining of mouth and anus Cornea and lens of eye Nervous system Sensory receptors in epidermis Adrenal medulla Tooth enamel Epithelium of pineal and pituitary glands MESODERM ENDODERM Notochord Skeletal system Muscular system Muscular layer of stomach and intestine 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 7 Developmental AdaptaBons of Amniotes • Amniotes • Embryos of birds, other repBles, and mammals • four extraembryonic membranes form around the embryo: • • • develop in a fluid-‐filled sac in a shell or the uterus chorion • funcBons in gas exchange • Contributes to the placenta in mammals Amnion amnion • encloses the amnioBc fluid • yolk sac • allantois • Allantois Embryo Amniotic cavity with amniotic fluid Albumen encloses the yolk Shell Yolk (nutrients) Chorion • disposes of waste products and contributes to gas exchange Yolk sac Mammalian Development • The eggs of placental mammals • Are small and store few nutrients • Exhibit holoblasBc cleavage • Show no obvious polarity • GastrulaBon and organogenesis • resemble the processes in birds • and other repBles • Early cleavage • is relaBvely slow in humans and other mammals Mammalian Development • Blastocyst • Formed at compleBon of cleavage • inner cell mass • Inside blasotcoel • A group of cells develops into embryo • and forms the extra-‐embryonic membranes • trophoblast • outer epithelium of the blastocyst • iniBates implantaBon in the uterus • and the inner cell mass of the blastocyst forms a flat disk of cells • As implantaBon is completed • gastrulaBon begins 8 Fig. 47-16-1 Endometrial epithelium (uterine lining) Uterus Inner cell mass Trophoblast Blastocoel Fig. 47-16-2 Expanding region of trophoblast Maternal blood vessel Epiblast Hypoblast Trophoblast Mammalian Development • epiblast cells • invaginate through a primiBve streak • to form mesoderm and endoderm • Placenta • allows for the exchange of materials • between the mother and embryo • is formed from the trophoblast • mesodermal cells from the epiblast • and adjacent endometrial Bssue • By the end of gastrulaBon • the embryonic germ layers have formed Expanding region of trophoblast Amniotic cavity Epiblast Hypoblast Yolk sac (from hypoblast) Extraembryonic mesoderm cells (from epiblast) Chorion (from trophoblast) Amnion Chorion Ectoderm Mesoderm Endoderm Yolk sac Extraembryonic mesoderm Atlantois 9 You should now be able to: 1. Describe the acrosomal reacBon 2. Describe the corBcal reacBon 3. DisBnguish among meroblasBc cleavage and holoblasBc cleavage 4. Compare the formaBon of a blastula and gastrulaBon in a sea urchin, a frog, and a chick 5. List and explain the funcBons of the extraembryonic membranes 10
