Early Development Review • Zygote = 1 celled stage • Cleavage=early cell division (no cell growth) • Morula = solid multicelled embryo • Blastula = “hollow” multicelled embryo • Blastocoel = space/cavity in blastula • Gastrulation - formation of germ layers • Gastrula - embryo gastrulating • Germ Layers - ecto-, endo-, & mesoderm Early Development Review • Neurulation - formation of nervous system from ectoderm • Neurula - embryo neurulating Cleavage and Blastula Zygote & Cleavage • Yolk (lecithin) = food storage • Animal pole = less yolky end of zygote • Vegetal pole = yolky end of zygote • Holoblastic cleavage = zygote divides completely (plesiomorphic for vertebrates) • Meroblastic cleavage = yolky region does not divide (apomorphic, evolved at least 3 times in vertebrates) Cleavage Blastula • Cavity is flat and “above” yolk in species with meroblastic cleavage. • Blastodisc = small cellular area “on” large yolk • In most therian mammals a blastocyst forms. Blastocyst = a blastula containing a distinct inner cells mass (stem cells) ds Amniota meroblastic cleavage & sn cr oc ak od es ili an s bi rd s liz ar m on tr em es m ar su pi al s eu th er ia tu rt le s b A ow m fin ia ca lv a TE LE O ST S he s st & urg pa e dd on le s ga fis rs h re ed fis M y mero. cleavage xin la i (h a m pr gfis meroblastic ey he s) s cleavage C ho n Ac dric tin ht op hy te s co ry el gi ac i an lu th ng fis s am he ph s ib ia M ns am m a R ep lia til ia Craniate Phylogeny meroblastic cleavage Class Actinopterygii meroblastic cleavage single, dorsal lung * holoblastic cleavage *blastocyst (with inner cell mass) Gastrulation • Hollow blastula “pushes” in. • Cells invaginate (pouch in), involute (fold in), or ingress (move in as individual to form a multi-layered embryo. • Forms germ layers. • Method of gastrulation varies among vertebrates. • Amount of yolk affects gastrulation. cells) Lancelet Gastrulation Frog Gastrulation Similar in lampreys, nonteleost actinopterygiians, lungfishes, & other amphibians Amniote Gastrulation • Primitive Streak = elongate region of ingressing cells in an amniote gastrula • Primitive streak gastrulation modified gastrulation for a large volume of yolk. Small disk (blastodisc) of cells on top of a large yolk. • Therian Mammals = little yolk but still retain primitive streak gastrulation. Chicken Blastula animal pole Primitive Streak Gastrulation Chicken Gastrulation Primitive Streak Posterior Chicken Gastrulation Node Primitive Streak Posterior Chicken Gastrulation Node Primitive Streak Posterior yx i n la i (h a m pr gfis ey he s) s C ho n Ac dric tin ht op hy te s co ry el gi ac i an lu th ng fis s am he ph s ib ia M ns am m a R ep lia til ia M Chicken Gastrulation Node Primitive Streak Posterior Fate Maps Craniate Phylogeny primitive streak gastrulation Neurulation • Mesodermal notochord signals overlying ectodermal epithelium to form hollow neural tube. • Neural tube J central nervous system. • Primary neurulation = ectodermal epithelium infolds anteriorly. • Secondary neurulation = occurs in the post-anal tail of (all) vertebrates; mesenchymal mesoderm cells form a rod-shaped mass that cavitates. Neurulation Primary Neurulation Primitive Streak Gastrulation Amniote Gastrulation Node Primitive Streak Posterior Amniote Neurulation Node Primitive Streak Posterior Neural Fold Amniote Neurulation Node Primitive Streak Neural Tube (brain) Neural Fold Posterior Vertebrate Ectoderm • Generalized Ectoderm = covers embryo. • Neurectoderm = cells that fold in to form neural tube (brain, spinal cord, retina, some nerves). • Neural Crest = migratory ectodermal tissue; forms in neurulation (pigment cells, “face” skeleton, ganglionic nerves, meninges, & chromaffin cells of adrenal glands). • Ectodermal Placodes = ectodermal epithelium thickens to form most of the sense organs (and some head nerve ganglia). Neural Crest Migration M yx i n la i (h a m pr gfis ey he s) s C ho n Ac dric tin ht op hy te s co ry el gi ac i an lu th ng fis s am he ph s ib ia M ns am m a R ep lia til ia Craniate Phylogeny secondary neurulation posteriorly neural crest; ectodermal placodes Vertebrate Mesoderm • Chordamesoderm = notochord. • Somites (Paraxial) = pouched mesoderm segments formed lateral to notochord. dermatome, sclerotome, & myotome • Intermediate Mesoderm = lateral & posterior pouched mesodermal segments. • Lateral Plate Mesoderm = ventral unsegmented mesoderm; around coelom. splanchnic – deep; along endoderm somatic – superficial; along body wall Vertebrate Mesoderm Somitomeres Somites Lateral Plate Mesoderm Intermediate Mesoderm Cephalochordate Mesoderm Formation During Neurulation Vertebrate Mesoderm Somitomeres & Somites • Somitomeres = small, less defined head somites. Vertebrate Neurula (section) somite notochord coelom gut intermediate mesoderm lateral plate mesoderm Vertebrate Mesoderm Vertebrate Neurula (section) somite notochord coelom gut intermediate mesoderm lateral plate mesoderm Vertebrate Embryo (section) neural crest cells sclerotome dermatome myotome splanchnic mesoderm gut intermediate mesoderm coelom lateral plate mesoderm somatic mesoderm Vertebrate Embryo (section) neural crest cells sclerotome dermatome myotome gut epidermal ectoderm coelom Vertebrate Embryo (section) pigment cell precursors sclerotome (neural crest) ganglionic neuron precursors dermatome myotome developing skin epidermis neural crest dermis (neural crest) gut coelom somatic (lateral plate) mesoderm Vertebrate Embryo (section) sclerotome horizontal septum dermatome myotome developing skin gut epidermis neural crest dermis coelom somatic (lateral plate) mesoderm Vertebrate Embryo (section) vertebra horizontal septum dermatome myotome developing skin gut epidermis neural crest dermis coelom somatic (lateral plate) mesoderm Vertebrate Embryo (section) vertebra horizontal septum dermatome myotome developing skin epidermis neural crest dermis gut coelom somatic (lateral plate) mesoderm Salmon Section Diagram Lamprey Section Human Thorax Section Mesentaries & “Membranes” • Septum = a “wall”-like division • Membrane = relatively thin sheet of tissue • Mesentary = a relatively thin twolayered sheet of mesothelium with connective tissue between within the coelomic cavity. (General usage) • Mesothelium = simple squamous coelomic epithelium derived from lateral plate mesoderm. Visceral Mesenteries • Dorsal Mesentery – (splancnic mesoderm) supports digestive tract (and blood vessel pathway) from dorsal body wall. [greater omentum/mesogaster, mesentery (proper), mesocolon] • Ventral Mesentery – (splancnic mesoderm) supports digestive tract from ventral body wall. – developmentally lost along most of body except in region of liver [lesser omentum (hepatogastric & hepatoduodenal “ligaments”) and coronal & falciform “ligaments”] Vertebrate Neurula (section) somite notochord gut intermediate mesoderm lateral plate mesoderm Vertebrate Embryo (section) splanchnic mesoderm gut somatic mesoderm lateral plate mesoderm Vertebrate Embryo (section) splanchnic mesoderm gut somatic mesoderm lateral plate mesoderm Vertebrate Embryo (section) coelom dorsal & ventral mesenteries (splanchnic lat. pl. mesoderm) gut somatic lateral plate mesoderm splanchnic lat. pl. mesoderm Vertebrate Embryo (section) coelom dorsal & ventral mesenteries gut somatic lateral plate mesoderm splanchnic lat. pl. mesoderm (splanchnic lat. pl. mesoderm) Vertebrate Embryo (section) coelom dorsal mesentery (splanchnic lat. pl. mesoderm) gut somatic lateral plate mesoderm splanchnic lat. pl. mesoderm Visceral Mesenteries Extraembryonic Membranes • Extraembryonic membranes = membranes that do NOT contribute to the adult body. Shed at birth/hatching. • Chorion in non-amniotes = acellular layer covering developing embryo. • Yolk sac in non-amniotes = membrane continuous with midgut, surrounds yolk mass; composed of endoderm, mesoderm, and ectoderm. ch or io n Non-Amniote Membranes Yolk Sac Yolk endoderm splanchnic mesoderm Amniotes • 4 extraembryonic membranes. • Yolk sac = endoderm and splanchnic mesoderm; contains yolk • Allantois = endoderm and splanchnic mesoderm; extension of posterior gut • Chorion = ectoderm and somatic mesoderm; surrounds “everything” • Amnion = ectoderm and somatic mesoderm; surrounds embryo Reptile Development embryo somatic mesoderm coelom endoderm splanchnic mesoderm ectoderm Yolk Reptile Development somatic mesoderm allantois splanchnic mesoderm coelom Reptile Development amnion wa st e yolk sac O2 CO2 nutrients chorion allantois Human Development • Humans exhibit typical eutherian mammal development. • Holoblastic cleavage leads to blastocyst • Implantation of blastocyst in uterus using a special tissue called trophoblast • Early formation of amnion. • Primitive streak gastrulation • Primary neurulation with secondary neurulation in post-anal tail Human Cleavage Human Early Development Human Blastocyst/Implantation Human Blastocyst/Implantation Human Gastrulation Human Neurulation Human Development Human Extraembryonic Memb. = Human Extraembryonic Memb. Human Extraembryonic Memb. Mesoderm Differentiation • Chordamesoderm – notochord • Lateral Plate Mesoderm somatic mesoderm splanchnic mesoderm • Intermediate Mesoderm • Somites & Somitomeres (Paraxial) dermatome sclerotome myotome Ectoderm Differentiation • Generalized Ectoderm • Neurectoderm (neural tube) • Neural Crest • Epidermal Placodes Endoderm