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)

Craniate Phylogeny Myxini (hagfishes) Chondrichthys Actinopterygii coelacanths amphibians lungfishes lampreys Mammalia Reptilia meroblastic cleavage mero. cleavage mero- blastic cleavage

Class Actinopterygii sturgeons & paddlefish Amia calva bowfin TELEOSTS reedfishes TELEOSTS gars meroblastic cleavage single, dorsal lung

Amniota lizards & snakes crocodilians montremes marsupials eutheria turtles lizards & snakes crocodilians birds *blastocyst (with inner cell mass) * holoblastic cleavage meroblastic cleavage

Gastrulation Hollow blastula “pushes” in. Cells invaginate (pouch in), involute (fold in), or ingress (move in as individual cells) to form a multi-layered embryo. Forms germ layers. Method of gastrulation varies among vertebrates. Amount of yolk affects gastrulation.

Lancelet Gastrulation

Frog Gastrulation Similar in lampreys, non-teleost 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 Posterior Primitive Streak

Chicken Gastrulation Node Posterior Primitive Streak

Chicken Gastrulation Node Posterior Primitive Streak

Chicken Gastrulation Node Posterior Primitive Streak

Fate Maps

Craniate Phylogeny Myxini (hagfishes) Chondrichthys Actinopterygii coelacanths amphibians lungfishes lampreys Mammalia Reptilia primitive streak gastrulation

Neurulation Mesodermal notochord signals overlying ectodermal epithelium to form hollow neural tube. Neural tube  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 Posterior Primitive Streak

Amniote Neurulation Node Neural Fold Posterior Primitive Streak

Amniote Neurulation Neural Node Tube (brain) Neural Fold Primitive Streak 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

Craniate Phylogeny Myxini (hagfishes) Chondrichthys Actinopterygii coelacanths amphibians lungfishes lampreys Mammalia Reptilia 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 inter- mediate meso- derm lateral plate mesoderm notochord coelom gut

Vertebrate Mesoderm

Vertebrate Neurula (section) somite inter- mediate meso- derm lateral plate mesoderm notochord coelom gut

Vertebrate Embryo (section) neural crest cells dermatome sclerotome myotome inter- mediate meso- derm gut splanchnic mesoderm somatic coelom lateral plate mesoderm

Vertebrate Embryo (section) neural crest cells dermatome sclerotome myotome gut epidermal ectoderm coelom

Vertebrate Embryo (section) pigment cell precursors (neural crest) ganglionic neuron dermatome sclerotome myotome developing skin coelom gut dermis epidermis neural crest somatic (lateral plate) mesoderm

Vertebrate Embryo (section) dermatome sclerotome myotome horizontal septum developing skin coelom gut dermis epidermis neural crest somatic (lateral plate) mesoderm

Vertebrate Embryo (section) dermatome vertebra myotome horizontal septum developing skin coelom gut dermis epidermis neural crest somatic (lateral plate) mesoderm

Vertebrate Embryo (section) dermatome vertebra myotome horizontal septum developing skin coelom gut dermis epidermis neural crest 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 two-layered 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 inter- mediate meso- derm lateral plate mesoderm notochord gut

Vertebrate Embryo (section) gut splanchnic mesoderm somatic lateral plate mesoderm

Vertebrate Embryo (section) gut splanchnic mesoderm somatic lateral plate mesoderm

Vertebrate Embryo (section) coelom splanchnic lat. pl. mesoderm somatic lateral plate mesoderm dorsal & ventral mesenteries (splanchnic lat. pl. mesoderm) gut

Vertebrate Embryo (section) coelom splanchnic lat. pl. mesoderm somatic lateral plate mesoderm dorsal & ventral mesenteries (splanchnic lat. pl. mesoderm) gut

Vertebrate Embryo (section) coelom somatic lateral plate mesoderm dorsal mesentery (splanchnic lat. pl. mesoderm) gut 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.

Non-Amniote Membranes chorion Yolk Yolk Sac 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 Yolk embryo coelom somatic mesoderm endoderm ecto- derm somatic mesoderm splanchnic embryo Yolk

Reptile Development allantois somatic mesoderm splanchnic coelom

Reptile Development O2 waste CO2 nutrients amnion yolk sac chorion allantois amnion yolk sac chorion O2 CO2 waste nutrients

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.

Development Review

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