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

Myx

ini (

hagf

ishes

)

lam

preys

Chondrichth

ys

Actin

optery

gii

coel

acan

ths

lungfis

hes

amphib

ians

Mam

mal

ia

Reptil

iam

ero

bla

stic

clea

vag

e

mero-blasticcleavage

mer

o. c

leav

age

Class Actinopterygii

meroblasticcleavage

sturg

eons

& pad

dlefis

h

gars bowfin

Amia

cal

va

TELEOSTS

reed

fishes

single, dorsallung

Amniota

montre

mes

mar

supia

ls

euth

eria

turtl

es

lizar

ds & s

nakes

croco

dilian

s

birds

meroblastic cleavage

holoblasticcleavage

*blastocyst (with inner cell mass)

*

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

animalpole

Primitive Streak Gastrulation

Chicken Gastrulation

Posterior

PrimitiveStreak

Chicken Gastrulation

Node

Posterior

PrimitiveStreak

Chicken Gastrulation

Node

Posterior

PrimitiveStreak

Chicken Gastrulation

Posterior

PrimitiveStreak

Node

Fate Maps

Craniate Phylogeny

Myx

ini (

hagf

ishes

)

lam

preys

Chondrichth

ys

Actin

optery

gii

coel

acan

ths

lungfis

hes

amphib

ians

Mam

mal

ia

Reptil

ia

primitivestreak

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

Posterior

PrimitiveStreak

Node

Amniote Neurulation

NeuralFold

Posterior

PrimitiveStreak

Node

Amniote Neurulation

NeuralTube(brain)

NeuralFold

Posterior

PrimitiveStreak

Node

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

Myx

ini (

hagf

ishes

)

lam

preys

Chondrichth

ys

Actin

optery

gii

coel

acan

ths

lungfis

hes

amphib

ians

Mam

mal

ia

Reptil

ia

secondary neurulation posteriorlyneural 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 endodermsomatic – superficial; along body wall

Vertebrate Mesoderm

Lateral Plate Mesoderm

Somites

Somitomeres

IntermediateMesoderm

Cephalochordate Mesoderm Formation During Neurulation

Vertebrate Mesoderm

Somitomeres & Somites Somitomeres = small, less defined

head somites.

Vertebrate Neurula (section)

notochord

coelom

somite

inter-mediatemeso-derm

lateralplate mesoderm

gut

Vertebrate Mesoderm

Vertebrate Neurula (section)

notochord

coelom gut

somite

inter-mediatemeso-derm

lateralplate mesoderm

Vertebrate Embryo (section)neuralcrestcells

dermatome

sclerotome

myotome

splanchnicmesoderm

somaticmesoderm

coelom

gut

inter-mediatemeso-derm

lateralplate mesoderm

Vertebrate Embryo (section)

coelom

neuralcrestcells

dermatome

sclerotome

myotome

gutepidermalectoderm

Vertebrate Embryo (section)

gut

pigment cell precursors(neural crest)

ganglionicneuron precursors(neural crest)

dermatome

sclerotome

myotomedeveloping

skin

dermis

epidermis

neural crest

coelom

somatic(lateral plate)

mesoderm

Vertebrate Embryo (section)

gut

dermatome

sclerotome

myotome

horizontalseptum

developingskin

dermis

epidermis

neural crest

coelom

somatic(lateral plate)

mesoderm

Vertebrate Embryo (section)

gut

dermatome

vertebra

myotomedeveloping

skin

dermis

epidermis

neural crest

horizontalseptum

coelom

somatic(lateral plate)

mesoderm

Vertebrate Embryo (section)

gut

dermatome

vertebra

myotome

horizontalseptum

coelom

somatic(lateral plate)

mesoderm

developingskin

dermis

epidermis

neural crest

Salmon Section Diagram

Lamprey Section

Human Thorax Section

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.

Mesentaries & “Membranes”

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)

notochordsomite

inter-mediatemeso-derm

lateralplate mesoderm

gut

Vertebrate Embryo (section)

splanchnicmesoderm

somaticmesoderm

gut

lateralplate mesoderm

Vertebrate Embryo (section)

gut

lateralplate mesoderm

splanchnicmesoderm

somaticmesoderm

Vertebrate Embryo (section)

gut

coelom

dorsal & ventralmesenteries(splanchnic lat. pl. mesoderm)

splanchniclat. pl. mesoderm

somaticlateralplate

mesoderm

Vertebrate Embryo (section)

gutdorsal & ventralmesenteries(splanchnic lat. pl. mesoderm)

coelom

splanchniclat. pl. mesoderm

somaticlateralplate

mesoderm

Vertebrate Embryo (section)

gutdorsalmesentery(splanchnic lat. pl. mesoderm)

coelom

splanchniclat. pl. mesoderm

somaticlateralplate

mesoderm

Visceral Mesenteries

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.

Extraembryonic Membranes

Non-Amniote Membranes

YolkYolkSac endoderm

splanchnicmesoderm

chor

ion

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

Amniotes

Reptile Development

coelom

endoderm

ecto-derm

somaticmesoderm

splanchnicmesoderm

embryo

Yolk

Reptile Development

coelom

allantois

somaticmesoderm

splanchnicmesoderm

Reptile Development

allantois

amnionyolk sac

chorion

nutrients

was

te

O2

CO2

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