Early Embryology

Front 1

zygote

Back 1

male and female pronuclei join

Front 2

zona pellucida

Back 2

membrane + ova

Front 3

Cleavage

Back 3

mitotic division without growth, occuring as it moves the uterine horn

Front 4

blastomers

Back 4

ball of cells

Front 5

morula

Back 5

ball of blastomeres
when morula reaches the uterine cavity the zona pellucida dissolves, influx of fluid creates a cavity in the morula and it is now called a blastocyst

Front 6

development of the blastocyst:
trophoblast:
Inner Cell Mass (ICM)

Back 6

= single layer of cells surrounds the blastocyst cavity. Will form an extraembryonic membrane (placenta), and will undergo further differentiation
ICM = at one of the blastocyst will form embryo

Front 7

6 days after fertilization

Back 7

blastocyst contacts uterine wall and beings inplantation
trophoblast differentiates into inner cytotrophoblast and outer invasive layer called syncytiotrophoblast, they both develop into placenta

Front 8

Formation of primative germ layers and cavities

Back 8

Amniotic fluid
hypoblast
epiblast
primative yolk sac

Front 9

Amniotic Cavity

Back 9

space develops b/w ICM and trophoblast

Front 10

hypoblast

Back 10

ICM cells facing blastocyst layer

Front 11

epiblast

Back 11

cells b/w the hypoblast and amniotic cavity segregate to this, all tissue from the embryo are derived from this

Front 12

Primative Yolk Sack

Back 12

hypoblast line the blastocyst forming this, lined by extraembryonic endoderm
epiblast cells line the amniotic cavity and form the extraembryonic ectoderm
hypoblast cells stream out and form the extraembryonic mesoderm layer surrounding the amnion and yolk sac
now have a small bi-layered embryonic disc situatied between the yolk sac and amnion

Front 13

Gastrulation (3 weeks po)

Back 13

Main Events:
1. formation of the primative streak
2. formation of 3 embryonic germ layers (trilaminer disc)
3. formation of the notochord

Front 14

formation of the primative streak

Back 14

epiblast cells migrate medially and form the middle thickening = primative streak,
cranial end of the streak = primative node - important for organizing center of embryo
depressions form the primative pit in the node and primative groove in the streak

Front 15

formation of the trilaminar disc

Back 15

epiblast cells migrate inwards at primative groove and pit and form new layer (3rd layer) b/w epiblast and hypoblast = embryonic mesoderm
some epiblast cells migrate in and replace hypoblast cells forming a new layer below the mesoderm and above the yolk sac = embryonic endoderm
surface epiblast cells = embryonic ectoderm

Front 16

Migrating mesodermal cells follow distinct migration routes and will form

Back 16

precordal plate
notochord
different layers of mesoderm

Front 17

precordal plate

Back 17

first cells to migrate, migrate cranially from primative pit

Front 18

notochord

Back 18

cells follow precordal plate cells, migrate cranially from pit and form medial cord of cells extending from pit to precordal plate

Front 19

cells migrating in at the primative streak form: (from cranial to caudal)

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paraxial mesoderm - most medial
intermediate mesoderm
lateral mesoderm
extraembryonic mesoderm - most lateral

Front 20

oropharyngeal (buccopharyngeal) membrane

Back 20

at cranial and caudal ends of the embryo, mesoderm cells do not migrate between endoderm and ectoderm. in these areas, the ectoderm is fused to the endoderm forming this membrane cranialy and the clocal membrane caudally

Front 21

Caudal Dysgenesis

Back 21

epiblast cells stop migration to soon and caudal end of the embryo does not develop properly

Front 22

sirenimelia

Back 22

most severe form of caudal dysegenesis

Front 23

Notochord Development

Back 23

cells migrate cranially from the primative pit and form midline cord of cells extending from the node to precordal plate. Primative pit extends into the cord creating a canal. Floor of canal fuses with endoderm forming the notochrodal plate, short transient neuroenteric canal
notochordal plate proliferates and infolds forming notochord. Notochord detaches from the underlying endodern
notochord is the primary inducer of differentiation of embryonic tissues. vertebral column forms around notochord and most of it degenerates except for a small portion that form the nucleus pulposus of ithe IV disc
it is a transient structure for the development of the neural tube

Front 24

notochordal plate

Back 24

fused layers degenerative leaving roof of canal

Front 25

neuroenteric canal

Back 25

short transient communication betwen the two divisions, yolk sac and amnion, if persists results in a rare defect where central canal of spinal cord communicates with intestines

Front 26

Primative Node

Back 26

genes in this area determine the organization of the embryo and development of the embryo axes cranial-caudal, dorsal-ventral, left-right

Front 27

Reversal of Laterality Sequences

Back 27

Partial: partial defects in left and right sideness
Situs Inversis: complete reversal of left and right

Front 28

Neuralation (complete by 4 weeks po): formation of the neural tube

Back 28

ectoderm over notochord (mesoderm) thickens and forms neural plate (neuroectoderm)
neural plate invaginates forming neural folds on either side of the neural grove
neural folds approach each other and fuse forming neural tube. closure starts in the cervical area and extends cranially and caudally
anterior and posterior neuropores: anterior closes first (25 days po), then posterior (28 po)
as neural folds approach each other, neural creast cells seperate from the creast of neural folds and form clusters dorsolateral to neural tube
ectoderm closes over the surface and mesoderm surrounds the neural tube
Mesoderm - vertebrae and meninges
Neural Tube - CNS
Neural Crest - PNS, meninges, head, mesenchyme, melanocytes, adrenal medulla

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Neural Tube Defects

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can occur if neural tube doesnt close or defects in induction of other tissues

Front 30

neaural tube does not close

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anencephaly - lack of brain
spina bifida cystica
rachischisis - most severe, no brain,loss of spinal cord

Front 31

neural tube closes but other tissue defects

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protrusion of the meninges = meningocele
protrusion of the meninges + neural tissue = encephalocele and myelomeningocele
vertebral defects = spina bifida occulta, least severe

Front 32

Mesoderm migration, organization, and its derivatives

Back 32

1. precordal plate cells - induce forebrain development
2. notochord
3. cardiogenic area - mesoderm cells that migrate around precordal plate, lie cranial to neural tube but will be drawn ventral caudally with embryo folding
4. paraxial mesoderm - closest to neural tube, gives rise to somites, in the cranial region referred to as somatomeres

Front 33

somite formation

Back 33

paraxial mesoderm proliferates and segregates
head region: remains loosely arranged = somatomeres
neural crest + somatomeres form head mesenchyme
head mesenchyme - pharyngeal arches, bones and CT of cranical facial structures
from occipital reigion to caudal region paraxial mesoderm segregates and forms paired somites on either side of the developing neural tube, by week 5 po 42-44 pairs of somites
somite mesoderm organizes further (from paraxial mesoderm)

Front 34

Scelrotome

Back 34

ventral medial region to- bone forming cells of vertebra and ribs

Front 35

Epimere

Back 35

dorsolateral region to - muscles, CT of the back proper muscles (dorsal muscles)

Front 36

hypomere

Back 36

dorsolateral region to - muscles of the CT body wall and extremities, ventral muscles and limbs

Front 37

dermatome

Back 37

dorsal most region to - dermis of skin

Front 38

Intermediate mesoderm

Back 38

urogenital organs, ducts and glands

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lateral (plate) mesoderm

Back 39

forms layer extending from intermediate mesoderm to edges of embryonic disc (starts out as a solid and then spaces appear)
spaces apprear in lateral mesoderm and seperate into 2 layers with a space inbetween
somatic (parietal) layer: lines inner body wall (under the mesoderm)
splanchnic (visceral) layer:gut organ muscles + CT , gut organ derivatives

Front 40

intraembryonic cavity (coelom)

Back 40

space betwen layers, body cavities

Front 41

Folding of the embryo

Back 41

folding of cranial and caudal ends occur simultaneously with later foldings and results in:
1. head and tail folds tuck under, cardiogenic area brought in ventrally
2. endoderm forms gut tube surrounded by splanchinic mesoderm, small caudal outpocket = allantosis, gut tube connected to yolk sac by vitrelline duct
3. amnion is pulled ventrally and surrounds entire embryo
4. "purse string" effect draws vitrillene duct, allantosis and body stalk together to form an umbillica ring