1A03 -EMBRYOLOGY

Ovaries

bilateral gonads

Secondary Oocytes

- gamete cells released in females during ovulation
- have only finished first meiotic division by time of release

Uterus

pear-like structure where embryonic implantation + growth occur

Gametes

- sex cells
- sperm and egg

Gonads

organs that produce gametes

Prenatal Development

- germinal period
- embryonic period
- fetal period

Initial Steps of Fertilization

1. sperm penetrates corona radiata

2. acrosomal reaction: sperm binds to ZP3 glycoprotein → digests zona pellucida

3. one sperm binds to integrin α6β1 → causes fast block to polyspermy

4. female nucleus undergoes 2nd meiotic division

5. pronuclei created (female from nucleus, male from sperm head)

6. fusion of haploid pronuclei → zygotes

Fast Block to Polyspermy

- depolarization
- prevents multi-sperm fertilization

Slow Block to Polyspermy

- release of calcium
- exocytosis of water
- oocyte shrinks
- zona pellucida denatures and ZP3 becomes inactive

Morula and Blastocyst Steps

1. zygote cleaves in zona pellucida (24 hours)

2. morula (12 cells) created around day 5

3. hatching occurs (morula free from zona pellucida)

4. fluid is secreted to form blastocyst around day 6

Trophoblast Layer

- outside layer
- forms embryonic membranes
- those in inner cell mass differentiate into cyto + syncytio

Blastocele

fluid-filled cavity

Inner Cell Mass

- region with more than one cellular layer
- contains embryo proper which becomes embryo

Totipotent and Pleuripotent

totipotent: one cell that can differentiate into every cell type

pleuripotent: can form most cell types but not all

Implantation

burrowing of blastocyst into uterine wall

Placenta

exchange of nutrients/waste between mother and embryo

Maternal Blood Supply (MBS)

connects to blastocyst to provide nourishment and oxygen, and remove waste

Endometrium

- wall closest to uterine cavity
- changes in thickness
- large blood supply

Cytotrophoblasts

- stay close to embryo proper
- separate embryo proper and MBS

Syncytiotrophoblasts

- farther from embryo proper
- invade endometrium
- digest blood vessels
- release hCG

Connecting Stalk

connect embryo proper and trophoblast shell

Lacunae

syncytiotrophoblasts that surround blood vessels and digest walls

Cytotrophoblast Chords

surround syncytiotrophoblasts and lacunae

Embryonic Mesoderm Cells

- build blood vessels on embryo side
- connects maternal and embryonic blood supplies

Chorionic Villi

- formed by cytotrophoblast chords
- fill in with fetal blood vessels
- surrounded by lacuna

Umbilical Cord

- connects chorionic villi to fetus
- has arteries and veins

Chorion

- barrier between maternal/fetal blood supply
- syncytiotrophoblast + basement membrane

Bilaminar Embryonic Disk

- 2 layers of cells in inner cell mass
- has epiblasts and hypoblasts between them

Epiblast Cells

- top layers
- become embryo
- has amniotic sac and cavity

Hypoblast Cells

- bottom layers
- forms extra embyronic tissue
- has yolk sac

Amniotic Sac

- cavity above epiblast
- in blastocele

Amniotic Cavity

- between epiblast and sac
- will fill with fluid

Yolk Sac

- cavity below hypoblast
- forms in blastocele

Gastrulation

- differentiation of epiblast cells
- day 13-14

1. epiblast elongates
2. cells move to form new layer

Layers Formed Through Gastrulation

endoderm: move through streak and replace hypoblast cells → digestive tract and derivatives

mesoderm: stay between epiblast and endoderm → blood/bones/muscles

ectoderm: don't migrate through streak → skin/nervous system

Primitive Streak

- thickened cell region formed during gastrulation
- has head (away from streak) and tail (close to streak) ends

Notochord

- formed on day 16
- solid cylinder of mesoderm
- involved in induction

Neural Plate

- formed on day 18 through induction
- becomes CNS
- thick layer of ectoderm

Formation of Neural Tube (Days 18-26)

neural folds: rise of lateral plates

neural groove: indentation between folds

crest of neural fold: ridges formed as groove/fold increase

neural crest cells: break off → form PNS and ANS

neural tube: become CNS (from crest merging)

Somites

- adjacent to neural tube
- form mesoderm
- become vertebral column, ribs, skeletal muscles

Embryonic Folding and Gut Formation (Day 20)

amniotic cavity folds around 3 germ layers

Longitudinal Fold

- fold along length of embryo
- lateral and median

Lateral Folds

- folds on either side of neural tube
- try to wrap around embryo

Median Plane Folds

amniotic cavity wraps around 3 germ layers in head and tail regions

Primitive Gut Tube

- from lateral folds
- formed from endoderm

Coelom

- from lateral folds
- pocket formed on sides of neural tube in mesoderm
- becomes body cavities

Hindgut and Foregut

hindgut: close to caudal end
foregut: close to cephalic end

Allantois

- pinched of region in yolk sac in connecting stalk
- will form part of bladder

Embryonic Folding and Gut Formation (Day 25)

- openings for mouth and anus form when membranes dissolve
- lateral folds pinch endoderm
- coelom increases in size

Yolk Stalk

narrow region from pinching of endoderm relative to yolk sac

Embryonic Folding and Gut Formation (Day 30)

- amniotic cavity is fully around germ layers
- ectoderm closed off
- embryo is curved

Evaginations

pockets that form different organ systems

Branchial Arches

- pouches in head region
- evaginations move into them

Gut Tube

oropharyngeal to cloacal membrane

Limb Bud Development (Day 24-28)

limb buds → digits

1. blood supply
2. cartilage skeleton
3. cardiovascular system
4. skeletal system
5. muscles + connection to nerve supply

Development of Skeleton, Muscle, Nerves

skeleton: from mesoderm or neural crest cells

muscle: from somites

nervous system: from neural tube and neural crest cells

Myoblasts

- cells differentiated from somites
- migrate to wherever muscle needs to be built
- become muscle fibres

Apical Ectodermal Ridge

- thickening of ectoderm
- stimulates outward growth of tissues

Development of the Circulatory System

1. cells in yolk sac become angioblasts

2. cells organize and form cavities where blood islands are

3. cavities get larger and fuse → tubing networks formed → become blood vessels

Angioblasts

- help build walls of blood vessels
- form blood islands

Blood Islands

- formed by angioblasts on mesoderm
- on surface of yolk sac and inside embryo

Endothelium

cells that line inside of blood vessels

Lumen

inside of a tube

Heart Development

1. two endothelial tubes become endocardial tubes

2. tubes fuse and bend

3. contractions begin at week 3

4. ventricles and atria move

5. sinus venous → right atria + sa node

6. primitive atria → atrium

7. bulbus cordis → right ventricle

8. ventricles leave + connect to pulmonary trunk or aorta

Interatrial Septum

septum secundum: closest to right atrium; inferior

septum primum: closest to left atrium; superior

* hole between septum is foramen ovale

Respiratory System (Day 28)

- begins as single mid-line evagination of foregut
- lung buds branch off developing trachea

Lung Bud

where respiratory system develops