Embryology and Development 3-4

transfer of materials between maternal and fetal blood

oxygen and nutrients in blood of mother's intervillous spaces diffuse across cell membranes into fetal capillaries of villi, waste products diffuse out in opposite direction

umbilical arteries

carry deoxygenated blood away from fetus

umbilical vein

carries oxygenated blood to heart of fetus

endoderm

inner germ layer

mesoderm

middle germ layer

ectoderm

outer germ layer

endoderm function

forms lining of digestive tract and other organs such as the respiratory system

mesoderm function

forms tissues such as muscle, bones, and blood vessels

ectoderm function

forms skin and nervous system

oropharyngeal membrane

future site of mouth, at head end of embryo

cloacal membrane

future site of anus, urinary, and reproductive tracts, at tail end of embryo

degradation of cloacal membrane

will degenerate in seventh week to form openings of anus and urinary and reproductive tracts

formation of neural tube

neural folds approach eachother which turns neural plate into a neural tube

neural tube function

forms brain and spinal cord

neural crest

formed form ectodermal cells

neural crest function

forms all sensory and post ganglionic neurons of peripheral nervous system and general connective tissue of head

embryonic folding

process which converts embryo from a flat two-dimensional disc to a three-dimensional cylinder

longitudinal (lateral) fold

forms the gut tube

primitive gut formation

lateral folds incorporate the dorsal part of the yolk sac into the embryo as the primitve gut when they move toward the midline

folds in median plane

makes a head and tail fold

head fold

brings developing heart and mouth into their eventual adult positions

tail fold

brings developing anus into its eventual adult position

location of germ layers after folding

after folding the endoderm is in the centre (gut), the ectoderm is on the outside (epidermis), and the mesoderm is in between

coelom

forms body cavities

formation of coelom

as lateral folds occur the coelom is formed

allantois

structure that moves into the connecting stalk

removal of yolk sac

lateral folding continues and once it goes all the way around the yolk sac is pinched off

tube connecting mouth and anus

a tube extends from the oropharyngeal membrane to the cloacal membrane, it connects the mouth and anus

evaginations

pockets that extend from the tube connecting the mouth and anus

evaginations function

forms the pharynx (throat), lungs, liver, anterior pituitary, thyroid gland, and pancreas

formation of bladder

part of allantois becomes the bladder

branchial arches

pockets in the ectoderm

branchial arches function

form the pharynx, auditory tubes, tonsils, thymus, and parathyroids

formation of pericardial cavity

coelom first fuses around the heart as the pericardial cavity

how does the heart get its shape

the heart cavity is created first and then it is filled

formation of pleural and peritoneal cavities

pericardial cavity expands to become the pleural and peritoneal cavities, all cavities eventually become separate

why is embryo curved when embryonic folding is complete

nervous system grows faster than everything else, as the nervous system grows it forces the embryo to curve, this is also why the head is so big at first - the brain is growing very fast

limb buds

beginnings of the growth of the limbs

day 28

arms and legs appear (arms appear first at about day 24)

apical ectodermal ridge

cap at ends of limb bud that stimulate growth of limbs in length

apical ectodermal ridge function

thickens ectoderm, stimulates outward growth

direction of limb tissue growth

limb tissue grows in proximal-to-distal sequence

apoptosis

pre programmed cell death

apoptosis function

get rid of webbing between fingers

when does separation of digits occur

week 7-8

sonic hedgehog protein

protein in humans which stimulates limb development, released form apical ectodermal ridge

order of germ layer development

first ecoderm makes skin, then mesoderm makes blood first then bones and then muscles

why does mesoderm make blood first

mesoderm makes blood first so everything can get oxygen and nutrients, then it makes bones, then muscles, then nerves come in

thaliodomide

drug which caused birth defects

effects of thalidomide

babies limbs didn't develop properly, depending on when the mother took the drug and for how long there were other developmental problems in other organ systems, often caused miscarriage because embryo would die

anti-angiogenesis

preventing formation of blood vessels - limbs could not grow outwards

how is thalidomide still used

thalidomide is still used to treat cancerous tumors to stop them from spreading

development of skeleton

develops from mesoderm or neural crest cells, 'loose cells' in mesoderm go to form part of the skull

how is skeleton built

cartilage skeleton is built first, then it becomes ossified through calcification

development of muscle

develops form somites

myoblasts

early embryonic cells that are derived from somites that develop into skeletal muscle fibers

why do muscle cells have lots of nuclei

because myoblast cells fuse together to make a muscle fiber

proliferation of muscle fibers

muscle fibers continue to proliferate and they will grow all the muscles in the body

nervous system development

derived form neural tube and neural crest cells

what does the neural tube derive

spinal cord, brain, and motor neurons

what do neural crest cells derive

sensory neurons

formation of circulatory system

formation of circulatory system begins just after gastrulation (day 15-16)

angiogenesis

formation of blood vessels

process of angiogenesis

begins in extraembryonic mesoderm in the yolk sac, connecting stalk, and chorion, begins at the yolk sac because it will join with the umbilical cord - circulation necessary here to get blood from mother to fetus

angioblasts

differentiated mesoderm cells which will become the walls of the blood vessels

blood islands

masses of angioblasts

development of blood vessels and primitive blood cells

blood islands will develop spaces in their centre's which will become the lumen of blood vessels, cells inside the spaces of the blood islands are pluripotent and they will develop into primitive blood cells

primitive blood cells

formed by stem cells from mesoderm

endothelial lining

cells that line blood vessels

creation of network of blood vessels for circulatory system

as development continues tubes that have been created begin to fuse together which creates a network

endothelial tubes

name of blood islands with spaces in the middle

formation of endocardial tubes

two of the endothelial tubes are signaled to turn into endocardial tubes via induction

formation of primitive heart tube

occurs in pericardial cavity, the two endocardial tubes fuse together to make the primitive heart tube

formation of primitive heart structures

bulges/dilations are created in the primitive heart tube which eventually form the primitive heart structures

sinus venosus

turns into superior and inferior vena cava tubes

primitive atrium

turns into left atrium and part of right atrium

primitive ventricle

turns into left ventricle

bulbus cordis

turns into right ventrible

when do primitive heart structures form

form when heart starts folding in on itself

movement of primitive atrium and sinus venosus

move to the back of he embryo and up

movement of primitive ventricle and bulbus cordis

move to the front of the embryo and down

when does the heart start contracting

by end of the third week

why does blood not flow through pulmonary artery in a fetus

fetal lungs are collapsed so blood does not need to flow through the pulmonary artery to the lungs

pressure build up in fetal heart

instead of blood being pumped from the right side of the heart to the lungs pressure builds up in the right side of the heart, the pressure pushes blood through two holes in the heart (septums)

foramen ovale

hole between two atria, composed of two septums

septum secundum

by the right atrium, hole on the bottom

septum primum

by the left atrium, hold on front

how do septums remain open

pressure keeps both septums open which allows blood to flow through, septums remain open until birth

interventricular septum

muscular wall between ventricles

why does blood bypass the lungs and liver in a fetus

blood is already oxygenated and filtered of toxins in the mother before it reaches the fetus, liver usually filters blood from the gut after you eat but this is not necessary in the fetus

ductus arteriosus

tube leading from pulmonary artery to aorta

foramen ovale function

divers flow of blood from lungs

ductus arteriosus function

divers flow of blood from lungs

ductus venosus

tube leading from liver to inferior vena cava

ductus venosus function

diverts flow of blood from liver

why does deoxygenated and oxygenated blood mix in a fetus

blood from the mother needs to get tot he vena cava via the umbilical cord (oxygenated blood), the vena cava usually carries deoxygenated blood so oxygenated and deoxygenated blood mix together

when does foramen ovale close

after birth air enters infant's lungs which inflates them and force blood into the pulmonary arteries, the increased pressure on the left side of the heart and decreased pressure on the right forces the septum secundum and septum primum to close which causes the foramen ovale to close

fossa ovalis

former foramen ovale

ligamentum venosum

former ductus venosus (ductus venosus degenerates and turns into a ligament), extends from liver to vena cava

ligamentum teres

former umbilical vein (umbilical vein degenerates and turns into a ligament)