Embryo and fetal development

BHCS1002

Embryonic period

• lasts 2-9 weeks

• at the end of the period, most organs will nearly be formed

• inner cell mass becomes flat plate of cells = Bilaminar embryonic disc

the disc contains of 2 cell layers:

1) upper layer → epiblast

2) lower layer → hypoblast

Extraembryonic membranes

1) amnion liquid-filled sac protects and buffers developing embryo

2) chorion. functions as vascularized gas exchange site next to the shell

3) yolk Sac. extension of the hypoblast. used in early nutrient absorption. loses significance as placenta takes on this role

4) Allantois early blood formation and contributes to formation of umbilical cord

Ectoderm (outer layer)

skin, central nervous system peripheral nervous system

mesoderm (middle layer)

muscle, connective tissue, notochord, bones and cartilage, circulatory system urogenital system

endoderm (inner layer)

will become: gut, liver, lungs pancreas, salivary glands

notochord

Inductive signalling 

establishment of the embryonic axis

mesoderm differentiation

axial skeleton formation

cell fate determination 

patterning of the surrounding tissue 

nucleus pulposes formation 

Neurulation

• process of neural tube formation

• begins in 3rd week

• the neural plate ectoderm as induced by the notochord and subsequent development of the neural ectoderm to form neural folds and the neural tube

mesodermal Somites

at day 18/19 some mesoderm forms paired blocks (somites) in a craniocaudal sequence 

somites are arranged laterally to notochord and neural tube appear along its whole length

eventually form structures like the vertebrate(backbone) and skeletal muscles of the neck and trunk as well as eg. dermis 

Somites become

• sclerotome - vertebrae and ribs

• myotome - skeletal muscles

• dermatome - dermis

• syndetome - tendons and ligaments

• myoseptum - organisation and segmentation of skeletal muscles

• scleromyotome - both muscles and connective tissue 

GIT development

• formation of the gut tube

• foregut, midgut and hindgut division

• liver pancreas and gallbladder forms

• rotation and herniation 

• mesenteries and vascularisation

• differentiation of GI structures

• finalisation and maturation 

blood

• blood vessels and cells form as blood islands on outer surface of yolk sac

• 6 weeks post fertilisation, liver takes over as main site of blood production

• spleen also site of haemopoiesis. role of spleen persists in foetus and after birth

• in foetus and adult bone marrow contirbtes to blood cell formation

new vessel formation begins with the degradation of the endothellial basemnt membrane by proteases in an existing capilary or venule

elongation of the tbe continues until it reaches another capilary with which it connects allowing blood to circulate

initial cardiac development

2 weeks after fertilisation, the embryo receives oxygen and nutrients via diffusion 

soon after the embryo becomes too large and requires a circlation system

at the end of second week blod vessels begin to form

the heart will be the first organ to reach functioning status

paired, longitudinal endothelial-lined channels- the endocardial heart tubes develop during 3rd week and fuse to form primordial heat tube

by end of third week blood will be circulaating and heart beating

by 5 weels the heart beat can b detected by doppler ultrasongraphy

Growth of the heart tube

• myocardium secretes cardiac jelly throughout process - separating the myocardium and endocardium.

• cardiac jelly is an acellular gelantinous matrix

• cardiac tube lengrhened by growth/ addition of myocardium to outflow pole (from the second heart field)

• looping of hert tube creates different regions: sinus venosus, atrium, PLV, PRV, Conus arteriosus, truncus arteriosus

looping of the tubular heart

• following fusion of heart tubes, initial C looping occurs = ventral bending and rightward rotation 

• rotation of the heart tube to the right

• dextral looping, curvature of dorsal midline on left side

• translocation of inflow region cranially - s looping

looping controlled by PitX2 and lefty signaling - controlling left - right body axis 

Respiratory system

• in the 4th week post fertilisation lungs appear as a single pouch from foregut 

• a ventral groove forms = laryngotracheal groove. this diverticulum ‘buds-off’ gut

• during 5th week the diverticulum enlarges and divides into 2 further buds

• this are the primary bronchi, which produce more buds: 3 on RHS and 2 on LHS

embryonic urinary system 

Kidneys develop from mesoderm 

at 3 weeks post fertilisation, mesoderm in ‘neck’ region differentiates into pronephros (early kidney)

this is a degenerate structure which never functions as an excretory organ in humans

phronephros is followed by the mesonephros

Mesonephros

• Mesonephros - temporary excretory structure, functional by 5th week

• has two components: mesonephric ducts and nephron units. the ducts discharge waste into hindgut(forms cloaca)

• Mesonephrons is replace by metanephros

• few ducts and tubules persist but only in the male, as part of reproductive system

The Cloaca and Bladder

as mesonephros develops rear of the hindgut enlarges to form the cloaca.

a joint urinary genital and digestive region

cloaca is split by a septum into 2 digestive part (rectum) and urogenital part (urethra)

the allantois is a sac associated with cloaca

it extends into umbilical cord and part of the allantois enlarges to contribute to the bladder 

kidney formation

ureter forms at a junction of mesonephros with cloaca: its distal end enlarges and branches to form ducts of adult kidney

this is the metanephros

urogenital sinus part of cloaca forms rest of the urinary system: bladder and urethra

at 36 weeks post fertiliation each kidney has its adult number of nephrons which is like >1 million nephrons

the foetal period

• developmental stage from week 10 to birth

• differentiation and growth of tissues occurs

• developmental changes vital to make organs and tissues functional

• period of dramatic growth

• foetus is about 8g at the start of the 3rd monht but grows to 3400g at birth

foetal measurements

• external characteristics useful for measuring foetal age and expected delivery date

• crown- rump length (CRL) measured by ultrasound accurately determines foetal age until end of 3rd month 

• foetal head measurements as well as femur and foot lengths also used to evaluate age and growth of foetus 

system maturation

all organ systems present by the end of 9 weeks but few functional. (excepption heart and blood vessels)

eyelids remain shut until 5-7 months gestation

ear ossicles can’t vibrate prior to birth 

cerebrum and cerebellum immature at birth

reproductive system immature until puberty

Foetal viability

viability → the ability of fetus to survive

foetuses weighing les than 500g at birth are unlikely to survive

foetuses at a developmental stage of less than 22 weeks are consdered inivable

foetuses born at 22-28 weeks have difficulty surviving, due to immature respiraroty and central nervous systems

Early foetal events

external genitalia of males and females similar until foetal form well established (week 12)

erthreopoiesis decreases in liver and begins in spleen by the end of the 12th week

by the end of the 12th week, primary ossification centres appear in skeleton (skull and long bones)

urine formation by kidney begins between 11-12 weeks (placenta main waste organ).

Amniotic fluid

initially derived from the maternal tissue fluid and maternal plasma diffuses across the amniochorion membrane

before the fetal skin is keratinased fluid can also enter amniotic cavity via foetal skin

also secreted across foetal respiratory tract

foetus contributes urine to amniotic fluid after 1th week (main source after 16 weeks)

fluid is dynamic and 95% replaced daily

amniotic fluid composition

99% is water also contains undissolved foetal matter such as shed cells

other constituents: organic and inorganic salts, protein, sugars, fats, enzymes, hormones

meconium may also be present if foetus is stressed during birth

fluid can be removed by amniocentesis and foetal cells and substaced can be examined

importance if amniotic fluid

1) permits symmetrical growth of the embryo

acts as a barrier to inection

permits normal foetal lung development 

prevents adherence of amnion to embryo

cushions embr against injuries

helps control foetal body tempature 

enables the foetus to move freely 

sceoond trimester

ossification takes place rapily and bones are visible by ultrasound at 16th week

foetal movements start being felt by week 17-20

brown fat forms in key locations

important thermogenesis role in neonates

releaes heat by fatty acid oxidation

foetal skin

until 30 weeks fetus appears red due to thin skin and absenc of subcutaneous fat

skin covered in greasy material = vernix caseosa

fatty secretion from sebaceous glands with dead epidermal cells. protects delicate foetal skin from abrasions, chapping and amniotic flid

20 week foetuses also covered in fine downy hair or lanugo which is lost before birth

third trimester

substantial weight gain

from 24 weeks on surfactant produced in lungs

a phospholipid fluid acts to maintain patency of developing alveoli in lungs

babies born before 28weeks may suffer respiaroty distress

if immenent premature birth is likely, steroid administration may help mature lungs

at 38 weeks

breathing movements move amniotic fluid into lungs, triggering hiccups

meconium accumulates in the bowel

kidneys are fully functional 

fat deposits continue to accumulate

little space left in uterus for movements

circulatory changes in neonate

series of structural changes need to occur..

when lungs are stretched at first breath hanges occur in levels of PCO2 and also in blood levels of bradykinin and prostaglandins

these changes lead to a shut down of:

ductus venosus ductus arteriosus and umbilical vessels as well as closure of foramen ovale

foetal vessels subsequently become ligaments

Foetal Blood

embryonic and foetal erythrocytes are intially nucleated but at birth only 5-10% have nuclei

foetal blood has higher haemoglobin concentration than adult blood

foetal haemoglobin (HbF) has a different structure and a higher affinity for oxygen

(2 gamma chains instead of 2 beta chains)

this increases oxygen- carrying capacity