Embryology

pre-embryonic period (1st 3 wks)

· fertilisation forms zygote

· zygote undergoes cleavage

· into a number of cells called blastomeres

· forms morula

· 3 days after fertilization

· 16 blastomeres

· cavity forms to convert morula into blastocyst

· consists of

· inner cell mass

· blastocyst cavity

· trophoblast (outer cells)

· implants into uterus

· trophoblast proliferates and erodes maternal sinusoids

· establishes uteroplacental circulation

· blastocyst cavity becomes yolk sac

· amniotic cavity appears between trophoblast and inner cell mass

· inner cell mass becomes embryonic disc

· disc consists of

· ectoderm continuous with amniotic cavity on dorsal surface

· endoderm continuous with yolk sac on dorsal surface

· primitive streak appears at caudal end of ectoderm

· migrates between ectoderm and endoderm

· forms mesoderm

· trilaminar embryo produced

Axial skeleton

Notochord

· day 16

· primitive knot is cranial thickening of primitive streak

· knot forms notochordal process

· runs down middle of embryonic disc

· primitive pit develops in primitive knot

· extends into notochordal process to form notochordal canal

· notochordal process fuses with underlying endoderm

· floor breaks down so notochordal canal becomes continuous with endoderm

· remnant of notochordal process becomes notochord plate

· notochordal plate infolds to form ntochord

· endoderm becomes continuous again

Neural tube

· day 18

· embryonic ectoderm thickens over notochord

· forms neural plate

· called neuroectoderm

· gives rise to CNS

· neural plate invaginates

· forms neural groove and neural folds

· fuses to form neural tube

· some cells do not fuse and form neural crest

· gives rise to parts of PNS

· spinal sensory ganglia, autonomic ganglia, pigment cells, Schwann cells

Somites

· day 20

· mesoderm along notochord divides into 44 paired bodies

· called somites

· give rise to axial skeleton and musculature

· somites differentiate into

· ventromedial sclerotome

· dorsolateral dermatomyotome

· sclerotome gives rise to

· bones, cartilage, ligaments

· dermatomyotome gives rise to

· skeletal muscles

· dermis

embryonic period (4th to 8th wks)

· folding occurs

· forms cylindrical embryo

· dorsal yolk sac incorporated into embryo

· forms primitive gut

· germ layers differentiate

· beginnings of organ systems established

· ectoderm

· CNS, PNS, pituitary

· skin and appendages

· mesoderm

· cartilage, bone, muscle

· heart and blood vessels

· kidneys, gonads, spleen

· lining of body cavities

· endoderm

· lining of GI, GU and respiratory tracts

· thyroid, liver, pancreas

· most critical time for development

Axial skeleton

· starts at 4th wk

· cells from sclerotome migrate

· ventromedially to form vertebral bodies and intervertebral disc

· dorsally to form neural arches

· ventrolaterally to form body wall

Ventromedial

· sclerotome cells surround notochord

· each sclerotome consists of

· loose cranial cells

· dense caudal cells

· some of caudal cells form intervertebral disc

· remaining caudal cells and adjacent cranial cells form centrum of vertebra

· vertebra is intersegmental structure

· forms from 2 adjacent sclerotomes

· notochord

· disappears where surrounded by vertebral body

· becomes nucleus pulposis in intervertebral disc

Appendicular skeleton

· starts at 4th wk

Limb buds

· limb buds develop as small elevations of ventrolateral body

· arm buds then leg buds

· consist of

· mesenchyme from somatic mesoderm

· covering layer of ectoderm

· ends differentiate

· flatten into hand and foot plates

· digits differentiate at margins

· direction of limbs changes

· initially directed caudally

· extend ventrally at 7th wk

· then rotate on longitudinal axis

· rotate in opposite directions and to different degrees

· start with

· flexor aspect ventral

· extensor aspect dorsal

· preaxial border cranial

· postaxial border caudal

· upper limbs

· rotate laterally through 90^o

· elbow comes to lie dorsally

· lower limbs

· rotate medially through 90^o

· knee comes to lie ventrally

Bones

· cartilage develops by intramembranous ossification in 5th wk

· mesenchymal chondrification starts at 6th wk

· form hyaline cartilage models of skeleton

· primary ossification centres have developed in nearly all bones by 12th wk

Dermatomes

· spinal nerves have initial segmental arrangement

· elongation of limbs maens that not all reach skin

· results in ventral and dorsal axial lines

· where discontinous dermatomes meet

foetal period (9th wk to birth)

· characterised by

· rapid body growth

· differentiation of organ systems

· less vulnerable to teratogens