Embryology

Question 1

3 Stages of Embryonic Development

Answer 1

• pre-embryonic: conception-day 14
• embryonic: day 14-end of 8th week
• fetal: end of 8th week to birth

Question 2

Pre-embryonic

Answer 2

• fertilization
• ovum released from ovary
• fertilization of ovum divides as it moves down the uterine tube into uterus
• morula (solid ball of cells) forms and then forms a cavity inside
• cells are now called blastocyst (hollow ball)
• blastocyst cavity: will become yolk sac which nourishes embryo until placenta is fully formed
• outer layer: trophoblast will become placenta
• inner layer: inner cell mass will become embryo; develops into embryonic disc which has 2 cell layers (ectoderm, endoderm, and a third layer later forms between as the mesoderm)
• blastocyst implants into endometrium
• mother doesn’t know she’s pregnant

Question 3

Embryonic Stage

Answer 3

• organs formed
• ectoderm: sensory organs, epidermis, and nervous system
• mesoderm: dermis, muscles, skeleton, excretory and circulatory systems
• endoderm: gut, liver, pancreas, and respiratory system
• neurulation occurs: folding process where neural plate forms neutral tube

Question 4

Fetal Stage

Answer 4

-nervous system develops more and myelination begins

Question 5

Formation of Nervous System

Answer 5

• occurs during embryonic stage
• two phases: phase 1-neural tube forms (day 21), develops into brain and spinal cord; phase 2-brain formation, begins when neural tube closes

Question 6

Neurulation

Answer 6

• neural plate: thickening of ectoderm, forms on surface of embryo and extends from head to tail, floor of amniotic sac and therefore bathed in amniotic fluid
• neural folds: 3rd week after fertilization
• neural tube: commences in neck region
• neuropores: closed off by end of 4th week, superior closes at 27 days, inferior closes at 30 days, closure coincides with establishment of blood supply
• neural crest: cells from neural folds; becomes cranial and spinal nerves and schwann cells

Question 7

Mantle

Answer 7

• by day 26 neural tube differentiates into 2 layers
• mantle layer: inner wall, contains cell body and becomes gray matter
• marginal layer: outer wall, contains processes of cells whose bodies are in gray and develops into white matter: axons and glial cells

Question 8

Neural Tube Defects

Answer 8

• anterior neural tube defect: anterior neuropores fail to close results in anencephaly
• posterior neural tube defect: posterior neuropores fail to close results in spina bifida or myelomeningocele

Question 9

Neurulation Pt 2

Answer 9

• walls of neural tube thicken to become the brain and spinal cord
• lumen becomes ventricular system and central canal

Question 10

Spinal Cord

Answer 10

• dorsal tube=alar plate: sensory neurons, receive dorsal nerve roots growing in from spinal ganglia, becomes dorsal horn
• ventral tube=basal plate: motor neurons, ventral nerve roots, becomes ventral horn

Question 11

Brain Vesicles

Answer 11

• prosencephalon: forebrain-made up of telencephalon (cerebral cortex, corpus stratum) and diencephalon (thalamus, hypothalamus, optic outgrowth)
• mesencephalon: midbrain-4th week=primitive brain folds at mesencephalon
• rhombencephalon: hindbrain made up of metencephalon (pons, cerebellum) and myelencephalon (medulla)

Question 12

Ventricular System

Answer 12

• neural canal dilates and forms ventricular system (next 4 parts)
• hemispheres=lateral ventricles 1 and 2
• diencephalon=3rd ventricle
• midbrain=cerebral aqueduct, for CSF
• hindbrain=4th ventricle anterior to pons, posterior to cerebellum
• choroid plexus: secretes CSF

Question 13

Cerebral Hemispheres

Answer 13

• by 14th week the frontal, parietal, temporal, and occipital lobes are identifiable
• insula only grows so much
• superior operculum (upper hat) covers insula top and inferior operculum covers bottom
• by 28th week the lateral, central and calcarine sulci are noticeable

Question 14

Hydrocephalus

Answer 14

• water on the brian
• excessive CSF in ventricular system
• imbalance between production and absorption of CSF
• congenital aqueductal stenosis
• obstruction of foramina from 4th ventricle to subarachnoid space: arnold-chiari malformation=herniation of part of cerebellum and brainstem into foramen magnum
• meningitis: adhesions may compromise CSF flow
• increased fluid leads to increased pressure on neural structures
• thinning of the bones of calvaria, prominence of forehead, atrophy of cerebral cortex and white matter, and compression of basal ganglia and diencephalon
• tx: early identification, pressure sensitive catheter or shunt into internal jugular vein
• shunt: can go into jugular vein or into peritoneum; can get blocked can lead to H/A, vomiting, memory impairment

Question 15

Microcephaly

Answer 15

• normal or slightly small calvaria
• fontanelles close during early infancy and sutures close during first year, brain is underdeveloped
• gross mentally impairments
• suture lines between skull plates not closed until 12-14 months

Question 16

Anencephaly

Answer 16

-failure of rostral neuropore to close during 4th week
-forebrain abnormal and calvaria defective
-due to abnormal structure and vascularization the nervous tissue undergoes degeneration
-brain is spongy mass consisting mostly of hindbrain structures with no bone or skin covering
-lethal
-accounts for 1/2 of severe neural tube defects: most common severe anomaly seen in stillborn fetuses
-causes: genetic factors; high exposure to nickel, chromium, lead, mercury; higher in areas where depleted uranium and man-made isotopes; can be induced in rats by teratogenic agents
-prevention: folic acid .4 mg/day before pregnancy
-dx: ultrasound, alpha-fetoproteins in amniotic fluid
fetal counterpart to serum albumen, increased in neural tube defects, decrease in down syndrome

Question 17

Naturally Occurring Folate

Answer 17

• beans and legumes
• citrus fruits and juices
• wheat bran and other whole grains
• dark green leafy vegetables
• poultry, pork, shellfish
• liver

Question 18

Arnold-Chiari Malformation

Answer 18

• most common congenital anomaly involving the lower brainstem and cerebellum
• structural defect of the cerebellum with herniation of the medulla and part of cerebellum through foramen magnum
• may cause hydrocephalus
• occurs 1+/1000 births and is frequently associated with spina bifida and syringomyelia
• S&S: neck pain, balance problems, muscle weakness, numbness of other abnormal feelings in arms or legs, dizziness, vision problems, difficulty swallowing, ringing or buzzing ears, hearing loss, vomiting, insomnia, depression, or H/A made worse by coughing or straining
• hand coordination and fine motor skills may be affected

Question 19

Spina Bifida

Answer 19

• nonfusion of embryonic halves of vertebral arches during 4th week
• types: spina bifida occulta, spina bifida cystica (with meningocele, or with myelomeningocele), spina bifida with myelocele

Question 20

Spina Bifida Occulta

Answer 20

• usually clinically unimportant
• occurs in L5 or S1 in about 10% of otherwise normal people
• small dimple with tuft of hair
• small percentage may have functional deficits
• vertebra don’t close completely

Question 21

Spina Bifida Cystica

Answer 21

• cystlike sac: covered by skin or a thin, easily ruptured membrane; most common in lumbar region
• causes: combo of genetic and environmental factors-DM and anticonvulsant medication, fever or external temperature during neural tube development, obesity
• 75% have latex allergies
• 1/1000 births
• 10% cyst in meningocele
• 90% cyst is myelomenigocele

Question 22

Spina Bifida with Meningocele

Answer 22

• protrusion of meninges through a defect in the vertebral arch
• varying levels of functional deficits depending on lesion site and extent of herniation
• high alpha-fetoproteins

Question 23

Spina Bifida with Myelomeningocele

Answer 23

• protrusion of the meninges and spinal cord through a defect in vertebral arch
• marked neurologic deficit inferior to the level
• more serious than meningocele
• paralysis=lower limb, bladder and bowel
• learning disability common
• prevention: folic acid prior to conception

Question 24

Spina Bifida with Myelocele

Answer 24

• most severe form of spina bifida
• neural folds have remained open
• CSF leaks out
• clinical outlook poor

Question 25

Cranium Bifidum

Answer 25

-herniation of brain and/or meninges

Question 26

Factors Affecting Brain Development

Answer 26

• genetic: down syndrome; brachycephaly (flat head on back)
• maternal infection: congenital rubella-mental retardation; toxoplasmosis-microcephaly, hydrocephalus, cerebral calcification
• plagiocephaly: flat head on one side
• environmental factors: radiation (cell depletion), steroids, chemical substances-alcohol (microcephaly), smoking (smaller frontal lobes and cerebellar volumes), marijuana (impairs nerve development and guidance-permanent cognitive deficits, concentration disorders, hyperactivity)