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ABPN 2022 > Embryology > Flashcards

Embryology Flashcards

1
Q

Main embryonal layer giving rise to the nervous system

A

Ectoderm

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2
Q

Formation of the neural plate

A

Notochord (layer of mesoderm cells) induces ectoderm to form neural plate; the notochord becomes the vertebral column

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3
Q

Neurulation

A

The proliferation and migration of ectodermal cells and invagination, folding, and fusion of the neural plate in a specific pattern.
Occurs at 3-6 weeks gestation.
Involves the formation of a midline groove along which lateral margins form. Fusion begins at center so that there is an anterior and posterior neuropore; f_usion reaches anterior first then posterior, resulting in the neural tube._

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4
Q

Ventral versus dorsal notochord differentiation

A

Ventral neural tube differentiation is driven by the sonic hedgehog protein (of the mesoderm).
Dorsal neural tube differentiation is driven by the lateral epidermal ectoderm.

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5
Q

Abnormal rostral fusion vs abnormal caudal fusion of the notochord

A

Rostral fusion - affects the anterior notochord, resulting in encephalocele and anencephaly
Caudal fusion - affects the posterior notochord, resulting in spina bifida

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6
Q

Specification

A

Segmentation of the neural tube into three parts: prosencephalon, mesencephalon, and rhombencephalon
Occurs at 5-6 weeks of gestation

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7
Q

Prosencephalon

A

Becomes:

  1. ) The telencephalon, which becomes the cerebral hemispheres and basal ganglia/lateral ventricles
  2. ) The diencephalon, which becomes the thalamus and the hypothalamus and retina/third ventricle
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8
Q

Mesencephalon

A

Gives rise to the midbrain/cerebral aqueduct

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9
Q

Rhombencephalon

A

Becomes the metencephalon→ pons, cerebellum/upper part of the fourth ventricle

And myelencephalon→ medulla/lower part of the fourth ventricle

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10
Q

Disorders of specification

A

Septo-optic dysplasia

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11
Q

Formation of the peripheral nervous system

A

Arises from neural crest cells that form from the neural tube that occurs after it fuses

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12
Q

Neural crest cells give rise to:

A

PNS
Chromaffin tissue of the adrenal medulla and the melanocytes

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13
Q

Disorders of primary neurulation

A

Occurs within 1st month of gestation/3-6 weeks of gestation

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14
Q

Disorders of prosencephalic development

A

2nd month of gestation/5-6 weeks

2/2 issues with specification

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15
Q

Formation of the corpus callosum

A

During the 5th week of gestation, the commissural plate develops within the lamina terminals and is a place for axonal processes to decussate. The corpus callosum is fully developed by the 17th week of gestation.

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16
Q

Formation of the cerebral hemispheres

A

The hemispheres form from an initially single layer of epithelium in the region surrounding the lateral ventricles, known as the ventricular zone. Cells in this layer are pluripotent, frequent divide, and are radially oriented, in contact with the pial surface and the ventricle. Cell layers form subsequently.

  • 5th week gestation: marginal layer superficial to ventricular zone with cytoplasmic processes from the ventricular zone
  • The subventricular zone than forms between the ventricular and the marginal zone; both subventiruclar and ventricular zones contain mitotic cells.
  • Specialized cells - radial glia - have cell bodies in the ventricular zone and send processes that extend all the way to the pial membrane in the cortical surface. During cortical development, new neurons migrate in waves from the zone of origin along a scaffold formed by these processes. There is 1 wave at 8 weeks, 1 at 12 weeks, with the leak of neuronal migration between 12-20 weeks but can be seen up to 26 weeks. The cortex develops in an inside-out pattern, such that cells that migrate earlier form deeper structures and cells that migrate later form more superficial structures. Makes sense bc deep structures are fundamental structures to basic f(X)
  • All 6 layers of cortex are identifiable by 27 weeks of gestation.

5-27 weeks

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17
Q

Categories of malformations of cortical development

A
  1. ) Cell proliferation: some forms of megalencephaly and focal cortical dysplasia
  2. ) Cell migration: lissencephaly - agyria, pachygyria and subcortical band heterotopia, cobblestone complex malformations, heterotopias
  3. ) Cortical organization: polymicrogyria, focal cortical dysplasia with normal cell types, microdysgenesis, and schizencephaly
  4. ) NOS
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18
Q

Focal cortical dysplasia with abnormal cell types

A

This is associated with disorders of neuronal proliferation, vs FCDs with normal cells that are disorders of cortical organization. Abnormal cell types include dysmorphic, enlarged neurons and balloon cells with abnormal lamination. The type of FCD characterized by balloon cells, result from the proliferation of abnormal cells in the germinal matrix. This is a common pathology in focal epilepsy that is often medically refractory.

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19
Q

Microcephaly and macrocephaly

A

Defined as head circumference >2 SDs below the mean/above the mean

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20
Q

Megalencephaly

A

Brain weight >2 SDs above the mean

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21
Q

Hemimegalencephaly

A

Disorder of cell proliferation
Enlargement of only one brain hemisphere is a brain malformation with hamartomatous growth of a cerebral hemisphere and frequently associated with cortical malformations. It is a disorder of neuronal proliferation.
*Hamartoma: a local malformation made up of an abnormal mixture of cells and tissue

While a choristoma is an excess of tissue in an abnormal situation (e.g., pancreatic tissue in the duodenum)

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22
Q

Lissencephaly

A

Malformation of cortical development from abnormal migration, resulting in impaired formation of gyri. It is characterized by the presence of reduced cortical gyration and in the most severe form agyria, resulting in a smooth brain. There are often 4 (or fewer) cortical layers as opposed to the usual 6.
- Miller-Dieker syndrome (MDS): four-layer or classic lissencephaly, where the cortex is smooth and may be thick but consists of only four layers
- Classification schemes:
[] Formerly type 1 including MDS and other variants and type 2 including cobblestone lissencephalies
[] A newer classification scheme groups them into lissencephaly variants 4,3,2 layers according to the number of layers, in which MDS is included as a 4 layer variant, cobblestone cortical marformations, microlissencephaly spectrum, and other lissencephalies
Patients typically have severe psychomotor retardation and seizures

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23
Q

Subcortical band heterotopia

A

Disorder of cell migration
AKA double cortex, in which there is a relatively normal cortex with 6 layers with an underlying band of white matter underneath which is a band of gray matter

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24
Q

Cobblestone malformations

A

Classified within the spectrum of lissencephalies and formerly known as lissencephalies type 2
They are neuronal migration disorders in which the cortical gray matter has reduced number of gyro and sulci that appear like cobblestones. There is reduced and abnormal white matter, and the cerebellum and brainstem may be abnormal or hypoplastic. _Microscopically, the cortex has no recognizable layers and is dysplastic and thic_k.

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25
Q

Schizencephaly

A

Disorder of cortical organization
Most common in the perisylvian region
In closed-lip schizencehaly, the cerebral cortical walls on either side of the lip are in contact vs in open-lip, the two walls are separated by CSF
Resulting cavity is lined by gray mater, and you typically do not seen the expected astrocytic gliosis that you might in porencephaly

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26
Q

Betz cells

A

UMNs of the nervous system - large cells found in the primary motor cortex (layer V)

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27
Q

Cell of origin of CNS

A

Ectoderm, neural tube

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27
Q

Layers of the neocortex

A 
Layer I (most superficial, covered by the Pia) - molecular layer
Layer II - external granular cell layer
Layer III - external pyramidal cell lyer
Layer IV - internal granular cell layer
Layer V - internal pyramidal cell layer
Layer VI - multiforme layer

MGP/GPM

Corticocortical efferents arise from layer III and project to II and III
Layers I, IV, and VI receive the majority of thalamic efferents
Layer V gives rise to corticostriate projections - location of Betz cells
Layer VI gives rise to corticothalamic projections

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28
Q

Cell of origin of PNS

A

Ectoderm, neural crest cells

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29
Q

Cell of origin of vertebral bodies

A

Mesoderm of notochord

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30
Q

Holoprosencephaly

A

Failure of prosencephalon to form telencephalon and diencephalon and subsequently divide into cerebral hemispheres and other structures. Olfactory bulb and tracts develop from the prosencephalon after hemispheres divide. Problem during 4-8 weeks of gestation. Specification at 5-6 weeks.
Alobar holoprosencephaly: most severe, cerebral hemispheres are almost completely fused w/ absence of the interhemispheric fissure and corpus callosum, single midline ventricle, variable dysgenesis and fusion of the thalamus, hypothalamus, and basal ganglia
Semilobar holoprosencephaly: parts of the posterior hemispheres may be separated by a fissure.
Lobar holoprosencephaly, only the most anterior portions of the hemispheres are not separated, and there is partial agenesis of the corpus callosum, but the splenium and genu are present.
Associated with arrhinencephaly (agenesis of the olfactory bulb and tract) that can also occur in isolation in less severe forms (e.g., Kallmann’s an X-linked dominant disorder, w/anosmia 2/2 arrhinencephaly) and hypogonadism), midline face deficits
Genetics: Trisomy 13, 18, AD/AR forms with sonic hedgehog gene implicated →ventral differentiation

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31
Q

Neuronal proliferation

A

Megalencephaly and focal cortical dysplasia

32
Q

Neuronal migration

A

*Polymicrogyria and schizencephaly listed under cortical organization for Cheng-Ching

33
Q

Risk factor for neural tube defects

A

Maternal diabetes, folate deficiency, and use of AEDs with pregnancy

34
Q

Craniorachi_schisis_ totalis

A

Key findings in craniorachischisis
Head – anencephaly (absence of the brain and cranial vault).
Covering – no skin covering residual brain tissue, spinal cord tissue, or cranial vault (calvarium).
Spine – open (rachischisis) might be limited to the cervical spine, but the open defect can extend to the thoracic spine or even lumbar or sacral spine (craniorachischisis totalis).

35
Q

Anencephaly

A

The absence of a major portion of the brain, skull, and scalp that occurs during embryonic development.

36
Q

Myeloschisis

A

The severe form of a neural tube defect where the open neural tube appears as a flattened, plate-like mass of nervous tissue with no overlying membrane.

37
Q

Encephalocele

A

Encephalocele is a sac-like protrusion or projection of the brain and the membranes that cover it through an opening in the skull.

38
Q

Myelomeningocele

A

The spinal canal is open along several vertebrae in the lower or middle back. The membranes and spinal nerves push through this opening at birth, forming a sac on the baby’s back, typically exposing tissues and nerves.

39
Q

Myelocystocele

A

A rare neural tube defect characterized by cystic dilatation of the central canal of the spinal cord, herniating posteriorly through a dorsal spinal defect. The malformation can occur anywhere along the spinal cord but appears to be more frequent in the posterior cervical and the lumbosacral region. It may be an isolated anomaly or be associated with other defects, including anorectal and genitourinary anomalies, or sacral agenesis.

40
Q

Diastematomyelia

A

Diastematomyelia is a rare developmental deformity in which the spinal cord is separated into two parts by a rigid or fibrous septum. The deformity is often accompanied by abnormal development of the vertebrae.

41
Q

Meningocele

A

A protrusion of the meninges through a gap in the spine due to a congenital defect.

42
Q

Arnold Chiari

A

Ray: AC I >3 mm in length

Most common: >5 mm, but controversial

Low-laying tonsils < 3-5 mm, non-pathological variant

43
Q

Dandy Walker malformation

A
44
Q

Holoprosencephaly

A

Differentiation of cortical structures drives the differentiation of facial structures to right and left
A proboscis is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In invertebrates, the term usually refers to tubular mouthparts used for feeding and sucking. In vertebrates, a proboscis is an elongated nose or snout.

45
Q

Agenesis of the corpus callosum

A

Corpus callosum develops anteriorly to posteriorly, so partial agenesis of the corpus callosum effects the splenium; myelination of the corpus callosum occurs from posterior to anterior, so if you had a myelination disorder, it would be more likely the anterior to be affected
Guides anterior to posterior organization of the cingulate gyrus, causing it to organize outward to the surface, causing appearance of radial gyri

46
Q

Schizencephaly

A
47
Q

Hydranencephaly

A

Diffuse destruction of cortex by infection or a vascular process
Skull is still there so this is not anencephaly

48
Q

Periventricular leukomalacia

A

MC in utero vs germinal matrix hemorrhage is after birth

49
Q

Germinal matrix hemorrhage

A

Germinal matrix - pluripotent cells that give rise to the layers of the cortex

50
Q

Congenital aqueductal stenosis

A
51
Q

Lissencephaly genetics

A

Big ones to know

LIS1 - AD, posterior predominant

DCX - X-linked, male with lissencephaly, female with subcortical band heteropia

52
Q

Holoprosencephaly genetics

A
53
Q

Agenesis of corpus collosum

A

Aicardi, FG, Mowat Wilson

54
Q

Polymicrogyria

A

Disorder of cortical organization

Bifrontal - GPR65, AR

55
Q

Schizencephaly

A

Bilateral or unilateral
Some familial cases, EMX2

56
Q

Diastematomyelia

A

Diastematomyelia is splitting of the spinal cord into two portions by a midline septum

Diplomyelia is also distinguished by the presence of two central canals each surrounded by gray and white matter as in a normal spinal cord.

When the split does not reunite distally to the spur, the condition is referred to as diplomyelia, which is true duplication of the spinal cord.

57
Q

Diplomyelia

A

Duplication of the spinal cord, distinguished from diastematomyelia by the presence of two central canals each surrounded by gray and white matter as in a normal spinal cord.

When the split does not reunite distally to the spur, the condition is referred to as diplomyelia, which is true duplication of the spinal cord.

58
Q

Sacral agenesis

A

Absence of the whole (or in some cases parts of the) sacrum, occurs with other urogenital, gastrointestinal, and spinal cord abnormalities.
Associated with maternal insulin-dependent diabetes. Autosomal dominant forms associated with homeobox gene mutations
Clinical manifestations: mild motor deficits to severe sensory and motor deficits and bowel and bladder dysfunction.

59
Q

Spina bifida occulta and variants

A

Tuft of hair wo neurological s(x)s = SBO
Tuft of hair w/neurological s(x)s = Occult spinal dysraphism

60
Q

NTD risk factors

A

F>M
Maternal folate deficiency
Retinoic acid
AEDs - Depakote, Carbamazepine
Maternal diabetes
Hx of pregnancy with child with NTD

61
Q

Detecting NTD

A

Prenatal ultrasonography is used in detecting NTDs and characterizing them. Prenatally, in the setting of most NTDs, serum maternal α-fetoprotein level is elevated. Elevations in amniotic fluid acetylcholinesterase levels also occur and, combined with an elevated α-fetoprotein, increase sensitivity and specificity in prenatal screening.

Prenatal MRI is also done in some cases to assess the extent of the abnormality, aiding in prognostication of neurologic function in life. When an NTD is detected with screening, and/or other abnormalities are found on ultrasonography, fetal karyotyping to assess for trisomy 13, 18, and others may be obtained to aid in prognosis and diagnosis.

62
Q

Syringomyelia vs hydromyelia

A

Syringomyelia: fluid-filled cavity wi the spinal cord that is separate from the central canal and lined by gliotic tissue
Hydromyelia: enlargement in the central canal itself, and the cavity wall is, therefore, lined by ependyma.

63
Q

Chiari III/IV

A

Chiari III malformation is cerebellar herniation into a cervical or occipital encephalocele. Chiari IV malformation is the term previously used to describe cerebellar hypoplasia but is no longer used.

64
Q

Molar tooth sign

A

Secondary to cerebellar vermis hypoplasia with fourth ventricular enlargement, a large interpeduncular fossa, and abnormal superior cerebellar peduncles

Molar tooth appearance results from a lack of normal decussation of superior cerebellar peduncular fiber tracts which in turn leads to enlargement of the peduncles, which also follow a more horizontal course.
The absence of crossing fibers also leads to a reduction in the anteroposterior diameter of the midbrain and deepening of the interpeduncular cistern.

Classically associated with Joubert, also seen in COACH

65
Q

Joubert’s syndrome

A

AR

Present with developmental delay (hypotonia), ataxia, oculomotor abnormalities (oculomotor apraxia and sometimes blindness - for example, Leber congenital amaurosis, with vision loss from rod and cone dystrophy), and respiratory difficulties (unusual breathing patterns, central apneas)

66
Q

Congenital aqueductal stenosis

A

Narrowing of the cerebral aqueduct connecting the third and fourth ventricle: may be a disorder of neurulation, related to downregulation of genes in the vertical axis of the neural tube, and associated with abnormal development of the dorsomedial septum of the midbrain.
There are various causes of congenital aqueductal stenosis, genetic or acquired. There is an X-linked form associated with pachygyria. It may occur in association with holoprosencephaly or Chiari II malformation. Acquired causes include congenital infections such as cytomegalovirus or mumps virus, or a variety of tumors such as ependymomas or hamartomas.

67
Q

Septic-optic dysplasia

A

Hypoplasia or absence of the septum pellucidum, optic nerve and optic chiasm hypoplasia, dysgenesis of the corpus callosum and anterior commissure, and fornix detachment from the corpus callosum. Arrhinencephaly (agenesis of only the olfactory bulb and tract) and/or hypothalamic hamartomas may be associated. Other less commonly associated abnormalities: cerebellar vermis defects and hydrocephalus. Can also be associated with lobar holoprosencephaly and other malformations of cortical development.
Clinical manifestation: vision loss, ataxia when the cerebellum is involved, s(x)s of hydrocephalus when present, and endocrinologic disturbances: from panhypopituitarism to isolated hormone deficiencies.
Mutations in the transcription factors HESX1, homeobox, and SOX genes may be implicated in this disorder.

68
Q

Miller-Dieker

A

Classic lissencephaly where 4 layers of cortex form
Formerly known as lissencephaly type 1
Associated with microdeletions on chromosome 17 in the LIS1 gene
X-linked lissencephaly: 1.) DCX gene, encodes for the protein doublecortin, involved in microtubule organization and stabilization, presents as lisssencephaly in males and subcortical band heterotopia in females 2.) X-linked lissencephaly with abnormal genitalia in male patients 2/2 ARX gene mutation, encodes for transcription factor involved in nonradial migration of cortical interneurons.
Presents with microcephaly, atypical facies (micrognathia low-set ears, thin upper lip, short nose with upturned nares, prominent forehead, bitemporal hollowing, and other features), GDD, hypotonia and later spasticity, and intractable seizures.

69
Q

Subcortical band heterotopia - specifically male vs female genetics

A

2/2 mutation in DCX gene on chromosome X in females. DCX -> protein doublecortin:microtubule organization and stabilization.
Same mutation -> classic lissencephaly (smooth brain, agyria, or pachygyria) n males.
Difference in manifestations from lyonization (random X inactivation) in females (XX), such that normal migration cannot occur in neurons in which the mutated gene is not compensated by the normal homologous copy, and neurons with normal copy of the gene can migrate to the cortex, resulting in a double cortical band.
Subcortical band heterotopia: some patients with mosaic mutations of LIS1.
Clinically: intractable seizures, intellectual impairment and other neurologic deficits

70
Q

Cobblestone malformations

A

Formerly lissencephaly type 2
Associated with a spectrum of autosomal recessive syndromes including Walker–Warburg syndrome (most severe cobblestone appearance, more apparent frontally), muscle–eye–brain disease (intermediate severity), Fukuyama muscular dystrophy (least severe but with more severe muscle disease with some elevation in CK). All 3 share features of microcephaly, global developmental delay, epilepsy, hypotonia, and evidence of muscular dystrophy.
Cobblestone malformations with muscle involvement have reduced glycosylation of α-dystroglycan, a component of the dystrophin-associated glycoprotein complex.
Some genes: POMT1, POMT2, LARGE, FKTN, FKRP, and POMGNT1 (involved in glycoslation of dystroglycan) but poor genotype-phenotype correlation

71
Q

Heterotopia

A

A heterotopia is a cluster of abnormally located neurons that are otherwise normal in morphology.

72
Q

Polymicrogria

A

Disorder of cortical organization
Can be unilateral or bilateral, generalized, perisylvian (MC), predominantly frontal, etc The perisylvian form is the most common.
Occurs as part of various syndromes, either sporadically or in familial forms (has been seen with Zellweger)
Epilepsy and developmental delay are common

73
Q

Layers of cortex

A

The six layers of the neocortex are
1) molecular layer (layer I, most superficial, covered by the pia) - receive thalamic efferents
2) external granular cell layer (layer II) - corticocortical efferents project here
3) external pyramidal cell layer (layer III) - corticocortical efferents arise and project here
4) internal granular cell layer (layer IV) - receive thalamic efferents
5) internal pyramidal cell layer (layer V) - gives rise to corticostriate projections (from cortex to striatum), Beta cells
6) multiform layer (layer VI, deepest layer, overlying the subcortical white matter) - receive thalamic efferents and gives rise to corticothalamic projections (from cortex to thalamus)
Pyramidal cells in layers of cortex with efferents, so III, IV, VI
Granular (or stellate) neurons are smaller cells (≤15 micrometers) that function predominantly as cortical interneurons and predominate in regions involved in sensory function or integration (secondary association cortices, etc.).

74
Q

Cell of origin for the Microglia and Dura

A

MesoDerm

75
Q

Cell of origin for the Microglia and Dura

A

MesoDerm

76
Q

Layers of neural tube

A

The inner layer is called the ependymal layer and becomes the ependymal lining and the choroid plexus epithelium; the mantle layer becomes the gray matter; the marginal layer becomes the white matter.

77
Q

Vesicles and brain regions

A
78
Q

Sulcus limitans

A 
Divides alar (dorsal to sulcus limitans) from basal plate (ventral to sulcus limitans) along the walls of the ventricular system
Remains in the adult tissue in the brainstem (along side of 4th ventricle) and diencephalon (as the hypothalamic sulcus, which divides thalamus from hypothalamus)
_In general, separates the cranial nerves into motor (basal: SHH) and sensory (alar) nuclei: LEP_

ABPN 2022 (26 decks)

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