Embryology Unit 2 Exam TQs

What are the parts of the integumentary system?

Skin, hair, nails, mammary glands (modified sweat glands)

What are the layers of the skin? What germ layers do they arise from?

Epidermis, from ectoderm

Dermis, from mesoderm

What are the two early layers of the epidermis?

Periderm (keratinized dead cells, protective)

Basal (makes new cells)

What is the vernix caseosa?

A whitish, greasy substance formed by secretions of sebaceous glands and exfoliated cells which covers the skin of the fetus

What are the two functions of the vernix caseosa?

1. Protect fetal skin from amniotic fluid

2. Lubricates fetus for parturition (birthing)

What are epidermal ridges?

Genetically determined, unique patterns which lead to fingerprints. They form the basis of fingerprint analysis (dermatoglyphics) and can indicate chromosomal abnormalities like Trisomy 21

What are the 4 qualities of albinism?

1. Autosomal recessive trait

2. Lack pigment in skin, hair, and retina

3. Normal number of melanocytes present

4. Lack the enzyme tyrosinase (part of melanin production pathway)

What are arrector muscles (arrector pili)?

Muscles which elevate hair shafts and are responsible for goosebumps, responding to sympathetic activation

What are lanugo?

Fine hairs which are early to form, holding the vernix caseosa in place

When do fingernails reach the tips of the digits? When do toenails reach the tips?

32 weeks, 36 weeks

What is indicated if a newborn’s nails don’t reach the tips of the digits?

Prematurity

What is the mammary crest/ridge?

The band-like thickening of the epidermis which gives rise to mammary glands (specialized sweat glands)

What structure in the nipples is identical in males and females at birth?

Lactiferous ducts; in males, remain rudimentary for life

What is polythelia?

Having an extra nipple, more common in females

What is polymastia?

Having an extra breast, typically just inferior to the normal breast (can occur anywhere on the mammary crest)

What is an inverted nipple?

When a nipple fails to evert; causes latching problems for an infant. Can be from birth (harmless) or invert after eversion sometime in adulthood (sign of breast cancer)

What are somites?

A series of blocks of paraxial mesoderm which separate during the 3rd week of development

What are the sclerotome and the dermomyotome? What do they give rise to?

Sclerotome: ventromedial, lead to vertebrae and ribs

Dermomyotome: dorsolateral, lead to muscle and dermis

What is mesenchyme?

A loosely woven embryonic connective tissue

• Derived from sclerotomes by end of week 4

• Derived from somatic mesoderm, leading to neural crest cells

What does mesenchyme develop into?

Fibroblasts, chondroblasts and osteoblasts

What is osteogenesis?

Bone formation

How does bone form from mesenchyme?

Intramembranous ossification: directly within a membranous sheath; flat bones of skull and clavicle

Endochondral ossification: first differentiates to a hyaline cartilage model, then ossify; most bones

What are the two parts of the skull?

1. Neurocranium: encloses the brain

   1. Membranous neurocranium: forms skull vault (calvaria; frontal bone, parietal bone, mandible, maxilla)

   2. Cartilaginous neurocranium: forms base of skull (hyoid, thyroid cartilage, sphenoid bone, occipital bone)

2. Viscerocranium: forms the face

Why is the skull a common site for birth defects?

Mesenchyme forming the skull is primarily derived from neural crest cells, which migrate (makes them susceptible to teratogens)

What is the large “soft spot” of the infant head? The smaller soft spot? The two pairs?

Larger is the anterior fontanelle; between the frontal (metopic), coronal, and sagittal sutures

Smaller is the posterior fontanelle; between the sagittal and lamboid sutures

Anterolateral and posterolateral fontanelles

Why are fontanelles and sutures important for parturition?

Allow the fetal cranium to mold to the birth canal

What is craniosyntosis? What are the two key types?

Prenatal fusion of at least one cranial suture

1. Scaphocelphaly: sagittal suture is fused, skull is long/narrow/wedge-shaped, most common

2. Brachycephaly: coronal suture is fused, skull is high and tower-like

What is acrania?

Complete or partial absence of the neurocranium, often associated with major defects of the vertebral column and partial absence of the brain (meroencephaly); usually incompatible with life

When do the mesenchymal cells of the sclerotome migrate to surround the notochord?

Week 4

What are the 2 parts of the sclerotome?

Cranial part: loosely packed cells

Caudal part: densely packed cells, form the annulus fibrosis (alternate with myotomes)

What forms the vertebral centrum of a given vertebra?

The caudal half of one sclerotome and the cranial half of the sclerotome most caudal to it

What are the 3 primary centers of ossification for each vertebra?

Centrum: forms part of vertebral body

Neural arches: surround neural tube, give rise to vertebral arch and spinous/transverse processes

What are the 5 secondary centers of ossification for each vertebra after puberty?

Tip of the spinous process

Tips of the transverse processes

Annular epiphyses (sup. and inf.)

What is Klippel-Feil Syndrome characterized by?

Short neck, fused cervical vertebrae, restricted neck movement, low hairline posteriorly, abnormalities of cerebellum/brain stem

What is a hemivertebra?

One of the two chondrification centers of the vertebral column fails to appear, resulting in only one half of the vertebral body forming; the most common cause of congenital scoliosis

What are accessory ribs? What are the two types?

Ribs originating elsewhere from the thoracic region

1. Lumbar ribs: most common, usually produce no symptoms

2. Cervical ribs: usually on C7, may produce neurovascular symptoms (compress brachial plexus/subclavian artery), in 0.5-1% of people

What does the sternum develop from?

Two sternal bars in the somatic mesoderm of the ventral body wall

What is pectus excavatum?

Variation which displays a concave depression of the lower sternum, arising from overgrowth of lower costal cartilages (push sternum inward); more common in boys

What is pectus carinatum?

“Pigeon chest,” where the chest juts out

In long bones, which part forms from the primary ossification center? When does this begin to occur?

The diaphysis/shaft, begins in week 8

Which bone undergoes ossification first? How does it ossify?

Clavicle, intramembranous initially (finishes with endochondral)

Which bone is second to undergo ossification? How does it ossify?

Femur, endochondral

What portions of a long bone are formed from the secondary centers of ossification?

Epiphyses

Which centers of ossification appear after birth?

Secondary centers of ossification

What is achondroplasia?

The most common type of dwarfism, due to a disturbance of endochondral ossification

• Leads to a normal-sized head with a bulging forehead and indented nasal bridge, with short and bowed limbs due to disturbances of the epiphyseal plates

• Autosomal dominant trait

• Rate increases with paternal age

Which germ layer gives rise to most of the muscular system? What are the exceptions?

Mesoderm

1. Eye, smooth muscle sphincter and dilator muscles (neuroectoderm)

2. Sweat and mammary glands’ smooth muscle (surface ectoderm)

What are the three types of muscle derived from?

1. Skeletal, somites (paraxial mesoderm)

2. Smooth, splanchnic mesoderm around gut

3. Cardiac, splanchnic mesoderm around heart tube

When do most skeletal muscles develop? When are they all formed?

Before birth, finished by the end of the first year

What are myotomes and their divisions?

Dorsolateral derivatives of somites, giving rise to skeletal muscles

Epaxial division (epimere) located dorsally, gives rise to extensor muscles of neck/vertebral column/sacrococcygeal ligaments

Hypaxial division (hypomere) located ventrally, gives rise to different muscles by region

What are the primary rami responsible for innervating the myotome divisions?

Posterior primary ramus: epaxial division

Anterior primary ramus: hypaxial division

What muscles derive from the hypaxial divisions of the myotomes?

• Cervical: scalene, geniohyoid, infrahyoid, prevertebral

• Thoracic: intercostals, anterior abdominal wall

• Lumbar: quadratus lumborum

• Sacral, coccygeal: pelvic diaphragm, external anal sphincter, perineal m. (underlying external genetalia)

What do the hypaxial divisions lead to in the thorax? In the abdomen?

External, internal, and innermost intercostals

External oblique, transversus abdominis

What do the pharyngeal arch muscles do? Which muscles are those?

Face and visceral functions

1. Mastication (chewing)

2. Facial expressions

3. Pharynx (swallowing)

4. Larynx (speech, controlling airway)

Where do the tongue muscles derive from?

Occipital myotomes 2-4 (first disappears)

Where do the smooth muscles derive from?

Splanchnic mesoderm surrounding the organ endoderm

When does heart muscle appear?

Fourth week of development

How do skeletal and cardiac muscle fibers develop?

Skeletal: fusion of myoblasts

Cardiac: growth of single cells which attach by intercalated discs

What are Purkinje fibers?

Larger diameter cardiac muscle cells with fast-conducting gap junctions, forming the conduction system of the heart (controls rhythm)

How much time separates appearance of upper limb buds and lower limb buds?

2 days

What are the parts of a limb bud?

Apical ectodermal ridge (on surface)

Central core of mesenchyme (from mesoderm, induced to growth and development by apical ridge)

What is the most critical period for limb development?

24-36 days after fertilization

When do digital rays appear within the hand plate? The foot plate?

Hand: week 6

Food: week 7

What are digital rays formed from? What about the webbing?

Digital rays: condensed mesenchyme

Webbing: loose mesenchyme (degrades through apoptosis)

What is syndactyly? What are the two kinds?

Fusion of digits.

• Cutaneous: webbing does not degenerate, most common in foot

• Osseous: fusion of bones when notches between digital rays fail to form; autosomal dominant trait

What is polydactyly?

Extra digits, often under-developed and lacking normal musculature; may be inherited as a dominant trait or induced by teratogens

What is the weight-bearing bone of the forearm?

The radius (homologous to tibia, more often broken in a colles fracture than the ulna)

How do the limbs rotate? Where are the extensor muscles located?

• Upper limb rotates 90 degrees laterally, elbows point dorsal with extensors on dorsal side

• Lower limb rotates 90 degrees medially, knees point ventral with extensors on ventral side

When do motor axons grow into limb buds?

During 5th week, innervate ventral and dorsal muscle masses

Which axons follow the course of motor axons?

Sensory axons

What are amelia and meromelia? How was this discovered?

Amelia is the complete absence of a limb; meromelia is a partial absence. Known from the thalidomide crisis of 1957-62

What is talipes equinovarus?

Also known as club foot, the most common musculoskeletal deformity (1 in 1000) in which the foot is rotated medially and inverted

• May be related to oligohydramnios

• All anatomical structures are present

• Usually treated by physiotherapy, casting, and taping

What is congenital hip dysplasia?

Hip capsule is lax at birth, acetabulum is shallow, femoral head is small; dislocation of hip usually occurs after birth

• 1 in 1500 births

• More common in girls

What happens to the notochord in week 3?

Induces overlying ectoderm to form the neural plate

Which somites form the brain? Which form the spinal cord?

Brain: the cranial 2/3 of somites (cranial to 4th pair)

Spinal cord: caudal 1/3

When does fusion of the neural tube begin, and at which pair of somites?

Begins in the early 4th week at the 5th pair of somites

What are the two openings of the neural tube?

Rostral neuropore at cranial end, closes at day 25

Caudal neuropore at caudal end, closes at day 27

How does the spinal cord develop?

At the caudal to 4th pair of somites, the walls of the neural tube thicken until the neural canal is reduced to the narrow central canal

What is the order of neuroepithelial cell formation?

1. Apolar neuroblasts (become neurons)

2. Glioblast/spongioblast (become glia)

3. Ependyma

What are the three zones in the developing spinal cord?

1. Ventricular zone: innermost, contains neuroepithelial cells

2. Intermediate zone: contains neurons

3. Marginal zone: outermost, becomes the white matter

How do glioblasts differentiate?

They become astroblasts (astrocytes) and oligodendroblasts (oligodendrocytes)

What is a glioblastoma?

An aggressive and deadly brain tumor which is incredibly difficult to remove, arising from glia

What two types of cells comprise the macroglia?

Astrocytes and oligodendrocytes

How do ependymal cells form?

After neuroblasts and glioblasts differentiate, neuroepithelia in the ventricular zone differentiate and line the central canal

How do microglia arise?

Derived from mesenchyme, they originate in the bone marrow and migrate to the CNS late in the fetal period via blood vessels; they are phagocytic

What does the sulcus limitans divide the developing spinal cord into?

Basal plate: ventral, forming the ventral and lateral gray columns (motor)

Alar plate: dorsal, form dorsal gray columns (sensory)

What do the spinal (dorsal root) ganglia develop from?

Neural crest cells

How do pseudounipolar neurons form?

Afferent (sensory) only, they develop within spinal ganglia in a bipolar manner; the processes later unite to form the T-shape

• Peripheral processes grow outward in spinal nerves to form the sensory endings in somatic/visceral structures

• Central processes grow into the spinal cord to form the dorsal roots of spinal nerves

What germ layer gives rise to the dura mater? What gives rise to the pia arachnoid?

Dura mater comes from mesoderm, pia arachnoid comes from neural crest cells

How does the spinal cord termination position change during development?

8 weeks = coccyx

24 weeks = S1 vertebra

Birth = L2/L3 vertebra

Adult = L1/L2 intervertebral disc

What is the filum terminale (internum)?

The thread-like extension of the pia mater from S2 to Co1, reflecting the original termination of the spinal cord

What is multiple sclerosis?

A demyelinating disease of the CNS

When does myelination begin and end?

Begins in late fetal period, ends after 1 year of life

Which cells form myelin in the CNS and PNS?

CNS = oligodendrocytes

PNS = Schwann cells

What is spina bifida occulta?

“Hidden,” the most mild form, resulting from failure of the two halves of the neural arch to unite

• 10% of population, most common at L5/S1

• Indicated externally by a dimple with hair growing on it

What is spina bifida cystica?

A severe form, involving protrusion of meninges/spinal cord through defects in the vertebral arches as a sac-like cyst

• Several types with varying degrees of neurological deficit

• Symptoms depend on type, position, and extent of lesion

What are the three subdivisions of spina bifida cystica?

1. With meningocele: cyst contains meninges and CSF, cord and nerve roots are normal

2. With meningomyelocele: cyst contains spinal cord and spinal nerve roots

3. With myeloschisis: most severe type, cord is open because neural folds failed to fuse

   • Cord is a flattened mass of nerve tissue, seen externally

   • Results in permanent paralysis of lower extremities

What are some causes of neural tube defects?

Deficiency in folic acid (B9) or B12; prevented through eating fortified foods, dark leafy greens, and animal products

What are the primary and secondary brain vesicles?

1. Prosencephalon (forebrain)

   1. Telencephalon (cerebral hemispheres)

   2. Diencephalon (thalamus, hypothalamus, epithalamus)

2. Mesencephalon (midbrain)

3. Rhombencephalon (hindbrain)

   1. Metencephalon (pons, cerebellum)

   2. Myelencephalon (medulla oblongata)

TeDi is a Pro, Rhom Met Mye, THES 3 Di, 4th Place Can Matter, MMC #4 <3

What are the two flexures of the brain in week 5? Which third flexure comes later?

Midbrain flexure, cervical flexure (at junction of hindbrain/spinal cord)

Later, the pontine flexure (opposite direction, at junction of met-/myelencephalon)

Like the spinal cord, which 3 secondary brain vesicles arise from the alar and basal plates, separated by a sulcus limitans?

The mes-, met-, and myelencephalon