Week 7, Thursday

Introduction to Embryology ppt - last few slides we didn’t finish Monday

Notochordal Development:

• early 3rd week

• Some of the mesodermal cells migrate cranially from the primitive node and pit, and form the notochordal process

• The notochordal process develops between the ectoderm and endoderm

• The notochordal process develops a lumen= notochordal canal

• Openings develop in the floor of the canal, forming the notochordal plate

• The notochordal plate infolds to form the notochord

  • The primordial axis around which the vertebral column forms

• As the vertebral bodies form, The plate will degenerate and some portions with form the Nucleus Pulposus of each IVD

Neurulation:

• the formation of neural plate, neural folds and neural tube

• The neural plate appear as a thickening of the ectoderm

• A longitudinal neural groove forms in the plate

• The groove is flanked by neural folds

• Fusion of these grooves form the neural tube (primordial of the CNS)

  • As the neural tube separates from the surface ectoderm, neural crest cells form the neural crest between the newly developed neural tube and the surface ectoderm

  • Then the neural crest soon separates into left and right parts that migrated to the Dorsolateral aspects of the neural tube

  • These 2 parts of the neural crest give rise to the sensory ganglia of the spinal and cranial nerves

• Collection of CNS nerve cells is called a Nuclei

• The neural tube becomes the spinal cord

  • Neural tube defects occur usually in the time period before a woman finds out they are pregnant

  • Women should start taking prenatal vitamins before they start trying to get pregnant

Condensation of the mesoderm:

• at the time the notocaord is developing, the mesoderm is forming 3 distinct regions:

  • Paraxial mesoderm

    • Around the axis

  • Intermediate mesoderm

  • Lateral mesoderm

  • Each region is destined to be something different

Sacrococcygeal Teratoma

• Germ cell tumor derived from remnants of the primitive streak

• Contains tissue from all three germ layers in incomplete stages of differentiation

• Most common tumor in newborns

  • Incidence of 1 per 35,000 live births (80% are females)

• Most are benign

• Usually surgically excised and prognosis is good

Chordomas

• Benign and malignant tumors derived from vestigial remnants of notochoral tissue

• 1/3 occur at the base of the cranium

• Slow growing

• Malignant forms infiltrate bone

1/3 of boards is usually embryology

Embryology- the Somite system and vertebral column ppt

The notochord degenerates as the bodies of the vertebrae form, but portions of it persist as the nucleus pulposus of each IVD

• This was already stated, but it is very important

Somite formation:

• starts around Day 18

• Paraxial mesoderm begins to form discrete segmented blocks (somites) in a cranial to caudal sequence

• 42-44 somites include:

  • 4 occipital

  • 8 cervical

  • 12 thoracic

  • 5 lumbar

  • 5 sacral

  • 8-10 coccygeal

• The first occipital and last 5-7 coccygeal somites eventually disappear

• Paraxial mesoderm condensing to form somite

• 3 types of mesoderm:

  • Paraxial

  • Intermediate

  • Lateral plate

• Each Somite organizes into 3 parts:

  • Dermatome

  • Myotome

  • Sclerotome

• Cells from sclerotome part migrate around the notochord and neural tube

  • They form the ribs and vertebrae

• Dermatome cells migrate to form the dermis of the skin

• Myotome cells migrate to form 2 parts:

  • epimere: a dorsal mass that gives rise to the deep back muscles - dorsal rami

  • Hypomere: a ventral mass that gives rise to trunk, limb, superficial back muscles - ventral rami

• Folding lateral to medial creates abdominal and thoracic cavities

• Folding into fetal position is cranium to caudal, and then we lose our tail

Clinical correlation:

• The level at which a somite develops is retained in its Dermatome, Myotome and sclerotome throughout life

• The deltoid by muscle is innervated by the auxiliary nerve

• The axillary nerve contains fibers from the C5 & C6 spinal nerves

• The deltoid is primarily innervated by C5 fibers but some C6 fibers also innervate the Deltoid

• This indicates that the deltoid muscle primarily developed front he myotonic at the 5th cervical level

Formation of the vertebral column:

• Axonal spouts grow into surrounding sclerotome mesenchyme and end in the Myotome mesenchyme

  • Sclerotome cells form vertebral bodies while Myotome cells migrate to form muscles, and the axon is right behind then to form neurons

• As this occurs the cranial segment of each sclerotome recombines with the caudal segment of the next superior sclerotome to form a vertebral rudiment

• This re-combination also places the myotomes BETWEN, as opposed to opposite. The sclerotome enabling the future muscles to act on the vertebral column

• This also results in nerves exiting between the vertebrae

Normal vs blocked vertebrae:

• if the cells dont migrate the way they are supposed to, they are called a “blocked vertebrae”.

  • Patient wont have the same ROM as they could

  • You can adjust above and below it, but you can’t adjust it

  • Basically looks like two bodies that are fused with a large SP

Formation of the IVD

• form from sclerotome cells and notochord

• “Mesenchyme cells are like loosely packed cells”

• Where the future vertebral body will form, the notochord degenerates and is replaced by sclerotome Mesenchymal. The centrum (body) of the vertebra is formed by sclerotome cells

• Between vertebrae the notochord expands and becomes the nucleus Pulposus, which is the gelatinous center of the disc

• The annulus Fibrosus forms from invading sclerotomal cells and consists of circular fibers that surround around the nucleus pulposis

• “If it has to do wit the spine it will be on boards” - Gillam

Stages of Vertebral Development:

• blastemal or pre-cartilaginous stage

  • Migration and organization of sclerotome mesenchyme forms a basic vertebral pattern around the notochord and neural tube; formation of the IVD

• Cartilaginous Stage

  • Chondrification centers form as the sclerotome cells differentiate into chondroblasts and begin creating the cartilage vertebral model

• Ossification stage

  • Bone replaces cartilage beginning around prenatal week 7-8 and continuing until the 25th postnatal year

Malformations:

• Spina Bifida = unfused vertebral arch, can present with different severities:

  • Bulging area with displaced spinal cord

  • Open spinal cord

Ossification of the spine ppt

Stages of human development:

• the developing human is called a Conceptus

• The period of development or pregnancy is referred to as the gestation period which on average lasts for 280 days

• There are 2 distinct period of gestation:

  • Embryonic Period:

    • From fertilization through week 8

    • Conceptus is called an embryo

  • Fetal period:

    • From week 9 through birth

    • Conceptus is called a fetus

Ossification of vertebrae:

• vertebrae begin to develop during the embryonic period as mesenchymal condensations around the notochord

• Later, these mesechymal bone models chondrify to form cartilaginous vertebrae

• Vertebrae usually ossify towards the end of the embryonic period (8th week)

Ossification centers of typical vertebrae

• begin at specific ossification centers within cartilaginous bone models

• These centers can be classified as primary and secondary ossification centers

• Typical vertebrae develop 3 primary and 5 secondary centers of ossification

Primary ossification centers:

• 1 for the vertebral body (centrum)

• 1 for each of the neural arches

  • Fuse with the body anteriorly, and in the midline posteriorly at the spinous process

• With ossification, the cartilage begins to turn to bone from the primary center

Secondary ossification centers:

• 1 at the tip of each TP

• 1 at the tip of each SP

• 1 for each of the. 2 ring-shaped annular epiphysis that cap the superior and inferior aspects of the vertebral bodies

Epiphyseal Growth plate

• Each secondary center is separated from the primary center by an epiphysial growth plate

Ossification Periods:

• primary ossification centers for bodies appear first in the Lower T/S and upper L/S regions at 9-10 weeks in utero

• These centers proceed cranially and cuadally from there

• Primary centers for neural arches appear in the C/S and upper T/S segments during the 8th-9th week in utero

• All 3 primary ossification centers are distinct at birth

• The halves of the neural arches fuse wit each other during the st year of life, whining with the lumbar region

• The arches fuse wit the bodies a few years later

• The costal elements fuse with the neural arches, not the bodies

• The 5 secondary centers appear after puberty and fuse with primary centers by the age of 21

  • In males, the superior and inferior annular epiphysis begin to fuse with eh body at ~17-18 y/o and is complete by age 25

• T4 & T5 are the last to be completely ossified

Ossification of Atlas:

• Has 2 primary ossification centers, one for each side

  • These form the arches and the lateral masses

  • The 2 sides fuse at ~ 4 years of age

• Secondary ossification centers develop at the tips of both TPs, and on the anterior and posterior tubercles

• Has the same ossification centers as all other typical vertebrae

• Also has 2 primary centers for the dens

• The body begins to ossify by the 3rd-4th month in utero

• All primary centers fuse by the age of 6-7 years

• An annular epiphysis for the inferior aspect of the body appears by puberty

Sacral Ossification:

• The sacrum forms through the fusion of 5 sacral vertebrae, which ossify similar to other typical vertebrae

• 2 secondary centers form each Auricular surface

• The 1st segment begins to ossify by the 9th week in utero, and this process proceeds caudally

• IVDs exist between sacral segments until the bodies fuse beginning at ~20 years of age

• Synostosis begins between S4 & S5, and proceed cranially

• The process may not be completed until 33 years of age

Coccygeal Ossification:

• 4 primary center of ossification develop, one for each segment

  • 1st segment begins to ossify soon after birth

  • 2nd segment begins to ossify by age 5-10

  • 3rd segment begins to ossify by age 10-15

  • 4th segment begins to ossify by age 14-20

• All segments fuse with one another by 25-30 years of age

1st bone in the body to ossify is the Clavicle

The midterm will be on everything up to this point, and since we are caught up, we will discuss the midterm in length on Monday next week