What is Ovum Factor?
Signal from the zygote at 3-5 hours telling the mother's immune system not to attack it
What is EPF?
Early Pregnancy Factor. Released by the zygote at 24 hours, this protein further suppresses the mother's immune system.
What two signals are absolutely necessary for implantation to take place?
Ovum factor and EPF
When does implantation occur?
6 - 10 days
What is the bilaminar disc and when does it form?
Forms at 12 days. It is the source of all future tissues in the body, including MSK tissues.
What comprises the bilaminar disc (i.e., why is it called bilaminar)?
It is two layered. Has an epiblast cellular layer and a hypoblast cellular layer
What occurs at 15 days?
1. Epiblast cells detach and migrate to the primitive streak 2. When they arrive, they submerge and fan out between the two layers to become the endoderm, mesoderm, and ectoderm. This migration and sorting is called gastrulation
Deep groove and portal for the rapidly moving cells to enter, submerge beneath the epiblast layer, and stream cranially. Displacing the hypoblast cells, these migrating cells become the mesoderm.
Acts on the migrating cells via specific factors, and after the modified cells reach their destinations, they attach, grow, and become specific organs of the body.
Common condition caused by EtOH uptake during gastrulation
Holoprosencephaly - Forebrain fails to develop into two hemispheres, usually leading to death before birth. Those that live will range in symptoms, but most have defects in the face and brain along with brain function.
Common condition associated with taking seritonin reuptake inhibitors for depression and axiety
Caudal Dysgenesis - Malformation/lack of formation of tailbone or lower vertebrae
During development, what do we see at 21 days?
2 distinct layers: Ectoderm, mesoderm, and endoderm
Where does the MSK system develop from SPECIFICALLY?
Paraxial and lateral plate mesoderm (which later becomes mesenchyme) + neral crest cells
What are somitomeres?
Pairs of tissue blocks formed from the loose masses of paraxial mesoderm alongside the neural tube. Some are destined for the head region and some eventually become the somites of the body wall and limbs
What is longitudinal folding, when does it occur and what does it cause?
Longitudinal folding occurs at Days 17-28 and involves bringing the structures of the heart ventrally. Differential growth will further position these structures in the right places.
What is lateral folding, when does it occur?
Lateral folding occurs simultaneously to longitudinal folding at 15-21 days. It involves layers migrating ventrally to pinch off part of the yolk sac which forms the alimentary canal, and also forms the body wall.
What is the segmentation clock?
Cyclic waves of expression and suppression from extremely specialized genes
Stages of Somite progression:
1. Start as initially solid clumps of mesodermal cells 2. Hollow out, migrate and differentiate into MSK precursors of bone, muscle, connective tissues and skin
Becomes the ventral muscles of the limb and body wall
Becomes the dorsal muscles of the back
Becomes the skeleton
Becomes the dermis of the neck, back, and trunk
Two meanings for Dermatome:
1. Part of a somite 2. Strip of skin innervated by one spinal segment
Developmental layers that make up the axial skeleton
Sclerotome, which si the paraxial and lateral plate mesoderm and the cephalad part of the neural crests which makes up the brain case
Folic Acid supplements during development can help with what?
Preventing spina bifida occulta
What is meningocele?
Interruption of the development process causing the midline to not fully close
Thought to be related to an overgrowth of the rib cartilages
When do limb buds appear?
When does limb rotation occur?
Upper limb rotations at 7 weeks?
Rotate laterally so that the thumbs are positioned on the radial side and the elbows point posteriorly (palms up)
Lower limb rotations at 7 weeks?
Rotate medially so that the great toes are medial and the knees point anteriorly (soles down)
What programs limb growth?
AER - Apical Ectodermal Ridge
What does the AER do SPECIFICALLY?
Induces outward growth along the limb axis and is present by day 29.
What is the "progress zone?"
Subjacent to the AER, this strip contains coordinating cells.
Processes of Endochondral Ossification:
1. Mesenchymal cells condense into chondrocytes which move into patterns to form the shape of the future bone. 2. Angiogenesis occurs 3. Epiphyseal plates are formed 4. Osteoblasts differentiate and bind in a pattern, leading to production of the osteoid which is then mineralized into bone.
What causes syndactyly?
When apoptosis between the AER segments fails, leading to fusion of the fingers.
What is intramembranous ossification?
Ossification process of the skull. Radiating bone spicules coalesce into flat bones
What is the neurocranium?
What is the viscerocranium?
What forms the viscerocranium?
1st and 2nd pharyngeal arches - A core of neural crest cells and somatic and lateral plate mesoderm
Specifics on Limb rotation Progression
At 5 weeks, upper and lower limbs have formed as finlike appendages pointing laterally and caudally
At 6 weeks, limbs bend anteriorly, so elbows and knees point laterally, palms and soles face the trunk
At 7 weeks, upper limbs have undergone 90 degree torsion about their long axes, but in opposite directions [elbows point caudally, knees cranially]
At 8 weeks, torsion of lower limbs results in twisted or "barber pole" arrangement, resulting in birth positions