neurulation Flashcards
derivatives of ectoderm
- Skin (epidermis)
- Epidermal glands (sweat, sebaceous, mammary)
- Hair, nails
- Eye: lens, corneal epithelium
- Inner ear labyrinth
- Anterior pituitary
- Tooth enamel
- Neural tissue (Neuroectoderm) is a large, specialized derivative of ectoderm.
- Brain, Brainstem & Spinal Cord
- Eye: retina, pigment epithelium, optic nerve, part of iris.
- Pars nervosa of pituitary gland
derivatives of endoderm
- Original endoderm (hypoblast) replaced by mesodermal cells
- Epithelium of alimentary canal (stomach, SI, LI, etc.)
- Parenchyma (functional cells) of Pancreas & liver
- Thyroid, parathyroid
- Thymus
- Lung and trachea (epithelial lining)
- Middle ear epithelium
- Urinary bladder and Urethral epithelia
- Vaginal epithelium
- Note, endoderm forms epithelial surfaces, but the supporting elements of any given organ are derived from its mesoderm.
derivatives of mesoderm
• Somites:
• Differentiation:
o Begins from undifferentiated mesoderm
o Early somite contains a somatocoele
o Differentiates into 2 layers; dermomyotome & sclerotome
• Dermomyotome (includes hypaxial myotome): Dermis of skin, ventral skeletal muscle (hypaxial muscle), dorsal skeletal muscle (deep back, epaxial myotome)
o All muscles of body come from this!! They migrate to final location starting at 5th week.
• Sclerotome: Vertebral Bones and intervertebral disks (and ribs)
o The rest of the skeletal tissue and body skeleton arise from the somatopleure of the lateral plate mesoderm
- Intermediate mesoderm “nephrotome”:
- Between lateral plate and the somite
- Gives rise to: kidney, ureter, ductus deferens and gonads.
• Lateral plate mesoderm (2 parts):
• Somatic mesoderm (“somatopleure”) gives rise to
o Bones, cartilage, fascia and other CT around periphery of body.
o Associated vasculature and connective tissue
• Splanchnic mesoderm (“splanchonopleure” gives rise to
o Visceral smooth muscle
o Fascia and connective tissues of viscera
o Associated vasculature
o Associated with endoderm
• Neural Crest:
• Arises from neural folds
• Gives rise to:
o Melanocytes of skin.
o All parts of peripherial nervous system, including ganglion cells, support cells, Enteric Nervous System,etc.
o In and near head can form bone, cartilage, and other CT of the pharyngeal arches; odontoblasts, and ear ossicles.
o Adrenal medulla (NOT adrenal cortex).
o Bulbar ridges (spiral septum) of the Heart, including semilunar valves.
o Meninges.
• Muscles of the head are from somitomeres (NOT from somite myotome)
- Septum transversum*
- Migrates with heart during embryo folding
- Separates thorax from abdomen early in development
- Participates in venous cardiac development
- Final derivative is a part of the central tendon of diaphragm.
development of vertebrae
Vertebrae arise from sclerotome, which is the most medial part of the somite.
Each vertebral segment derives from a part of two separate somites.
These come together to form a single vertebra.
The intervertebral disk derives from mesoderm remaining behind after mesodermal migration
joint development
Bones develop all together in linear sequence, not as isolated bones that join together (which you may not get from the traditional pictures of bone development)
Joints develop between bones even as the bone itself is forming and developing.
Outside of bone=periosteum
cartilage has perichondrium form over it shared between adjascent bones. From single perichondrium get membrane where joint space is. Form connected by this tissue- joint=natural outgrowth of bone formation
timing of endochondral bone development
Mesenchymal migration, 4th-5th week.
Condensation of mesoderm: 5th-6th week.
Chondrification, generally beginning 6th week.
Primary ossification usually starts 7-12 weeks (note, a few bones don’t develop until later, some not until after birth).
Secondary ossification centers usually begin from birth to 9th year, depending on specific bone.
Fusion of epiphyseal plates, usually 18th to 25th year.–> no more growth
epiphyseal plates=cartilage/bone interface==> subject to separation here
Intramembraneous bones of skull: from cranial neural crest.
Axial skeleton: base of skull (endochondral bones), vertebrae, ribs, clavicle: from somite origin.
Appendicular bones: Endochondral bone from Somatoplure.
differentiation sequence of the hypaxial myoblasts
Dermomyotome=from epaxial myoblasts
delamination (separates from this layer) to get myoblasts.
Then hypoblast migrates and divides–> proliferation
cells limited to dermomyotome=multiply many many times before get to destination–>determination=
differentiation=fuse into myoblasts.
muscle development
- Arises from dermamyotome of somite.
- Myoblasts arise at about 5th week and begin to migrate within mesoderm. Nerve axons follow the muscles at this time.
- Fuse into myotubes at final site; these fuse into skeletal muscle fibers.
- Migrating myoblasts → fusion of myoblasts → primary myotube → myotube growing to secondary myotube → mature skeletal muscle fiber.
Muscles migrate during the 5th week, and move along to the distal end as it forms.
Early muscle primordia are not fully differentiated and appear as mesodermal mases condensed superficial to bony cartilagina.
The start proximal then move distal, beginning with the postaxial side first, then the preaxial side.
(extensors migrate before flexors)
upper extremity development
muscle will differentiate into deep and superficial layers (except triceps)
tendons develop from surrounding mesoderm
muscles attach to tendons after they begin differentiation
tendons appear to arise from insertion and migrate proximally to muscle, which the muscle attached to its proximal cartilage/bone
Fine developmental processes in the forearm–>complex structures with many different parts.
Require a lot of chemical elements for differentiation at different times
lot of variation in these mm because so much going on that more likely to screw up (myoblasts)
