DEVELOPMENT OF THE SOMITES

1. DEVELOPMENT OF THE SOMITES • By 3rd week paraxial mesoderm segments into somitomeres. • Somitomeres first appear in the cephalic region of embryo then the cephalocaudal direction. • In the head they develop in relation to the neural plate and contribute majority of the head mesenchyme. • From the occipital region, somitomeres becomes organised into somites. • The first pair of somites appear in the cervical region at about 20th day, • subsequent somites appear in craniocaudal order • about 3 somites per day till 5th week • about 42 – 44 pairs are present – somite period

2. neural tube somite notochord sonic hedgehog? Somite Differentiation

3. Sonic hedgehog • secreted morphogen involved in patterning of a variety of structures and organs in vertebrates • Important in development of the floor plate • Also in the events that pattern and configure the shape and size of the central nervous system.

4. somites

5. Estimation of age

6. Segmentation of Somites • 4th week, cells of ventral and medial walls of somites loose their compactness and become polymorphous. • shift position to surround the notochord. • These cells are called Sclerotome . • They give rise to mesenchyme (a loosely woven tissue), that will surround the spinal cord and notchord to form vertebral column. • The dorsal somite wall is now called dermamyotome which gives rise to cells called myotome which provides musculature for its own segment.

8. neural tube somite notochord sonic hedgehog? Somite Differentiation

9. dermamyotome sclerotome Somite Differentiation

10. Somites Development • Somites establish the segmental nature of the body. • They differentiate into three layers. • Sclerotome: Forms the vertebral bone. This originates from paraxial mesoderm. • As cells migrate outward, each sclerotome splits into inferior and superior halves. • The inferior half of one sclerotome merges with the superior half of the next sclerotome to form the respective vertebrata. • Two somites from each side, form a total of FOUR DIFFERENT SOMITES, contribute to the formation of each vertebrata. • When adjacent sclerotomes merge, the remaining notochord forms the nucleus pulposus. The rest of the notochord degenerates. • Dermamyotome, - myotome (medial part) and dermatome (most lateral part)

11. Neural tube development ~S (a) At 18 days after conception the embryo begins to implant in the uterine wall. It consists of 3 layers of cells: endoderm, mesoderm, and ectoderm. Thickening of the ectoderm leads to the development of the neural plate (inserts). (b) The neural groove begins to develop at 20 days.

12. Neural Tube • As the notochord develops the embryonic ectoderm overlying it thickens to form the neural plate. • by18th day, the neural plate becomes depressed along the midline to form a neural groove. • portion of the ectoderm is called neuroectoderm, • with the formation of neural groove, elevated portion of the ectoderm forms the neural fold. • The tips of the neural fold are lined by neural crest cells. • theyare continuous with the surface of the ectoderm.

13. Formation of neural tube. • neural groove deepens and the edges of the neural fold grow towards each. • They fuse to form a tube – neural tube. • The fusion is at first in the cervical region and then extends in both directions. • two openings temporary appear at the cephalic and caudal ends. • The cranial opening is called anterior – neuropore while the caudal is posterior neuropore.

16. Formation contd • The amniotic fluid circulates through the neuropore and provides nutrition to the neural tube till vascular system becomes well established in the embryo.

17. Subdivision of neural tube • divided into a cranial enlarged portion and caudal tubular part. • the cranial part, develops 3 dilatation forming the primary brain vesicle -prosencephalon -mesencephalon -Rhombencephalon • The prosencephalon (forebrain) forms cerebral hemisphere • Mesencephalon - midbrain • Rhombencephalon - Brain stain and cerebellum • The caudal tubular part forms spinal cord

18. Neural tube development (c) At 22 days the neural groove closes along the length of the embryo making a tube. (d) A few days later 4 major divisions of the brain are observable – the telencephalon, diencephalon, mesencephalon, and rhombencephalon.

19. Neural tube development

20. Neural Plate Folding Movement of epidermis towards midpoint forces edges of neural plate up and together Dorsal Neural plate anchored to epidermis Dorsolateral hinge point Cells here become wedge shaped (induced by notochord) Epidermis Neural plate Neural plate anchored to notochord Medial hinge point Notochord Ventral

21. Neural Plate Folding Neural plate cells at the medial hinge point Microtubules elongate – cells become columnar Actin filament bundles contract – narrowing the cell apex – like a drawstring bag

22. Homophilic aggregation Red  epidermal cells Green  mesodermal cells Blue  neural plate cells

23. Cadherins E E and P N N and P E and N Neural Plate Folding Neural folds meet and adhere Cells at this junction form neural crest Epidermis Notochord Closure not simultaneous Neural Crest Cells Regional differences Neural Tube Closed tube detaches – change in adhesion molecule expression

24. Neural tube formation

25. Origin of neural crest cells • Neural crest cells can form from the dorsal side of the closed neural tube • Epidermal and neural plate/tube interactions may generate crest cells • Co-culturing plate and epidermal cells produces crest cells • Culturing plate cells with BMP4 or BMP7 also produces crest cells

26. Neural crest cell differentiation NGF – cells now unable to respond to glucocorticoids FGF promotes competency to respond to NGF signal Bipotent precursor Pluripotent neural crest cell NGF-competent cell Sympathetic neuron Chromaffin precursor cell Chromaffin cell Glucocorticoids inhibit neural differentiation Glucocorticoids promote accumulation of chromaffin-specific enzymes

27. Neural Crest • Cranial (Cephalic) - head region forms cranial neurons, face skeleton, tooth primordia, thymus • Trunk - posterior to the head melanocytes, peripheral neurons Cardiac - near somites 1-3; large heart arteries and some throat skeleton near somites 18-24 - chromaffin cells of adrenal gland • Vagal and Sacral – Parasympathetic ganglion in gut

29. Embryonic Membranes • Amnion – epiblast cells form a transparent membrane filled with amniotic fluid • Provides a buoyant environment that protects the embryo • Helps maintain a constant homeostatic temperature • Amniotic fluid comes from maternal blood, and later, fetal urine

30. Embryonic Membranes • Yolk sac – hypoblast cells that form a sac on the ventral surface of the embryo • Forms part of the digestive tube • Produces earliest blood cells and vessels • Is the source of primordial germ cells

31. Embryonic Membranes • Allantois – a small outpocketing at the caudal end of the yolk sac • Structural base for the umbilical cord • Becomes part of the urinary bladder • Chorion – helps form the placenta • Encloses the embryonic body and all other membranes