الفهرس 
Introduction and Historical ReviewOnly 14 pages are availabe for public view
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Abstract
In the present work, the development of the neural crest cells in seven developmental stages (0.7, 1.0; 1.5; 2.0; 3.0; 4.2 and 6.0 nun total length) of the teleost fish, Xiphophorus helleri is studied and compared with other vertebrates. Moreover, three developmental stages (3.0, 4.2 and 6.0 nun total length) of the swordtail - platyfish hybrids [used as melanoma - bearing fish] are studied also to show the fonnation of melanoma.
The study is based on photomicrographs from stained serial paraffin transverse sections (5-7 in thicicness) through the head and tntnk regions of the studied specimens. Scanning electron micrographs of some specimens were also used.
In the morphological description, the site, time of segregation and migratory pathways of the neural crest cells have been explained. Moreover, the changes in the distribution of the extracellular matrix components;collagen,neutral polysaccharides, non - sulfated and sulfated glYcosaminoglycans during the migration of the neural crest cells, their differentiation and the formation of the melanoma are also considered.
The study of the development of the neural crest cells of Xiphophorus helleri revealed the following results :
1- The neural crest cells are recognized early at the dorso-lateral sides , of the neural keel or the brain.
2-The formation; segregation and migration of the neural crest cell, take place after complete formation of the neural keel in an antero - posterior direction.
3-At stage 9 (1.0 inm total length), the neural crest cells are mainly pleiomorphic and not oriented which makes them easily distinguished from the shape of the brain cells.
4-At the optic region, the neural crest cells migrated earlier (0.7mm total length), than the otic region (1.0 mm total length), and the olfactory and trunk regions (1.5 mm total length).
5-The migration of the neural crest cells at the head and trunk regions ” occurred in three migratory pathways. The first pathway is ventral along the brain and notochord in the head, and in the space between the neural tube and notochord, from one side, and the somites on the other side lin the trunk region. The second pathway is lateral in the sub-ectodermal space. The third pathway, contralateral pathway, is across the dorsal midline of the fore-brain, mid-brain and the neural tube at the anterior part of the trunk region.
6-The migration of the neural crest cells increased in agreement with the widening [i.e. expansion] of the migratory space.
7-Both ventral and lateral migrations of the crest cells are rapid and intensive at the Mink level, compared with the head level.
8-Non - sulfated glycosaminoglycans, collagen and, of a lesser extent, sulfated glycosaminoglycans are found in the early migratory spaces and their lining basement membranes at the head (1.0 min total length) and trunk regions (1.5 mm total length).
9-Non-sulfated glycosaminoglycans are increased in the crest cell im&atory spaces while sulfated glycosaminoglycans are nearly absent 1,1 these spaces.
10- Collagen material increased in the crest cell migratory pathways and their lining basement membranes at the trunk level (2.0 nun total length). Collagen probably plays an important role in crest cell migration at the trunk level. •
11.- In stage 11 (2.0 nim total length), many crest cells are probably participated in the formation of the presumptive sympathetic ganglia and urinary ducts. Other crest cells are aggregated laterally in the wall of the gut, besides, some crest cells invaded the mesodermal wall of the gut (4.2 rnm total lengt) and may contribute to the formation of the presumptive adrenal medulla and enteric gan-gliairespectively. Non - sulfated glycosaminoglycans and collagen are the main matriX components that probably facilitate the crest cell invasion and differentiation into the above mentioned stnictures. However,sulfated glycosaminoglycans and neutral polysaccharides are not seen to play such a role.
12-In stage .13 (3.0 mm total length), sulfated glycosaminoglycans and neutral polysaccharides are increased, with the cessation of the crest cell migration. They probably reduce the crest cell migration and provoke their arresting and homeing.
13-In stage 13 (3.0 mm total length), the crest cells are probably contributed to the formation of the pigmented layer of retina besides the sclera in stage 15 (4.2 mm total length). They are also participated into the formation of the presumptive corneal endothelium, stromal fibroblasts and iris melanophores as well as the inner and outer meninges in stage 16 (6.0 mm total length). Non - sulfated glycosaminoglycans are the main extracellular matrix (ECM) components that may facilitate the neural crest cell migration and invasion to form the corneal endothelium and non - sulfated
glycosaminoglycans and collagen are in the meninges. Sulfated . glycosaminoglycans may provoke the arresting of the crest cells to
form the iris melanophores.
14- The cranial neural crest cells are aggregated at Lettaiii levels to form the presumptive proximal ganglionic primordia of 5th (trige_ minal) 7th (facial) 9th (glassopharyngeal) and 1 Oth (vagus) cranial nerves. Likewise, the trinik neural crest cells are aggregated to form the presumptive dorsal root ganglia of the spinal nerves.Neutral polysaccharides are seen to be the predominant matrix components that probably provoke the crest cell aggregation to form these structures and stromal fibroblasts while. glycosaminoglycans and collagen do not share such a role.
15-In stage 16 (6.0 mm total length), glycosaminoglycans are intensed in
the matrix of some cartilages that form the chondrocranium and may
facilitate the neural crest cell invasion into them.while collagen and
neutral polysaccharides are not seen to share in this process.
16-Some crest cells are observed near the notochord in the intermyotomal spaces, while other crest cells invaded strongly the presumptive lateral line placodes. Thus, they may contribute to the formation of the Schwann sheeth cells and the neuromasts, respectively. Non - sulfated glycosaminoglycans may play the prominant role during the early development of the above mentioned stnictures while sulfated glycosaminoglycans, collagen and neutral polysaccharides are not seen
to share in this process.
The study of migration of the neural crest cells in the swordtail - ptatyfish hybrid larvae and their invasion into the integument, show many
differences, compared with swordtail fish, Xiphophorus helleri, of the same work and revealed the following results :
1-The cranial crest cells migrated largely in the wide sub-ectodermal
space.
2-At several areas of the skin. of the head, trunk and dorsal fin levels, the abundance of the melanocytes deposition is observed.
3-Certain crest - derived melartocytes (i.e. naevus cells) exhibited
focal proliferation and accumulation at the jtmction of the epidermis
and dermis to form the ”junctional naevus”. Some of the previously
described cells invaded the dermis to form the ”developing
compound naevus”.
4-Melanocytes (i.e. naevus cells) are increased in size and number, They
disordeity invaded the epidermis and Me adjacent tissues
astizedj. Also, the nuncio nodule projects above the skin
These changes represented the formation of malignant melanoma.
5-The distribution and intensity of sulfated and non - sulfated glycosaminoglycans and collagen undergo some changes in the hybrid .skin., compared to swordtail fish, and probably provoke intensive invasion of the crest cells into the skin leading to the melanoma formation.
6-Non-sulfated glycosaminoglycans and collagen are characteristically intensed in the hybrid skin during the invasion of the crest cells. However , sulfated glycosaminoglycans are uniquely absent in this skin . This probably gives a great facility to great numbers of crest cells’ to invade the most layers of the skin leading to the
melanoma formation.