CHAPTER SUMMARY

VOCABULARY FLASH CARDS

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Chapter Summary

In vertebrates, the ectoderm will form the epidermis and the nervous system. The nervous system is derived from the portion of the ectoderm that overlies the dorsal mesoderm after gastrulation [fig. 6.1]. During neural induction, the dorsal mesoderm, which was derived from the Spemann organizer, induces the overlying ectoderm by releasing chemical signals that induce neural fates and suppress epidermal fate [fig. 6.2]. The region of ectoderm that receives these chemical signals will become the nervous system, and this region is called the neural plate. The rest of the ectoderm will become epidermis. Some of the chemical signals secreted by the organizer have been identified, such as Noggin and Chordin [fig. 6.3]. These molecules are commonly referred to as BMP antagonists because they interfere with BMP signaling by binding to secreted ligands and blocking ligand-receptor binding interactions.

The neural plate undergoes morphogenesis and forms the neural tube during the process of neurogenesis. During neurogenesis different regions of the neural plate will undergo different types of morphogenetic movements [fig. 6.4]. Cells at the border between the neural plate and the epidermis will undergo apical constrictions, forming the neural folds. The cells within the neural plate change shape from a cuboidal epithelium to a columnar epithelium. The neural crest cells delaminate from the epithelium and become migratory. The entire neural region undergoes convergent extension movements, lengthening the embryo along the anterior to posterior axis.

It is important to understand that the neural plate is organized and different regions of the neural plate have different characteristics [fig. 6.5]. The neural plate is organized in the anterior-posterior direction by the time gastrulation is complete, and this organization can be visualized by the change in homeotic gene expression along this axis of the embryo. The neural plate is also organized in a medial-lateral dimension, which prefigures the dorsal-ventral organization of the neural tube [fig. 6.6].

The dorsal-ventral organization of the neural tube is probably a consequence of lateral BMP signaling originating from the epidermis, and the BMP is probably being antagonized by molecules secreted from the notochord [fig. 6.7]. The region of neural tissue adjacent to the notochord which becomes the floor plate and which expresses sonic hedgehog (Shh) as does the notochord. The region of neural tissue farthest from the notochord becomes dorsal neural tube. This region will begin to express BMPs, and will also express dorsal specific neural markers, such as msx, pax3, and pax7. The region of neural tissue in between the floor plate and dorsal neural tube will become the ventral neural tube and will express ventral neural markers like pax6.

Regionalization of the neural tube is very important for the proper specification of neurons [box 6.1], ganglia, and a special population of cells called the neural crest cells [fig. 6.8]. Neural crest cells are a migratory population of cells derived from epithelial cells at the border of the neural plate and prospective epidermis. These cells are multipotent, which means they can differentiate in to many different cell types. The neural crest cells remain multipotent as they migrate through the anterior portions of the somite and become committed to a cell fate only after reaching a destination. The neural crest cells form many derivatives depending on where they migrate to in the embryo [table 6.1]. The multipotency of neural crest cells is limited however, depending on whether they originate from head or trunk regions of the embryo [fig. 6.9].

Regionalization of the neural tube also has an effect on cell proliferation, creating different regions of the neural tube: the innermost ependymal zone, the medial mantle zone, and the outermost marginal zone [fig. 6.10]. The ependymal zone will become a source of migrating neuroblasts. The mantle zone will become the grey matter, and the marginal zone will become the white matter of the nervous system. Proliferation is also effected by peripheral target size, meaning that if a population of neurons is connected to a target (a limb muscle for example) then that population will survive and proliferate [fig. 6.11]. This effect is due to two factors: the prevention of apoptosis and the influence of nerve growth factors [fig. 6.12].

The regionalization of the nervous system in vertebrates is influenced by the anatomical segmentation of the vertebrate body plan. Segmentation of the vertebrate nervous system is seen early in development in the form of neuromeres and later in development in regionalization of the brain [fig. 6.13]. The brain is divided into the forebrain (the prosencephalon), the midbrain (the mesencephalon), and the hindbrain (the rhombencephalon). The rhombomeres are derived from the rhomencephalon and are the most overtly segmented regions of the brain. The spinal chord is also overtly segmented, and the segmentation of the spinal chord into discreet ganglia is largely influenced by the adjascent somites [fig. 6.14]. The segmentation of the nervous system is essential, because different neural systems develop from different regions of the brain. For example, eyes will develop from the diencephalon, which is derived from the forebrain [fig. 6.15].

Eye development is an excellent example of morphogenesis, induction, and terminal differentiation. First, a region of the diencephalon will undergo morphogenesis to form an outpocket called the optic vesicle and optic stalk. The optic vesicle will begin to invaginate to from the optic cup and induce the overlying ectoderm to form the lens placode. The lens placode will then invaginate into the optic cup, and the portion of the optic cup contacting the lens will become the retina. The induction of the lens, which formerly was believed to be induced solely by the optic cup, is now known to involve signals from several adjacent tissues including heart, head mesoderm and optic cup [fig. 6.16]. Eye development involves the expression of so-called executive, or master control, genes like eyeless/pax6. These genes are expressed during the eye development of many different species and, when misexpressed, can induce ectopic eye formation [fig. 6.17]. The lens placode is but one example of ectodermal structures that develop together with the central nervous system [fig 6.18]. These cranial ectodermal placodes are important for the development of the sense organs and the sensory system. The major placodes are the lens placode (eye), otic placode (ear), and nasal placode (nose). Other placodes contribute to cranial nerve ganglia. Many placodes do not solely contribute to the development of a certain structure but also receive important contributions from neural crest cells [table 6.2]. The regionalization and coordination of the brain vesicles, ectodermal placodes, and neural crest cells are important for the form and function of the vertebrate central nervous system [fig. 6.19].






 

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