Neural Tube Formation: Primary and Secondary Neurulation Mechanisms

Formation of the Neural Tube There are two major ways of forming a neural tube. In primary neurulation, the cells surrounding the neural plate direct the neural plate cells to proliferate, invaginate, and pinch off from the surface to form a hollow tube. In secondary neurulation, the neural tube arises from a solid cord of cells that sinks into the embryo and subsequently hollows out (cavitates) to form a hollow tube. The extent to which these modes of construction are used varies among vertebrate classes. Neurulation in fishes is exclusively secondary. In birds, the anterior portions of the neural tube are constructed by primary neurulation, while the neural tube caudal to the twenty- seventh somite pair (i.e., everything posterior to the hindlimbs) is made by secondary neurulation (Pasteels 1937; Catala et al. 1996). In amphibians, such as Xenopus, most of the tadpole neural tube is made by primary neurulation, but the tail neural tube is derived from secondary neurulation (Gont et al. 1993). In mice (and probably humans, too), secondary neurulation begins at or around the level of somite 35 (Schoenwolf 1984; Nievelstein et al. 1993).

Formation of the Neural Tube There are two major ways of forming a neural tube. In primary neurulation, the cells surrounding the neural plate direct the neural plate cells to proliferate, invaginate, and pinch off from the surface to form a hollow tube. In secondary neurulation, the neural tube arises from a solid cord of cells that sinks into the embryo and subsequently hollows out (cavitates) to form a hollow tube. The extent to which these modes of construction are used varies among vertebrate classes. Neurulation in fishes is exclusively secondary. In birds, the anterior portions of the neural tube are constructed by primary neurulation, while the neural tube caudal to the twenty-seventh somite pair (i.e., everything posterior to the hindlimbs) is made by secondary neurulation (Pasteels 1937; Catala et al. 1996). In amphibians, such as Xenopus, most of the tadpole neural tube is made by primary neurulation, but the tail neural tube is derived from secondary neurulation (Gont et al. 1993). In mice (and probably humans, too), secondary neurulation begins at or around the level of somite 35 (Schoenwolf 1984; Nievelstein et al. 1993)

Formation and shaping of the neural plate The process of neurulation begins when the underlying dorsal mesoderm (and pharyngeal endoderm in the head region) signals the ectodermal cells above it to elongate into columnar neural plate cells (Smith and Schoenwolf 1989; Keller et al. 1992 ). Their elongated shape distinguishes the cells of the prospective neural plate from the flatter pre-epidermal cells surrounding them. As much as 50% of the ectoderm is included in the neural plate. The neural plate is shaped by the intrinsic movements of the epidermal and neural plate regions. The neural plate lengthens along the anterior-posterior axis, narrowing itself so that subsequent bending will form a tube (instead of a spherical capsule). In both amphibians and amniotes, the neural plate lengthens and narrows by convergent extension, intercalating several layers of cells into a few layers. In addition, the cell divisions of the neural plate cells are preferentially in the rostral- caudal (beak-tail; anterior-posterior) direction (Jacobson and Sater 1988; Schoenwolf and Alvarez 1989; Sausedo et al. 1997; see Figures 12.2 and 12.3). These events will occur even if the tissues involved are isolated. If the neural plate is isolated, its cells converge and extend to make a thinner plate, but fail to roll up into a neural tube. However, if the border region containing both presumptive epidermis and neural plate tissue is isolated, it will form small neural folds in culture (Jacobson and Moury 1995; Moury and Schoenwolf 1995).

Primary neurulation The events of primary neurulation in the chick and the frog are illustrated in Figures 12.3 and 12.4, respectively. During primary neurulation, the original ectoderm is divided into three sets of cells: (1) the internally positioned neural tube, which will form the brain and spinal cord, (2) the externally positioned epidermis of the skin, and (3) the neural crest cells. The neural crest cells form in the region that connects the neural tube and epidermis, but then migrate elsewhere; they will generate the peripheral neurons and glia, the pigment cells of the skin, and several other cell types.

MESODERM DEVELOPMENT Majority of body structures are mesodermal in origin. Notochordal Mesoderm rapidly rounds up and separates from lateral mesoderm, forming a discrete cylinder = notochord. Notochord is much reduced or obliterated in most adult vertebrates, but forms the center around which vertebral formation occurs. Lateral Mesoderm Amphioxus Mesoderm forms paired series of segmentally arranged blocks = somites. From their initiation, somites have a cavity inside = coelomic cavity.

Fig 5.7 (b) Notochordal and lateral mesoderm in Amphioxus

MESODERM DEVELOPMENT Lateral Mesoderm Vertebrates Initially there is no segmentation of mesoderm; instead forms as a continuous sheet without a central cavity. Mesodermal differentiation occurs from dorsal midline outward into 3 divisions, each extending the entire length of the body trunk. Differentiation always occurs head-to-tail. The 3 divisions are: 1. Next to neural tube and notochord = Epimere (somites). Thicken and subdivide on either side to form longitudinal rows of blocks. This is the first indication of segmentation in vertebrate embryos. Proliferation and differentiation occurs within somite forming: Sclerotome = portion surrounding notochord and neural tube Dermatome = outermost portion near skin ectoderm Myotome = middle portion between and ventral to sclerotome and dermatome

MESODERM DEVELOPMENT 2. Lateral and ventral to somites is a relatively small region of mesoderm, known as intermediate mesoderm or Mesomere. This may show segmentation similar to somites. 3. Beyond mesomere region, extending ventrolaterally is a sheet of mesoderm known as lateral plate mesoderm or Hypomere. Apart from cyclostomes, there is no segmentation in this region. Coelomic cavity forms within lateral plate mesoderm, dividing it into: Somatopleure = external mesoderm + ectoderm Splanchnopleure = internal mesoderm + endoderm

Figs 5.11 & 5.16 Mesoderm divisions in Amphibians and Mammals

Organogenesis/Differentiation Once the mesoderm divisions are set up, then ontogenetic development proceeds to embryonic differentiation to adult body. What causes this differentiation? Induction = process by which developmental fate of cells is determined

Induction Classic Experiments of Spemann (1920s) won Nobel Prize Took piece of presumptive neural plate from early gastrula stage, transplanted to ventral region of another early gastrula normal development, prospective neural ectoderm becomes skin ectoderm. Similar experiment with late gastrula transplanted neural ectoderm becomes neural ectoderm regardless of transplant site. Transplant dorsal lip (prospective notochordal mesoderm) of early gastrula to ventral region in another embryo induces formation of a secondary embryo involving both host and transplanted tissues. Conclusions: Some change took place between early and late gastrula that determined fate of cells Dorsal lip responsible for inducing shift in direction of differentiation of host tissue. Dorsal lip plays a central role in determining the craniocaudal axis of the embryo = Primary Organizer.

Spemann Expt - Transplanted dorsal lip (prospective notochordal mesoderm) of early gastrula to ventral region in another embryo induces formation of a secondary embryo involving both host and transplanted tissues.

Primary Induction Definition = induction with primary importance in determining cranio-caudal axis, also the first of many inductive interactions. Two types of inductive interactions: Instructive = directs differentiation of cells along a certain path Permissive = allows differentiation when given a proper stimulus

What is the Mechanism of Induction? Classic Triturus (Salamander) experiment Dorsal Lip (Inducer) cultured for 24 hr across nitrocellulose membrane with pores of specific diameter from undifferentiated ectoderm Ectoderm differentiates to become neural tissues. If undifferentiated ectoderm cultured in the absence of dorsal lip, it develops into unspecialized skin ectoderm. EM analysis of membrane after culture showed no cellular processes between dorsal lip and ectoderm. Conclusion = a diffusable substance responsible for induction.

What is the Mechanism of Induction? Chemical nature of diffusable substance? Not known with certainty Purified active ingredients from various inducers turn out to be proteins or glycoproteins Also some evidence that changes in ratio of bound/free ions within cells of early gastrula may influence induction.

What is the Mechanism of Induction? Other inductive interactions between cells can result from Cell-to-cell direct physical interactions usually between molecules located on the cell surface Cell-to-cell communication of signals through gap junctions For induction via diffusable substances or direct physical interactions, cell membrane receptors on the induced cells are required

SUMMARY The full developmental pathway is dependent upon the genetic and biochemical capacities of the induced cells + the full inductive capabilities of the inducing tissue. Many inductive interactions occur throughout development and influence gene expression and the production of specific gene products and cell migration, among other processes Precise mechanisms remain uncertain in most cases.

Neural crest: a transient structure composed of cells originally located in the dorsal most portion of the neural folds and closing neural tube. Neural folds: bilateral elevated lateral portions of the neural plate flanking either side of the neural groove. Neural groove: a midline ventral depression in the neural plate. Neural plate: that portion of the dorsal ectoderm that becomes specified to become neural ectoderm. Neuraxis: the brain and spinal cord. In developmental terms the term refers to the neural tube, from its rostral to caudal end. Neuroblast: an immature neuron. Neuroepithelium: a single layer of rapidly dividing neural stem cells situated adjacent to the lumen of the neural tube (ventricular zone). Neuropore: open portions of the neural tube. The unclosed cephalic and caudal parts of the neural tube are called anterior (cranial) and posterior (caudal) neuropores, respectively. Neurotrophic factors: proteins released from potential targets that promote or inhibit neuronal survival. Neurulation: the process by which neural plate develops into a neural tube. Roof plate: analogous to floor plate but on the dorsal surface of the neural tube. Primary neurulation: development of the neural tube from neural plate. Secondary neurulation: development of the neural tube from mesenchyme caudal to the posterior neuropore (tail bud). Sonic hedgehog (Shh): secreted paracrine factor that induces specific transcription factors. Made by notochord and floor plate. Spina bifida: a birth defect resulting from an unclosed portion of the posterior neural tube or subsequent rupture of the posterior neuropore soon thereafter. Transcription factors: activate genes encoding proteins.