DEVELOPMENTAL BIOLOGY Organogenesis Anita Grover Reader Department of Zoology Zakir Hussain College Jawahar Lal Nehru Marg Delhi – 110 002 Key words: Rudiment, neurulation, hinge points, cadherin, epimere, choroid plexus, vesicle, placode, palisading, visceral layer, parietal layer, coelom. ORGANOGENESIS (DEVELOPMENT OF ORGANS) Organogenesis is a crucial phase in development, in which the embryo finally becomes a fully functional organism, capable of independent survival. In this chapter we shall first consider the development of some of the organ rudiments from the three germ layers: ectoderm, mesoderm and endoderm. Then we will study the development of certain organs like eye, ear and heart in detail including central nervous system. I. EARLY VERTEBRATE DEVELOPMENT UPTO ORGAN RUDIMENTS During the development of vertebrate body, the different regions in the three germ layers of the gastrula segregate from each other to form the rudiments of future organs and tissues. Most of the rearrangements of the germ layers to form organ rudiments involve transformation in the epithelial cell sheets. Epithelial cells undergo a variety of folding and spreading movements, which are as follows: Local thickening of epithelium: Thickening of epithelium is known as palisading. Palisading occurs due to the elongation of single cells (fig. 4.1) as can be seen in the formation of neural plate and ectodermal placodes such as lens, ear and nasal rudiments. Folding of epithelium: Epithelium can take several forms of folding such as inward, outward or linear folds. When the epithelium bends inwards into the embryo or into a cavity, it is called invagination, whereas if the epithelium bends outwards from the surface of the embryo, it is known as evagination. Folding along a line i.e. linear folds give rise to a groove (fig. 4.2). The formation of neural tube is by linear folding. The formation of lens vesicle or otic vesicle from their respective thickenings illustrates inpocketing or infolding of the epithelium to form pouches (fig. 4.3). Folds or pouches may undergo modifications to form branched structures. The formation of various glands depends on the folds appearing at the epithelial outpocket (fig. 4.4.). Folding or bending of a sheet may change shape of epithelial cells (fig. 4.5). The narrowing of columnar epithelial cells at the apical end results in formation of pyramidal cells. This in turn results in the differences in the surface area on the two ends of the epithelium and the bending of the entire sheet. 2 3 Separation of epithelial layers: This takes place by the appearance of cervices, which enlarge to form cavities. Normally the cervices appear either parallel to the surface of the epithelium or perpendicular to it. In the first case, the epithelial layer splits into two layers lying on top of the other. The original cervice may be increased due to secretion of fluid into it and becomes a spacious cavity. This type of epithelial splitting is seen in chick and other vertebrates that forms the parietal and visceral layers of the lateral plate mesoderm, and coelomic cavity between them . The second type of splitting, which is perpendicular to its surface, can be seen during the development of mesodermal somites. Flattening and spreading of epithelial layer: This occurs during epiboly of presumptive ectoderm during amphibian gastrulation. In this process there is spreading of cells. The prospective ectoderm spreads towards dorsal mid line to cover the area left vacant by convergence of neural epithelium . Spreading may be accompanied by a change in cell shape such as thinning and flattening of individual cells (fig. 4.6). Dissociation of epithelium into individually migrating cells: Dissociated cells from the epithelial layer may move from one location to another either over long distances as in case of neural crest cell migration or over short distances as in the formation of hair or feather germ. In addition to various modifications to the epithelial cell sheets, selective death of cells plays an important role in shaping various structures of developing embryo e.g. during brain and limb development cell death is seen in certain regions. Differentiation of germ layers Ectoderm The ectodermal layer essentially separates into epidermal ectoderm, neural ectoderm and neural crest: the fate of this layer is shown in fig.4.7. Formation of neural tube When the gastrulation is near completion the presumptive area of the nervous system becomes differentiated from rest of the ectoderm. The cells in this part of the ectoderm become columnar in shape and this region of the embryo is called neural plate. The process by which neural plate forms neural tube; is called neurulation and an embryo undergoing such changes is referred to as neurula. There are two ways by which neural tube is formed - primary neurulation and secondary neurulation. 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 called the neural 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 mode of construction of neural tube varies among different vertebrate classes. In fishes, neurulation is exclusively secondary. In birds, the anterior portions of the neural tube are formed by primary neurulation whereas the neural tube caudal to the 27th somite pair (posterior to hind limbs) is made by secondary neurulation. In amphibians ( Xenopus), most of the neural tube in tadpole is made by primary neurulation except the tail neural tube, which is formed by secondary neurulation. In mice and may be in humans too, the neural tube posterior to the level of somite 35 is derived by secondary neurulation. 4 5 Primary neurulation The process of primary neurulation appears to be similar in amphibians, reptiles, birds and mammals. The presumptive area of the nervous system differentiates from the rest of the ectoderm to form a neural plate. Cells of the neural plate elongate and arrange themselves as columnar epithelium. During this process the embryo lengthens along the anterio – posterior axis. At the same time, the edges of the neural plate are thickened and raised above the general level as ridges called neural folds. Neural folds elevate further resulting in the formation of the neural groove between them. Neural folds meet each other in the mid dorsal line and fuse to form neural tube beneath the overlying ectoderm. Embryo at this stage is called neurula. Fig 4.8 depicts the stages in the formation of neural tube in amphibians. The cells at the dorsal most portion of the neural tube become the neural crest cells. They lie between the dorsal part of the neural tube and dorsal epidermis. The neural crest cells undergo extensive migration and form the autonomic nervous system, melanocytes and parts of skull etc. (see fig 4.7). In amphibians, the formation of neural tube occurs simultaneously along the entire length of the embryo. Fig 4.9 shows the different stages of neurulation in amphibian embryo. In birds, reptiles and mammals even as the neurulation has begun in the anterior part of the embryo, the posterior region is still in the process of gastrulation. At a time when the neural folds are just about to form in the posterior region, the neural folds have already started fusing to form the neural tube in the anterior region (fig 4.10). 6 Mechanism of Primary Neurulation Primary neurulation involves four distinct stages. i. formation of the neural plate ii. shaping of the neural plate iii. bending of the neural plate to form neural groove and iv. closure of the neural groove to form neural tube i & ii Formation and shaping of neural plate: The process of neurulation is initiated 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 & Schoenwolf, 1989; Keller et al, 1992). These elongated cells are the cells of the presumptive neural plate and thus become different from the cells of the epidermis, which remain more or less flat and arranged as a stratified epithelium. About 50 % of the ectoderm is included in the neural plate. The shaping of the neural plate is attained by the intrinsic movements of the epidermal and neural plate regions. The neural plate lengthens along the anterior – posterior axis and becomes narrow (fig 4.11a). iii. Bending of the neural plate: The bending of the neural plate involves the formation of hinge regions. In birds and mammals, the cells at the mid-line of the neural plate are called medial hinge point (MHP) cells (fig. 4.11b). They are derived from the portion of the neural plate just anterior to Hensen’s node and from the anterior mid-line of Hensen’s node. The MHP cells become anchored to the notochord beneath them and form a hinge, thereby forming a furrow at the dorsal mid-line. The notochord induces the MHP cells to decrease their height and to become wedge shaped (Van Straaten et al 1988; Smith and Schoenwolf 1989). The cells lateral to MHP undergo a change to form two other hinge regions called dorsolateral hinge points (DLHP). DLHP are anchored to the surface ectoderm of the neural folds therefore the neural plate remains attached to the rest of the ectoderm (fig. 4.11c). These cells increase in their height and become wedge shaped. Both microtubules and microfilaments are involved in these changes. Two main forces are involved in bending of the neural plate a) formation of hinges, which act as a pivot and directs the rotation of the cells around it and b) the movement of the presumptive epidermis towards the mid-line of the embryo and anchoring of the neural plate to the underlying mesoderm.