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Morphogenesis the process by which bodily form is established Morphogenesis The key embryonic processes that contribute to morphogenesis are: Cell division Changes in cell shape Folding of cell layers Cell migration Cell death (apoptosis) Cell-to-cell communication Cell differentiation Embryonic disc The cells of the inner cell mass of the blastocyst become organized into two hollow spheres in contact with each other and attached to the inside of the trophoblastic shell. One of the inner spheres encloses the amniotic cavity, and the other encloses the yolk sac cavity. Where the two inner spheres contact each other there is a twolayered embryonic disc. Cell division • Cell division continues throughout the prenatal period. • Most of these divisions are mitotic • Dividing cells - and hence the embryo - are very sensitive to a variety of chemical and physical agents. Changes in cell shape Shape changes have a central role in morphogenesis. • They are produced by the co-ordinated activity of the cytoskeleton. The cytoskeleton consists of microtubules and different types of microfilaments. • Microtubules assist in elongation of cells and movement of structures such as the nucleus and chromosomes. • Microfilaments can draw structures closer together. Images of cells stained to show the cytoskeleton Microfilaments and microtubules Epithelium • one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. • Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. • Functions of epithelial cells include secretion, selective absorption, protection, transcellular transport and detection of sensation Folding of cell layers When shape changes occur within cells forming an epithelium, the epithelium will buckle and fold. This occurs frequently during morphogenesis, and can give rise to tubular and spherical forms. Examples are seen in the formation of the central nervous system and the lens of the eye. Folding of an epithelium Cell migration During morphogenesis, many cells migrate within the embryo. • Mesoderm cells move and aggregate to form structures such as the somites (paired structures). • Neural crest cells arise from the ectoderm alongside the developing neural tissue and migrate amongst the mesodermal cells. Cell death Some embryonic cells have to die for normal development to occur. • Eg. normal development of the fingers and toes depends on death of the cells forming webs between them. • The cells die without initiating an inflammatory response • The process of this selective cell death is called apoptosis. Cell-to-cell communication Cell-to-cell interactions co-ordinate development and influence gene-selection. • Some regions of the embryo are induced to develop in the correct way by chemical signals from neighbouring tissues, eg: the lens of the eye. Differentiation All the cells of the embryo are derived from the zygote, and they have the same genetic information in their nuclei. • However, during development differences arise between cells. • Different genetic characteristics are expressed in each cell type. • As cells differentiate, they lose developmental potential and become more specialized. Embryonic disc Initially, the embryonic disc has two layers ectoderm and endoderm. Then a third layer called the mesoderm is formed by invagination of ectodermal cells through the primitive streak. The three layers are referred to as the germ layers - they give rise to all the organs and systems of the baby’s body Germ layers Contributions by germ layers to parts of the body • Some of the mesodermal cells aggregate to form the midline notochord, a precursor of the axial skeleton. • The ectoderm over the notochord thickens to form the neural plate. • The embryonic disc remains two-layered in the region of the oral and cloacal membranes. – The cloacal membrane covers the urogential region of the embryo Folding of neural plate The neural plate begins to fold, forming the neural groove. Closure to form the neural tube begins in the future midbrain region, and then extends cranially and caudally. Neural crest cells migrate away from the site of closure and later take numerous roles within the body, forming peripheral neurons, Schwann cells, melanocytes, and head and neck structures. Sections through the neural plate as it closes to become the neural tube Formation of somites Mesodermal cells form up alongside the neural tube and notochord to form segmental somites. Later, the somites will contribute to skeletal elements, muscles, and dermis. This segmental origin is apparent in the pattern of spinal nerves, vertebrae, ribs, nerve supplies, and dermal layers in the completed body. The three main zones of somites – • sclerotome (yellow), • myotome (brown), • dermatome (purple)