Abstract
Embryology is a branch of science that is related to the formation, growth, and development of embryo. It deals with the prenatal stage of development beginning from formation of gametes, fertilization, formation of zygote, development of embryo and fetus to the birth of a new individual. Two basic processes involved are growth and differentiation. These lead to formation of various tissues and organs in body specialized to perform specific functions. Neuroembryology is related to the development of central nervous system (brain and spinal cord) and peripheral nervous system (spinal, cranial, and autonomic nerves) in the body. These tissues develop from neural tube and neural crest cells. In this chapter we have described the origin and various stages of development of a multicellular, highly complex, and specialized nervous system from a single-celled zygote.
Keywords
Brain, Differentiation, Embryology, Growth, Nerves, Nervous system, Neural tube, Neuron, Spinal cord
Outline
Formation of Zygote 41
Formation of Blastocyst 41
Formation of Embryonic or Germ Disc 42
Formation of Definitive Notochord 44
Development of Nervous System 45
References 50
Embryology is a branch of science that is related to the formation, growth, and development of embryo. It deals with the prenatal stage of development beginning from formation of gametes, fertilization, formation of zygote, development of embryo and fetus to the birth of a new individual. Two basic processes involved during conversion of a single-celled zygote to a complex, multicellular organism are growth and differentiation . Growth occurs by increase in cell number (cell division and multiplication) or cell size. On the other hand, cell differentiation is a complicated process in which the cell acquires special characteristics to perform specific functions. These lead to the formation of various tissues and organs assigned to perform specific functions.
Formation of Zygote
The germ cells or gametes (sperm and ovum) are specialized haploid cells (with 23 unpaired chromosomes in human). Fertilization results in union of the gametes (i.e., fusion of sperm with ovum, Fig. 2.1 ) to form an undifferentiated, mononucleated, diploid cell (with 23 pairs or 46 chromosomes) called z ygote . The fertilization usually takes place in the ampulla or lateral third of fallopian tube. After fertilization, the fertilized egg travels down the fallopian tube to reach the uterus.
Formation of Blastocyst
The single-celled zygote ( Fig. 2.2 ) divides repeatedly by mitotic division thereby retaining the same number of chromosomes (i.e., 46 chromosomes) in each of the two daughter cells. The cells so formed are called blastomeres , and the process of division is called cleavage ( Fig. 2.2 ). Thus a single-celled zygote results in the formation of a mass of cell called morula (16- to 32-celled stage). The inner cells of the morula called the inner cell mass gives rise to embryo proper, and the outer layer of cells called the outer cell mass forms the covering of embryo and contributes to formation of placenta. As the cells of morula continue to divide, fluid from uterine cavity enters the intercellular spaces between the inner and outer cell mass. Later the intercellular spaces fuse to form a single cavity called blastocele , and this stage of embryo is called blastocyst . The cells of the inner cell mass are pushed to one side of blastocyst and this side of blastocyst is known as the embryonic pole . The cells of the inner cell mass are called embryoblast cells . The cells of outer cell mass flatten and form the wall of blastocyst and are called trophoblast cells ( Fig. 2.2 ). The trophoblast cells covering the embryonic pole have the property to invade the epithelial cells of uterine mucosa and thus get attached to uterus.
After fertilization in the fallopian tube, as the fertilized egg (zygote) divides repeatedly to form morula, it travels down the fallopian tube to reach the uterine cavity. The morula reaches the uterine cavity on the third to fourth day of fertilization. On day 5, blastocyst is formed, which adheres to uterine mucosa on the sixth day of fertilization and gets implanted in the uterus.
Formation of Embryonic or Germ Disc
During the second week of development, the cells of the inner cell mass (embryoblasts) differentiate and organize into two epithelial layers—the inner layer of cuboidal cells or hypoblast on the ventral surface that faces the blastocyst cavity and the outer layer of columnar cells or epiblast on the dorsal surface. These layers together form the bilaminar germ disc or embryonic disc ( Fig. 2.3 ). The trophoblasts start forming the placenta. Fluid begins to collect between the cells of outer layer (epiblast cells) and the trophoblasts and forms a fluid-filled cavity known as amniotic cavity . The epiblast cells proliferate and migrate to line the roof of amniotic cavity. These cells are called amniogenic cells . Similarly, the hypoblast cells facing the blastocyst cavity proliferate and migrate to line the blastocyst cavity to form the yolk sac ( Fig. 2.3 ).
During the third week of embryonic development, bilaminar germ disc is converted to trilaminar germ disc with the formation of the three primary germ layers— ectoderm , mesoderm , and endoderm . This process is called gastrulation , which begins with the appearance of primitive streak (characterized by narrow median groove with slight raised margins) on the outer surface (epiblast) of the embryonic disc. At the cranial end of this streak there is a primitive node (Hensen’s node), the center of which presents a depression called primitive pit ( Fig. 2.4A ). The cells of the epiblast migrate toward the primitive streak, get detached from the epiblast layer, and come to lie underneath it ( Fig. 2.5 ). This is called invagination. Some of these invaginated cells displace the hypoblast cells from the endoderm while others migrate to occupy the space between the epiblast and hypoblast (endoderm) cell layers to form the third germ layer—the intraembryonic or secondary mesoderm . The remaining cells of epiblast forms the ectoderm. The three germ layers thus formed give rise to all the tissues and organs in the embryo.
Formation of Definitive Notochord
At the cephalic end of the germ or embryonic disc, some of the endodermal cells thicken to form an oval plate called the prechordal plate ( Fig. 2.4B ). The prechordal plate decides the cephalic end of the embryo. The intraembryonic mesoderm extends between the ectoderm and endoderm over the entire embryonic disc except at two sites—one in the region of prechordal plate and the other caudal to primitive streak. At these sites, the endoderm is closely adherent to overlying ectoderm without mesoderm in between forming two bilayered membranes—the buccopharyngeal membrane (cranially) and the cloacal membrane (caudally) ( Fig. 2.4 ). Buccopharyngeal membrane is the site for future oral opening and cloacal membrane for anal opening.
The embryonic disc grows more at the cephalic end than the caudal end because of continuous migration of cells from primitive streak and primitive node in the cephalic direction. This causes the rounded embryonic disc to become elongated with broad cephalic and narrow caudal end ( Fig. 2.4B ). The primitive streak regresses after the third week and finally disappears. The primitive pit surrounded by cord of cells extends in cephalic direction from primitive node to the prechordal plate in midline and lies between the ectodermal and endodermal layers. This canalized cellular cord is called notochordal process ( Fig. 2.4B ). This process cannot extend beyond the prechordal plate as the endoderm and ectoderm are firmly adherent to each other here. The cells in the floor of the notochord canal fuse with the endoderm cells beneath it (which forms roof of yolk sac) and subsequently both group of cells disappears in craniocaudal direction. Thus, the yolk sac communicates with amniotic cavity through primitive pit. This temporary communication between the two cavities is called neurenteric canal , which later gets closed ( Fig. 2.6A ). The notochord process now forms a notochordal plate along the roof of the yolk sac ( Fig. 2.6B ). Later this plate folds along its long axis and separates from the roof of yolk sac, which is now lined by endoderm. This chord of cells is known as definitive notochord ( Fig. 2.6C ).
