3. Regulation of gene expression

The cell fates can be mapped by identifying cells using reporter genes, fluorescent probes and other markers. The gene expression can be regulated at several levels

1. How different genes may be transcribed?

This is important because an embryonic injury when the cells are pluripotent may be lethal or may not have any effect at all. But when the cells cross the phase of specification and determination an injury invariably results in a structural anomaly. For example, exposure to rubella virus causes loss of cells in fetal lens and results in congenital cataract and microopthalmia.

Prenatal life can be divided into three main stages, i.e., pre-embryonic, embryonic and fetal. The pre-embryonic phase is the period during which the small collection of cells gets differentiated to form three germ layers, i.e., ectoderm, mesoderm and endoderm by the process of gastrulation. The body axes, i.e., anteroposterior, dorsoventral, left and right also established

The pre-embryonic and embryonic phases are the times during which a single cell progresses

The final fetal stage after 8 weeks leads to rapid overall growth and maturation of the embryo into a viable human fetus. The integration of complex phenomena, which leads to the formation of a viable infant, has generated interest in understanding the molecular and structural

Various comparative and evolutionary studies have been done to understand this fascinating developmental phenomenon. Many experiments were done to trace cells during their development. Transparent embryos were observed and even living cells stained with vital dyes were used to observe their fate. Later radiographic labels and autoradiography techniques were used. Grafting experiments using quail cells into chick embryos at early stages of development were some of the pioneering efforts in monitoring cell fates that lead to origin of different organs and tissues. Other grafting experiments were grafting the primitive node from its normal position on the body axis to another site could induce formation of a second body axis.

It was also concluded that a piece of tissue from posterior axial border of one limb if grafted to the anterior border of another limb then digits on the host limb are duplicated. This posterior signaling was called zone of polarizing activity and now the signaling molecule is identified as Sonic Hedgehog. Thus the advent and progression in the field of molecular biology has led to better understanding of embryology of normal and abnormal developmental processes.

The cell fates can be mapped by identifying cells using reporter genes, fluorescent probes and

other markers. The gene expression can be regulated at several levels

during gastrulation. The embryonic phase lasts from 5 to 8 weeks.

to form the organ primordia which is the first 8 weeks of human development.

2. Phases of prenatal life

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aspect of this process [1].

3. Regulation of gene expression


Fertilization is the process of union of male and female gametes and it occurs in the ampullary part of the uterine tube. Out of 100–200 million spermatozoa deposited in the female genital tract only a single spermatozoon penetrates through the corona radiates and zona pellucida. Sperm cell membrane fuses with the oocyte membrane.

The penetration of sperm induces cortical and zona reactions, which prevents further sperm binding and penetration. This also leads to the resumption and completion of second meiotic activation of oocyte. The results of fertilization lead to restoration of diploid number of chromosomes, determination of gender of the zygote and initiation of cleavage (Figure 1).

The zygote undergoes repeated mitotic divisions through the process of cleavage. As it attains an eight-cell stage it undergoes compaction and segregates into an inner cell mass and outer cell mass. Inner cell mass, which is the embryoblast, forms the embryo proper and outer cells mass forms the trophoblast. With accumulation of fluid in the embryo the embryoblast and trophoblast separate from each other and a blastocyst is formed (cyst = fluid filled cavity). The zona pellucida disintegrates and the embryo is implanted.

By Day 8 of fertilization the blastocyst gets partially embedded into the endometrial stroma (Figure 2). The inner cell mass differentiates into hypoblast layer and a layer of tall columnar

Figure 1. (a) Oocyte after ovulation. (b) Spermatozoa penetrate the oolemma. (c) Syngamy with male and female pronuclei seen. (d) Chromosomes are arranged in the equatorial plane. (e) Chromosomes are migrating towards poles. (f) Two-cell stage.

Figure 2. (a) Blastocyst stage. (b) Morula formation. (c) Blastocyst formation. (d) Blastocyst on 6th day of development showing trophoblast cells of blastocyst penetrating the uterine mucosa.

cell which is epiblast. This also leads to formation of two fluid filled cavities in the blastocyst, the primitive yolk sac and the amniotic cavity (Figures 3 and 4).

inward movement called invagination. This process is controlled by Fibroblast growth factor 8

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Fibroblast Growth Factor 8 down regulates E cadherin, which is the binding protein of epiblast cells thus helping in cell migration and invasion into underlying layer. Cells that are specified by regulation of Brachyury T expression form the mesoderm layer. FGF 8 also controls this expression. Some of the epiblast cells will even replace the hypoblast Layer and proliferate to form the embryonic endoderm (Figures 6 and 7). Cells, which continue to remain in epiblast,

Figure 5. Dorsal view of germ disc at 14 days showing epiblast and hypoblast. A primitive node is formed at the cephalic

(FGF 8) synthesized by the cells of primitive streak.

end and a primitive streak is seen at the caudal end.

Figure 4. Bilaminar embryonic disc at 14 days in the implantation site in endometrium.

The most important event during the third week of gestation is gastrulation by which all three germ layers are formed, i.e., ectoderm, mesoderm, and endoderm. Gastrulation starts with the formation of primitive streak on the epiblast (Figure 5).

At the cephalic end of the streak a small primitive pit develops which the primitive node surrounds. The epiblast cells of the primitive streak undergo proliferation and migration by

Figure 3. Blastocyst in the endometrial cavity at 9 days.

Figure 4. Bilaminar embryonic disc at 14 days in the implantation site in endometrium.

cell which is epiblast. This also leads to formation of two fluid filled cavities in the blastocyst,

Figure 2. (a) Blastocyst stage. (b) Morula formation. (c) Blastocyst formation. (d) Blastocyst on 6th day of development

The most important event during the third week of gestation is gastrulation by which all three germ layers are formed, i.e., ectoderm, mesoderm, and endoderm. Gastrulation starts with the

At the cephalic end of the streak a small primitive pit develops which the primitive node surrounds. The epiblast cells of the primitive streak undergo proliferation and migration by

the primitive yolk sac and the amniotic cavity (Figures 3 and 4).

formation of primitive streak on the epiblast (Figure 5).

Figure 3. Blastocyst in the endometrial cavity at 9 days.

showing trophoblast cells of blastocyst penetrating the uterine mucosa.

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inward movement called invagination. This process is controlled by Fibroblast growth factor 8 (FGF 8) synthesized by the cells of primitive streak.

Fibroblast Growth Factor 8 down regulates E cadherin, which is the binding protein of epiblast cells thus helping in cell migration and invasion into underlying layer. Cells that are specified by regulation of Brachyury T expression form the mesoderm layer. FGF 8 also controls this expression. Some of the epiblast cells will even replace the hypoblast Layer and proliferate to form the embryonic endoderm (Figures 6 and 7). Cells, which continue to remain in epiblast,

Figure 5. Dorsal view of germ disc at 14 days showing epiblast and hypoblast. A primitive node is formed at the cephalic end and a primitive streak is seen at the caudal end.

Figure 6. Cross section through the cranial region of primitive streak at 15 days showing invagination of epiblast cells.

Figure 7. (a) Sagittal section of a day 17 embryo showing most cranial part of notochord. (b) Schematic cross section through the region of notochord plate. (c) Schematic view showing definitive notochord.

give rise to the embryonic ectoderm. Thus all the three germ layers are gradually formed in the epiblast and hypoblast and the process of gastrulation is complete [2].

During and before gastrulation another important event, which occurs, is the establishment of body axes—anteroposterior, dorsoventral and left—right (Figure 8). As already discussed the primitive streak with the primitive node at its head end determines the anteroposterior axis of the developing human embryo [3]. The anterior visceral endodermal cells at the anterior end of embryonic disc expresses genes and transcription factors like OTX2, LIM1 and HESX1 and also signaling proteins like Cerberus and Lefty which decreases the primitive node activity at head

Figure 8. Dorsal view of the embryonic disc showing gene expression patterns responsible the laterality in the embryo (om: oropharyngeal membrane, cm: cloacal membrane, N: notochord, pm: paraaxial mesoderm, im: intermediate mesoderm, lpm: lateral plate mesoderm, eem: extraembryonic mesoderm. (a) FGF8 secreted by the node and primitive streak establishes expression of nodal gene which leads to TGFα family on the left side. (b) Expression of nodal and LEFTY on the left side of the ventral aspect of neural tube and this is mediated by serotonin monoamine oxidase (MAO) on right side breaks down 5-hydroxy traptamine (5HT). (c) Dorsal view of the primitive streak and fate map of epiblast cells.

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Figure 8. Dorsal view of the embryonic disc showing gene expression patterns responsible the laterality in the embryo (om: oropharyngeal membrane, cm: cloacal membrane, N: notochord, pm: paraaxial mesoderm, im: intermediate mesoderm, lpm: lateral plate mesoderm, eem: extraembryonic mesoderm. (a) FGF8 secreted by the node and primitive streak establishes expression of nodal gene which leads to TGFα family on the left side. (b) Expression of nodal and LEFTY on the left side of the ventral aspect of neural tube and this is mediated by serotonin monoamine oxidase (MAO) on right side breaks down 5-hydroxy traptamine (5HT). (c) Dorsal view of the primitive streak and fate map of epiblast cells.

give rise to the embryonic ectoderm. Thus all the three germ layers are gradually formed in the

Figure 7. (a) Sagittal section of a day 17 embryo showing most cranial part of notochord. (b) Schematic cross section

Figure 6. Cross section through the cranial region of primitive streak at 15 days showing invagination of epiblast cells.

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During and before gastrulation another important event, which occurs, is the establishment of body axes—anteroposterior, dorsoventral and left—right (Figure 8). As already discussed the primitive streak with the primitive node at its head end determines the anteroposterior axis of the developing human embryo [3]. The anterior visceral endodermal cells at the anterior end of embryonic disc expresses genes and transcription factors like OTX2, LIM1 and HESX1 and also signaling proteins like Cerberus and Lefty which decreases the primitive node activity at head

epiblast and hypoblast and the process of gastrulation is complete [2].

through the region of notochord plate. (c) Schematic view showing definitive notochord.

end of the embryo. Nodal which is a protein of transforming growth factor beta family establishes and maintains the primitive streak.

after exhaustive experiments in genetic expression studies, but still the processes are not clear. The developmental gene families identified in vertebrates and mammals show sequence homology with developmental regulators of fruit fly Drosophila. The developmental genes produce proteins called transcription factors, which regulate RNA transcription from DNA template, by binding to regulatory DNA sequences to form complexes, which induces transcription by RNA polymerase. Thus gene expressions can be controlled by transcription factors thus in other words they can switch genes on and off. The variety of processes taking part during development include induction during which extracellular signals change fate of one type of cells to another, segmentation, migration, differentiation and apoptosis or programmed cell death. The protein signaling molecules identified to be involved in the above processes during development across specific belong to Transforming growth factor family,

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Cell signaling plays a very important role in in normal embryogenesis. The receptor tyrosine kinase (RTK) signaling phosphorylates tyrosine residues and is activated by ligands and thus brings about diverse functions during development. Certain ligands, which activate RTK, are fibroblast growth factor (FGF), epidermal growth factor, TGF, Platelet derived growth factor, etc. The intracellular cascade of reactions that follows once binding of ligands activates the receptors has been studied in vitro experiments and is believed to bring about multiple

The TGF-beta family of signaling molecules constitutes at least 33 members of this cytokine family. They are involved in cross talk between cells thus establish cellular communication. These extracellular signaling polypeptides are transduced to regulate other gene expression. These signaling molecules bind to the receptors on the cell surface brings about a cascade of reactions and activation of receptor kinases which leads to translocation of complexes into

The TGF-beta family can be grouped into (1) the BMPs and (2) TGF-beta s, activin, nodal, and myostatin. These groups of signaling molecules are actively involved in various cellular and developmental processes like cell cycle, cell migration, cell size, gastrulation and axis specification and various metabolic processes. BMP signaling is also involved in dendritogenesis and

The end of third week also marks the beginning of segmentation of the paraxial presomitic mesoderm into somatomeres proceeding rostrocaudally. By the end of fifth week there are about 42–44 pairs of somites divided into occipital, cervical, thoracic lumbar and coccygeal pairs. The Wnt, Notch and FGF signal molecules play a vital role in the formation of somites.

nucleus of cells and thus either activate or suppress the target gene expression.

(TGF-beta), Wingless (Wnt) family and the Hedgehog (HH) family.

5. Cell signaling

axonal transport.

6. Segmentation

changes though not fully understood [4].

Another important protein of Transforming growth factor beta is Bone Morphogenic Protein 4, which is secreted throughout the embryonic disc. This is important for ventralising mesoderm for kidney, blood and body wall development through the formation of lateral plate and intermediate mesoderm. The primitive node acts as an organizer and inhibits action of Bone Morphogenic Protein 4 by promoting other genes. Chordin, noggin and follistatin antagonize the action of BMP4. This results in dorsalisation of cranial mesoderm to form notochord, somites and somatomeres.

Brachyury T gene regulates the dorsal mesoderm formation in middle and caudal regions of the embryo. Laterality, i.e., left -right -sidedness is development by the interaction of signal molecules and genes. Genes regulating left side development are well known like LEFTY-2, Nodal that up regulates PITX2, which is a transcription factor. A neurotransmitter serotonin (5HT) also plays an important role in establishing left sidedness in the embryo. Defects of Lefty-2 and PITX2 and 5 HT can lead to defects like dextrocardia, situs inversus and cardiac abnormalities.

The cells of the epiblast, the regions of streak and node from they migrate determines the fate and the type of mesoderm they will eventually form. The cells which migrate from cranial end of node form the notochord and prochordal plate. The cells that proliferate and migrate from lateral edges of the node and cranial part of primitive streak form the paraxial mesoderm. The cells that proliferate and migrate from middle of primitive streak form the intermediate mesoderm. The cells migrating from caudal part of the streak form the lateral plate mesoderm. The cells from the caudal most part of the streak contribute to extra embryonic mesoderm. Thus the fate map of development of various organ systems is established at the time of gastrulation itself [2].
