**6. Discussion**

The aim of this study was to uncover how social media usage by SGMY during the period of identity concealment can contribute to the development of a resilient SGM-identity. This study used the following definition for a resilient SGM-identity: the ability to maintain a positive SGM-identity, and find resources to do so, living

*Developing a Resilient Sexual and Gender Minority Identity Online: The Importance of Social… DOI: http://dx.doi.org/10.5772/intechopen.108208*

in a society where that identity is seen as norm-deviating. The empirical results showed that social media offers several possibilities for SGMY in developing a positive SGM-identity during the period of identity concealment. This study adds to the body of research on SGM-identity development through social media. This study also goes beyond it by bringing insights together with exploring SGM-identity development through a resilience framework. How the results relate to the development of a resilient SGM-identity will be discussed in this section, using the above definition as a guide. The discussion will end with practical implications, strengths and weaknesses of this study, and recommendations for future research.

#### **6.1 Social media as a resource**

Due to a lack of information, representation, and connections experienced offline, SGMY turn to social media to find labels, recognition, understanding, community, and to feel free instead of vigilant. Additionally, results show that through social media, SGMY can find places where they can be their true selves, before disclosing their SGM-identity to people in their offline environment. This finding shows that social media can offer identity development experiences during the period of identity concealment, as is shown in other studies as well [5, 15, 17, 20, 22]. Furthermore, this study offers new findings by showing that SGMY can develop a positive SGM-identity online before sharing this information with others offline. As online experiences are not necessarily reflected offline, a focus on social media usage by SGMY provides further support for D′Augelli's model [34] in which the fluidity of SGM-identity development is central. Furthermore, without social media, difficulties might be found in accessing to community—and with that community-based resilience—during the period of identity concealment, as it requires identification with a social network [44]. This study showed that with the bridging possibilities offered by social media, SGMY have access to community-based resilience before having to disclose their identity to people in their offline environment.

The possibility to have these experiences online is ensured by the social media affordance accessibility. The other affordances, anonymity and asynchronicity, provide safety and comfort in these online experiences. These findings build upon previous research [3, 19, 21, 22, 29]. With anonymity, SGMY can privately develop their SGM-identity and find support online, without a need for explanation or fear of judgment from people in their offline environment. This allows them to become comfortable with their identity, before having to share it with others, thus bridging the period of identity concealment. Asynchronicity can offer SGMY comfort in their online search as they are in control of with whom, what, and when they share information about themselves.

#### **6.2 Developing a positive SGM-identity**

Social media can promote a positive SGM-identity in several ways, reflected in an increase of SGM-identity acceptance, comfort, and pride. In the realization and confirmation of SGM-feelings feelings of being different and incongruence due to internalized expectations of hetero- and gender-normativity are diminished with information, representation, and connections found on social media. Additionally, the access to information on social media can accelerate the process of self-concept clarity for SGMY. Furthermore, the representation of diverse SGM-identities on social media can be considered vital for SGMY with less commonly known SGM-identities,

because knowledge and understanding of these identities are often more lacking in the offline environment. The above findings are in line with previous studies, showing the importance of information, representation, and connections for SGMY in developing self-concept clarity [15, 20, 35, 44].

Building upon findings from previous research on SGM-identity development through social media, it was found that information, representation, and connections can increase the self-esteem of SGMY through normalization and validation of SGMidentities, diminished feelings of loneliness, and increased feelings of pride and hope. This process reflects the increase of individual resilience [3, 17, 19–22]. Moreover, identifying with a label enables SGMY to become more secure in its identity. The SGM-identity can then be described as the foundation of who they are. This can eventually diminish the need to hold strictly onto that label, which can generate a freeing feeling. The influence of input through information, representation, and connections is supported by theories on developing self-concept clarity and positive self-esteem [16, 30, 31].

### **6.3 Coping with negativity in the offline environment**

Social media can be defined as safe spaces for SGMY because they allow them to create their own SGM-positive bubble, by following accounts and finding content where diversity is celebrated. With that, SGMY can use social media as an escape to a supportive community online, in order to cope with negativity in the offline environment. Previous studies have also found support for using social media as an escape [15, 44]. Additionally, an increased positive SGM-identity was found to result in SGMY being able to cope with (the risk of) negativity in their offline environment. This was portrayed in SGMY wanting to share their SGM-identity with people in their offline environment, despite possible risks of negativity. Additionally, similar to the findings of Craig et al. [15], it was found that finding community on social media, relates to wanting to fight back for this community, by educating people in the offline environment. This reflects a way of SGMY navigating the society in which their identity is seen as norm-deviating, for which willpower was found through social media.

#### **6.4 Practical implications**

Two practical implications can be formulated. First, studying the opportunities of social media leads to insights into the needs of SGMY growing up in society today. Indeed, the importance of having information, representation and connections, and the experienced lack thereof in the offline environment, shows that changes in the offline environment are needed. This could be done by incorporating more inclusive sex education in primary and high schools, in which SGM-identities, and the fluidity thereof, are represented. Additionally, more schools should have explicit safe spaces for the SGM-community, such as GSAs, where they can find belonging. However, it is not only the duty of schools to create more inclusive environments for SGMY, this responsibility needs to be recognized society-wide, from a local to a national level.

Second, it is important to emphasize the opportunities it offers, as a risk-based discourse on social media in research and society leads to a misrepresentation of social media, and with that stigmatization of social media users, which is undesirable [18]. Especially when already stigmatized people use it as a means to attain a positive identity [17]. Although the offline environment should be better aligned with the developmental needs of SGMY, social media can simultaneously still offer them

*Developing a Resilient Sexual and Gender Minority Identity Online: The Importance of Social… DOI: http://dx.doi.org/10.5772/intechopen.108208*

opportunities that the offline environment cannot explained by the affordances of social media. Indeed, via social media, the SGM-community goes beyond the borders of cities and countries and introduces SGMY to community-based resilience and a supportive community worldwide.

#### **6.5 Strengths, weaknesses, and recommendations for future research**

This study has several strengths and weaknesses. To start, a diverse sample based on age, sexual-, and gender identity was included in this study. With that, voices of those that are otherwise often underrepresented in research [7], were heard in this study, including asexual, pansexual, unlabeled, non-binary, and transgender youth. No cisgender males with a sexual minority identity participated in this study. To be more representative of SGM-people as a group, it is important to include their perspectives as well. The intention of this study, however, was not to make generalizable conclusions but to increase understanding of developing a resilient SGM-identity through of social media.

Furthermore, no attention was paid to intersections between SGM-identities and non-SGM-identities, such as race, and class. These intersections, however, might offer interesting insights as these identities intersect to create unique experiences for each individual, based on institutional power structures [26]. Thus, intersectionality of identities should be paid attention to in future research by selecting participants based on the intersections of identities. Another recommendation is to undertake a comparison study between two age cohorts of SGMY, to see whether the influence of social media has changed over time. Indeed, many participants articulated that they see changes between themselves and their younger siblings, regarding information, representation, and connections available in the offline environment. While interesting for future research, it, even more importantly, gives hope for future generations to know that they will find understanding and community, and feel free to be themselves, both online and offline.

### **7. Conclusion**

In this study, identity development of SGMY was studied through a framework of resilience. To do so, the term a 'resilient SGM-identity' was proposed: the ability to maintain a positive SGM-identity, and find resources to do so, living in a society where that identity is seen as norm-deviating. The aim of proposing this term was to better capture the dynamic nature of identity development. The results of this study show that this term indeed can be used to study the identity development of SGMY, given the obstacles they may experience due to society's persistent negative attitudes towards SGM-identities.

It was found that finding information, interaction, representation, and community on social media, helps SGMY develop self-concept clarity and positive self-esteem, resulting in a positive SGM-identity. Moreover, as a result of three affordances of social media, anonymity, asynchronicity and accessibility, this process can take place while SGMY have not yet come out to the people in their offline environment. This period of identity concealment is previously often characterized as a period of isolation, in which access to social and emotional support is limited, accompanied with the risk of developing psychopathology [4, 10]. Although it is not to be forgotten that parental and offline peer support and acceptance are important

for SGMY, with the help of social media the need for this might be delayed, allowing them time to come to peace with their SGM-identity. Furthermore, social media fostered individual- and community-based resilience, reflected in an increase of self-acceptance, identity pride, a sense of belonging and wanting to fight back for the SGM-community. These characteristics helped SGMY navigate their (hostile) offline environment, resulting in them wanting to be their authentic selves, despite the risk of adversity.

Overall, the online community on social media platforms can be identified as a crucial factor in the development of a resilient SGM-identity for SGMY. Being part of the online community can be seen as a feeling of belonging, a knowing that there are people who will support you unconditionally, rather than an active membership. The results showed that the online community played various roles in the lives of SGMY during different phases of their identity development process. During the formative phases of their identity, SGMY relies on the community for the confirmation and normalization of their feelings. Whereas later in time, they may feel the need to stand up for the community.


**3.4 Statistical summaries**

*Advanced Evapotranspiration Methods and Applications*

**66**

**Figure 15.**

*runs.*

**Figure 14.**

the baseline run 1 by about 19% on average.

Growing season (March–September) ET produced by the time-integration is plotted in **Figure 18** for the Nebraska study area for runs 3, 2, and 5 versus run 1. Agreement was strongest between baseline run 1 and run 2 that used images from both Landsats from path 29 only. Growing season ET produced from path 29 using only Landsat 7 images only had the worse correlation with r2 = 0.64. Growing season ET produced from path 29 images, tended to overestimate ET according to

*Monthly ET averaged from the 1500 sample pixels for the Nebraska study area for the five time-integration*

*Plots of average integrated ETrF for August from model run 2 (two Landsats on path 29), run 3 (two satellites on path 30), run 4 (Landsat 5, only on path 29), and run 5 (Landsat 7, only on path 29) versus ETrF produced*

*from the baseline model run 1 for the central Nebraska analysis area for year 2002.*

**Table 3** summarizes monthly average ET for the four time-integration runs for the Idaho study area and root mean square error (RMSE) for the 1500 sampled fields. RMSE was relatively high for run 2 (both satellites for path 39 only),

**Figure 16.**

*Monthly ETrF averaged over the 1500 sample pixels for the Nebraska study area for the five time-integration runs.*

**Figure 17.**

*ET for April–October growing season for 1500 sampled locations for the Idaho study area for the timeintegration runs 2, 3, and 4 versus run 1 and (lower right) averages over all 1500 sampled fields. Also shown in the lower right is reference ET summed over the April–October period.*

#### **Figure 18.**

*Total ET for May–September growing season of 2002 for 1500 sampled locations for the Nebraska study area for time-integration model runs 3, 2, and 5 versus model run 1. Also shown in the lower right is total growing season ET averaged over all samples and reference ET from two AWDN stations (right two columns).*

#### **Table 3.**

*Average monthly ET over the 1500 sampled fields in the Idaho study area for the four runs and RMSE, percentage error and reference ET.*

**69**

*\**

**Table 5.**

**Table 4.**

*Influence of Landsat Revisit Frequency on Time-Integration of Evapotranspiration…*

dates so that important inflection points in ETrF curves can be obtained.

**Table 5** provides monthly average ET, RMSE, percentage error (RMSE error in relation to ETr), and total monthly reference ET for the 1500 pixel sample locations in the Nebraska study area. RMSE was high for all model runs for May and June, exceeding 19%, with a maximum RMSE of 104% for model run 2 for May. Model run 2 (both Landsats from path 30 only) had lower error for June compared to May

Both paths 849 0.00 0.60 Path 39 778 8.30 0.50 Path 40 838 1.25 0.53 Path 40\_L5 834 1.74 0.53

*Growing season ET (April–October 2000) averaged over 1500 sampled fields in the Idaho study area for the four*

**Average** Both paths 34 159 194 156 123

**RMSE** Both paths 0 0 0 0 0

**% error\*** Both paths 0 0 0 0 0

**ETr (mm)** 184 259 232 173 158

*Average monthly ET, RMSE, percentage error, and total monthly reference ET for the 1500 pixel sample*

Path 30 130 169 210 152 123 Path 29 90 178 199 153 120 Path 29\_L5 only 59 164 197 151 123 Path 29\_L7 only 94 197 205 164 93

Path 30 104 19 31 10 6 Path 29 61 45 8 6 7 Path 29\_L5 only 52 45 14 8 8 Path 29\_L7 only 64 63 19 14 32

Path 30 56 7 13 6 4 Path 29 33 17 3 3 4 Path 29\_L5 only 28 17 6 5 5 Path 29\_L7 only 35 24 8 8 29

**ET (mm) % Diff ETrF**

**Average ET, RMSE, and Total ETr are in mm—Year 2002 May June July August Sept**

exceeding 20% of reference ET for 5 of 7 months. RMSE for runs 3 and 4 had less error than run 2 for 2 months, even though run 4 utilized only seven image dates (from Landsat 5 and path 40) in the integration. This indicates the importance of timing of images to identify key inflection points in the ETrF curves and to capture special events such as wetting events from irrigation and rain or from water stress or cuttings, as in the case of alfalfa hay. **Table 4** summarizes growing season comparisons for ET among the four runs. The runs that used images from path 40 only compared to within 2%, when averaged over all 1500 fields, to the baseline run. This outcome is likely due to the timing of path 40 images relative to the combined run as compared to path 39. This shows the high value of a high density of image

*DOI: http://dx.doi.org/10.5772/intechopen.80946*

ETr 1422

*runs and percent differences from the base run 1.*

*Percentage error is RMSE error relative to ETr.*

*locations for the Nebraska study area.*


# **Markov and Semi-markov Chains, Processes, Systems and Emerging Related Fields**

Edited by Panagiotis-Christos Vassiliou and Andreas C. Georgiou Printed Edition of the Special Issue Published in *Mathematics*

www.mdpi.com/journal/mathematics


**4. Production, modeling, and characterization of supercapacitors**

Supercapacitors fill a wide area between storage batteries and conventional capacitors. Both from the aspect of energy density and from the aspect of power density this area covers an area of several orders of magnitude. Materials, construction, production technology, and test methods are chosen depending on the part of the area covered by a certain type of supercapacitor.


9. Limb development

The process of development of limbs is well understood as the genes controlling the growth, patterning and signaling pathways for its development is well conserved from Drosophila to mammals. The developing limb is divided into a proximal stylopod, middle zeugopod and distal autopod. In humans the stylopod becomes the arm or thigh, the zeugopod becomes the forearm or leg and the autopod changes into the hand or foot. The limb buds appear on Day 26 for upper limb and 1 or 2 days later for lower limb. The limb bud to begin with consists of a mesenchyme core as an extension from the parietal layer of lateral plate mesoderm which leads to the formation of bones and connective tissues and it is covered with a layer of cubical ectoderm. The positioning of the limb buds depend on the correct functioning of HOX genes, which control the expression of FGF8, and the latter determines the limb type, i.e., fore or hind limb. The apical

Figure 9. Molecular regulation of limb development. Fibroblast growth factor 10 and AER initiate limb growth. These are induced by bone morphogenetic protein S (RA is retinoic acid, ZPA is zone of polarizing activity, AER is apical endodermal ridge).

ectodermal ridge (AER), which is a localized area of thickening on the surface ectoderm covering the limb bud, produces growth signals such as FGF4 and FGF8, which result in growth in proximal-distal axis. A collection of cells at the posterior margin of limb bud near body wall also undergoes proliferation to form the zone of polarizing activity which produce morphogen retinoic acid which in turn expresses SHH (Sonic Hedge Hog) which plays a huge role in determining the anteroposterior axis for limb development (Figure 9). Bones of the limb are derived by endochondral ossification of the parietal layer of the lateral plate mesoderm which is determined by HOX genes (HOXA and HOXD), which regulate the type, and shape of the bones. Digits are formed due to organized apoptosis of the AER into separate ridges and ultimately by interdigital apoptosis. HOXD13 mutations result in synpolydactyly, which is a dominantly inherited condition. Hand foot genital syndromes results from mutations of HOXA13 and presents hypospadias in males and bilateral thumb and great toe hypoplasia [6]. Dorsalization of the limb bud is under the control of genes like radical fringe and Wnt7a. Holt Oram syndrome an autosomal dominant inheritance is due to mutation of T-box 5 gene and is characterized by forelimb abnormalities and cardiac defects thus T-box genes determine the limb identity by activating FGFs. Another gene is the PitX, which contribute in forming hind limb characteristics [6].

#### 10. Head and neck formation

genitalia does not fuse and develop into normal female genitals. Normal sexual differentiation is completed by 14 weeks though the testis descends at a much later part of pregnancy [1].

Different cells in an embryo express different sets of genes at different times. A common set of genes is present in all cells at all times and is called as the "House-keeping genes." These genes carry out the normal functions of a cell. Speciality genes are the special genes that enable a cell to carry out special functions. The genes responsible for embryonic body plan Homeobox

The gene expression is regulated by three sets of proteins called as the promoter, silencer and enhancer proteins. These proteins can modify the gene expression and define a particular cell type. There are also signaling proteins that facilitate the development of adjacent areas in a

The SRY gene on the Y chromosome produces testis determining factor, which in turn activate the SOX9, and steroidogenesis factor (SF1) and it stimulates development of Sertoli and Leydig cells. WNT4 is the master gene for ovarian development. It inhibits SOX9 and up regulates DAX1. WNT4 along with other genes promotes formation of cortical cords and causes regres-

The extracellular growth factors regulate cell division and differentiation by the process of signal transduction and thus mutations in genes, which produce these factors and receptors, have been implicated in cancer development and also in developmental anomalies. The Fibroblast growth factor receptors (FGR) play an important role in embryogenesis. Mutations in FGR have been associated with two groups of disorders of skeletal system, which broadly include the craniosynostosis syndromes and achondroplasias. Genes such as Fibrillin-I and Elastin code for proteins that are needed for arranging microfibrils in the matrix and thus the

The process of development of limbs is well understood as the genes controlling the growth, patterning and signaling pathways for its development is well conserved from Drosophila to mammals. The developing limb is divided into a proximal stylopod, middle zeugopod and distal autopod. In humans the stylopod becomes the arm or thigh, the zeugopod becomes the forearm or leg and the autopod changes into the hand or foot. The limb buds appear on Day 26 for upper limb and 1 or 2 days later for lower limb. The limb bud to begin with consists of a mesenchyme core as an extension from the parietal layer of lateral plate mesoderm which leads to the formation of bones and connective tissues and it is covered with a layer of cubical ectoderm. The positioning of the limb buds depend on the correct functioning of HOX genes, which control the expression of FGF8, and the latter determines the limb type, i.e., fore or hind limb. The apical

sion of medullary cords and prevents the tunica albuginea to develop [2].

mutations are associated with Marfans' syndrome [3].

9. Limb development

8. Gene expression during development

genes, proto-oncogenes and PAX genes.

84 Genetic Diversity and Disease Susceptibility

particular organ.

The most important feature in development of head and neck is the formation of pharyngeal arches resembling the gills of the fish. These arches are bars of mesenchyme tissue appearing at fourth or fifth week of development with an outer covering of ectoderm and inner layer of endoderm at the cranial most part of the primitive gut. The arches are separated from each other by ectodermal clefts and form out pouching in the lateral wall of developing pharynx as endodermal pouches. The mesoderm which lies in the center of the arches form the arch arteries and musculature and the skeletal elements are formed from the neural crest cells, which migrate early from hindbrain region. The FGF signaling plays an important role in determining the migration and fate of neural crest cells in the arches, endodermal pouch formation and differentiation [7]. Though it was previously thought that the neural crest cells are important for pharyngeal arch development but nowadays it is shown that the endoderm of the pharyngeal apparatus acts as the principal organizer and thus it is proved that neural crest cells are not necessary for pharyngeal arch development. The out pocketing of the endoderm is significant for pharyngeal arch development and regulated by TBX1 and RA signaling [8]. The most well-known condition due to faulty development of third and fourth pouches is the DiGeorge syndrome which results due to submicroscopic chromosome deletion 22q11 and loss of approximately 30 genes. Treacher Collins is hypoplasia of the maxilla and mandible with coloboma of the lower eyelid, cleft palate, hearing impairment has an autosomal dominant inheritance affecting the first arch and results due to mutation in TCOF1 gene [1].

transcription factor NKX2 which is a homolog of the gene tinman that regulates heart development in Drosophila, is induced in the endoderm overlying the splanchnopleuric mesoderm for human heart development. NKX2 is considered as the master gene for human heart development and its expression is brought about by BMP activity and WNT inhibition. The venous portion of the heart is specified and formed under influence of retinoic acid and cardiac

Developmental Genetics and Preimplantation Genetic Diagnosis

http://dx.doi.org/10.5772/intechopen.77965

87

The cephalocaudal and lateral folding of the embryo gives rise to formation of primitive gut tube which is endoderm lined yolk sac cavity incorporated into the embryo by fourth week of development. The retinoic acid (RA) concentration varies in this primitive gut and it is the highest in colon region and least in that part of this gut tube which give rise to the pharynx. This variation of RA concentrations causes expression of certain transcription factors, which leads to the specification of the gut tube to develop the parts. SOX2 is transcription factor for development of esophagus and stomach, PDX1 is for duodenum development, CDXC is for small intestine, and CDXA is for large intestine and rectum. The interaction of the epithelial and underlying mesenchyme components of the gut follows the initial patterning and brought about by sonic hedgehog expression throughout the gut. The transition of the foregut to midgut and thus hind gut derivatives and the change in the epithelial characteristics are brought about by nested expressions of HOX genes which are activated by sonic hedgehog to bring about an orchestrated development of the gut. The secretion of FGFs from cardiac mesoderm and BMPs from the septum transversum leads to the hepatic development. The FGFs inhibits the activity of inhibitors and thus specifies the development of hepatic endoderm by a negative feedback mechanism as suppressing the inhibitors brings about the induction. The sonic hedgehog expression in gut endoderm, which is destined to form dorsal pancreatic bud, is suppressed in the gut endoderm by FGF2 and activin. The ventral bud is induced by visceral mesoderm, which up regulates expression of pancreatic and duodenal homeobox gene (PDX). Two types of PAX genes specify the endocrine cells. PAX4 and PAX6 genes specify the insulin, somatostatin, and pancreatic polypeptide secreting cells whereas PAX 6 genes specify

The urogenital system develops from a common mesodermal ridge, i.e., the intermediate mesoderm. The intermediate mesoderm gives rise to three overlapping kidney systems the pronephros, the mesonephros and the metanephros. The metanephros, which appears by fifth week, gives rise to the definitive kidney in humans. The mesonephric duct of the intermediate mesoderm gives rise to an ureteric bud which forms the collecting part of the kidneys the renal pelvis, calyces, 1–3 million collecting tubules. The interaction of this ureteric bud from the mesonephric duct with the mesenchyme of the metanephric blastema is the key factor in determining the initiation of kidney development. WT1 is the transcription factor expressed by the mesenchyme of metanephros, which increases the sensitivity of the metanephros to interact with the ureteric bud and respond to it. The branching and growth of ureteric bud is under control of production of Glial derived neurotrophic factor (GDNF) and Hepatocyte growth factor produced by the mesenchyme of metanephros which bind to the receptor on

looping is due to expression of PITX2 in lateral plate mesoderm.

the glucagon secreting cells [5].

13. Urogenital system

### 11. Ciliary development

The cilia on the apical surface of the cells have an important role in human embryological development deciding cell specification, axis determination and pathology of human congenital disorders. A variety of congenital defects like polycystic kidney, laterality defects, nervous system defects, and retinal degeneration are associated with ciliopathies thus proving that cilia can serve as signaling factor in embryonic development. Cilia plays an important role in Hedgehog patch smoothened Gli pathway and thus it regulates the Gli protein activity. The cilia transition zone a short segment just above the basal body acts as an essential gatekeeper for movement of protein in and out of the cilia. Cilia also play an important role in movement of fluid in various confined embryonic spaces, which may act as a developmental shear force [9]. The congenital polycystic kidney disease, which results in numerous cyst formations, can be inherited as autosomal dominant or recessive. In autosomal recessive type the cysts are formed in the collecting tubules and it is a progressive disorder though less common but highly progressive and leads to kidney failure early in infancy and childhood. The autosomal dominant type is more common (1/500 to 1/1000) results in cyst formation in all segments of the nephron is less progressive and does not result in renal failure till adulthood, The above two types of conditions are related to the mutation of genes that code ciliary protein and thus fall under the group of disorders the ciliopathies which are creating a lot of interest in recent times. Bardet-Biedl syndrome and Meckel-Gruber syndrome are characterized with renal cysts and other features are also some of the ciliopathies recently identified [2].

#### 12. Reciprocal interactions

Organogenesis results due to reciprocal interaction between the differentiated epithelial cells and the underlying mesenchyme, which is brought about by signaling molecules. The transcription factor NKX2 which is a homolog of the gene tinman that regulates heart development in Drosophila, is induced in the endoderm overlying the splanchnopleuric mesoderm for human heart development. NKX2 is considered as the master gene for human heart development and its expression is brought about by BMP activity and WNT inhibition. The venous portion of the heart is specified and formed under influence of retinoic acid and cardiac looping is due to expression of PITX2 in lateral plate mesoderm.

The cephalocaudal and lateral folding of the embryo gives rise to formation of primitive gut tube which is endoderm lined yolk sac cavity incorporated into the embryo by fourth week of development. The retinoic acid (RA) concentration varies in this primitive gut and it is the highest in colon region and least in that part of this gut tube which give rise to the pharynx. This variation of RA concentrations causes expression of certain transcription factors, which leads to the specification of the gut tube to develop the parts. SOX2 is transcription factor for development of esophagus and stomach, PDX1 is for duodenum development, CDXC is for small intestine, and CDXA is for large intestine and rectum. The interaction of the epithelial and underlying mesenchyme components of the gut follows the initial patterning and brought about by sonic hedgehog expression throughout the gut. The transition of the foregut to midgut and thus hind gut derivatives and the change in the epithelial characteristics are brought about by nested expressions of HOX genes which are activated by sonic hedgehog to bring about an orchestrated development of the gut. The secretion of FGFs from cardiac mesoderm and BMPs from the septum transversum leads to the hepatic development. The FGFs inhibits the activity of inhibitors and thus specifies the development of hepatic endoderm by a negative feedback mechanism as suppressing the inhibitors brings about the induction. The sonic hedgehog expression in gut endoderm, which is destined to form dorsal pancreatic bud, is suppressed in the gut endoderm by FGF2 and activin. The ventral bud is induced by visceral mesoderm, which up regulates expression of pancreatic and duodenal homeobox gene (PDX). Two types of PAX genes specify the endocrine cells. PAX4 and PAX6 genes specify the insulin, somatostatin, and pancreatic polypeptide secreting cells whereas PAX 6 genes specify the glucagon secreting cells [5].