The development of male germ cell occurs in three phases, which includes the self-renewal of spermatogonial cells by mitosis followed by the reduction division of spermatocytes to form spermatids and subsequent transformation to spermatozoa by spermiogenesis (Yao et al., 2015). A schematic representation of spermatogenesis along with the timeline has been mentioned in Figure 1. The entire process of spermatogenesis is completed in around 35 days in mice whereas it takes almost 65 days in human. The spermatogonial stem cells (SSCs) arise from primordial germ cells (PGCs) that mark the initial stage of male germ cell formation. The committed SSCs undergo meiosis to form the haploid spermatocytes (Manku and Culty, 2015). The SSCs undergoes continuous mitosis to replenish its pool in the testis and to generate the progenitor populations for the generation of sperm. Any abnormalities in the self-renewal and progenitor generation, ultimately leads to failure in spermatogenesis. The linker chaperone, Testis-specific nuclear autoantigenic sperm protein (tNASP) is indispensible for cell cycle regulation in spermatogonial cells. The higher expression of linker histone H1T coincides with the tNASP expression in proliferating spermatogonial cells in mice. The association of tNASP-H1T complex with the Heat Shock Protein 90 Alpha Family Class A Member 1(HSP90AA1) facilitates the translocation of cytoplasmic NASP to the nucleus (Alekseev et al., 2005) where it associates with Chromatin Assembly Factor 1A subunit (CHAF1A) in a multi-chaperone complex for G1/S progression in spermatogonial cells (Groth et al., 2005). In human and mouse, another H3/H4 chaperone, CCG1 interacting Factor II (CIA-II) was identified to be profoundly expressed in the testis and proliferating cells. However in mice, mCIAII expression is restricted to pre-meiotic and meiotic germ cells. It is proposed that this chaperone could possibly be involved in the recruitment of testis-specific histones for the nucleosome assembly in spermatogonial cells (Umehara and Horikoshi, 2003). Though, the presence of CIAII is evident in male germ cells, its distinctive function in spermatogonial cell remain to be explored.

The next phase is the commitment of male germ cells to meiosis, which is critical for the generation of spermatozoa in the testis. Growth factors and hormones are responsible for the initiation and tight regulation of meiosis in cells, which are committed to spermatogenic lineage. The replicative histone H3 (H3.1 and H3.2), the non-replicative variants H3.3 and its chaperones are involved in regulation of different events of spermatogenesis. The incorporation of testis specific H3 variant, H3T allows germ cells to enter meiosis by providing the open chromatin structure (Ueda et al., 2017). Mammalian H3.3 have two isoforms H3.3A and H3.3B, which are expressed until the pre-meiotic and meiotic-prophase spermatocytes respectively (Bramlage et al., 1997). In mice, the perturbation of H3.3A and H3.3B impairs male fertility (Couldrey et al., 1999; Hödl and Basler, 2009; Bush et al., 2013; Tang et al., 2015). The increase in expression of H3.3 variant in the testis coincides with the expression of the corresponding chaperones. This observation is in agreement with the higher expression of H3.3 chaperone, Anti-silencing Factor B (ASF1B) in the proliferative compartment of progenitor cells. Upregulation of ASF1B in male germ cell directly or indirectly enhances the transcriptional activation of Stra8 (Stimulated by Retinoic Acid) gene, which regulates the meiotic entry and mitotic/meiotic shift. Retinoic acid and STRAT8 signaling primarily regulate the mitotic to meiotic shift (Anderson et al., 2008). Hence, the loss of ASF1B function in mice results in delayed meiotic entry (Messiaen et al., 2016). Despite being reproductively compromised, Asf1b knockout mice are viable.

Meiosis begins with the pairing of homologous chromosomes, which is followed by the formation of synapse for the exchange of genetic material by crossing over and recombination. Recombination is critical for the establishment of synapse between the homologous chromosomes for the exchange of paternal alleles. This event take place during prophase I which is divided into sub-stages: leptotene, zygotene, pachytene and diplotene. The synapse formation completes at pachytene stage. The chromosomes are prone to meiotic errors during the cross over and recombination. The meiotic check point thereby ensures the fidelity of the chromosomes and the DNA damage repair and this surveillance mechanism is active in the pachytene spermatocytes. The synpased region involves in the recombination whereas the unsynapsed region becomes transcriptionally silenced. The silencing of unsynapsed region prevents the error-prone crossing over events by the process called meiotic silencing of unpaired chromosomes (Kelly and Aramayo, 2007). Most likely, this event affects X and Y-chromosomes as they have the least homology. In consequence, they form XY body. The histone H3.3 chaperones, Histone Cell Cycle Regulator A (HIRA) and Death Domain Associated Protein (DAXX) are detected in the XY body. Both the chaperones are reported to be involved in the global H3.3 incorporation at the time of Meiotic sex chromosome inactivation (MSCI) in late pachytene spermatocytes for the formation of XY body (van der heijden et al., 2007; Rogers et al., 2004; Szenker et al., 2011). MSCI refers to the transcriptional silencing of the genes in the X and Y-chromosomes followed by the condensation of the chromatin for the XY body formation. MSCI is critical for the meiosis to proceed. Any impairment in this process leads to meiotic arrest at pachytene stage and apoptosis of spermatogonial cells (Handel, 2004). The histone variants H2A.X and macroH2A1.2 are associated with the chromatin condensation in XY body in mice (Fernandez- Capetillo et al., 2003). For the reorganization of synapsed chromosomes in the pachytene spermatocyte, the histone variants and their respective chaperones participates in the nucleosome assembly and DNA repair events. The linker histone H1T present in the pachytene spermatocytes are incorporated throughout the chromosomes (van der Heijden et al., 2007). tNASP-H1T has been found to be associated with the synaptonemal complex in the pachytene spermatocytes for the meiotic progression. The synaptonemal complex is a protein structure that is formed between the two homologous chromosomes during meiosis I in eukaryotes. It is mediated through synapsis and recombination and is absolutely critical for the crossover. Heat Shock protein Family A (Hsp70) Member 2 (HSPA2) is a lateral component of synaptonemal complex, which provides a link between the synapsed chromosome and the cell cycle in the pachytene spermatocyte. Interaction of tNASP-H1T with HSPA2 enhances its ATPase activity and facilitates the translocation of the HSPA2-tNASP-H1T complex to synaptonemal complex. There it associates with the cell cycle regulator Cyclin Dependent kinase 1 (CDK1/CDC2), which is required for G2 to M transition in prophase I (Alekseev et al., 2009).

Following meiosis, the spermatids undergo morphogenesis to form spermatozoa by the process called spermiogenesis. The initial remodeling of the spermatid genome is established by the genome-wide histone H4 hyperacetylation (Meistrich et al., 1992) to facilitate the open chromatin configuration for the subsequent displacement of somatic histones. The first phase of the process is marked by the conversion of histone-based chromatin to highly specialized testis-specific arginine-rich protamine based chromatin (Goldberg et al., 1977), which is a unique feature in eukaryotes. The second phase is the crosslinking of protamine by the formation of disulphide bonds in late spermatid for the exit (Golan et al., 1996). The protamine-associated genome contributes to the hydrodynamicity and condensation of chromatin and facilitates its packaging into the nucleus during sperm head formation in spermiogenesis. The histones are evicted from the nucleosome concomitant with the incorporation of transition proteins (TNPs) followed by its replacement by protamines. The histone H4 hyperacetylation in elongating spermatids establishes the open chromatin configuration for the histone displacement. The reduced level of H4 acetylation causes impaired fertility in humans and mice (Sonnack et al., 2002; Fenic et al., 2004). The canonical histones (core histones and linker histones) and the somatic histone variants are replaced by testis-specific histone variants. H3T is testis-specific and synthesized in spermatogonia and is expressed in early spermatids (Trostle-Weige et al., 1984). The reciprocal expression of testis-specific histone variant TH2A/TH2B controls the histone–nucleoprotamine transition during spermiogeneisis. TH2A gradually declines by the condensation of spermatid nucleus while TH2B replaces the canonical H2B in condensing spermatid (Montellier et al., 2013; Shinagawa et al., 2015). In mice, double knockout of Th2a/Th2b result in sterile male with very few sperms in epididymis (Shinagawa et al., 2015). However, the Th2b null mice show normal spermiogenesis and fertility suggesting that the double knockout phenotype is probably contributed either by Th2a alone or the synergistic effect with Th2b. The linker histones and their variants expressed in male germs cells are also critical in the preparation of the chromatin structure for histone to protamine transition. In D. melanogaster, CAF1-P180/P75 subunits and tNASP is associated with the histone-protamine exchange during the sperm chromatin assembly (Rathke et al., 2010; Doyen et al., 2015). The protamine based toroidal chromatin structure is different from the usual somatic nucleosomal array and contributes to increased compaction of the sperm chromatin making it inaccessible for replication or transcription (Doyen et al., 2013).

Due to the easy accessibility and limited ethical concern of different model system, development studies are mostly performed in lower mammals and hence the discussed content hereby include mouse and few lower organisms as the model system. But an interesting study on male infertility in human, revealed that deregulation of many DNA repair and apoptosis modulating genes associate with human male infertility (Cheung et al., 2019). Among them, Aprataxin PNK Like Factor (APLF), a core histone and macroH2A chaperone having DNA repair activity is remarkably under expressed in infertile males suggesting its essentiality in male fertility (Cheung et al., 2019). However, the mechanism by which APLF regulates male fertility remains elusive.

It is evident from the studies, performed majorly in mice, that an intricate network of histone chaperones, histone variants and histone modification guides the compaction and relaxation of sperm chromatin at different stages of spermatogenesis thereby guiding and regulating the entire process (Figure 1).

The process of oogenesis involves the development of oocytes from PGCs by meiotic recombination and genome reorganization followed by its activation at the time of fertilization (Figure 2). PGCs proliferate in the ovary by mitosis to form the oogonia, which then subsequently develop into primary oocytes. The oogonia undergoes first meiosis and gets arrested at diplotene stage of prophase 1, lasts until puberty in human whereas in lower organisms like Xenopus, it lasts for over a year (Carlson, 1996), and completes only before ovulation. During the arrest, the folliculogenesis event is initiated resulting in the formation of primary follicle. The resumption of meiosis I lead to the breakdown of nuclear envelope in the germinal vesicle (GV) and formation of secondary oocyte and release of the first polar body. The secondary oocytes thus formed goes through second meiotic division and progresses only to metaphase II and get arrested. Upon fertilization, secondary oocyte completes meiosis II division, retaining all the maternal cytoplasm for the initial development of the embryo. The fertilization competency in oocyte is established through global transcriptional changes and de novo DNA methylation. The onset of methylation marks coincide with the primary to secondary oocyte transition and is completed at GV stage, when the oocyte becomes transcriptionally inert (Tomizawa et al., 2012).

In mammals, primordial oogonial cells have less abundant oocyte specific-linker histones and the expression is remarkably increased in primary oocyte substantiating its vital role in mitotic to meiotic transition (Furuya et al., 2007; Pan and Fan, 2016). The replacement of histones by the oocyte-specific histone variants is critically important for transcriptional regulation and oocyte reprogramming (Nashun et al., 2015). Histone variant H3.3 is required for female fertility (Hödl and Basler, 2009; Sakai et al., 2009). Similar to spermatogenesis, continuous H3.3–H4 turnover and the expression of HIRA are critical for the transcriptional regulation of genes for developmental progression. Mouse primordial oocyte-specific knockout of Hira results in a significant increase in oocyte death and aberrant chromosome structure due to increased DNA accessibility, segregation defects, and responsiveness to DNA damage upon reduced H3 and H4 replacement (Nashun et al., 2015). Continuous reduction in H3.3/H4 level in consequence of HIRA depletion leads to aberrant transcriptional profile and increased sensitivity to DNaseI. This results in the accumulation of more DNA damage as evidenced by an increased level of γ-H2AX and upregulation of DNA-responsive genes. Additionally, de novo methylation are drastically reduced in HIRA-depleted oocytes. Methylation at CpG islands are greatly reduced in Hira depleted oocytes (Nashun et al., 2015). The H3.3 deposition in oocyte chromatin cannot be compensated by other H3.3 histone chaperones including ATRX/DAXX. Basically, Hira mutant oocytes fail to undergo cleavage to 2-cell stage embryo when developed parthenogenetically (Lin et al., 2014).

The second meiotic arrest occurs at the metaphase II in the pre-ovulatory oocyte. The nuclei-germinal vesicle (GV) displays a unique chromatin configuration to confer the oocye with meiotic potential and fertilization competence. GV oocytes become transcriptionally inert by global transcriptional repression and methylation. During this time, the oocytes rely on the stored maternal factors for sustaining the meiotic resumption and further stages until pre-implantation. GV oocyte is characterized by the nuclei condensation and formation of nucleolus like body (NLB). These specialized structures reassemble into pronuclei and form nucleolus bodies after fertilization. Nucleophosmin 2 (NPM2) is involved in the formation of NLB in GV oocytes in mouse. Npm2 knock out oocyte demonstrated aberrant heterochromatin formation outside nucleolus and exhibited a reduction in clearing acetylated nuclear H3. These mice are viable but fertility is compromised in females. In Xenopus and Zebrafish, the transcript variants, Npm2, Npm2a, and Npm2b are expressed predominantly in early oocytes and they take part in cortical granule (CG) formation during folliculogenesis. NPM2 deficiency impairs the egg activation in stage II follicle (Cheung et al., 2018), but the mechanism by which NPM2 affects the oocyte competence is yet to be deciphered. Similar to the Npm2 knockout, loss in fertility along with impaired gonad development has been observed in Asf1b female knock out mice (Messiaen et al., 2016). ASF1B regulate the process of meiotic entry of oocytes and hence its loss results in altered oocyte pool and finally affect fertility in the female knockout mice.

The heterochromatin composition at the chromocentre of preovulatory oocytes is preserved by ATRX/DAXX complex (Baumann et al., 2010). ATRX recruits transcriptional regulator DAXX at pericentric heterochromatin in the pre-ovulatory oocytes. The loss of maternal function of ATRX at the metaphase II stage affects female fertility presumably by impairing the chromosome modification, condensation, and segregation of chromosomes. Lack of ATRX function in matured MII oocyte resulted in misaligned chromosomes, chromosome lagging and in some instances led to the premature anaphase onset with the formation of chromosome bridge. Consistent with this observation, ATRX ablated oocytes exhibited reduced level of H3 phosphorylation (H3S10) at pericentric heterochromatin region, which is responsible for proper condensation and chromatin cohesion and centromere stability during meiosis II. Additionally, maternal ATRX ablation affects the transmission of aneuploidy and genomic stability in oocytes (Baumann et al., 2010). Upon sperm entry, the MII arrested oocyte completes the meiosis II and gets activated for the preparation of embryo development.

Interplay of epigenetic factors of histone chaperones and histone variants significantly contribute to the condensation or decondensation of oocyte chromatin thereby regulating not only the different stages of oogenesis (Figure 2) but also towards the competency of the oocyte for the next stage of fertilization. Gametogenesis sets the tone for the union of the gametes to form the zygote and hence the first stage of development.

After gastrulation, the embryo undergoes an event called neurulation that involves the development of the nervous system. During neurulation, the ectoderm differentiates into dorsal ectoderm, that becomes specialized to cells present on the outer surface of the body, and neural ectoderm which gives rise to the nervous system of the embryo. Adult stem cells present in the nervous system, called neural stem cells, proliferate and give rise to neurons and glia through a well-regulated process called neurogenesis. The proliferation and differentiation of neurons from the Neural Progenitor Cells (NPCs), specification of the subtypes, migration of immature neurons to their respective destination, maturation, and establishment of neuronal connectivity are precisely regulated key events in neural development.

Several studies have demonstrated the role of histone H3, H3 variants, and chaperones in neurogenesis. H3.3 incorporation maintains the level of H3K27me3 at the promoters of genes that regulate the development of neural tube mesenchyme and ear in mice (Banaszynski et al., 2013). Interestingly, histone chaperone ASF1, HIRA, and DAXX associated with H3.3 incorporation have been implicated in diverse event of neural development. In post-mitotic neurons, the replication-independent deposition of the histone H3.3 variant by the HIRA complex is a key nucleosome replacement mechanism regulating gene transcription. HIRA regulates the ß-catenin pathway, which plays an important role in regulating the developmental program and can direct progenitors to proliferate or differentiate (Zechner et al., 2003). HIRA expressed in embryonic cerebral cortex and NPCs, recruits H3K4 trimethyltransferase, SET Domain protein 1A (SETD1A), at the ß-catenin promoter to increase H3K4me3 level resulting in increased ß-catenin expression. Hence, perturbation of Hira leads to restricted proliferation of NPCs, increase in terminal mitosis and cell cycle exit and ultimately leads to premature neuronal differentiation (Li and Jiao, 2017). Depletion of another H3.3 chaperone, ATRX in NPCs causes replicative stress and DNA damage at telomeres and pericentric heterochromatin in mitotically active neurons (Watson et al., 2013). DAXX appeared to be crucial for corticogenesis as it associates with Calcium-dependent dephosphorylation mediated H3.3 loading and transcription upon neuronal activation (Michod et al., 2012).

In mammals, Nap1 encodes for three neuron-specific genes Nap1l2, Nap1l3, and Nap1l5, whereas other transcript variants are ubiquitous. Among them NAP1L1 has been implicated in mouse brain and neocortical development in particular (Qiao et al., 2018). In mice, NAP1L1, is expressed in the developing cerebral cortex and NPCs. However, upon differentiation, the expression progressively declines in neural progenitors. Nap1l1 knockdown mice exhibit premature neuronal differentiation and decrease in progenitor pool suggesting its role in corticogenesis from E13.5 to E16.5. In addition, NAP1L1 also promotes radial glial cell proliferation. Not only NAP1L1 modulates neuronal differentiation but also the cell cycle exit in neuronal progenitors. Mechanistically, NAP1L1 regulates the Ras Association Domain Family Member 10 (RASSF10) expression by promoting SETD1A mediated H3K4 methylation on the Rassf10 promoter, hence controls NPCs differentiation (Qiao et al., 2018). In Drosophila, depletion of neuron-specific NAP1, NAP1L2, leads to embryonic lethality from the mid-gestation phase exhibiting ectoderm defects, and the neural tube defects are attributed by enhanced proliferation of neural precursors (Rogner et al., 2000; Lankenau et al., 2003).

Similar to NAPLl1, depletion of NAP1L2 in mice not only affects the differentiation but also enhances the proliferation and apoptosis of the cells. The binding of NAP1L2 with H3 and H4 in the nucleus of neuronal cells increases the H3 acetylation (H3K9/14) at the Cyclin dependent kinase inhibitor 1C (Cdkn1c) promoter. The expression of CDKN1C increases during neuronal differentiation that correlates with the specific recruitment of NAP1L2 for histone acetylation (Attia et al., 2007).

Recently a study has investigated the functional implication of H2A histone variant H2A.Z in embryonic neurogenesis (Shenet al., 2018). Interestingly, H2a.z knockdown phenotype in mice appeared to be the same observed for H2A-H2B chaperone NAP1 transcript variants. Depletion of H2A.Z promotes proliferation of neural progenitors and inhibits neuronal differentiation. However, the H2a.z knockout mouse exhibits abnormal dendrites with cortical development. H2A.Z recruits methyl-transferase, SETD2 to enhance H3K36me3 levels at NK2 Homeobox 4 (Nkx24) promoter hence increasing its expression, which ultimately regulates cortical neuron expansion (Shenet al., 2018). Moreover, in mice, the core histone chaperone binding protein Acidic Leucine rich Nuclear Phosphoprotein 32 (ANP32) family, comprising of ANP32A, ANP32B, and ANP32E, is specifically involved in supply and eviction of H2A.Z and have a subtle role in embryogenesis. Anp32b ^-/- mice exhibit mild and sporadic craniofacial abnormalities, possess expanded ventricle and inner ear cavities, and palate closure defects suggesting its implication in brain development during embryogenesis (Reilly et al., 2011). Similarly, depletion of H2A.Z.1 subdues gliogenesis and results in reduced astrocyte differentiation in mice. It also interacts with histone chaperone ASF1A to regulate H3K56ac, which in turn modulates the expression of Folate Receptor Alpha 1 (FOLR1), a signal-transducing component of the JAK-STAT pathway, thereby regulating gliogenesis (Su et al., 2018).

Embryonic hematopoiesis occurs in two distinct waves - primitive and definitive hematopoiesis which takes place in the yolk sac and dorsal aorta or equivalent structure respectively. The primitive wave produces primitive erythrocytes for oxygen transport and hence critically important for the survival of the embryo. During the first wave, in the yolk sac, the myeloid cells are also produced, which migrates to CNS and skin to differentiate to microglia and Langerhans cells. On the other hand, definitive wave generates multipotent hematopoietic progenitors from mesoderm derivatives. These cells are produced once in a lifetime. Different lineages of blood cells are produced continuously from the pool of hematopoietic stem cells (HSCs). HSC expansion and differentiation occurs in fetal liver, subsequently the progenitor exit and migrates to spleen for further differentiation into myeloid and lymphoid subset. The progenitors develop from hemogenic endothelium (HE), which undergoes endothelial to hematopoietic transition to give rise to HSCs.

HIRA regulates angiogenesis in endothelial cells by H3.3 mediated K56 acetylation on genes implicated in angiogenesis (Dutta et al., 2010). Interaction of HIRA with the chromatin remodeler Brahma-Related Gene 1 (BRG1) regulates the transcription factor Runt-related Transcription Factor 1 (RUNX1) by the incorporation of H3.3 in the enhancer element of Runx1 and regulates the downstream targets Spi-1 Proto-Oncogene/Hematopoietic Transcription Factor (Pu.1), Growth Factor Independent 1 Transcriptional Repressor (Gfi1), Gfi2 and GATA Binding Protein 2 (Gata2) to generate hematopoietic stem cells from hemogenic endothelium differentiated from mouse ESCs (Majumder et al., 2015). Furthermore, the erythroid lineage differentiation factors including Erythroid Kruppel Like Factor (EKLF), GATA1, and ß-globin are regulated by HIRA. EKLF recruits HIRA to the β-globin locus for the incorporation of H3.3 to retain the open chromatin state by forming HIRA/ASF1A/H3.3/EKLF complex (Soni et al., 2014). Recent study has shown that HIRA can dictate differentiation of Chronic Myeloid Leukemia cells to megakaryocytes via MKL1/GATA2/H3.3 axis (Majumder et al., 2019). Conditional knockout of Hira in hematopoietic cells exhibits a dramatic reduction in HSCs in the bone marrow resulting in thrombocytopenia, lymphocytopenia, and anemia. Massive depletion of T and B-lymphocytes, monocytes and platelets, and RBCs is observed in Hira knockout mice. However, the fetal hematopoiesis is normal, although they demonstrate impaired reconstitution capacity due to defective chromatin accessibility (Chen et al., 2020). Additionally, HIRA has been implicated in heart development by regulating the troponins (Troponin I2, TNNI2, and Troponin T3, TNNT3) involved in cardiac contractility and EPH Receptor A3 (EPHA3), a vital gene for the fusion of muscular ventricular septum and the atrioventricular cushions. Conditional knockdown of HIRA in cardiogenic mesoderm in mice leads to defects in ventricular and atrial septation, surface edema, and embryonic lethality (Dilg et al., 2016).

Interestingly, a recent study has demonstrated the requirement of NAP1L3 in the maintenance and differentiation of HSC. NAP1L3 is highly expressed in mouse hematopoietic stem cells concerning the downstream progenitors proposing its potential function in primitive hematopoiesis. The perturbation of these genes impairs colony-forming capacity both in vitro and in vivo after transplantation. In agreement with this, downregulation of human NAP1L3 in umbilical cord blood derived HSCs induces G0 arrest of cell cycle and affects the expression of E2 transcription factor (E2F) and MYC targets as well as Homeobox A cluster 3 (HOXA3) and Homeobox A cluster 5 (HOXA5) (Heshmati et al., 2018). Zygotic depletion of NAP1L in Xenopus, affects α-globin expression in ventral blood island and result in reduction in transcript level of precursor genes Erythroid differentiation factor (SCL) and XAM1 (ortholog of RUNX1) (Abu-Daya et al., 2005).

Other histone chaperones including the P60 subunit of CAF1 (CHAF1B), which assembles histones H3.1/H4 heterodimers at the replication fork during the S phase and DEK, a nuclear phosphoprotein have been implicated in the maintenance of adult hematopoiesis. DEK expression is enhanced by inflammatory stimuli and it also increases the self-renewal and repopulating capacity of hematopoietic stem cells and subdues the number of hematopoietic progenitor cells. Perturbation of these chaperones results in hematological malignancies in adults (Capitano and Broxmeyer, 2017; Volk et al., 2018).

Skeletal muscles are derived from myogenic progenitor cells (MPCs) and are regulated by a family of highly related transcription factors together known as the myogenic regulatory factors (MRFs) (Berkes and Tapscott, 2005). It includes Myogenic Factor 5 (MYF5), MyoD, myogenin, and MYF6 (also known as MRF4). They function along with the MEF2 family of MADS-box transcription factors, which lacks any myogenic activity alone (Molkentin et al., 1995; Wang et al., 2001; Blais et al., 2005). During muscle differentiation, proliferating myoblasts exit the cell cycle concomitant with muscle-specific gene activation to differentiate to multinucleated myotubes (Sartorelli and Caretti, 2005).

The transcription factors, MYOD and Myocyte Enhancer Factor 2 (MEF2) recruit ATP-dependent chromatin remodellers and histone modifying enzymes to activate the muscle-specific genes. In proliferating myoblasts in vitro, HIRA enhances the transcriptional activity of MEF2, which is opposed by the Calcineurin Binding Protein 1 (CABIN1), which is a component of the HIRA-containing complex. CABIN1 via its N-terminal domain interacts with C-terminus of HIRA and result in the suppression of MEF2/HIRA mediated transcription. The repression of MEF2/HIRA is removed by the reduction in the level of CABIN1 upon differentiation of myoblasts. The association of ASF1 with HIRA enhances the MEF2 transcriptional activity thereby regulating other muscle-specific genes like myogenin (Yang et al., 2011).

The myogenic activation of MyoD is mediated by replication independent histone deposition. The incorporation of non-replicative H3.3 on the promoter and regulatory elements of MyoD by HIRA and ASF1B enables chromatin to become more permissive for RNA polymerase II to access for the transcriptional activation (Yang et al., 2011). The same group has investigated that the phosphorylation of serine (S) 650 in human HIRA mediated by AKT Serine/Threonine Kinase 1 (AKT1) is key in regulating muscle gene transcription during cellular differentiation (Yang et al., 2016). Transcriptional regulation of MyoD is also mediated by the coordinated interplay of histone chaperone-PolII-demethylase complex. It has been reported that histone chaperone SPT6 in association with PolII and H3K27 demethylase KMD6A, counteracts the repressive mark H3K27me3 on Myod for its activation. The depletion of SPT6 in Zebrafish embryos fails to undergo myogenic differentiation due to the enrichment of H3K27me3 on the Myod gene (Wang et al., 2013).

Histone chaperone can associate with long noncoding RNA (lncRNA) in regulating constitutive heterochromatin establishment during myogenesis where chromocenters form clusters progressively, which lead to heterochromatin compartment establishment (Park et al., 2018). The muscle-specific lncRNA, ChRO1 associate with DAXX/H3.3 and bind to ATRX, forms a stable complex at chromocenters for the elevation of satellite RNA and subsequently the fusion of chromocenters and establishment of heterochromatin compartment (Park et al., 2018).

Our insights on embryonic development are often from studies performed in vitro in the ESCs. The source for an embryo proper to develop into an entire organism lies in a distinct set of cells called the inner cell mass, which is present during the blastocyst stage of the pre-implantation development. The pluripotent nature of those cells has been widely exploited in vitro by the generation of ESCs. To avoid ethical concerns, scientists have generated pluripotent stem cells similar to ESCs independent of an embryo. So, it would be important to have a look at how histone chaperones could regulate this character of cells, which forms the basis of the development of an entire embryo.