**1. Introduction**

Spermiogenesis involves several steps and finally differentiate to A spermatogonia from stem cells [1]. Self renewal of stem cells was necessary for production of sperm cells throughout the life. C-Kit and SCF play an important role in determining the germ cell fate. Retinoic acid receptor plays a crucial role in proliferation of germ cells. Expression of Proteins like PP2A is reduced in later stages of the spermiogenesis and differentiation of A- spermatozoa in to B spermatozoa occurs through various intermediate stages like Apr, Aal, A1–4 and finally to Bspermatozoa. Commitment to B- spermatozoa was followed by meiosis-I and ii leading to formation of round spermatids. During the separation of chromosomes, cohesins hold the sister chromatids and disassemble after the homologous recombination leading to separation of chromosomes, defects in which leads to stage IV arrest. The round spermatids undergo several morphological and cytological changes like elongation of sperm head, acrosome formation and shedding of the cytoplasm. During the process, several signalling mechanisms like PDK-Akt pathway, GDNF-GFR1 pathway, mTOR pathway, and MAP kinase pathways operate proceeding to spermiogenesis.

## **2. Protamines are originated from histone H1**

In humans and mammals histones was replaced by either protamines or histone variants in sperm cell. But recent discovery of protamine P1 show both N- terminal and C- terminal sequence similarity between each other, and where as histone variant precursor resembled to that of protamine P2. Histones are basic proteins rich in Arg and Lys residues where as protamines in Styela contain polyarginine tracts than compared with histones.

Sperm condensation is one of the important criteria to be considered during assisted reproductive technologies in order to prevent genetic defects in upcoming generation. Spermatid formation involves 12–14 steps, and finally elongation of nuclear material leads to elongated sperms with acrosome. In sperm, histones are replaced by protamines and in humans histone variants are present leading to sperm condensation. Disulfide bonds contribute additional stability to the condensation as it results in interlinking of protamines [2, 3]. Certain PTMs of histones was also one of the epigenetic modifications required for condensation. Decondensation of Sperm chromatin takes place after sperm penetration in to oocyte which leads to exposure of sperm DNA [4] to oocyte reducing agents like glutathione. As known, glutathione reduces disulfide bonds between the protamines and relaxing the sperm DNA. From the previous reports, agents like SDS, EDTA reduces the disulfide bonds in combined state. Glycosaminoglycans like heparin sulphate binds to receptors on sperm membrane and promotes decondensation instead with nonsulfated form heparin (**Figure 1**).

### **Figure 1.**

*Flow cytometry of spermatozoa showing DNA fragmentation. Where (a) is control, (b) is testes treated with Metosartan (c) with RNaseA +aspirin and (d) with RNaseA+metosartan.*

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*Understanding the Epigenetic Modifications in Sperm Genome*

Acrosome and mitochondrial sheath formation occurs at 6 and 16 steps of the sperm biogenesis. Acrosome vesicles are secreted from the Golgi vesicles and form acrosomes cap around the nucleus. Defect with vesicle fusion or vesicle proteins leads to abnormalities in sperm head. Mitochondria is the major energy reserve of the sperm and the movement of mitochondria occurs through IMT pathways. Finally the sperms shed their cytoplasm by forming a cytoplasmic droplet around the neck region and remains attached to Sertoli cells until they become mature. Apical epistatic interaction involves holding of sperms at Sertoli junctions through the help of adherens proteins like NECTIN, Integrin- laminin and cadherincatenin. Loss of Protein leads to premature release of sperms from Sertoli cells into the lumen of seminiferous tubules and abnormalities leads to vice versa of the same.

H1T1 was one of the variant of linker histone found during pachytene of the cell division. H1T2 was the other linker variant found in apical pole of spermatids and responsible for male fertility. In case of humans HilS1 is necessary for condensation in elongated spermatids. Certain Core histone variants like TH2A and TH2B are found to be elevated during elongation of spermatids and whereas the normal core histones show low amounts of expression. Gene knock outs of TH2A and TH2B induced male infertility but PTMs on core histones compensate the loss of these variants in the spermatids [5]. TH2A was found to be present at transcriptional start

Recently, humans contain histone variants instead of canonical histones which are subjected to acetylation at H4K5, H4K8, H4K12 and H4K16. In Drosophila H3 is also acetylated especially at H3 K9 by Plant domain containing protein PYGO2 and responsible for open confirmation of chromatin. Presence of histone chaperones as identified recently in Drosophila named CAF1 (Chromatin Assembly Factor) and in humans was normal Heat shock protein variant namely HSPA2 (HSP 70.2). These chaperones acts as chromatin remodellers and responsible for exchange of transition proteins with protamines. ATP dependent remodeller proteins that act as histone chaperones was not known up to now, but above mentioned are some of the

Histone modifications like phosphorylation, acetylation and ubiquitination play an important role in spermiogenesis [8]. Phosphorylation is found to be common on the four core histone proteins, and acetylation was mostly seen in H4, whereas ubiquitination was with respect to H2A and B. Phosphorylation and acetylation leads to neutralisation of charges on histones whereas ubiquitylation causes Rnf 8 mediated recruitment of transition proteins and found to be normal in gene knock out animals with respect to meiosis and repression of expression in germ cells.

site [6] and where as TH2B is required during leptotene of the cell division. Other histone variant is with core histone H3 which include H3.3, H3T and CenP. H3.3 is present at the stage of spermatids [7] and where as H3T was specifically expressed in testes and both differ by 5 amino acid residues from core histone H3.1. H3.3 represents actively transcripted regions where as H3.1 represents transcriptionally repressive regions and may be involved in replacement of Histones by protamines. H3.1 may be required for maintaining the repression of X and Y chromosomes where as H3.1 is involved in activating the genes required for transi-

*DOI: http://dx.doi.org/10.5772/intechopen.88506*

**3. Testes-specific histones**

tion protein and protamine synthesis.

remodellers that help in chromatin remodelling in testes.

**4. Various histone modifications during spermiogenesis**

Acrosome and mitochondrial sheath formation occurs at 6 and 16 steps of the sperm biogenesis. Acrosome vesicles are secreted from the Golgi vesicles and form acrosomes cap around the nucleus. Defect with vesicle fusion or vesicle proteins leads to abnormalities in sperm head. Mitochondria is the major energy reserve of the sperm and the movement of mitochondria occurs through IMT pathways. Finally the sperms shed their cytoplasm by forming a cytoplasmic droplet around the neck region and remains attached to Sertoli cells until they become mature. Apical epistatic interaction involves holding of sperms at Sertoli junctions through the help of adherens proteins like NECTIN, Integrin- laminin and cadherincatenin. Loss of Protein leads to premature release of sperms from Sertoli cells into the lumen of seminiferous tubules and abnormalities leads to vice versa of the same.
