**1. Introduction**

The development of a human being begins with fertilization, a process by which two highly specialized cells, the spermatozoon from the male and the oocyte from the female, unite to give rise to a new organism, the zygote.

### **1.1 Oogenesis**

In humans, the ovaries begin to form during embryonic formation in the first few weeks of the first trimester of pregnancy. It starts at the yolk sac, where the primordial germ cells originate, followed by migration along the hindgut to the gonadal ridge around the fifth and sixth week of the embryonic development [1, 2].

At around the sixth-week post-conception, the ovary contains around 26,000 oogonia. The number of oogonia increases by invasion and local proliferation, whereby the ninth week the number of oogonia would be around 250,000 in each ovary [3].

At around this stage of development, the first oogonia of the ovary enter meiosis, although most oogonia continue the mitotic cycle up to the time of the initiation of meiosis [4]. At the start of meiosis, the oogonia lose their ability to divide mitotically and will be described as oocytes [5]. The mitotic division of the oogonia finishes around the 20th week of gestation, where the formation of new oocytes ends [6].

The early development of the ovary and the relation between oogonia and somatic cells is a delicate process. Interfering with ovarian formation during this critical period may have consequences on the number of oocytes a girl is born with and thus on her fertility later on in life [7–9].

### **1.2 Spermatogenesis**

From puberty to old age, male germ cells originate at the seminiferous tubules from a self-renewing stem cell pool. This spermatogenic process is a cascade of developmental stages that provide the mechanism of successful spermatogenesis [10].

There are intratesticular and extratesticular hormonal regulatory mechanisms for successful spermatogenesis in the testicular Leydig cells and the intertubular space, where thin septula divide the parenchyma into about 370 conical lobules. These lobules contain the seminiferous tubules, Leydig cells, and other cellular elements [10, 11].

The seminiferous tubules are coiled loops with two ends that open in the rete testis [11], where their secreted fluid is delivered to the epididymis. The seminiferous tubules consist of germinal epithelium and the peritubular tissue that include different developmental stages of germ cells, namely spermatogonia, primary and secondary spermatocytes, and spermatids that are located in the Sertoli cells [12].

The Sertoli cells have a specialized germinal epithelium in a basal and an adluminal zones, called "tight junctions" that form the blood-testis barrier of the testis. The germ cells pass through this barrier to the adluminal compartment, thus avoiding the possibly diffused nextraneous substances. With the advancement in age, Sertoli cells exhibit increasing amounts of lipid droplets as an indicator of the testicular "biological clock" [13].

Other functions that are attributed to Sertoli cells include nutrition of the germ cell, delivery of spermatids to the tubular lumen; a process that is described as spermiation; production of endocrine and paracrine factors that play a role in spermatogenesis, and secretion of androgen-binding protein to help maintain the duct system [14].

The process of germ cell development during spermatogenesis passes through various stages that include spermatogoniogenesis, meiosis, maturation of spermatocytes, spermiogenesis, and spermiation [15–19].

Spermatocytes go through meiosis with its associated change in chromatin configuration after spermatogonial division. These cells go through two divisions during meiosis and are called primary spermatocytes before the first division and secondary spermatocytes before the second division [20].

Spermiogenesis begins after spermatocytes complete two quick successive meiotic reductive divisions to produce haploid round spermatids. During cytodifferentiation of spermatids, there is condensation of the nuclear chromatin, formation of the acrosome cap, and the development of flagellum, to enable them to leave the germinal epithelium as the process of spermiation takes place [19–20].

Leydig cells surround the testicular capillaries and secrete androgens, including testosterone. Testosterone activates the hypophyseal-testicular axis, masculinizes the brain, initiates and maintains spermatogenesis, and commands the differentiation of the male genital organs and secondary sexual characteristics [21].

Furthermore, Leydig cells have neuroendocrine activities added to their endocrine role as they express serotonin, catecholamine-synthesizing enzymes, neurohormones, cell adhesion molecules, components of the renin-angiotensin system, growth factors, and their receptors [19, 22].

#### *The Quantum Theory of Reproduction – How Unique is an Individual? DOI: http://dx.doi.org/10.5772/intechopen.105769*

In addition, Leydig cells are involved in autocrine and paracrine regulation mechanisms of the testes and are considered a part of the general neuroendocrine cell system, and their main regulator is the luteinizing hormone of the pituitary gland [22, 23].

The kinetics of spermatogenesis that exists throughout the reproductive life of man is due to the large reservoir of stem cell in the seminiferous tubules. The continuous process of spermatogenesis features cell differentiation and migration from the basal to the adlumin of the germinal epithelium [24–26].

In all parts of the germinal epithelium, there is a 16-day cycle of standard differentiation processes. This "space of time" is called "cycle of the seminiferous epithelium." The production of an A type spermatogonium to become a mature spermatid requires 74 days. Mature spermatids leave the germinal epithelium as spermatozoa and pass through the epididymis. This additional transport takes another 12 days. Thus, the complete spermatogenetic cycle from spermatogonium to mature spermatozoa takes around 86 days [27–28].

Spermatozoa, the products of spermatogenesis are unique in their shape and function making them capable of progression through the female genital tract to meet the oocyte at the lateral end of the Fallopian tubes. At this point, the acrosome reaction takes place. This enables the spermatozoon to pass through the zona pellucida of the female gamete and to get into the cytoplasm and merge with the pronucleus of the zygote [28].

The efficiency of spermatogenesis is questionable. Germ cell loss (oligozoospermia), percentage of malformed spermatozoa (teratozoospermia), and motility problems (asthenozoospermia) in the ejaculate can be extremely high. A high percentage of the developed germ cells are lost by apoptosis and degeneration. Only a fraction of the male germ cells reaches the ejaculation including a high percentage of malformed gametes. Thus, only around 10% of the spermatogenetic potential might serve the reproductive process [28].

The fecundability of the human race is compared poorly with laboratory animals. The mean elongate spermatid-Sertoli cell ratio is 3–4 for the human germinal epithelium versus 12 in rats [28]. The daily rate of spermatozoa production in humans is around 3–4 million per gram of testicular tissue. Accordingly, a higher number of ejaculate spermatozoa are expected in relation to the 20 million spermatozoa per ml as considered a normal value by the WHO [29].

Recent observations report a recent decline of sperm counts in the ejaculates of healthy individuals. This might be due to detrimental prenatal factors including hormones and their metabolites in the drinking water that may adversely affect the different internal and external processes of spermatogenesis in the seminiferous tubules [30].

The intrinsic factors include testosterone, neuroendocrine substances, and growth factors (IGF1, TGFβ, NGF) that represent an independent intratesticular regulation of spermatogenesis. The extrinsic influence is provided by the pulsatile secretion of gonadotropin-releasing hormone by the hypothalamus and the gonadotrophins of the pituitary gland. Other factors include nutrive substances, drugs, different toxic substances, and radiation that may adversely affect testicular function [30–34].
