Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Urogenital System
Reproductive System
Female Reproductive Organs - Germ Cells. Spermatogenesis and Oogenesis

Male Germ Cells — spermatozoa — are motile cells about 70 µm in length. A spermatozoon typically consists of a thickened, rounded HEAD and a thin, long tail. The head contains a nucleus preceded by a specialized Structure called the acrosome. The acrosome contains a set of Enzymes capable of dissolving the egg Cell membrane during Fertilization. The tail contains contractile elements (bundles of fibrils) that propel the spermatozoon forward. As sperm travel through the male efferent ducts, they are mixed with fluid secretions from the accessory sex glands, namely the Seminal Vesicles, prostate, and Bulbourethral Glands. This forms a fluid medium known as semen, which carries the spermatozoa. The lifespan and fertilizing capacity of human spermatozoa range from several hours to two days.

Female Germ Cells — ova (egg cells) — are spherical and relatively large (up to 150 µm in diameter). Each egg cell contains a nucleus and an abundant Cytoplasm that, alongside standard cellular Organelles, holds protein-lipid inclusions (yolk) and Glycogen required to nourish The Cell. In humans, because the embryo develops intrauterinely and is nourished directly by the maternal Organism, there is no evolutionary need for massive yolk reserves within the egg. Consequently, the cytoplasm of a human egg cell contains very few yolk and carbohydrate inclusions. The egg typically expends its nutrient reserves within 12–24 hours post-ovulation. If fertilization does not occur within this window, the egg cell perishes.

The human egg cell is enveloped by two protective membranes. The inner layer is the oolemma, which serves as The Plasma Membrane of the ovum. Surrounding the oolemma externally is a layer of follicular cells that protect the egg and exhibit hormone-producing activity.

Follicular cells synthesize and secrete Female Sex Hormones, known as estrogens.

Germ cell development takes place within the Gonads. Spermatozoa are formed in the male Testes, while egg cells develop in the female Ovaries. The developmental cycle of spermatozoa is termed Spermatogenesis, and that of the egg cell is called oogenesis (from the Greek sperma — seed, ovum — egg, and genesis — development).

The Development of germ cells culminates in their readiness for fertilization (syngamy) and subsequent embryonic development. The preparation of male and female germ cells for fertilization involves far more than acquiring specific morphological features. Throughout spermatogenesis and oogenesis, complex transformations occur in both precursor cells and immature germ cells. Through a specialized form of Cell Division called Meiosis, the chromosome number in both spermatozoa and egg cells is halved (reduced) from a double (diploid) set to a single (haploid) set. Instead of the standard diploid Complement of 46 Chromosomes found in all other human cells, each mature germ cell (spermatozoon or ovum) contains a single set of 23 chromosomes.

Spermatogenesis. Spermatozoa are produced in men throughout their entire active reproductive life span. The development and maturation of mature spermatozoa from their precursors, spermatogonia, takes approximately 70–75 days. This process occurs within the walls of the convoluted seminiferous tubules of the Testis. Initially, spermatogonia—whose total number in a single testis reaches up to 1 billion—proliferate intensively via mitotic division, thereby increasing the pool of new spermatogonia. Subsequently, a fraction of these spermatogonia retain their division capacity to maintain the stem cell population, while others undergo two successive meiotic divisions. As a result, each such spermatogonium, possessing a diploid (double) chromosome set ($n = 46$), gives rise to four spermatids. Each spermatid receives a haploid (single) chromosome set ($n = 23$). These spermatids gradually transform into spermatozoa through a complex structural remodeling process, elongating to form a thickened head and a thin, long tail. At the apex of the sperm head, a condensed body known as the acrosome is formed; it contains enzymes that break down the protective envelope of the female germ cell (ovum) upon contact. This mechanism is essential for the spermatozoon to penetrate the egg. If the acrosome is underdeveloped or absent, the spermatozoon is incapable of penetrating and fertilizing the ovum.

Newly formed spermatozoa enter the lumen of the seminiferous tubules. Aided by fluid secreted by the tubular walls, the sperm gradually migrate toward the Epididymis, which also Functions as a sperm reservoir. The volume of produced spermatozoa is immense: 1 ml of semen contains up to 100 million sperm. These motile cells advance through the testicular tubules at a speed of approximately 3.5 mm per minute. Within the female reproductive tract, spermatozoa maintain viability for 1–2 days, actively migrating toward the oocyte via chemotaxis.

Oogenesis. Unlike male germ cells, female germ cells multiply and increase in number during the Embryonic and Fetal Selection/3.html">Stages of development, while the female fetus is still in utero. This process forms primordial follicles located in the deep layers of the ovarian cortex. Each primordial follicle contains a female germ cell—an oogonium—surrounded by a single layer of follicular cells. Oogonia undergo repeated mitotic divisions, transforming into primary oocytes, which remain dormant in the girl's ovaries until Puberty. By the onset of puberty, the ovaries contain approximately 300,000 primary oocytes. Each primary oocyte, measuring about 30 µm in diameter, together with its surrounding two layers of follicular epithelium, constitutes a primary follicle.

During puberty and throughout reproductive life, the majority of primary oocytes in young women undergo atresia and die. Over the course of a woman's lifetime, only 400 to 500 egg cells ultimately mature.

During maturation, the primary oocyte undergoes the stages of meiosis. Meiotic division yields a secondary oocyte, which already possesses a single (haploid) chromosome set ($n = 23$), and a small polar body containing an identical set of chromosomes ($n = 23$). Concurrently, primary follicles develop into secondary follicles. Fluid accumulates within these follicles, and two distinct layers—the oolemma and a layer of follicular cells—form around each secondary oocyte. Consequently, the secondary follicle evolves into a vesicular (tertiary) follicle filled with follicular fluid.

The diameter of a mature vesicular follicle can reach up to 1 cm. In a mature female, typically 1 or, less commonly, 2 follicles mature simultaneously each cycle. The remaining follicles undergoing growth at that time regress through a process known as atresia. The remnants of these degenerated and non-matured follicles form structures designated as atretic bodies.