BASICS OF MEDICAL BIOLOGY - 2012

Biological Features of Human Reproduction. Gametogenesis. Meiosis. Fertilization. Reproduction as a Mechanism Ensuring Genetic Continuity Across Generations

Reproduction, or procreation, is the fundamental property of organisms to replicate their own kind, thereby ensuring the continuity of generations and the persistence of life. Various Forms of reproduction have emerged in the course of evolution, which can all be broadly categorized into two main types: Selection/8.html">Asexual and sexual reproduction.

Asexual reproduction occurs via non-sexual (somatic) Cells. This form of reproduction is widespread among plants and lower animals. Regardless of the level of Organization, asexual reproduction involves a single parent, and the resulting offspring are genetically uniform. Organisms may reproduce using either a single non-sexual Cell (monocytogenic) or a group of such cells (polycytogenic).

Forms of asexual monocytogenic reproduction:

1. Cell Division (binary fission or fragmentation in prokaryotes; mitotic division in eukaryotes) results in The formation of two daughter cells from a single parent Organism. It is predominant in prokaryotes and Protozoa, but also occurs in Multicellular Organisms (Annelids, jellyfish).

2. Endogony is a form of intracellular division. For example, Toxoplasma (an intracellular parasite) forms two daughter cells via endodyogeny.

3. Schizogony is intracellular multiple fission. Initially, The Nucleus divides repeatedly, after which a portion of the Cytoplasm surrounds each nucleus, and The Cell breaks apart into multiple cells (corresponding to the number of nuclei). Typically, 8, 16, or 32 daughter cells are formed. Schizogony is characteristic of the malaria parasite (Plasmodium).

4. Budding is an uneven division characterized by the formation of a cellular outgrowth (a bud) into which one of the nuclei produced during karyokinesis migrates. The bud grows and subsequently detaches from the parent cell.

5. Sporulation is a reproductive process utilizing specialized cells called spores, found in Sporozoans and lower plants. Spores are also formed by Bacteria, but their function is survival under unfavorable conditions rather than reproduction.

Forms of asexual polycytogenic reproduction:

1. Vegetative Reproduction is reproduction through a group of cells or body parts, whereby a whole organism regenerates from a fragment of the plant. This type of reproduction is characteristic of Higher Plants and primitive animals such as Sponges, Coelenterates, Flatworms, and annelids.

2. Gemmation (budding) is a reproductive mode in which sponges and coelenterates develop outgrowths (buds) on their bodies that grow, develop, and eventually detach.

3. Strobilization is the transverse division of an organism into segments, typical of certain invertebrates. For instance, in jellyfish, the polyp grows and then undergoes transverse fission to produce young medusae (ephyrae), which detach and begin an independent life.

4. Fragmentation is a mode of reproduction via the Separation of specific body parts. Some annelids develop constrictions along their bodies and subsequently divide into fragments that grow into new individuals.

5. Polyembryony is a form of vegetative reproduction where an embryo splits into parts, each developing into a separate organism. Polyembryony occurs in Hymenoptera (parasitic wasps) and in mammals such as armadillos. The formation of monozygotic twins in humans and other mammals also falls into this category of phenomena.

6. Sporulation is a reproductive form characteristic of higher plants. It is based on a spore-forming type of Meiosis followed by The Development of a haploid generation that bears the reproductive Organs.

Conjugation is a sexual process involving the EXCHANGE OF GENETIC material (recombination) between two organisms, resulting in offspring with recombined hereditary traits derived from both parents. It is found in bacteria and Ciliates. The transfer of genetic material between donor and recipient bacteria occurs through specialized outgrowths called sex pili. Through conjugation, the multiple drug resistance (R) factor, which confers bacterial resistance to numerous Antibiotics, can be transferred. Paramecia, for instance, connect via their peristomal grooves during conjugation. The macronucleus (large nucleus) of each paramecium disintegrates, while the micronucleus (small nucleus) undergoes two successive divisions to form four nuclei. Three of these disintegrate, and the fourth divides mitotically into two nuclei: a stationary one and a migratory one. The paramecia then exchange migratory nuclei. Within each paramecium, its own stationary nucleus fuses with the foreign migratory nucleus to form a synkaryon. The synkaryon divides to regenerate the macronucleus and micronucleus. Following conjugation, the paramecia separate and resume reproduction by fission through a cycle of asexual mitotic divisions that increases the population of ciliates.

Copulation is a sexual process involving the fusion of two cells. The forms of copulation include isogamy, anisogamy, and oogamy. Isogamy is the fusion of two morphologically identical but physiologically distinct gametes. Anisogamy is the fusion of two dissimilar gametes: a larger female gamete (macrogamete) and a smaller male gamete (microgamete), both of which are motile. Oogamy is the fusion of two unequal gametes: a large, non-motile egg cell (ovum) and a small, motile spermatozoon. In multicellular organisms, sexual reproduction relies exclusively on oogamy.

Class="center">Human Sex Cells

The process of gamete (sex cell) formation is called gametogenesis. There are two types of sex cells: male (spermatozoa) and female (egg cells or ova). Egg development takes place in the female Gonads (Ovaries), whereas spermatozoa are produced in the Testes. Typically, ova and spermatozoa are produced by separate organisms—females and males—which differ from one another in A number of traits, a phenomenon known as Sexual Dimorphism. The traits distinguishing one sex from the other are divided into Primary and secondary. Primary sex characteristics refer to the reproductive organs, while secondary characteristics encompass all other manifestations of sexual dimorphism.

When both male and female sex cells develop within a single individual, that organism is termed a hermaphrodite. Hermaphroditism is characteristic of many lower invertebrates, such as flatworms, annelids, and Mollusks. As a pathological condition, true hermaphroditism also occurs in humans. It predominantly arises due to embryonic developmental disorders despite a normal set of sex Chromosomes (XX or XY) in all somatic cells. Cases of sex chromosome mosaicism have also been described, where some somatic cells possess an XX karyotype while others have an XY karyotype.

Sex cells (gametes) are highly differentiated cells. In the course of evolution, they have adapted to perform unique, specialized Functions: ensuring the transmission of Genetic information from generation to generation during sexual reproduction. Their nuclei contain the complete hereditary information necessary for the development of an organism. However, since their other physiological functions differ, The Structure of sex cells varies accordingly.

Sex cells differ from somatic cells in having: 1) a single (haploid) set of chromosomes;

2) a drastically altered nuclear-cytoplasmic ratio: in egg cells, the nuclear-cytoplasmic ratio is low (abundant cytoplasm), whereas in spermatozoa it is high (cytoplasm is almost entirely absent); 3) a significantly reduced metabolic rate; 4) a complete lack of reproductive capacity: if an egg cell is not fertilized, it dies, whereas somatic cells can, under certain conditions, give rise to a new organism; 5) germ cells exhibit significantly greater diversity than somatic cells due to Crossing-over during their formation.

Spermatozoa (sperm cells) are extremely small (52–70 µm in humans), motile male gametes capable of Fertilization, and are vastly smaller than egg cells. They are produced in massive numbers (millions). A mature spermatozoon consists of a HEAD, neck, midpiece (middle piece), and tail (Fig.). The head is almost entirely occupied by the nucleus, which contains a haploid set of chromosomes (23)—specifically, 22 autosomes and one sex chromosome (either X or Y). Positioned above the nucleus is the acrosome, containing hydrolytic Enzymes (hyaluronidase, mucinase) necessary to penetrate the egg cell membranes. The neck contains two centrioles: a proximal and a distal one.

The proximal centriole (located closer to the nucleus) participates in forming the mitotic spindle of the fertilized egg cell. The axial filament of the tail originates from the distal centriole. The midpiece is widened and contains Mitochondria arranged in a spiral around the axial filament, supplying energy for the tail. Propelled by the tail (flagellum), spermatozoa can move through human seminal fluid at speeds up to 5 cm/h. They are capable of swimming against fluid flow (rheotaxis) and toward chemical substances secreted by the egg cell (positive chemotaxis). During intercourse, a male releases approximately 200 million spermatozoa. All spermatozoa carry a uniform negative electrical charge, which prevents them from clumping together.

Spermatozoa perform the following functions:

- fertilization and Formation of the zygote;

- transmission of The Genome to offspring;

- stimulation of the development (Cleavage) of the fertilized egg cell.

Egg cell (ovum) is a non-motile female gamete that, upon fertilization by a spermatozoon, forms a zygote that gives rise to a new generation. The human egg cell is spherical, with a diameter of approximately 130–200 µm, and is surrounded by several membranes:

1. Corona radiata (radiate crown). The outer follicular membrane formed by follicular cells and intercellular substance. Because both the follicular cells and the egg cell form microvilli, this layer appears striated. At the surface, accumulations of follicular cells form the cumulus oophorus, which disappears after fertilization.

2. Zona pellucida (transparent zone). A clear membrane formed by the secretions of follicular cells and the oocyte (mucopolysaccharides).

3. Vitelline membrane — a thin, transparent membrane composed of Glycoproteins. It ensures species Specificity and facilitates sperm attachment.

4. Plasma Membrane. A typical cell membrane bearing microvilli on its surface. Its primary function is The regulation of substance transport.

The egg cell contains all typical cellular Organelles as well as a range of substances essential for embryonic development (Fig.), including nutritive material (yolk). The nucleus contains a haploid set of chromosomes (23)—22 autosomes and one X chromosome. A cortical layer of granules lies just beneath The Plasma Membrane; the contents of these granules participate in forming the fertilization membrane. The human egg cell is secondarily oligolecithal (low yolk content) and isolecithal (yolk is evenly distributed throughout the cytoplasm). The primary function of the egg cell is the development of a complete organism.

Fig. 15. Mammalian spermatozoon.

a) general view; b) diagram: 1 - head; 2 - neck; 3 - tail; 4 - acrosome; 5 - nucleus; 6 - centrosome; 7 - mitochondrial spiral; 8 - axial filament;

Fig. 16. Mammalian egg cell.

1 - cytoplasm; 2 - nucleus; 3 - membrane; 4 - follicular cells.

Gametogenesis

Gametogenesis is the process of formation of sex cells (gametes). It is subdivided into Spermatogenesis and oogenesis (Fig.).

Spermatogenesis is the process of forming male sex cells, or spermatozoa. It takes place in the male gonads (testes). Each Testis consists of numerous seminiferous tubules, which in turn comprise several layers representing successive STAGES OF SPERM development. Spermatogenesis includes four distinct phases:

1) multiplication;

2) growth;

3) maturation;

4) differentiation.

The initial cells for spermatogenesis are spermatogonia—round cells with relatively large nuclei containing a diploid set of chromosomes (2n4c - 46 chromosomes). In The First stage, known as the proliferation stage, spermatogonia actively multiply via mitosis. The proliferation zone is located along the periphery of the seminiferous tubule. Some spermatogonia migrate into the growth zone, which lies just beyond the proliferation zone. Here, the Second Stage of spermatogenesis takes place—the growth stage, during which spermatogonia cease dividing, increase in size, and transform into primary spermatocytes (2n4c - 46 chromosomes). Beyond the growth zone, closer to the lumen of the tubule, lies the maturation zone. This is the site of the Third Stage of spermatogenesis—the maturation stage. This period is characterized by two successive divisions known as meiosis. During the first meiotic division (reductional division), a primary spermatocyte divides into two secondary spermatocytes, each containing a haploid set of chromosomes (1n2c - 23 chromosomes). During the second meiotic division, each secondary spermatocyte divides into two spermatids (1n1c - 23 chromosomes). In total, four spermatids are formed. They are round in shape and contain a large amount of cytoplasm. The spermatids migrate closer to the tubule lumen, where they develop into spermatozoa. This is the Fourth Stage of spermatogenesis—the spermiogenesis (differentiation) stage, during which immotile, non-descript cells transform into specialized, motile male gametes. Several key structural transformations occur in the cell during this period:

- spermatids shed almost all of their cytoplasm, leaving the head of the mature spermatozoon virtually devoid of it;

- the nucleus becomes highly compact;

- the Golgi apparatus forms the acrosome;

- one distal centriole forms the flagellum;

- mitochondria arrange themselves into a mitochondrial spiral.

As a result of spermatogenesis, one diploid spermatogonium yields four haploid spermatozoa.

Spermatogenesis is a continuous process that begins at Puberty and continues throughout a man's life, gradually slowing down in old age. In humans, one cycle of spermatogenesis takes approximately 70 days.

Oogenesis is the development of Female Germ Cells—ova (egg cells). It takes place within the follicles of the female gonads, the ovaries. Oogenesis comprises 3 stages (the differentiation stage is absent) (fig.):

1) proliferation;

2) growth;

3) maturation.

The starting cells are the diploid Cells of the embryonic ovaries—oogonia (2n4c - 46 chromosomes). In the first stage, the proliferation stage, oogonia multiply by mitosis. In the second stage, the growth stage, oogonia increase in size and transform into primary oocytes (2n4c - 46 chromosomes). The third stage is the maturation stage. Two meiotic divisions occur during this period. In the first meiotic division (reductional division), the primary oocyte divides into two cells of unequal size: a large secondary oocyte, which contains the yolk and almost all of the cytoplasm, and a small polar body (the first polar body), which contains little more than the nucleus. Both of these cells are haploid (1n2c - 23 chromosomes). During the second meiotic division (equational division), the secondary oocyte divides into two similarly unequal cells: a large ovum (1n1c - 23 chromosomes), which contains all the yolk and cytoplasm, and a small second polar body (1n1c - 23 chromosomes). The first polar body may also divide, producing two polar bodies, each with a haploid set of chromosomes. Thus, from a single initial diploid cell (oogonium), oogenesis produces four haploid cells: one large ovum and three small polar bodies that eventually degenerate. The unequal division of oocytes has profound biological significance: in this way, the egg cell receives not only a haploid set of chromosomes, but also all of the yolk and cytoplasm, along with CYTOPLASMIC INHERITANCE.

Unlike spermatogenesis in males, which begins at puberty, oogenesis in females starts in the embryonic ovaries even before birth. During prenatal development, oogonia proliferate via mitosis, grow, and transform into primary oocytes (approximately 7 million cells). A primary oocyte enters the first meiotic division but does not complete it. Meiosis arrests at the diplotene stage of prophase I. Instead of entering diakinesis, primary oocytes enter the dictyotene stage, which is unique to oogenesis and does not occur in spermatogenesis. At this stage, meiosis is suspended for many years—anywhere from 12–15 to 45–50 years. This prolonged arrest is widely believed to be the underlying cause of chromosome nondisjunction errors during meiosis, leading to an increased incidence of Chromosomal Disorders in children born to older mothers. By the time of birth, a female's ovaries contain about 2x10⁶ follicles, with the primary oocytes arrested in the dictyotene stage. Upon the onset of puberty, stimulated by pituitary luteinizing hormone, one follicle matures each month. During maturation, the primary oocyte exits the dictyotene stage, enters diakinesis of prophase I, and completes the suspended meiotic division, yielding a secondary oocyte and the first polar body. At the moment of ovulation (the rupture of the follicular and ovarian walls, releasing the oocyte into the Abdominal cavity), the secondary oocyte is stalled at metaphase II and will not resume division unless it encounters a spermatozoon in the fallopian tube. Upon fertilization, the secondary oocyte completes the second meiotic division, producing a mature ovum and a second polar body. Over a woman's entire reproductive lifetime, approximately 450 follicles reach maturity, while the rest undergo degeneration.

Fig. 17. Spermatogenesis and oogenesis (diagram).

Meiosis

Meiosis (from Greek meiosis — reduction) is a specialized type of cell division that halves the chromosome number (2n—>n). This process is accompanied by the redistribution (recombination) of genetic material between homologous chromosomes. As a result of meiosis, one diploid cell gives rise to four haploid cells. Meiosis occurs during gametogenesis at the gamete maturation stage. It consists of two consecutive divisions occurring in rapid succession: the first meiotic division, or reductional division (from Latin reductio — reduction), and the second meiotic division, or equational division. Each of these divisions comprises 4 phases: prophase, metaphase, anaphase, and telophase. The Phases of the first meiotic division are denoted by the Roman numeral I, and those of the second by the Roman numeral II. DNA Synthesis occurs only once—during interphase I, which precedes prophase I.

Prophase I is the longest and most complex phase in terms of cellular events. It is subdivided into 5 stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.

Leptotene is the stage of long, thin, lightly coiled chromosomes that resemble tangled threads. Thickening regions called chromomeres are visible along the length of the chromosomes. Chromomeres represent sites of tightly compacted DNA whose size and distribution are species-specific.

Fig. 18. The meiotic cycle.

1–4 — prophase of the first meiotic division; 5 — anaphase I; 6 — telophase I; 7 — prophase of the second meiotic division; 8 — anaphase II; 9 — metaphase II; 10 — mature gametes.

Zygotene is the stage of pairing of homologous chromosomes, during which the chromomeres of one homolog precisely align with the corresponding chromomeres of the other. This process is called synapsis (pairing), and the resulting pair of joined homologous chromosomes is known as a bivalent.

Pachytene is the stage of thick threads. Homologous chromosomes pair up to form bivalents, the number of which equals the haploid number (23). Each bivalent comprises four chromatids. Bivalents shorten and thicken due to the coiling of chromosomes. At this time, the conjugated chromosomes intertwine, resulting in a mutual exchange of segments between homologous chromosomes—a process known as crossing-over.

Diplotene is the stage of double threads, during which the chromosomes of a bivalent partially pull apart from one another, as if repelling each other. This repulsion begins in the centromeric regions and extends toward the ends. It becomes clearly visible that each bivalent consists of two chromosomes, and each chromosome consists of two chromatids. A bivalent contains four chromatids and is referred to as a tetrad. Intercrossed chromatids are visible at the points of crossing-over. The region where the chromatids cross is called a chiasma. The number of chiasmata corresponds to the number of crossovers. Chiasmata hold homologous chromosomes together within a single bivalent until anaphase.

Diakinesis is the stage during which homologous chromosomes continue to repel each other, yet remain joined at their ends. The shortening and thickening of chromosomes continue. Other changes also take place: the nucleoli disappear, the nuclear envelope breaks down, and the mitotic spindle begins to form.

Metaphase I. The bivalents (23) align along the equatorial plane, forming the metaphase plate. Chromosome coiling reaches its maximum. Unlike in mitosis, the centromere of each chromosome is connected to only one pole of the cell, but the centromeres of homologous bivalent chromosomes are always linked to opposite poles.

Anaphase I. Rather than individual chromatids separating from each other within every chromosome, whole homologous chromosomes begin to move from each bivalent toward the poles. This occurs because, unlike in mitosis, the centromere does not divide and the chromatids do not part. A haploid set of chromosomes (23) is formed at each pole, meaning that chromosome reduction takes place. The chromosomes remain two-chromatid.

Telophase I. Two daughter cells are formed. The nucleus of each contains a haploid set of chromosomes (n), but since the chromosomes are two-chromatid, the DNA amount corresponds not to the haploid set (c), but to the diploid set (2c).

The second meiotic division follows a short interphase II, during which DNA synthesis does not occur, bringing the chromosome number and DNA amount into balance. It proceeds similarly to mitosis. Prophase II is brief. In metaphase II, the chromosomes (23) align along the equatorial plane, and the centromeres divide. In anaphase II, the centromeres split, sister chromatids become daughter chromosomes, and they migrate to opposite poles. In telophase II, four cells are formed. Each of them contains a haploid set of chromosomes and the corresponding amount of DNA.

Biological Significance of Meiosis:

1. Meiosis is an evolutionary adaptation in living forms that ensures the constancy of the diploid chromosome number. This is achieved by halving the chromosome number in gametes. Upon the fusion of haploid gametes (fertilization), the diploid set is restored in the zygote (n+n=2n), and all somatic cells of the organism that develop from it via mitosis will possess the diploid chromosome number.

2. Meiosis ensures genetic diversity in gametes through the following mechanisms: 1) crossing-over in prophase I produces novel allele combinations (genetic recombinations) in chromosomes and, consequently, in germ cells; 2) the independent assortment of non-homologous chromosomes in anaphase I generates various combinations of paternal and maternal chromosomes in the gametes. The potential number of such combinations is determined by the formula 2ⁿ, where n is the number of chromosome pairs. In humans, this equals 2²³=8388608. The independent assortment of non-homologous chromosomes forms The basis of Mendel's third law and, alongside crossing-over, serves as a source of combinatorial Variability, which is crucial for evolution.

Meiotic Disorders

When chromosome segregation is disrupted, their number in gametes changes (genomic Mutations). For instance, if the chromosomes of a single pair fail to segregate, one gamete will receive 24 chromosomes and another 22. When a gamete containing 23 chromosomes fuses during fertilization with a gamete containing 24 or 22 chromosomes, an organism with 47 chromosomes (trisomy) or 45 chromosomes (monosomy) is produced. Non-disjunction can occur during both meiosis I and meiosis II, and it may affect either autosomes or sex chromosomes. As a result of meiotic disturbances, germline mutations arise. The causes include The impact of mutagenic agents and endogenous factors on spermatogenesis and oogenesis. The frequency of meiotic errors increases with maternal age (35 years and older). The consequences include spontaneous miscarriages and the birth of children with developmental defects, such as Congenital Heart defects, Nervous system disorders, cleft lip, cleft palate, and others. This correlation is particularly evident in Down syndrome, which has a chromosomal basis (trisomy 21, translocation, or mosaicism).

Fertilization

Fertilization is the fusion of a spermatozoon and an ovum, resulting in the formation of a zygote (a fertilized egg) that gives rise to a new organism. The Significance of fertilization lies in restoring the diploid chromosome set in the zygote. The resulting organism inherits traits from both parents, which enhances hereditary variability. Fertilization is a crucial stage of sexual reproduction.

Fertilization is preceded by insemination, which ensures the meeting of gametes in animals. There are Two Types of insemination: external and internal. External insemination occurs in many aquatic animals. In this case, eggs and sperm are released into the external environment, where their fusion takes place. Internal insemination is characteristic of terrestrial species, where environmental conditions do not support the survival and encounter of gametes externally. With this form of insemination, spermatozoa are introduced into the female reproductive tract during mating.

In humans, fertilization takes place in the ampullary region of the Uterine tube. This section of the tube is the widest and lies close to the Ovary. Following ovulation, the ovum—at the secondary oocyte stage—is passively swept into the uterine tube by fluid currents generated by the peristaltic contractions of the tube and the beating of epithelial cilia. Spermatozoa reach the site of fertilization thanks to contractions of the Uterus and uterine tube, as well as their own motility. A spermatozoon becomes capable of fertilizing an ovum only after spending several hours (about 7 hours) within the female reproductive tract. During this time, it undergoes activation (capacitation). Millions of spermatozoa coat The surface of the oocyte in what is known as the contact phase of fertilization. The contact between the spermatozoa and the oocyte triggers the acrosomal reaction in the sperm and the cortical reaction in the oocyte. Enzymes (such as hyaluronidase) are released from the sperm acrosomes, dissolving the intercellular contacts (hyaluronic acid) among the follicular cells of the corona radiata as well as the material of the zona pellucida. This exposes the oocyte plasma membrane. One of the many spermatozoa penetrates the oocyte (monospermy), after which the fertilization membrane rapidly forms through the action of cytoplasmic cortical granules, shielding the oocyte from The entry of additional sperm (preventing polyspermy). Upon sperm entry, the oocyte completes its second meiotic division, producing a mature ovum and a second polar body, which degenerates. The nuclei of the sperm and egg transform into the male and female pronuclei, respectively, which migrate toward each other and fuse (syngamy). The replicated maternal and paternal chromosomes form a joint metaphase plate. The first mitotic division of the zygote occurs, leading to the formation of a two-celled embryo. Pregnancy begins at the moment of fertilization.

Disorders of Fertilization

The fertilization process is influenced by numerous factors. Spermatozoa retain their fertilizing capacity for only 24–48 hours after ejaculation. During intercourse, an average of 3 ml of semen is ejaculated into the female Vagina, containing roughly 350 million spermatozoa. If their count drops below 60 million per 1 ml, the fertilizing capacity of the semen decreases. Fertilization failure can also stem from morphological abnormalities in the sperm. The pH of the female reproductive tract significantly impacts fertilization: in an acidic environment, spermatozoa rapidly lose motility and agglutinate. The optimal time window during which an egg can be successfully fertilized is approximately 12–24 hours.

The consequence of impaired fertilization is infertility. One method of overcoming it is In vitro fertilization (IVF) of human ova (externally, yielding "test-tube babies"). Oocytes are retrieved surgically following hormonal superovulation of the ovaries. The oocyte is then placed in a culture medium and mixed with sperm. Following fertilization, the embryo is transferred into the uterus (roughly 48 hours later), where subsequent development proceeds until birth. The success rate of this Procedure is approximately 20%.

Characteristics of Human Reproduction in Relation to its Biosocial Nature

The capacity for fertilization emerges after puberty. Signs of the onset of puberty in humans include the first pollutions (involuntary seminal emissions) in boys (around 13–14 years of age) and the first menstruation in girls (around 12–13 years of age). Sexual maturity is reached at 16–18 years of age in females and 18–20 years in males. Reproductive capacity persists in women until 45–50 years of age (and occasionally longer), and in men into advanced old age. A mature testis continuously produces vast numbers of spermatozoa (around 200 million), while a sexually mature ovary periodically (once every lunar month) releases an ovum at the secondary oocyte stage. The Specific features of human spermatogenesis and oogenesis were discussed above.

According to the WHO, a woman's true reproductive (childbearing) period spans from 20 to 40 years of age, because prior to 20 a woman is not yet sufficiently physically and psychologically prepared for childbirth and child-rearing, whereas after 40 the risk of fetal developmental abnormalities rises sharply. Humans feature an extended period of Embryogenesis, which is linked to the development of a complex nervous system. Following birth, there is prolonged contact between the child and the mother, which is vital for upbringing. An extended period of puberty prevents early reproduction and the premature diversion of energy toward rearing offspring. Consequently, humans gain time for prolonged learning. A number of traits are governed by human social nature: humans exercise control over their instincts, romantic love develops on a sexual foundation, and families are formed. Utilizing specialized Methods, humans have learned to control and regulate birth rates. This has led to The Emergence of a specialized medical field known as reproductive medicine.