Human Anatomy and Physiology - I. V. Gaivoronsky 2011

Anatomy of the Reproductive System. Human Reproductive Function and Development
Human Development

Human development is divided into prenatal (before birth) and postnatal (after birth) periods. Prenatal development begins at the moment of Fertilization and continues until the birth of the fetus. This period lasts 40 weeks. Postnatal human development begins after birth and lasts until natural death.

The prenatal period. This period is preceded by sexual intercourse, during which a man ejects 2–3 ml of semen into the woman's Vagina, containing about 400–600 million spermatozoa (150–200 million per 1 ml). A spermatozoon consists of a HEAD, neck, and tail (Fig. 11.13). Thanks to the active Movements of the latter, the spermatozoon propels itself through the female reproductive tract. This process requires a large expenditure of ATP and the active functioning of numerous Cell/35.html">Mitochondria. The majority of spermatozoa perish in the acidic environment of the vagina, as well as during passage through the uterine cavity and fallopian tubes. The advancement of spermatozoa depends on the condition of the female reproductive tract and can take from 5 to 30 hours. The most favorable conditions for progression occur in the female reproductive tract a few days before ovulation and 1–2 days after it. The egg cell remains capable of fertilization for 24 hours following ovulation. Only a single spermatozoon penetrates the egg's membrane, which immediately thereafter becomes completely impermeable to other sperm. The latter perish within the female reproductive tract after 2–3 days.

Fertilization occurs after a spermatozoon penetrates the egg cell. This is The process of fusion of the nuclei, each with a haploid (half—23) set of Chromosomes, from the egg and the sperm. As a result, a zygote is formed—a single-celled Organism with a diploid (complete—46) set of chromosomes. The zygote contains genes from both the maternal and paternal sides. The Genome of the zygote may contain either XX or XY sex chromosomes. In the first case, the new organism will develop as female, and In the second, as male. In the event of simultaneous maturation of two follicles, their ovulation, and the subsequent fertilization of two eggs, two fetuses may develop—so-called fraternal twins. Sometimes, during the Cleavage stage, two identical blastomeres derived from a single fertilized egg develop into two completely identical fetuses—identical twins.

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Fig. 11.13. Structure of a spermatozoon (schematic):

I — head; II — neck; III — tail; 1 — acrosome; 2 — mitochondria

Prenatal development is subdivided into the initial (implantation) period (the 1st week of development); the Embryonic period, or Embryogenesis (from the 2nd to the 8th week); and the fetal period (from the 9th week until birth).

During the seven days of the initial period, the zygote moves into the uterine cavity along the lumen of the fallopian tube due to the contractions of the ciliated epithelium and the muscular layer of its wall. At this time, the process of zygote cleavage takes place, during which a spherical multicellular structure resembling a mulberry—known as a morula—is formed from a single cell. On the 5th–6th day after fertilization, a fluid-filled cavity forms within the morula, at which point the future embryo is called a blastocyst, and the developmental phase is termed the blastula. The outer Cells of the blastocyst constitute extraembryonic material referred to as the trophoblast. The inner cell layer gives rise to the embryo and extraembryonic Organs: the chorion, amnion, and yolk sac. Upon entering the uterine cavity on the 7th day, the blastocyst uses the trophoblast to dissolve the uterine mucosa, burrows into its thickness, and attaches to the uterine wall—the implantation phase.

The Embryonic period of prenatal development includes the Histology/21.html">Formation of extraembryonic organs—the chorion, amnion, yolk sac, and allantois—as well as The Development of the primordia of all Tissues and Organs of the future infant.

The implanted embryo receives Nutrition and oxygen from the uterine mucosa via clusters of outer blastocyst cells that grow into it—the trophoblast villi. Subsequently, the cells of the trophoblast villi form the chorion, the primary external envelope of the embryo (Fig. 11.14), which lies superficial to the amnion and later contributes to The formation of the Placenta. During the 1st month of prenatal development, it plays a vital role in nourishing the developing embryo and eliminating Metabolic waste products. The amnion is a delicate membrane filled with a specialized Amniotic Fluid. The embryo floats in this fluid, which provides a watery environment essential for survival and protects it against mechanical trauma. In the human embryo, the yolk sac contains virtually no yolk and does not serve as a nutrient reservoir. Its primary role is hematopoiesis (Blood formation); additionally, primary Germ Cells form within its wall and subsequently migrate to the gonadal primordia.

The cellular mass of the adjacent walls of the amniotic and yolk vesicles gives rise to the embryonic disc, from which the embryo—the future infant—develops (Fig. 11.15). During the embryonic period, the cells of the embryonic disc differentiate into three distinct layers: the ectoderm, endoderm, and mesoderm. This developmental phase is known as the gastrula. These three germ layers give rise to all the organs and Tissues of the organism: the ectoderm serves as the source for the epithelium of the Oral Cavity and Pharynx, the anal canal, the olfactory epithelium, the conjunctiva, The Nervous system, the Skin epithelium, and its derivatives (sweat and Sebaceous Glands, Hair, and Nails); the mesoderm forms skeletal Muscles, bones, Connective Tissue, and the circulatory and lymphatic systems; the endoderm forms the epithelium of the digestive, respiratory, and urinary systems, as well as exocrine glands and certain Endocrine glands. By the end of the 3rd week, Organogenesis begins in the embryo: the nervous, digestive, circulatory, and other systems start to form. On the 5th week, the limb buds for the upper and lower extremities appear. Between the 6th and 8th weeks, facial features take shape, and the eyes shift from the lateral surface to an anterior position. By the end of the 8th week, organogenesis is complete, and The Heart begins to beat. The embryo begins to resemble a human, measuring 4 cm in length and weighing 5 g.

Fig. 11.14. Uterus. 4th week of Pregnancy (schematic):

1 — body of the uterus; 2 — embryo; 3 — chorion; 4 — ISTHMUS OF THE uterus; 5 — cervix of the uterus

Fig. 11.15. Development of the human embryo:

a — 4th week; b — 5th–6th week; c — 7th–8th week; 1 — chorionic villi; 2 — amnion; 3 — chorion; 4 — allantois; 5 — yolk sac; 6 — embryo

By the end of the 1st month of pregnancy, Metabolic exchange between the embryo and the maternal organism via the chorionic villi becomes insufficient; at this point, the embryo develops the allantois, which represents a finger-like pouch projecting from the embryonic gut. Blood Vessels grow along the allantois toward the chorion, participating during the 3rd week of pregnancy in the Formation of the placenta—a temporary extraembryonic organ that secures The connection between the embryo and the maternal organism. It is fully formed by the end of the 3rd month of prenatal development and appears as a disk embedded in the uterine mucosa. From this moment onward, the embryo is referred to as a fetus.

Throughout the fetal period, rapid growth takes place, accompanied by the definitive maturation of the STRUCTURE AND Functions of the fetal organs and systems. By the end of the 2nd month, the head and trunk are differentiated; by the end of the 3rd month, the limbs. In the 5th month, the fetus begins to move. By the end of the 6th month, the maturation of Internal Organs is complete. By the 8th month, the fetus is viable, though it still requires the conditions provided by the intrauterine environment. At 40 weeks of prenatal development, the fetus weighs at least 2500 g and measures at least 45 cm in length (Fig. 11.16). The ratio of head length to body length at this stage is 1:4.

Fig. 11.16. Human fetus: 9th month of development:

1 — amnion; 2 — endometrium; 3 — cervix of the uterus; 4 — amniotic fluid; 5 — umbilical cord; 6 — maternal part of the placenta; 7 — fetal part of the placenta

The fetus is attached to the placenta by the umbilical cord, a cord measuring 40 cm in length. The umbilical cord contains blood vessels through which blood flows from the fetus to the placenta and back. The placenta comprises two parts: fetal and maternal. The fetal part consists of the branching chorion with blood vessels growing into it. The maternal part consists of a modified, highly vascularized layer of the uterine mucosa. Within the placenta, the Blood vessels of the fetal part come into close contact with those of the maternal part, but there is no direct blood flow between mother and fetus, as this would lead to severe consequences for both. To prevent blood mixing, a placental barrier exists, which regulates The transfer of substances from the maternal portion of the placenta to the fetal portion and vice versa.

The placenta also functions as an endocrine gland, secreting Hormones (such as progesterone) that prevent the thickened uterine mucosa from sloughing off, suppress menstruation, and maintain the fetus within the uterus throughout the entire pregnancy.

During implantation, tissue and organogenesis, and the formation of the placenta, the embryo is highly vulnerable to adverse factors: narcotic substances, nicotine, certain medications, infectious diseases, ionizing radiation, and alcohol. Maternal alcohol consumption during pregnancy increases the likelihood of delivering premature infants who exhibit developmental delays and struggle to adapt to the external environment. Many such children perish at an early age.

Pregnancy culminates in childbirth, a complex physiological process during which the fetus and fetal membranes are expelled from the mother's body. Labor is divided into three stages: cervical dilation, fetal expulsion, and the placental stage. During The final stage, following the birth of the baby, the placenta and fetal membranes are delivered through the birth canal.

Postnatal development. Throughout the postembryonic period, humans grow and develop before eventually undergoing physical decline and Aging. Chronological age is determined by the number of years a person has lived. Concurrently, psychological development also takes place; psychological age does not always align with physiological age. Psychological development encompasses the evolution of an individual's perspective on the world across various life stages.

The pre-pubertal period of human development begins after birth and extends to 10–12 years of age. The first 4 weeks postpartum constitute the neonatal period, while the span from the 4th week until the end of the first year of life is known as infancy.

During the pre-pubertal period, children of both sexes exhibit few external anatomical differences, with the exception of the external genitalia.

Puberty spans from 10 to 16–18 years of age. It is characterized by accelerated somatic development and maturation, preparing the body for reproductive function. Elevated levels of Sex Hormones drive the Progressive development of secondary sexual characteristics. The onset, expression, and intensity of physical and psychological changes vary significantly between boys and girls during puberty.

In boys, the highest rates of linear growth and weight gain occur around the age of 13. The peak height velocity is observed between 12.5 and 15.5 years, amounting to approximately 10 cm per year. Hormonal stimulation also leads to the enlargement of the Testes and Penis; pubic hair appears without the distinct horizontal upper border characteristic of females. Axillary and facial hair emerges, accompanied by the enlargement of the laryngeal cartilages and the subsequent process of voice-breaking ("voice cracking"). By age 15, mature spermatozoa appear in the semen, and nocturnal emissions—involuntary ejaculations of semen during Sleep—occur for the first time.

Puberty in girls begins at age 10 and continues until 15–16 years. This phase is marked by rapid growth spurts in height and body mass, with peak height velocity typically occurring between 10.5 and 13.5 years of age. Pubic hair begins to grow around 10–12 years, followed by axillary hair, and the Mammary Glands develop. Concurrently, the Skeletal System grows intensively, the pelvic bones widen while the shoulders remain narrow, and the reproductive organs increase in size. Menarche, or the first menstruation, occurs between 12 and 14 years of age. The year following menarche is often characterized by relative sterility because initial menstrual cycles are not always ovulatory. By the end of puberty, a girl's Menstrual cycle becomes fully established, and her body is biologically prepared for sexual maturity and its specific functions: pregnancy, childbirth, and subsequent Lactation.

Currently, the GROWTH AND DEVELOPMENT of children are subject to secular trend acceleration—a process marked by the accelerated psychological and physical maturation of the organism. As a result, today's adults average about 10 cm taller than their counterparts a century ago.

The reproductive period in men varies in duration and does not precisely mirror that of women. Male fertility is established roughly midway through puberty, whereas the Male Climacteric (andropause) is more gradual and elusive than female menopause. Unlike in women, the onset of the male climacteric varies widely among individuals. Externally, it manifests as progressive erectile difficulties and a diminished interest in sexual intimacy. According to researchers, up to 50% of men over the age of 70 maintain sexual activity.

Senescence begins in both men and women at age 75. In men, this stage is accompanied by a decline and eventual cessation of reproductive capability. In individuals of both sexes, this period involves a reduction in the psychological and physical adaptive mechanisms that help the body cope with environmental stressors.

Natural death is a physiological biological process resulting in the cessation of life due to the depletion of the body's psychological and physical adaptive mechanisms against environmental factors.

Review Questions

1. List the internal Male reproductive organs.

2. Name the anatomical PARTS OF THE Epididymis.

3. What are the distinct segments of the vas deferens?

4. Describe the structural features and functional purpose of the Prostate Gland.

5. Name the parts and describe the Anatomy of the penis.

6. List the sections, constrictions, and dilations of the male Urethra.

7. Characterize the internal and external FEMALE REPRODUCTIVE ORGANS.

8. Indicate the functional purpose of the Ovary.

9. Describe the anatomical position of the uterus within the lesser pelvis and name its parts.

10. List the tissue layers comprising the uterine wall.

11. Which ligaments are involved in fixing the uterus in place?

12. Name the parts and Functional Significance of the fallopian tube.

13. What is the menstrual cycle? Name its constituent phases.

14. What are the Regions of the Perineum?

15. List the main periods of human development.