Biological Sex and Reproduction PDF

Chapter 9: Reproductive Systems Lesson 9.1 **Biological Sex and Reproduction** Introduction In humans, **biological sex** is defined by physical characteristics such as: Chromosomal and genetic features (eg, sex chromosomes, genes involved in sex determination) A set of anatomical and physiolo...

Chapter 9: Reproductive Systems Lesson 9.1 **Biological Sex and Reproduction** Introduction In humans, **biological sex** is defined by physical characteristics such as: Chromosomal and genetic features (eg, sex chromosomes, genes involved in sex determination) A set of anatomical and physiological conditions (**sex traits**) that include internal and external reproductive organs, secondary sex characteristics (eg, body morphology, hair growth pattern, breast tissue distribution), and hormone levels and function The concept of biological sex is distinct from the concepts of gender and gender identity. **Gender** is a social construct based on norms, behaviors, and cultural roles associated with a particular biological sex. **Gender identity** refers to a person\'s inner sense of self as male, female, or another identity (eg, gender-fluid) and may or may not correspond with an individual\'s biological sex traits. Further information on the distinctions between biological sex, gender, and gender identity can be found in Behavioral Sciences Concept 44.2.01. 9.1.01 Biological Sex Differences **Primary sex determination** in humans is defined by the inheritance pattern of sex chromosomes from each parent. The inheritance of particular sex chromosomes typically initiates the differentiation of reproductive organs during embryonic development (Figure 9.1). A person who inherits two X chromosomes (ie, XX individual) usually develops female reproductive organs and sex traits, whereas a person who inherits one X and one Y chromosome (ie, XY individual) usually develops male reproductive organs and sex traits (see Concepts 9.2.03 and 9.3.03). **Figure 9.1** Primary sex determination in humans. Diagram of a parent and parent Description automatically generated Chapter 9: Reproductive Systems Although the composition of sex chromosomes is determined at fertilization, there is variation in human sexual development. Individuals may be born with or develop differences in sex traits (eg, reproductive anatomy, sex hormones, secondary sex characteristics) that vary from sex traits in individuals typically characterized as \"male\" or \"female.\" In some cases, individuals may display variations in sex chromosome numbers (eg, XXY). Alternatively, there may be variations in hormonal signaling pathways involved in differentiation of reproductive organs. For example, some individuals who inherit a Y chromosome may develop both male and female sex traits due to an inability to respond to androgenic hormones (eg, testosterone). Biologically, sex differences affect the functions of various body systems, such as susceptibility to certain diseases (immune system), pain processing (nervous system), and heart health (cardiovascular system). Taken together, these differences in biology can affect how an individual experiences disease and responds to treatment. 9.1.02 Human Reproduction Human offspring are produced via sexual reproduction, in which haploid (1*n*) gametes (ie, **sperm** and **oocytes**) are united to form a diploid (2*n*) cell. This diploid cell is known as a **zygote** and contains one set of chromosomes from each parent (Figure 9.2). To facilitate the process of sexual reproduction, female and male reproductive systems are involved in the production of sex hormones and gametes, and the facilitation of a delivery mechanism (ie, sexual intercourse) that brings gametes together. In addition, female reproductive systems are involved in the maintenance of a supportive environment in the female reproductive tract, where **fertilization** (ie, zygote formation) and **pregnancy** (ie, fetal development) can occur (see Chapter 10). Anatomical features of the male and female reproductive systems, gamete production, and hormonal control of reproduction are covered in subsequent lessons in this chapter Enter word / phrase to search UBook text Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 9: Reproductive Systems 318 Lesson 9.2 **Male Reproductive System** Introduction The male reproductive system participates in human reproduction by generating haploid gametes known as **sperm** and facilitating the delivery of these gametes to a female reproductive tract. The concepts in this lesson provide an overview of male reproductive anatomy, the process of gamete formation via **spermatogenesis**, and hormonal regulation of the male reproductive system. 9.2.01 Male Reproductive Anatomy The male reproductive system consists of internal and external structures that participate in producing sperm, producing and regulating sex hormones, and facilitating reproduction. As depicted in Figure 9.3, the following structures are associated with the male reproductive system: **Testes** (singular, **testis**) are a pair of reproductive structures (ie, **gonads**) where spermatogenesis and sex hormone production occur. The **scrotum** is an external sac that hangs below the penis and contains the testes. The scrotum maintains the temperature of the testes at 2--4 ºC below body temperature, as required for spermatogenesis. The **epididymides** (singular, **epididymis**) are a pair of long, tightly coiled tubes on the posterior surface of each testis where sperm undergo maturation and become motile. Mature sperm are stored in the epididymides until release. The **ductus (vas) deferentia** (singular, **ductus \[vas\] deferens**) are a pair of long muscular ducts that transfer mature sperm from the epididymides to the urethra. The **seminal glands** (also known as **seminal vesicles**) are a pair of accessory glands responsible for the production of seminal fluid, which contains chemicals (eg, fructose, prostaglandins) that provide energy and stimulate contractions to propel sperm through the male and female reproductive tracts. The **prostate gland** is an unpaired accessory gland surrounding the urethra between the bladder and penis. The prostate gland secretes prostatic fluid, which contains the enzymes necessary to prevent coagulation of sperm in the vagina. The **bulbourethral (Cowper\'s) glands** are paired accessory glands located inferior to the prostate gland. These glands secrete thick, alkaline mucus, which lubricates the tip of the penis. The [alkalinity](javascript:void(0)) of the mucus neutralizes acids to protect sperm from the acidic environment of the urethra. The **urethra** is a long duct that originates from the bladder and runs through the prostate gland and the shaft of the penis. The urethra is used to transport both semen and urine out of the body (at different times), functioning in both the male reproductive and excretory systems. The **penis** is an external erectile structure that contains the urethra, through which ejaculation of **semen** (ie, a combination of sperm and seminal fluid) occurs. Chapter 9: Reproductive Systems 319 **Figure 9.3** Anatomical structures of the male reproductive system. The testes, surrounded by an outer fibrous capsule, contain numerous compartments filled with hundreds of tightly coiled **seminiferous tubules**. A cross section of these tubules (Figure 9.4) shows developing sperm cells and supporting cells known as **Sertoli (nurse) cells**, which play a critical supportive and regulatory role (eg, providing nutrients, fluids, hormones) in sperm production. **Leydig cells** are interstitial endocrine cells found in spaces between adjacent seminiferous tubules (Figure 9.4). Leydig cells stimulate sperm cell development by secreting the [steroid hormone](javascript:void(0)) [testosterone](javascript:void(0)) in response to hormones that are released from the anterior pituitary gland (see Concept 9.2.03).  Chapter 9: Reproductive Systems 320 **Figure 9.4** Cross section of a seminiferous tubule. Semen (ie, ejaculatory fluid) is composed of sperm cells suspended in seminal fluid. The highest proportion of seminal fluid comes from the seminal glands, followed by the prostate gland and bulbourethral glands. Seminal fluid has various functions, including serving as a transport medium, supplying nutrients, providing chemicals that protect and activate sperm, and facilitating propulsion of sperm through the male and female reproductive tracts. A diagram of a cell Description automatically generated Chapter 9: Reproductive Systems 321 A summary of the major components of semen and the origin of each component is provided in Table 9.1. **Table 9.1** Major components of semen. 9.2.02 Spermatogenesis Generation of gametes (ie, spermatozoa) in the male reproductive tract occurs in the seminiferous tubules of the testes. During the fetal period, a population of stem cells known as **spermatogonia** undergo continuous rounds of [mitotic division](javascript:void(0)) to yield identical diploid (2*n*) daughter cells. After the fetal period, spermatogonia are growth arrested (ie, dormant) until puberty. At puberty, spermatogonial cell division resumes, yielding identical diploid daughter cells. After each division, one daughter cell differentiates, giving rise to a spermatocyte that enters meiosis to generate four haploid (1*n*) spermatozoa. The other daughter cell remains in reserve as an undifferentiated stem cell which maintains the spermatogonial population. The differentiation of spermatogonia to form mature, active sperm is called **spermatogenesis** (Figure 9.5). This process begins at puberty and typically continues throughout the lifetime of an individual. During spermatogenesis, a **primary spermatocyte** (a diploid daughter cell produced from a mitotic division of a spermatogonium) undergoes [meiosis I](javascript:void(0)) to form two haploid daughter cells known as **secondary spermatocytes**. Both secondary spermatocytes undergo [meiosis II](javascript:void(0)) to form four haploid **early spermatids**, which are nonmotile and do not yet have the characteristic morphology of mature sperm.  Chapter 9: Reproductive Systems 322 **Figure 9.5** Spermatogenesis. Diagram of a diagram of spermatozoa Description automatically generated Chapter 9: Reproductive Systems 323 The early spermatids proceed into a maturation phase in which they are transformed into mature sperm cells (ie, **spermatozoa**), as shown in Figure 9.6. Each early spermatid differentiates into an individual elongated sperm cell known as a **late spermatid**. During this process, additional structures (eg, acrosome, flagella, spiral arrangement of mitochondria) are formed and excess cytoplasm is removed, but no additional cell division takes place. Finally, the nonmotile spermatozoa are released into the lumen of the seminiferous tubules and pushed toward the epididymis. **Figure 9.6** Sperm maturation. The final steps of maturation occur in the epididymis. Over a period of 14 days, spermatozoa gain motility and become capable of fertilizing an oocyte. Mature sperm may be stored in the epididymis for several months. Over time, any unejaculated sperm are eventually reabsorbed at the epididymis lining. In humans, the entire process of spermatogenesis, from spermatogonium to spermatozoon, happens over 64--75 days and occurs continually beginning at puberty. Once spermatogenesis has been initiated (ie, in post-pubertal individuals), developing sperm in all stages of spermatogenesis may be observed in the seminiferous tubules. A mature spermatozoon is divided into three segments, as depicted in Figure 9.7: The **head** contains the nucleus and the **acrosome**, which encapsulates the tip of the nucleus. The acrosome is a flattened vesicular structure that originates from a specialized acrosomal vesicle in the Golgi apparatus. Specialized enzymes (eg, hyaluronidase, proteases) within the acrosome allow sperm to penetrate the zona pellucida (ie, thick extracellular matrix) of an oocyte during fertilization. The **midpiece** is packed with [mitochondria](javascript:void(0)) arranged in a spiral around the microtubules that form the tail. These mitochondria produce the ATP required for flagellum-driven sperm motility. The midpiece also contains a pair of centrioles anchored to microtubules, from which the tail originates. The **tail** (ie, flagellum) is a singular elongated structure composed of microtubules that originate from the centrioles of the midpiece (see Concept 5.3.06 for more information on eukaryotic flagella). Sperm motility in the female reproductive tract is powered by the action of the flagellum.  Chapter 9: Reproductive Systems 324 **Figure 9.7** A mature spermatozoon. 9.2.03 Hormonal Control of the Male Reproductive System During the early weeks of embryogenesis, an XY embryo begins to develop male sexual characteristics based on the balanced expression of genes and hormonal signals that activate testis development and repress ovarian development. Internal and external male reproductive structures are typically formed in the presence of these specific signals. In an XY individual, expression of *SRY* and other genes involved in sex determination typically initiate a cascade of events that result in the development of male reproductive organs. Developing Sertoli (nurse) cells begin to secrete **anti-Müllerian hormone (AMH)**, repressing the development of **Müllerian (paramesonephric) ducts**, from which female reproductive structures are derived. This repression allows androgens (eg, testosterone) secreted by the developing testes to promote development of male reproductive structures derived from **Wolffian (mesonephric) ducts** (Figure 9.8). A diagram of a sperm Description automatically generated Chapter 9: Reproductive Systems 325 **Figure 9.8** Differentiation of male sex organs during development. During infancy and childhood, sex hormone levels are low. At the onset of puberty, gametogenesis and secondary sexual development are regulated by the coordinated action of the hypothalamus, anterior pituitary gland, and testes, collectively known as the **hypothalamic-pituitary-gonadal (HPG) axis** (Figure 9.9). The hypothalamus secretes **gonadotropin-releasing hormone (GnRH)**, which stimulates the secretion of two gonadotropin hormones, **luteinizing hormone (LH)** and **follicle-stimulating hormone (FSH)**, from the anterior pituitary gland.  Chapter 9: Reproductive Systems 326 **Figure 9.9** Hormonal control of the male reproductive system. In **Leydig cells**, LH stimulates the production of androgens such as **testosterone**, the dominant male sex steroid hormone essential for spermatogenesis and development of male sex traits. FSH stimulates **Sertoli cells**, thereby promoting and supporting the process of spermatogenesis. Both testosterone and FSH are required to initiate and maintain spermatogenesis. The male HPG axis is regulated via two [negative feedback](javascript:void(0)) systems (Figure 9.9). In the first system, testosterone levels regulate secretion of GnRH and LH. High testosterone levels inhibit GnRH and gonadotropin secretion, repressing the production of additional testosterone. If testosterone levels fall too low, this repression is relieved and GnRH is again released by the hypothalamus, stimulating secretion of LH and FSH by the anterior pituitary. In turn, Leydig cells are stimulated to increase testosterone production. In a second negative feedback mechanism acting on the HPG axis, Sertoli cells regulate the rate of spermatogenesis by secreting the peptide hormone **inhibin**. Inhibin acts on the anterior pituitary to inhibit FSH secretion, thereby inhibiting Sertoli cell stimulation. A decrease in inhibin secretion allows FSH levels to rise, and Sertoli cells are again stimulated to resume the promotion of spermatogenesis. Diagram of a human body Description automatically generated