Biology p. 313- 327 Biological Sex and Gender Identity

• Biological sex in humans includes chromosomal and genetic features and anatomical and physiological conditions called sex traits
• Gender is a social construct, whereas gender identity is a person's inner sense of self

Biological Sex Differences

• Primary sex determination in humans occurs via inherited sex chromosomes
• Typically, sex chromosomes trigger the differentiation of reproductive organs during embryonic development
• Two X chromosomes (XX) usually results in female reproductive organs while one X and one Y chromosome (XY) usually results in male reproductive organs
• Sex chromosome composition is set at fertilization; however, variation exists in human sexual development
• Variations in sex chromosome numbers (e.g., XXY) also occur, along with variations in hormone signaling pathways and androgen sensitivity
• Individuals inheriting a Y chromosome may develop both male and female sex traits due to an inability to respond to androgenic hormones like testosterone
• Biological sex differences affect immunity, pain processing, and cardiovascular health

Human Reproduction

• Human offspring result from sexual reproduction between sperm and oocytes
• Human reproduction unites haploid (1n) gametes to form a diploid (2n) zygote
• Female and male reproductive systems work together via sex hormones and gametes
• A delivery mechanism (sexual intercourse) brings gametes together
• The female reproductive system supports fertilization and pregnancy within the reproductive tract

Male Reproductive Anatomy

• The male reproductive system generates sperm and delivers it to the female tract
• Spermatogenesis is the process of producing gametes
• Male reproductive anatomy includes internal and external structures
• Testes are gonads where spermatogenesis and sex hormone production occur
• The scrotum is an external sac that houses the testes 2–4 ºC below body temperature for spermatogenesis
• Epididymides are coiled tubes on each testis' posterior surface where sperm mature, become motile, and are stored
• Ductus (vas) deferentia are muscular ducts that transport mature sperm from the epididymides to the urethra
• Seminal glands (seminal vesicles) produce seminal fluid containing chemicals like fructose and prostaglandins that propel sperm
• The prostate gland surrounds the urethra between the bladder and penis and secretes prostatic fluid with enzymes that prevent sperm coagulation in the vagina
• Bulbourethral (Cowper’s) glands are inferior to the prostate gland and secrete thick, alkaline mucus that lubricates the penis tip and neutralizes acids
• The urethra transports both semen and urine out of the body
• The penis is an external erectile structure containing the urethra for ejaculation
• Testes contain seminiferous tubules where sperm are produced
• Sertoli (nurse) cells support sperm development
• Leydig cells are endocrine cells located between seminiferous tubules that stimulate sperm cell development by secreting testosterone
• Semen contains sperm cells suspended in seminal fluid from the seminal glands, prostate gland, and bulbourethral glands
• Seminal fluid serves as a transport medium, supplies nutrients, and provides chemicals that protect and activate sperm

Spermatogenesis

• Spermatogenesis occurs in the seminiferous tubules of the testes
• Spermatogonia, stem cells, undergo mitotic division during the fetal period
• Spermatogonial cell division resumes at puberty, yielding identical diploid daughter cells
• One daughter cell from spermatogonial cell division differentiates into a spermatocyte that enters meiosis to generate four haploid (1n) spermatozoa
• The other daughter cell remains as an undifferentiated stem cell to maintain the spermatogonial population
• Spermatogenesis is differentiation of spermatogonia to form mature, active sperm which begins at puberty and continues for life
• A primary spermatocyte undergoes meiosis I to form two secondary spermatocytes
• Secondary spermatocytes then undergo meiosis II to form four haploid early spermatids
• Early spermatids are nonmotile lacking the morphology of mature sperm
• Early spermatids transform into spermatozoa (mature sperm cells)
• Each early spermatid differentiates into an elongated sperm cell called a late spermatid
• The sperm cell gains additional structures and removes excess cytoplasm during spermatogenesis
• Spermatogenesis takes 64-75 days in humans once underway at puberty
• Developing sperm in all stages of spermatogenesis are observed in the seminiferous tubules

Sperm Anatomy

• A mature spermatozoon contains a head, midpiece, and tail
• The sperm head contains the nucleus and an acrosome encapsulating the tip of the nucleus
• The acrosome is a flattened vesicular structure from the Golgi apparatus that has specialized enzymes to allow sperm to penetrate the oocyte
• The sperm midpiece is packed with mitochondria that generate the ATP required for flagellum-driven sperm motility
• The midpiece contains a pair of centrioles anchored to microtubules, which create the tail
• The sperm tail (flagellum) is composed of microtubules from the centrioles of the midpiece

Hormonal Control

• In XY embryos, male sexual characteristics develop based on gene expression and hormonal signals
• Signals activate testis development and repress ovarian development
• Internal/external male reproductive structures are formed in the presence of these signals
• Sertoli (nurse) cells secrete anti-Müllerian hormone (AMH)
• Anti-Müllerian hormone represses the development of Müllerian (paramesonephric) ducts
• Androgens secreted by the developing testes promote development of male reproductive structures derived from Wolffian (mesonephric) ducts
• Gametogenesis and secondary sexual development are regulated by the hypothalamus, anterior pituitary gland, and testes, known as the hypothalamic-pituitary-gonadal (HPG) axis
• The hypothalamus secretes gonadotropin-releasing hormone (GnRH), stimulating the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary gland
• LH stimulates the production of androgens (e.g., testosterone) in Leydig cells
• Testosterone, is essential for spermatogenesis and development of male sex traits
• FSH stimulates Sertoli cells, promoting and supporting spermatogenesis

Negative Feedback

• The male HPG axis is regulated by two negative feedback systems
• Testosterone levels regulate secretion of GnRH and LH
• High testosterone levels inhibit GnRH and gonadotropin secretion, repressing additional testosterone production
• If testosterone levels are too low, GnRH is released, stimulating LH and FSH secretion by the anterior pituitary
• Sertoli cells secrete inhibin
• Inhibin acts on the anterior pituitary and inhibits FSH secretion, thereby inhibiting Sertoli cell stimulation
• Decreased inhibin secretion allows FSH levels to rise, and Sertoli cells subsequently resume promoting spermatogenesis

Explore the intricate relationship between biological sex and gender identity. Understand primary sex determination in humans and hormonal influences on sex trait development. Learn about variations in sex chromosome number and the roles of chromosomes and hormones in sexual development.