week 11 human bio lecture 1

Mitosis vs. Meiosis

• Understanding the difference between mitosis and meiosis is crucial for understanding reproduction.

Mitosis

• Mitosis is a type of cell division that results in two identical daughter cells, each having the same number and type of chromosomes as the parent cell.
• Essential for growth, development, and tissue repair in multicellular organisms.
• Parent cell starts with 46 chromosomes (2n).
• DNA replicates, resulting in two identical sets of chromosomes.
• The cell undergoes one round of cell division, evenly splitting the DNA between the two daughter cells.
• Each daughter cell ends up with 46 chromosomes (2n), identical to the parent cell.
• From one parent cell, two identical daughter cells are produced, each with 46 chromosomes.

Meiosis

• Meiosis is a type of cell division that reduces the number of chromosomes by half, producing four genetically diverse daughter cells (gametes: sperm or eggs).
• Crucial for sexual reproduction.
• Consists of two rounds of cell division.
• Parent cell (germ cell) has 46 chromosomes (2n).
• DNA replicates, resulting in two copies of each chromosome.
• The cell undergoes one round of cell division, resulting in two daughter cells, each with 2n chromosomes.
• These two daughter cells then undergo another round of cell division, splitting the DNA again, resulting in four daughter cells.
• Each of the four daughter cells has half the number of original chromosomes (haploid).
• From one germ cell, four daughter cells are produced in meiosis.

Gametogenesis and Sex Chromosomes

• Terms like "male reproductive system" and "female reproductive system" refer to biological structures, not gender identities.
• Biological sex refers to biological differences (internal and external anatomy, chromosomes, hormones) between males and females, distinct from gender identity.
• The reproductive system's main goal is reproduction through gametogenesis, the production of gametes.
• Gametogenesis and meiosis are closely related; meiosis is part of gametogenesis.
• Most cells are diploid (46 chromosomes), while gametes are haploid (23 chromosomes).
• When male and female gametes fuse (sperm and egg), the resulting zygote has a full set of chromosomes (46).
• Gametes come from germ cells, which undergo meiosis to reduce the chromosome number from 46 to 23.
• Each gamete contains 22 autosomes and one sex chromosome.
• The zygote will have 44 autosomes and two sex chromosomes.

Sex Chromosomes

• Two types: X and Y chromosomes.
• Determine the biological sex of the zygote.
• Two X chromosomes (XX) result in a biologically female zygote.
• One X and one Y chromosome (XY) result in a biologically male zygote.
• Biological females are XX, so all daughter cells produced after meiosis have an X chromosome.
• Biological males are XY, so two daughter cells have an X chromosome, and two have a Y chromosome.
• The sperm determines the biological sex of the offspring.
• Up to about six weeks gestation, the embryo has primitive gonads that can develop into either testes or ovaries.
• The presence or absence of the SRY gene on the Y chromosome determines the development of the gonads.
• If the SRY gene is present, the primitive gonads develop into testes; if absent, they develop into ovaries.

Intersex Variations

• About 2% of the population is intersex, with variations in genitals, chromosomes, or internal reproductive organs.
• Genitals may not match reproductive organs, or individuals may have traits of both sexes.
• Intersex variations are not considered a disorder or disease but a variation of the population.
• Causes can include inheriting more or fewer than two sex chromosomes, missing SRY gene, or changes in hormone levels or receptors during development.

Male Reproductive System Anatomy

• The reproductive system includes all organs related to offspring production.
• The main male reproductive organs are the testes, which have two major functions: sperm production (gametogenesis) and hormone secretion (androgens, mostly testosterone).
• Testes are located outside the body in the scrotum.
• Epididymis: Where mature sperm is stored (at the back of each testes).
• Vas deferens: A tube that goes up into the pelvic cavity, around the bladder, and merges with tubes from the seminal vesicles and prostate gland.
• Seminal vesicles and prostate gland supply fluids to the sperm, contributing to the volume of semen.
• Urethra: Formed by the merged tubes, travels through the penis.

Testes

• Located outside the body in the scrotum to maintain a lower temperature (2-3 degrees less than normal body temperature).
• Normal body temperature (37 degrees Celsius) is too high for spermatogenesis; the process occurs best at around 34 degrees Celsius.
• Microanatomy: Each testis is divided into 250-300 compartments.
• Seminiferous tubules: Located within each compartment; this is where sperm is produced.
• Two main cell types within the seminiferous tubules: Sertoli cells and spermatogonia.
• Spermatogonia: Germ cells of the testes.
• Sertoli cells: Provide metabolic and physical support to developing spermatogonia.
• Spermatogonia develop between Sertoli cells and migrate towards the lumen of the seminiferous tubules.
• Interstitial space: Outside the seminiferous tubules, contains connective tissue and Leydig cells.
• Leydig cells: Produce and secrete testosterone.

Sertoli Cells

• Held together by tight junctions, forming the blood-testes barrier.
• Blood-testes barrier: Maintains a special environment in the lumen of the seminiferous tubules to support sperm development.
• Protects sperm from the host immune system, as sperm are haploid and could be seen as foreign.
• Secrete fluid into the lumen of the seminiferous tubules.
• Secrete androgen-binding proteins to keep testosterone within the testes.
• Activated by testosterone from Leydig cells and follicle-stimulating hormone (FSH) from the anterior pituitary gland.

Epididymis and Vas Deferens

• Seminiferous tubules merge to form the epididymis, located at the posterior surface of the testes.
• Epididymis: A coiled tube where mature sperm are stored until ejaculation (if ejaculation doesn't occur within two weeks, sperm are reabsorbed).
• Epididymis continues to become the vas deferens, which travels up out of the scrotum and into the pelvic cavity.
• Vas deferens travels over the bladder and joins onto a duct branching off from the seminal vesicles, becoming the ejaculatory duct.

Seminal Vesicles and Prostate Gland

• Seminal vesicles produce a fluid containing nutrients (e.g., fructose for energy) and alkaline liquid to neutralize the acidic environment of the vagina.
• Seminal vesicles contribute about 70% of the volume of semen.
• Ejaculatory duct goes into the prostate gland, located just beneath the bladder.
• Urethra coming off the bladder also goes through the prostate gland.
• Within the prostate, the ejaculatory duct and the urethra merge.
• In males, the urethra serves two functions: removal of waste (urine) and ejaculation of semen.
• The prostate gland contributes 25-30% of the fluid in semen.
• The urethra travels along the length of the penis, and semen is ejaculated.

Hormonal Regulation

• Begins in the brain with the hypothalamus.
• The hypothalamus releases gonadotropin-releasing hormone (GnRH), a tropic hormone.
• GnRH acts on the anterior pituitary gland, triggering the secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
• FSH and LH primarily act in the testes.
• FSH binds to Sertoli cells and stimulates spermatogenesis.
• LH binds to Leydig cells and stimulates testosterone production.

Negative Feedback Loops

• Sertoli cells secrete inhibin, which inhibits the production and secretion of FSH by the anterior pituitary gland.
• Testosterone inhibits the production of GnRH, FSH, and LH by the hypothalamus and anterior pituitary.

Functions of Testosterone

• Stimulates spermatogenesis.
• Involved in the development of secondary male characteristics (facial hair, body hair, voice cracking).
• Helps build and maintain bone and muscle mass.
• Regulates libido.
• Helps regulate red blood cell production.
• Regulates fat distribution around the body.

Spermatogenesis

• Spermatogonia (germ cells of the testes) are located at the base of the seminiferous tubules.
• Spermatogonia undergo mitosis, producing two identical diploid daughter cells (46 chromosomes each).
• One daughter cell remains as a spermatogonium, replacing the original cell, while the other goes through spermatogenesis and meiosis to produce sperm.
• Males have a continuous supply of spermatogonia, allowing fertility for most of their lives.
• Spermatogonium goes through two stages of cell division in meiosis, migrating between Sertoli cells towards the lumen.
• Forms the classical shape of sperm with the head and tail, still attached to the Sertoli cell by the head.
• Sperm are not fully mature at this stage and cannot swim yet.
• Sperm become motile after reaching the epididymis, where they undergo a final maturation phase.

Sperm Structure

• Three main segments:
  • Head: Contains the nucleus with genetic material and the acrosome (contains enzymes and proteins for penetrating and fusing with the ovum).
  • Midpiece: Contains an abundance of mitochondria, which provide energy for swimming.
  • Tail: Provides the sperm the ability to swim.