Reproduction

• Reproductive system

  The multiple differences between the male and the female depend primarily on a single chromosome (The Y chromosome) and a single pair of endocrine structures the testes and the ovaries
• Differentiation of primitive gonads

  1. Genetically determined

  2. Testes or ovaries formation

  3. Male genitalia (penis, scrotum) formation dependent on functioning secreting testes

  4. In the absence of testicular tissue, development is female
• Gonads


  Remain quiescent until puberty when they are activated by gonadotrophins from the anterior pituitary
• After gonads are activated


  1. They secrete hormones that produce the characteristic male and female appearance

  2. Activate the female sexual cycle to begin (menstruation)

  3. Ovarian function in females regresses as they advance in age and sexual cycles cease at some point (menopause)

  4. Males retain ability to procreate though their gonadal function also declines
• Gonads


  Have a dual function - Gametogenesis and sex hormone production
• Gonads secrete


  In response to pituitary gonadotrophins FSH and LH
• Hormones secreted by testes

  • Large amounts of androgens (primarily) testosterone
  • Small amounts of oestrogen
• Hormones secreted by ovaries


  • Large amounts of estrogens
  • Small amounts of androgens
• Androgens


  Steroid sex hormones that cause masculinization
• Common androgens


  • Testosterone
  • Dihydrotestosterone (DHT)
  • Androstenedione
• Oestrogens


  Feminizing steroid sex hormones
• Other hormones secreted by ovaries

  • Progesterone
  • Relaxin
  • Inhibin B
• Testosterone


  The major androgen in males, responsible for regulating sex differentiation, producing male sex characteristics, spermatogenesis, and fertility
• Dihydrotestosterone (DHT) and androstenedione


  Of equal importance in male development. DHT in utero causes differentiation of the penis, scrotum and prostate. In adulthood, DHT contributes to balding, prostate growth, and sebaceous gland activity
• Androgens are commonly thought of only as male sex hormones, but females also have them, at lower levels. They function in libido and sexual arousal. Androgens are also the precursors to estrogens in both men and women
• Androgens are also secreted from the adrenal cortex and some of them are converted to estrogens in fat
• Embryology of the human reproductive system

  1. Week 1-6: Primitive gonad arises from genital ridge, identical in both sexes

  2. Week 7-8: In genetic males, cortex regresses, medulla develops into a testis. Leydig and Sertoli cells develop and testosterone and mullerian inhibiting substance are secreted. In genetic females, medulla regresses, cortex develops into an ovary. Embryonic ovary does not secrete hormones
• Internal genitalia development


  1. 7th week - Embryo has both male and female primordial genital ducts

  2. In female fetus, the mullerian duct system develops into uterine tubes and a uterus

  3. In male fetus, the wolffian duct develops into the epididymis and vas deferens
• External genitalia development


  1. Bipotential until week 8, then urogenital slit disappears forming male genitalia or persists forming female genitalia

  2. Functional testes secrete testosterone from Leydig cells which causes the development of the vas deferens and epididymis from Wolffian duct

  3. Testicular Sertoli cells secret MIS (Mullerian inhibiting factor) which causes regression of the Mullerian duct by apoptosis

  4. Testosterone metabolite dihydrotestosterone (DHT) induces the formation of the male external genitalia and secondary sex characteristics
• Indifferent stage of gonadal development


  It is impossible to distinguish between the male and female gonad. The gonads begin as genital ridges and initially do not contain any germ cells. In the fourth week, germ cells begin to migrate from the endoderm lining of the yolk sac to the genital ridges. Simultaneously, the epithelium of the genital ridges proliferates and penetrates the intermediate mesoderm to form the primitive sex cords. The combination of germ cells and primitive sex cords forms the indifferent gonad
• Testes development


  In a male embryo, the XY sex chromosomes are present. The Y chromosome contains the SRY gene, which stimulates the development of the primitive sex cords to form testis (medullary) cords. The tunica albuginea, a fibrous connective tissue layer, forms around the cords. A portion of the testis cords breaks off to form the future rete testis. The remaining cords contain germ cells and Sertoli cells. In puberty, these cords acquire a lumen and become the seminiferous tubules. Located between the testis cords are the Leydig cells which begin production of testosterone in the eighth week
• Ovaries development


  In a female embryo, the XX sex chromosomes are present. As there is no Y chromosome, there is no SRY gene to influence development. Without it, the primitive sex cords degenerate and do not form the testis cords. Instead, the epithelium of the gonad continues to proliferate, producing cortical cords. In the third month, these cords break up into clusters, surrounding each oogonium (germ cell) with a layer of epithelial follicular cells, forming a primordial follicle
• Indifferent stage internal genitalia

  • Mesonephric (Wolffian) ducts
  • Paramesonephric (Mullerian) ducts
• Male internal genitalia development

  In the presence of testosterone, the mesonephric ducts develop to form the primary male genital ducts. They give rise to the efferent ductules, epididymis, vas deferens and seminal vesicles. The paramesonephric ducts degenerate in the presence of anti-Mullerian hormone produced by sertoli cells in the testes
• Female internal genitalia development

  In the female, there are no Leydig cells to produce testosterone. In the absence of this hormone, the mesonephric ducts degenerate, leaving behind only a vestigial remnant - Gartner's duct. The absence of anti-Mullerian hormone allows for development of the paramesonephric ducts, which initially have three parts: Cranial - becomes the Fallopian tubes, Horizontal - becomes the Fallopian tubes, Caudal - fuses to form the uterus, cervix and upper 1/3 of the vagina. The lower 2/3 of the vagina is formed by sinovaginal bulbs
• External genitalia development

  1. Indifferent stage - Mesenchymal cells from the primitive streak migrate to the cloacal membrane to form a pair of cloacal folds. Cranially, these fuse to form the genital tubercle. Caudally, they divide into the urethral folds (anterior) and anal folds (posterior). Genital swellings develop either side of the urethral folds

  2. Male - Rapid elongation of the genital tubercle forms the phallus. The urethral folds are pulled to form the urethral groove, which extends along the caudal aspect of the phallus and closes over by the 4th month, forming the penile urethra. The genital swellings become the scrotal swellings, moving caudally to form the scrotum

  3. Female - The genital tubercle only elongates slightly to form the clitoris. The urethral folds and genital swellings do not fuse, but instead form the labia minora and labia majora respectively. The urogenital groove remains open, forming the vestibule
• Descent of the gonads

  1. Testes - As the body of the fetus grows, the testes become more caudal. They pass through the inguinal canal around the 28th week, and reach the scrotum by the 33rd week with the help of the gubernaculum and becomes the scrotal ligament . During their descent, the testes retain their original blood supply

  2. Ovaries - The ovaries initially migrate caudally in a similar fashion to the testes from their origin on the posterior abdominal wall. However they do not travel as far, reaching their final position just within the true pelvis. Gubernaculum forms ligaments here.
• Cells


  Tiny self-contained components that living things are made of
• DNA (deoxyribonucleic acid)


  Long and complex molecules inside each cell that carry genetic information
• Nucleotides


  Simple units that DNA is made of and line up in a particular order
• Genes


  Bits of DNA that store information for making proteins
• Genetic code


  Language used by DNA that lets organisms read the information in the genes
• Every cell in the same living thing has the same DNA, but only some of it is used in each cell
• A person has two copies of each gene, one from their mother, and one from their father
• Deoxyribose


  Sugar molecule that is part of the DNA molecule
• Phosphate group


  Part of the DNA molecule
• Bases in DNA


  • Adenine (A)
  • Cytosine (C)
  • Guanine (G)
  • Thymine (T)
• Nucleotide


  Unit made of a base, sugar, and phosphate that connects to make long chains of DNA
• Histones


  Proteins that DNA wraps around to form chromosomes
• Genome


  Full set of hereditary material in an organism, usually the combined DNA sequences of all chromosomes