embryology 3

• Embryology


  The study of the formation of life, how life begins, and how it develops into a fully formed and functioning organism
• Practical applications of embryology

  • Helps understand anatomy
  • Prevention of genetic diseases
  • Treatment of sterility
  • Cloning
• Chromosome


  A very long DNA molecule, which contains many genes and other regulatory proteins
• Chromosome components


  • Chromatid
  • Centromere
• Human somatic cell contains 23 pairs (diploid) of chromosomes, each pair one from father and one from mother
• Types of chromosomes


  • Autosomes
  • Sex chromosomes (XY)
• Gene


  A region of DNA that controls a hereditary characteristic
• Genome


  All DNA in a cell
• Trait


  A distinct character of an organism that is determined by specific genes on chromosomes and inherited from the father and the mother
• Types of traits


  • Dominant trait
  • Recessive trait
• Gametogenesis


  Conversion of primordial germ cells (PGCs) into male and female gametes (sperm, oocyte)
• Primordial germ cells (PGCs) formation and migration

  1. Formed in epiblast during 2nd week

  2. Move to yolk sac wall during 3rd week

  3. Migrate to gonads during 4th week

  4. Increase in number by mitotic division during migration and after arrival to gonads
• Teratomas


  Tumors that contain different varieties of tissue, indicating they arise from pluripotent stem cells, one cause is PGCs that have strayed from their normal migratory paths
• Types of cell division

  • Mitotic cell division
  • Meiotic cell division
• Mitotic cell division


  A process by which one cell divides and gives rise to two daughter cells that are genetically identical to the parent cell
• Stages of mitotic cell division

  1. Prophase

  2. Prometaphase

  3. Metaphase

  4. Anaphase

  5. Telophase
• Meiotic cell division


  Occurs in male and female germ cells, the cells replicate their DNA just before entering the division, so the germ cells contain double the normal amount of DNA
• Stages of meiotic cell division

  1. Pairing of homologous chromosomes (synapsis)

  2. Crossover and interchange of chromatid segments

  3. Chiasma stage

  4. Separation of homologous chromosomes

  5. Second meiotic cell division
• Importance of meiotic divisions

  • Genetic variability is enhanced through crossover and random distribution
  • Each germ cell contains a haploid number of chromosomes, so that at fertilization the diploid number of 46 is restored
• Result of meiotic cell division in male and female gametes

  • Primary spermatocyte gives rise to 4 spermatids, each genetically different, and containing haploid number of chromosomes
  • One primary oocyte results in one mature oocyte (22+X) and three polar bodies
• Causes of chromosomal abnormalities

  • Nondisjunction
  • Translocation
  • Deletion
• Nondisjunction


  Chromosome pair fail to separate and move into the same cell, resulting in one cell receiving 24 chromosomes and the other 22
• Types of translocations


  • Balanced
  • Unbalanced
• Examples of chromosome abnormalities

  • Down syndrome
  • Klinefelter syndrome
  • Turner syndrome
  • Angelman syndrome
  • Cri-du-chat syndrome
• Oogenesis


  1. Differentiation of primordial germ cells into oocytes

  2. At 4 months: Oogonia increase in number and some differentiate into primary oocytes

  3. At 7 months: Most oogonia transformed into primary oocytes, some start to degenerate

  4. At birth: No oogonia, each primary oocyte surrounded by follicular cells forming primordial follicle

  5. At puberty: Primary oocyte begins to grow, follicular cells change and proliferate
• Spermatogenesis


  1. At birth: Germ cells located in sex cords

  2. Shortly before puberty: Sex cords transformed into seminiferous tubules, PGCs differentiate into spermatogonial stem cells

  3. Type A spermatogonia divide mitotically, last division gives rise to type B spermatogonia

  4. Type B spermatogonia differentiate into primary spermatocytes, which undergo meiosis 1 and 2 to form spermatids

  5. Spermatids transformed into spermatozoa (spermiogenesis)
• Functions of Sertoli cells

  • Provide support and protection
  • Provide nutrition
  • Assist in the release of mature spermatozoa
• Stages of spermiogenesis


  1. Acrosome formation

  2. Nuclear condensation

  3. Flagellum formation

  4. Cytoplasm removal
• Sex cords


  Large pale structures surrounded by supporting cells of the testis
• Transformation of sex cords before puberty

  Sex cords acquire a lumen and are transformed into seminiferous tubules
• PGCs


  Primordial germ cells
• Differentiation of PGCs


  PGCs differentiate into spermatogonial stem cells, which will give rise to type A spermatogonia
• Development of type A spermatogonia

  Type A spermatogonia work as a continuous reserve of stem cells, with their last cell division giving rise to type B spermatogonia
• Development of type B spermatogonia


  Type B spermatogonia will differentiate into primary spermatocytes
• Meiosis in primary spermatocytes

  Primary spermatocytes enter a prolonged prophase, then rapidly complete meiosis 1 to form secondary spermatocytes, which immediately enter meiosis 2 to form spermatids
• Sertoli cells


  • Provide support and protection
  • Provide nutrition
  • Assist in the release of mature spermatozoa
• Spermiogenesis


  1. Transformation of spermatids into spermatozoa

  2. Acrosome formation

  3. Condensation of the nucleus

  4. Formation of neck, middle piece and tail

  5. Shedding of most of the cytoplasm
• Maturation of spermatozoa


  After fully formed, spermatozoa enter the lumen of the seminiferous tubules, then move toward the epididymis where they obtain full motility
• Regulation of spermatogenesis


  1. LH from the pituitary stimulates Leydig cells to secrete testosterone, which binds to Sertoli cells and stimulates spermatogenesis

  2. FSH stimulates testicular fluid production
• Abnormal male germ cells

  • Abnormal head
  • Abnormal tail
  • Abnormal size (giant or very small)
  • Joined