2.4

• Name the 2 types of gametogenesis:

  • Spermatogenesis to produce spermatozoa
  • Oogenesis to produce ova
• Outline the stages of spermatogenesis:

  • primordial germline cell -> spermatogonia-> primary spermatocytes -> 2 secondary spermatocytes -> spermatids -> 4 spermatozoa
• What is Gametogenesis?

  The production of gametes (sex cells)
• Types of gametogenesis

  • Spermatogenesis
  • Oogenesis
• What are Primordial germline cells?

  Diploid precursors to gametes
• Spermatogenesis

  1. Primordial germline cell

  2. Spermatogonia

  3. Primary spermatocytes

  4. Secondary spermatocytes

  5. Spermatids

  6. Spermatozoa
• How spermatids form


  1. Primary spermatocytes undergo meiosis to form 4 undifferentiated haploid cells

  2. Meiosis I: diploid secondary spermatocytes

  3. Meiosis II: haploid spermatids
• How spermatozoa form


  Spermatids differentiate to gain flagellum, acrosome & many mitochondria
• Oogenesis

  1. Primordial germ cell

  2. Oogonia

  3. Primary oocyte

  4. Secondary oocyte & polar body

  5. Ootid & polar bodies

  6. 1 ovum
• How primary oocytes form

  1. Primordial germline cells divide by mitosis to form oogonia

  2. Growth period of oogonia without further division forms primary oocytes, which remain in prophase I in follicles until puberty
• First meiotic division of oogenesis

  1. 1 secondary oocyte with all the cytoplasm forms

  2. 1 polar body buds off and sticks to the oocyte

  
• Second meiotic division of oogenesis

  1. Secondary oocyte divides into haploid ootid & polar body

  2. Polar body divides in 2

  3. Overall 3 polar bodies form, they degenerate as ootid develops
• How an ovum forms

  1. Meiotic division in utero halts at prophase

  2. Fertilisation triggers influx of Ca2+

  3. This stimulates meiosis II to form ovum
• Structure of an ovum

  • Outer layer of follicle cells
  • Zona pellucida: coating above cytoplasm prevents polyspermy & hardens when cortical granules release chemicals
  • Haploid nucleus: fertilisation restores diploid chromosome number
• Structure of a spermatozoon

  • Acrosome, head, midpiece, tail
• Fertilisation


  1. Enzymes in acrosome of sperm head digest zona pellucida

  2. Sperm head fuses with ovum cell membrane, allowing sperm nucleus to enter

  3. Cortical reaction causes zona pellucida to harden, preventing polyspermy

  4. Nuclei fuse, forming a diploid zygote
• Early embryo development

  1. Zygote

  2. Morula

  3. Blastocyst

  4. Implantation
• How a blastocyst forms

  1. Several mitotic divisions of zygote produce morula (solid ball of cells)

  2. Morula divides further then undergoes differentiation and cavity formation
• Structure of a blastocyst

  • Inner cell mass develops into embryo
  • Outer layer (trophoblast) develops into placenta
  • Fluid filled cavity (blastocoele)
• Implantation

  1. Blastocyst breaches surrounding jelly coat, so it can attach to endometrium

  2. Release of autocrine hormones & digestive enzymes triggers implantation into uterine wall

  3. Growing embryo gains nutrients and oxygen from endometrial tissue fluid