Gametogenesis

• Gametogenesis:

  • The production of gametes.
  • Spermatogenesis and oogenesis are specific forms of meiosis that vary significantly
• Spermatogenesis

  • Type of Gametogenesis where sperm are produced.
  • Occurs inside the seminiferous tubules of each tests 
  • Takes about 72 days and occurs continuously after puberty
• Seminiferous tubules

  • Lined with immature cells called spermatogoina
  • Diploid
  • At puberty they start dividing by mitosis —-> constant supply of new cells
  • Some of these daughter cells are pushed inward towards the centre of the tubule —-> growth
• Meiosis 1

  • These enlarged cells are called primary spermatocytes and undergo the first stage of meiosis to produce secondary spermatocytes
  • They are now haploid
• Meiosis 2

  • The second meiosis division divides each secondary spermatocytes into two spermatids
  • Therefore, 4 haploid spermatids are formed from meiosis from one diploid spermatogonium
• Final stage

  • The final stage of Spermatogenesis occurs when spermatids mature into spermatozoa
  • Here, much of the cytoplasm of the cell is lost and a tail containing contractile material forms.
  • The maturing spermatozoa are nourished during this stage by special cells that extend from the outer portion of the seminiferous tubule into the centre.
• Sperm

  • The head consists of a nucleus and a fluid filled vesicle called the acrosome which contains enzymes to breakdown the outer layer of the cell surrounding the egg during fertilisation.
  • The neck lies between the head and the middle piece
  • The middle piece contains mitochondria for ATP production and only a very small amount of cytoplasm (sperm get their nourishment from semen)
  • The tail is capable of contractile motions to propel the cell forward.
• Oogenesis

  • Type of Gametogenesis where ova are produced
• Primary oocyte

  • Before birth, millions of oogonia develop in the ovaries
  •  These cells are diploid ad divide by mitosis to produce the cells that will eventually develop into ova
  • By birth oogonia have undergone a growth phase to become primary oocytes. These cells have begun probate of the first meiosis division only.
  • Each of these primary oocytes is surrounded by a single layer of cells, forming a primary follicle.
• Oogenesis process

  1. Follicle growth and maturation begins

  2. Primary oocyte completes first meiosis division

  3. Produces secondary oocyte and first polar body

  4. Secondary oocyte commences second division but stops at metaphase 2

  5. Ovulation occurs and oocyte is expelled

  6. If fertilized, meiosis is completed
• First meiosis division

  • Produces two haploid cells
  • Cells are of unequal size
  • Larger cell is secondary oocyte
  • Smaller cell is first polar body
• Second meiosis division

  • Produces two haploid cells
  • Cells are of unequal size
  • Larger cell develops into ovum
  • Smaller cell is second polar body
• All polar bodies disintegrate
• Only one ovum is produced from each primary oocyte
• In comparison to Spermatogenesis, 4 sperm are produced