Cell Division: Mitosis, Meiosis and Gametogenesis

Cell Division

• Mitosis is a type of cell division where a single cell, known as the mother cell, divides to create two new, genetically identical daughter cells.
• Mitosis involves the DNA in the cell's nucleus splitting into two equal sets of chromosomes.
• Meiosis divides a diploid cell, which contains two sets of chromosomes, into haploid cells, each with a single set of chromosomes.
• Meiosis reduces the number of chromosomes by half through reduction divisions.
• Meiosis results in the production of gametes, which include sex cells like sperm and eggs.

Gametogenesis

• Gametogenesis is process of forming haploid gametes from diploid cells of the germ line (somatic cells)
• In males it is called spermatogenesis:
  • Sperms are formed in the testes.
  • Approximately 200 million sperms are produced daily throughout a male's life.
• In females it is called oogenesis:
  • Ova are formed in the ovaries.
  • Usually, one egg is produced each menstrual cycle and production stops after menopause.
• During early embryonic development, primordial germ cells (PGCs) migrate from the yolk sac's dorsal endoderm along the hindgut to the gonadal ridges.
• After reaching the gonadal ridge, primordial germ cells proliferate through mitosis and become known as gametogonia, where males become spermatogonia and females become oogonia.
• Progenitor cells are committed to a specific cell fate.

Spermatogenesis

• Spermatogenesis has three phases, which occurs in the seminiferous tubules:
  • Spermatocytogenesis is the proliferative phase.
  • Meiosis is the production of the haploid gamete.
  • Spermiogenesis is where spermatids mature into spermatozoa.
• Spermatogonia contain 46 chromosomes, and divides via mitosis.
• Primary spermatocytes contain 46 chromosomes.
• This process is called Meiosis and it creates additional spermatogonia.
• Secondary spermatocytes contain 23 chromosomes.
• Spermatids contain 23 chromosomes.
• Spermatozoa contain 23 chromosomes.
• Spermiogenesis involves extensive cell differentiation and cellular remodeling, without further division.
• During spermiogenesis, spermatids shed cytoplasm and form:
  • A head covered by an acrosome filled with enzymes.
  • A mid-piece containing the mitochondria.
  • A flagellum that drives motility.
• Sertoli cells provide fluid, nutrients, and the appropriate conditions for spermiogenesis.
• Sertoli cells "squeeze" mature spermatozoa into the lumen.
• Sertoli cells direct spermatogenesis in response to FSH and testosterone by releasing growth factors that act in a paracrine manner.

Oogenesis

• Before birth:
  • The primary oocyte is surrounded by flat follicular (granulosa) cells, forming what is called primordial follicles.
  • Primary oocytes begin prophase of meiosis I, but stop at the diplotene stage.
• At birth:
  • All primary oocytes have started prophase 1, but are arrested at the diplotene stage.
• At puberty:
  • Primary oocytes complete the first meiotic division just before ovulation, forming a secondary oocyte.
  • Secondary oocytes commence in the second meiotic division, and arrests at Metaphase.
  • The second meiotic division is completed only if the oocyte is fertilized.

Ovarian Follicles

• Primordial follicles contains a primary oocyte surrounded by a single layer of follicular cells.
• Before puberty, there are only primordial follicles.
• After puberty, the ovarian cycle begins under the influence of FSH.
• At the beginning of each cycle, a few primordial follicles begin to mature.
• Only one follicle becomes a tertiary follicle.
• Primodial follicles transition to primary follicles.
• Primary follicles transition to secondary follicle.
• Secondary follicles transition to tertiary or Graafian follicle during ovulation and mature into the corpus luteum.
• Follicular cells divide to form granulosa cells.
• Granulosa cells and the oocyte secrete a layer of glycoproteins on the surface of the oocyte, forming the zona pellucida.
• The ovarian connective tissue surrounding granulosa cells (stromal cells) forms the theca folliculi.
• The theca folliculi are organized into an inner layer of secretory cells, known as the interna, and an outer fibrous capsule called the theca externa.
• As part of the Ovarian Cycle
  • FSH secreted by the anterior pituitary causes maturation of follicles.
  • Thecal gland secretes estrogen.
  • When blood estrogen levels go up, FSH secretion stops and LH surge occurs.
• After ovulation, granulosa cells and the cells from the theca interna are vascularized by the surrounding vessels.
• Influenced by LH, these cells develop a yellowish pigment and transform into lutein cells, forming the corpus luteum.
• The corpus luteum secretes estrogens and progesterone.
  • In the event that the oocyte is unfertilized, the corpus luteum degenerates and becomes the corpus luteum of menstruation.
  • If the oocyte is fertilized, the corpus luteum persists and becomes the corpus luteum of pregnancy.