11.4.7 Skills: Annotating Sexual Reproduction Diagrams

Annotating the Seminiferous Tubule

• The testes contain many small tubes, or tubules, known as seminiferous tubules
• The production of sperm, or spermatogenesis, takes place in the lining of the seminiferous tubules
• An annotated diagram of spermatogenesis in the lining of the seminiferous tubule should show the following events
  1. Cells of the germinal epithelium divide by mitosis to produce diploid spermatogonia cells
  2. Spermatogonia grow and differentiate into primary spermatocytes
  3. Primary spermatocytes enter meiosis I, dividing to produce secondary spermatocytes
  4. Secondary spermatocytes complete meiosis II, producing immature sperm cells known as spermatids
  5. Spermatids associate with nearby Sertoli cells as they grow and differentiate
     • Sertoli cells are sometimes known as nurse cells
  6. Mature sperm cells, or spermatozoa detach from the Sertoli cells and leave the seminiferous tubule

Annotating the Ovary

• The production of ova, or oogenesis, begins in the ovaries of the female foetus before birth
• An annotated diagram of oogenesis taking place inside the ovary should show the following events
  1. During the first 7 months of foetal development, cells of the germinal epithelium divide by mitosis to produce diploid cells called oogonia
  2. During the second half of pregnancy a layer of follicle cells develops around the oogonia to form a primary follicle, which then enters meiosis I
     • Note that at this point the process pauses with meiosis I still incomplete
  3. Hormones released from puberty up until the menopause stimulate the growth of several primary follicles each month
     • The hormone that stimulates a primary follicle to mature is FSH
  4. One of these developing primary follicles completes meiosis I, dividing to form a single haploid secondary oocyte
     • Note that the second daughter cell produced during meiosis I receives very little cytoplasm and is known as a polar body
  5. The secondary oocyte enters meiosis II and leaves the ovary in the process of ovulation
     • Note that meiosis II will be completed and an ovum formed after a sperm cell enters the secondary oocyte
  6. The remains of the follicle develops into a corpus luteum, which degenerates if fertilisation does not take place

Most of the oogenesis process takes place in the ovaries

Annotating the sperm

• Sperm and ova are examples of specialised cells, meaning that their structure aids their function
• Special features of sperm cells that relate to function include
  • A haploid nucleus that can fuse with an ovum nucleus to form a diploid zygote
  • An acrosome containing digestive, or hydrolytic, enzymes to aid entry into the ovum
  • Many mitochondria for the release of energy to aid movement
  • A tail made of protein microtubules to aid movement

Sperm cells are specialised to enable movement towards and entry into the ovum

Annotating the Egg

• Special features of ova that relate to function include
  • A haploid nucleus that can fuse with a sperm cell nucleus to form a diploid zygote
  • A surrounding jelly layer, or zona pellucida, that can harden to prevent polyspermy (when the ovum is penetrated by more than one sperm, this can effect embryo development)
  • A series of vesicles, or cortical granules, containing digestive enzymes that are released into the zona pellucida to prevent polyspermy
  • A cytoplasm rich in nutrients for the developing embryo after fertilisation

Ova are specialised to prevent polyspermy and aid development of the embryo