Topic 2.4 Sexual reproduction in mammals

Question 1

Name the two types of gametogenesis

Answer 1

• Spermatogenesis to produce spermatozoa
• Oogenesis to produce ova

Question 2

What are primordial germline cells?

Answer 2

Diploid precursors to gametes

Question 3

Outline the stages of spermatogenesis

Answer 3

Primodial germline cell -> spermatogonia -> primary spermatocytes -> secondary spermatocytes -> spermatids -> 4 spermatozoa

Question 4

How do primary spermatocytes form?

Answer 4

Primordial germline cells of seminiferrous tubules of testes divide by mitosis, forming spermatogonia.

Growth period of spermatogonia without futher division forms primary spermatocytes.

Question 5

How do spermatids form?

Answer 5

Primary spermatocytes undergo meiosis to form 4 undifferetiated hapoid cells.

Meiosis I: diploid secondary spermatocytes

Meiosis II: haploid spermatids

Question 6

How do spermatazoa form?

Answer 6

Spermatids differenciate to gain flagellum, acrosome and many mitochrondria.

Question 7

Outline the stages of oogenesis

Answer 7

Primordial germ cell -> oogonia -> primary oocyte -> secondary oocyte and polar body -> ootid and polar bodies -> 1 ovum

Question 8

How do primary oocytes form?

Answer 8

Primordial germline cells in ovaries (and oviduct) divide by mitosis, forming oogonia.

Growth period of oogonia without further division forms primary ooytes, which remain in prophase I in follicles until puberty.

Question 9

What happens during the first meiotic division of oogenesis?

Answer 9

1 secondary oocyte with all the cytoplasm forms.

1 polar body buds off and sticks to the oocyte.

Question 10

What happens during the second meiotic division of oogenesis?

Answer 10

Secodary oocyte divides into haploid ootid and polar body.

Polar body divides in 2.

Overall 3 polar bodies form. They degenerate as the ootid develops.

Question 11

How does an ovum form?

Answer 11

Meitotic division in utero halts at prophase. Fertilisation triggers influx of Ca²⁺. This stimulates meiosis II to form ovum.

Question 12

Describe the structure of the ovum.

Answer 12

Outer layer of follicle cells.

Zona pellucida: coating above cytoplasm prevents polyspermy and hardens when cortical granules release chemicals.

Haploid nucleus: fertilisation restores diploid chromosome number.

Question 13

Describe the stucture of a spermatozoon

Answer 13

• Acrosome
• Nucleus
• Spiral-shaped mitochondrion
• Flagellum bound by plasma membrane

Question 14

Explain what happens during fertilisation

Answer 14

• Enzymes in acrosome of sperm head digest zina pellucida
• Sperm head fuses with ovum cell membrane, allowing sperm nucleus to enter.
• Cortical reaction causes zona pellucida to harden, preventing polyspermy
• Nuclei fuse, forming a diploid zygote

Question 15

Outline the stages of early embryo development

Answer 15

Zygote -> morula -> blastocyst -> implantation

Question 16

How does a blastocyst form?

Answer 16

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

Morula divides further then undergoes differentiation and cavtiy formation.

Question 17

Describe the structure of the blastocyst

Answer 17

Inner cell mass develops in embryo.

Outerlayer (trophplast) develops into placenta.

Fluid filled cavity (blastocoele).

Question 18

What happens during implantation?

Answer 18

1. Blastocyst breaches surrounding jelly coat, so it can attach to endometrium
2. Release of autocrine hormones and digestive enzymes triggers implantation into uterine wall
3. Growing embryo gains nutrients and oxygen from endometrial tissue fluid