2.4 Sexual reproduction in mammals

Question 1

Define gametogenesis

Answer 1

The process by which gametes are produced. In mammals egg cells are produced by oogenesis and sperm cells are produced by spermatogenesis.

Question 2

What is the germinal epithelium

Answer 2

A single layer of cells (epithelium) that undergoes regular mitosis producing other cells.

Question 3

What are the three phases which gametogensis in females and males share?

Answer 3

• multiplication (cells present in epithelium layer divide by mitotic division to produce many cells capable of becoming gametes.)
• growth (these cells undergo growth)
• maturation (involves meiosis and results in the formation of haploid gametes.)

Question 4

outline spermatogenesis

Answer 4

• germinal epithelium cell (2n) of seminiferous tubule multiply (repeated mitotic division) forms
• spermatogonia (2n) which grow to form primary spermatocytes (2n)
  **- **primary spermatocytes (2n) undergo the first mitotic division (meiosis I) to form secondary spermatocytes (n)
  **- **secondary spermatocytes (n) 2.4 undergo the second meitoic division (meiosis II) to form spermatids (n)
  **- **spermatids (n) grow and differentiate into mature spermatazoa

stared = maturation phase

Question 5

what are spermatids?

Answer 5

haploid cells
sperm without acrosome or flagellum

Question 6

outline oogenesis

Answer 6

• primordial germ cells/ germinal epithelium cells (2n) of ovary divide several times to form oogium (2n) (multiplication)
• only one oogonium (2n) continues to grow to form a primary oocyte (2n)
• The first meiotic division forms one secondary oocyte (n) and one polar body
• The second meiotic division forms one haploid ootid and one polar body
• The second meiotic division of the 1st polar body forms two polar bodies
• ## The polar bodies degenerate and die as the ootid develops.

Question 7

Describe the stoppinhg and starting of oogenesis.

Answer 7

development until primary oocyte happens in utero
each mentrual cycyle a few primary oocytes start to develop and complete meiosis I
Meiosis II is halted at the prophase until arrival of sperm

Question 8

What happens at and after ovulation?

Answer 8

Mature ovarian follicle bursts open to release egg cell (secondary oocyte) surrounded by follicle cells.
The remains of the mature ovarian follicle now develops into endocrine gland (corpus luteum)
corpuus luteum degenerates quickly if fertilisation does not occur.

Question 9

Describe the structure of the mature ovum

Answer 9

• follicle cells form a protective coating around the egg
• has a zona pellucida (protective coating that the sperm have to penetrate in order for fertilisation to occur. It’s main prupose is to prevent polyspermy
• cortical granules release substances that cause the zona pellucida to harden forming a tough fertillisation membrane
• ovum contains a haploid nucleus so that a full set of chromosomes is restored at fertillisation

Question 10

Describe the structure of spermatozoa

Answer 10

• Contain many mitochondria to provide energy for rotation of flagellum which allows the cell to move
• contains an acrosome which contains digestive enzymes which breaks down the zona pellucida and allow sperm to penetrate the egg
• haploid nucleus allows for a full set of chromosomes to be restored at fertillisation

Question 11

Describe fertilisation in humans

Answer 11

• the sperm head contacts the zona pellucida and the acrosome reaction occurs- enzymes digest the zona pellucida as the acrosome fuses with the cell membrane of the sperm and releases the digestive enzymes
• the sperm head fuses with the cell membrane of the egg cell, thus allowing the sperm nucleus to enter the egg cell
• the cortical reaction occurs which causes the zona pellucida to harden and prevents polyspermy
• the nuclei fuse and a full set of chromosomes is restores, thus forming a diploid zygote.

Question 12

What is a blastomere?

Answer 12

As the zygote is moved down the oviduct cell divisions occur (cleavage divisions) and result in new cells called blastomeres.

Question 13

Describe what happens after fertilisation

Answer 13

Zygote moves down oviduct and cleavage divisions happen resulting in blastomeres forming. No growth occurs at this stage,. Blastomeres get progessively smaller.
After about 4 days the zygote has beconme a solid ball of tiny blastomeres.
After this, a blastocyst forms - instead of being a solid ball it forms a hollow ball of cells, it has an outer layer of cells, now called trophoblasts, an inner cell mass of blastomeres and a fluid-filled bastocoel.