chapter 12

Question 1

describe the process of fertilisation

Answer 1

• when male ejaculates, sperm are released in vagina at entrance of uterus (insemination)
• sperm travel through cervix and body of uterus to uterine tubes, quickly reaching upper portion of uterine tubes. (through both swimming motion and muscular contractions of uterus and uterine tubes).
• at same time as step above, contractions of uterine tubes and beating of cilia transport ova towards uterus after ovulation.
• fertilisation occurs in uterine tubes when ovum is about 1/3 of way down the tube.
• head of sperm contains an enzyme capable of breaking down the acid holding corona radiata together. needs to be several thousand sperm.
• one sperm penetrates corona radiata and once through it encounters the zone pelluda.
• imitates acrosomal reaction, rereleasing digestive enzymes which break down the glycoprotien matrix giving sperm access to plasma membrane of oocyte.
• when plasma membrane of oocyte and spermatozoa fuse, the nucleus of sperm enters the ovum.
• entrance of one sperm into secondary oocyte stimulates the formation of a fertilisation membrane around the oocyte, preventing the entrance of anymore sperm.

Question 2

what happens once sperm has entered the oocyte

Answer 2

the tail is absorbed and head moves through cytoplasm in the form of the male pronucleus. entrance of sperm stimulates secondary oocyte to complete the second meiotic division .

the nucleus of the oocyte develops into the female pronucleus, fusing with male pronucleus to form a single nucleus.

fertilisation is complete and fertilised oocyte is called a zygote.

Question 3

what is blastocyst?

Answer 3

a hollow ball of cells that surround a cavity filled with fluid.

at one side of cavity is a group of about 30 cells called inner cell mass.

Question 4

explain inner cell mass

Answer 4

composed of stem cells that will differentiate into the different body cells to form the embryo.

Question 5

explain implantation

Answer 5

• blastocyst remains free within cavity of uterus for 2-3 days and then sinks into soft endometrium to become firmly attached to the wall of uterus.
• process enables blastocyst to gain nourishment for growth and development by absorbing nutrients from glands and blood vessels of uterine lining.

Question 6

what type of stem cell is the fertilised oocyte?

Answer 6

totipotent.

can create any type of cell necessary

Question 7

what type of stem cell are the cells of the inner cell mass?

Answer 7

pluripotent stem cells.

they’re able to give rise to many, but not all, cell types necessary for foetal development.

Question 8

what are the three primary germ layers?

Answer 8

ectoderm, mesoderm and endoderm

Question 9

explain ectoderm

Answer 9

the outermost germ layer. this forms the outer layers of the body such as the skin, hair and mammary glands as well as nervous system.

Question 10

explain the mesoderm

Answer 10

the middle germ layer. skeleton, muscles, connective tissue, heart, blood and urogenital tract form from mesoderm.

also allows formation of stomach and intestines.

Question 11

explain the endoderm

Answer 11

the innermost germ layer. forms lining of digestive system as well as lungs and thyroid.

Question 12

what are the 4 embryonic membranes?

Answer 12

amnion, chorion, yolk sack and allantois.

Question 13

explain the amnion

Answer 13

the first membrane to develop. by 8th day after fertilisation it surrounds the embryo, enclosing a cavity to which it secretes amniotic fluid.

the amnion expands as growth takes place. it usually ruptures just before childbirth, releasing amniotic fluid.

Question 14

explain what amniotic fluid serves as

Answer 14

to protect the embryo against physical injury by acting as a shock absorber, regulates temperature and allows foetus to move freely.

Question 15

explain the chorion

Answer 15

formed from outer cells of blastocyst together with a layer of mesodermal cells.

chorion surrounds the embryo and other three membranes. as amnion enlarges it fuses with inner layer of the chorion.

eventually chorion becomes main part of placenta.

Question 16

explain the yolk sack and allantois

Answer 16

not as important, however, they do form the outer structure of umbilical cord.

Question 17

what is the placenta?

Answer 17

an organ that forms both foetal and maternal tissues during the first three months of gestation.

it supplies nutrients to, and removes wastes from, the foetus.

it also serves as an endocrine organ, producing a number of the hormones necessary to maintain pregnancy.

Question 18

explain the development of the placenta

Answer 18

• foetal part of placenta begins forming when blastocyst is implanted in endometrium
• chorionic villi (contain numerous blood vessels) grow into endometrium. they become surrounded by pools of mothers blood.
• in this way the foetal and maternal blood do not mix as there are a few layer of cells separating the two blood supplies.
• the exchange of materials takes place by diffusion and active transport. o2 and nutrients diffuse from mothers blood into foetal blood and wastes diffuse from foetal into mothers blood.