Chapter 28 Human development

Question 1

Define the following terms:

1. Zygote
2. Conceptus
3. Embryo
4. Fetus

Answer 1

1. Fertilized egg
2. Developing offspring
3. Fertilization to 8 weeks
4. 9 weeks to birth

Question 2

Where does sperm penetration occur?

Answer 2

Sperm swim to uterus and are helped by contraction to the uterine tube where they penetrate the egg. First sperm are helping out, later sperm are more likely to fertilize. Only one sperm will fertilize (monospermy)

Question 3

What is the process in which the sperm penetrates the egg?

Answer 3

1. Aided by enzymes, sperm weave there way around granulosa cells. 2. Many sperm release acrosmal enzymes to digest holes in the zona pellicuda clearing a path to the oocyte membrane. 3. Sperm binds to oocyte sperm-binding receptors. 4. Sperm and oocyte plasma membrane fuse, sperm enters oocyte. 5. a) Na+ influx temporarily blocks other sperm from entering ooctye. b) cortical reaction - sperm receptors are clipped off and zona pellucida hardens.

Question 4

Explain the two types of blocking once a sperm has reached the oocyte

Answer 4

1. Fast block polyspermy. When a sperm reaches an oocyte, Na+ channels open releasing Na+ into the oocyte. This causes membrane potential to chance preventing other sperm from entering. This is a temporary block.
2. Slow block polyspermy- cortical reaction. Ca2+ surge causes release of enzymes into space below zona pellucida. Receptors are destroyed. Other bound sperm are detached and the zona pellucida hardens.

Question 5

Explain blastocyte formation

Answer 5

secondary oocyte completes meiosis 2, ovum and sperm nuclei for pronuclei. pronuclei ruptures, chromosomes combine forming a zygote. zygote undergoes mitosis for growth and development. forms a multicellular hallow cell called a blastocyte. Blastocyte has trophocytes which are flattened outer sphere cells– implantation and placenta formation. the inner cell mass is the embryo and endometrial cells nourish blasocyte. Implantation is when blastocyte attaches to endometrial wall and becomes an embryo. happens at 8-12 days after fertilization. the blastocyte produces hCG (human chorionic gonadotropin) which maintains the corpus luteum until the placenta can produce estrogen and progesterone.

Question 6

what is it called when embryo implants somewhere other than the uterus? where does this usually occur?

Answer 6

ectopic pregnancy. usually occurs in the fallopian tubes or abdomen

Question 7

What are the three primary germ layers formed during months 1 and 2 of embryonic development?

Answer 7

ectoderm= forms epidermis of skin and nervous system
mesoderm= skeleton, muscles and organs
endoderm= lining of digestive tract and respiratory tract.
after 2 months fetus= organs and organ system develops

Question 8

what are the extraembryonic membranes?

Answer 8

Yolk sac= forms part of guts and early vascular tissue
amnion= surrounds the fetus and is fluid filled
allantois= forms umbilical vessels
chorion- outer layer= contributes to placenta

Question 9

formation of placenta

Answer 9

chorion develops projections called chorionic villi. They have blood vessels and inter-digitates with vascularized endometrium. Maternal and fetal blood supplies never mix. they are connected by an umbilical cord which as two arteries for deoxygenated blood to the placenta and one vein for oxygenated blood to the fetus.

Question 10

What is the function of the placenta?

Answer 10

provides nutrients, antibodies, and oxygen, and removes carbon dioxide and wastes. the placenta responds to and produces hormones and can also transfer drugs and toxins.

Question 11

Describe the first trimester

Answer 11

1-3 months. zygote develops into an embryo. it has a large head but the body will catch up. by 7 weeks it has a heart beat and eye and limb buds form. all body systems are present.

Question 12

Describe the second trimester

Answer 12

4-6 months. organ systems increase in complexity, bones are distinct, movement can be felt, and is in fetal position

Question 13

Describe the third trimester

Answer 13

7-9 months. large increase in weight, all major organs are functioning except lungs, myelination of spinal cord begins and ossification.

Question 14

What is pregnancy called?

Describe the first trimester

Answer 14

Gestation.
1-3 months. zygote develops into an embryo. it has a large head but the body will catch up. by 7 weeks it has a heart beat and eye and limb buds form. all body systems are present.

Question 15

how does labour start and what are the 3 stages?

Answer 15

cervical stretch triggers oxytocin and causes contractions.
1. dilation: Cervix dilates and fetus moves towards cervical canal. 8-24 hours
2. Expulsion: cervix completes dilation and fetus emerges. takes about an our with increasing muscle contractions.
3 Expulsion of placenta: 15 mins.