The human life cycle Chapter 16-3

What are the stages of human development that occur before and after birth?

The zygote Once the zygote is formed it takes about 4 days for it to reach the uterus. The zygote then divides into 2 cells, then 4, then 8 and so on until it becomes an embryo; a hollow ball made up of hundreds of cells.

Zygote

Cell Differentiation DNA directs changes in cells from a general cell type (embryonic stem cells) to specialized cell types in the adult Until recently, scientists thought that once cells became specialized in the adult, the DNA could not be reprogrammed back to its undifferentiated state Based on: Scientific American, June 2004

The embryo Once the embryo attaches itself to the uterus it will take 8 weeks to become a fetus. First the hollow ball will grow inward forming new membranes.

Embryo

Membranes What is the amniotic sac? A membrane which surrounds the embryo that is filled with amniotic fluid to cushion and protect the developing baby.

Membranes What is the placenta? The link between the embryo and the mother. The blood vessels align with the mother’s so the embryo can receive nutrients, O2, and other important substances. It removes CO2 and other wastes. Next, an umbilical cord forms between the placenta and the embryo. It also contains blood vessels that link the embryo to the mother.

4 weeks Yolk sac: produces baby’s RBCs & delivers nutrients until the placenta takes over

5 weeks Embryo is about the size of a sesame seed

6 weeks The baby’s heart is beating! It will soon divide into the left & right chambers.

7 weeks Eyelid folds cover the baby’s eyes Tailbone: what looks like a tail is the formation of the tailbone The baby’s intestines begin to grow into the umbilical cord

8 weeks

9 weeks The embryo becomes a FETUS! The baby is the size of a kidney bean!

The fetus On week 9 the embryo becomes a fetus until birth. The brain is developing rapidly. The eyes are developing. It has fingers and toes. On week 12 the fetus is about 9 cm long and has a mass of about 26g.

The fetus Between week 13 and week 24 the tissues develop into recognizable shapes. Bones become distinct. A heart beat can be heard. A layer of soft hair grows over the skin. Muscles are growing. It is now about 20 cm long and has a mass of 700g.

13 weeks Baby has its own set of unique fingerprints; The head is more proportional to the body

Week 16 The circulatory system is fully developed; the heart pumps about 25 quarts of blood a day

The fetus Between week 25 and 36 the fetus prepares to live outside the mother’s body. The brain surface develops grooves and ridges. The lungs can carry out the exchange of O2 and CO2. The eyelids can open. It is now about 40cm long and has a mass of about 3kg. After about 36 weeks the baby is ready to be born.

Week 24 The mother’s uterus is about the size of a soccer ball; the baby’s lungs are developing the bronchial tubes; baby can hear well & other senses like taste are being developed.

Six Months (weeks 25 – 29) The baby weighs about a pound and a half. His wrinkled skin is starting to smooth out as he puts on baby fat.

25 weeks

Seven Months (30 – 33 weeks) By now, the baby weighs about 3 pounds and is more than 15 inches long. She can open and close her eyes and follow a light.

Eight Months (34 – 37 weeks) The baby now weighs about 4 3/4 pounds. His layers of fat are filling him out, making him rounder, and his lungs are well developed.

36 weeks

Nine Months (38 – 41 weeks) The average baby is more than 19 inches long and weighs nearly 7 pounds now, but babies vary widely in size at this stage

40 weeks

Birth What are the 3 stages of birth? Labor - strong muscular contractions of the uterus, that cause the cervix to enlarge, so the baby can fit through the opening. Delivery - during this stage the baby is pushed out of the uterus, through the vagina, and out of the mother’s body. Then the umbilical cord is clamped and cut, separating the baby from the mother. Afterbirth- about 15 minutes later contractions push the placenta and other membranes out of the uterus though the vagina.

Multiple Births What are the two types of twins? Identical twins- come from on zygote that splits into two identical embryos. They have the same inherited traits and are the same sex. Fraternal twins- develop from two different zygotes and may or may not be the same sex.

Infancy From birth to two years of age the human nervous and muscular systems become better coordinated. Birth - 3 months: the human can cry, suck, yawn, and blink. 3 - 7 months: it can hold its head up and reach for objects. 7 - 10 months: it can crawl. 10 -16 months: it begins to walk by itself and speak it’s first word. 16- 24 months: it can understand simple directions, play with toys, and feed itself.

Childhood From 2 to about 12 years of age the human becomes more active and independent. 3 - 6 years: the human learns to share and play with others, carry conversations, and express themselves clearly. 7 - 10 years: it thinks and cares more for others, friends become important. 10 -12 years: the human develops the need to fit in with others of their age group.

Adolescence The stage of development when children become adults physically and mentally. 9 -14 years: the human enters puberty, the period of sexual development in which the body becomes able to reproduce. 13 - 15 years: the teenager gradually becomes able to think and reason like an adult. Consider consequences. Problem solving-skills improve. Develop specific interests.

Adulthood Being grown up or mature. It is difficult to define adulthood. 16 years old: can start driving. 18 years old: can vote and join the armed forces. 21 years old: can consume alcohol. About 30 years old: aging begins. 45 - 60 years old: skin becomes wrinkled, eyes loose ability to focus on close objects, hair looses color, muscle strength decreases. 65 - Death: less efficient organ action. Adults can slow the aging process by maintaining a healthy diet and exercising consistently.

Credits http://www.babycenter.com/pregnancy-fetal-development-index