Development and Inheritance

I. The gradual modification of physical and physiological characteristics during the period from conception to physical

maturity is called development. A. An overview of topics in development

 1. Development involves 2 things- the division and differentiation of cells and the changes that produce and modify anatomical structures.

 2. Development begins with fertilization or conception.

3. Development is divided into periods characterized by specific anatomical changes.

 4. Embryological development considers the events that occur in the first 2 months after fertilization.

 5. The study of these events in the developing organisms or embryo is called embryology.

6. After 2 months of development the embryo becomes a fetus.

 7. Fetal development begins at the start of the ninth week and continues to birth.

 8. Together, embryological and fetal development is referred to as prenatal development.

  9. Postnatal development begins at birth and continues to maturity, when the aging process begins.

 10. Inheritance refers to the transfer of genetically determined characteristics from generation to generation.

11. Genetics is the study of the mechanisms responsible for inheritance.

B. Fertilization 

1. Fertilization involves the fusion of two haploid gametes to produce a zygote with 46 chromosomes.

C. An overview of prenatal development

 1. The time spent in prenatal development is known as the period of gestation, or pregnancy.

 2. Gestation occurs within the uterus over a period of 36-40 weeks.

3. Prenatal development is divided into 3 trimesters.

 4. The first trimester: all the basic components of each organ system appear.

5. The second trimesters: the development of organs and organ system.

 6. Third trimester is fetal growth.

D. The first trimester 

1. By the end of the first trimester the fetus is about 3 in long and weighs about .5 oz.

 2. Four general events occur in the 1st trimester: cleavage, implantation, placentation and embryogenesis.

3. Cleavage is a series of cell divisions that subdivided the cytoplasm of the zygote.

4. The first cleavage division produces two identical cells called blastomeres.

 5. The first division is completed in about 30 hours after fertilization.

6. After 3 days of cleavage, the embryo is a solid ball of cells, this stage is called the morula.

 7. Over the next 2 days, the blastomeres form a blastocyst, a hollow ball with an inner cavity known as the blastocoele.

8. See table 20.1 the fates of the primary germ layers.

E. At fertilization, the zygote is still 4 days away from the uterus.

 1. It arrives in the uterine cavity as a morula, and over the next 2-3 days, blastocyst formation occurs.

 2. Implantation begins as the surface of the blastocyst closest to the inner cell touches and adheres to the uterine lining.

3. If fertilization occurs anywhere else in the body it is called an ectopic pregnancy.

F. Formation of the extra embryonic membranes.

 1. The first extra embryonic membrane to appear is the yolk sac.

 2. The yolk sac is present 10 days after fertilization. The yolk sac becomes an important site of blood cell formation.

3. The amnion is composed of ectoderm and mesoderm.

 4. It encloses fluid that surrounds and cushions the developing embryo and fetus.

  5. The chorion is created .

G. Placentation 

1. The placenta is a temporary structure in the uterine wall that provides a site for diffusion between the fetal and maternal circulatory systems.

2. Placentation or placenta formation takes place when blood vessels form in the chorion.

 3. The fetus remains connected to the placenta by the elongate umbilical cord, which contains the allantois, umbilical blood vessels and the yolk sac.

H. Placental Hormones 

1. The placenta acts as an endocrine organ.

 2. The placental hormones produced

include human chorionic gonadotropin (hCG), progesterone, estrogen, human placental lactogen, placental prolactin and relaxin.

 

3. hCG appears in the maternal bloodstream soon after implantation has occurred.

 4. The presence of hCG in blood or urine samples is a reliable indication of pregnancy.

5. hCG maintains the uterine lining and as a result menstruation does not occur.

6. If there is no hCG, a woman would get her period.

 7. Human placental lactogen (hPL) and placental prolactin helps prepare the mammary glands for milk production.

8. Relaxin is a hormone secreted by the placenta.

 9. Relaxin (1) increases the flexibility of the pubic bones allowing the pelvis to expand during delivery. (2) causes the dilation of the cervix, making it easier for the fetus to enter the vaginal canal. (3) suppresses the release of oxytoxin by the hypothalamus and delays the onset of labor contractions.

I. Embryogenesis

1. The process of forming an embryo is called embryogenesis.

 2. It begins as folding and differential growth.

3. The first trimester is a critical period for development, because events during the first 12 weeks establish the basis for organogenesis, the process of organ formation.

 SEE PAGE 624-625!!!!!!

II. The second and third trimesters

A. By the end of the first trimesters, the rudiments of all the major organ systems have formed.

1. Over the next 3 months, the fetus will grow to a weight of about 1.4 lbs.

 

2. During the second trimester, the fetus, encircled by the amnion, grows faster than the surrounding placenta.

3. During the third trimester, the basic components of all the organ systems appear, and most become ready to fulfill their normal functions.

4. The rate of growth starts to decrease, but the third trimester is when the baby gains the most weight.

 5. In the last 3 months of gestation, the baby gains about 5.7lbs, for a full term weight about 7lbs.

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B. Pregnancy and maternal systems 

1. The developing fetus is totally dependent on the maternal organ system for nourishment, respiration and waste removal.

 2. These functions must be performed by maternal systems in addition to their normal operations.

3. The mother must absorb enough oxygen, nutrients and vitamins for herself and her fetus and she must eliminate all of the generated wastes.

 4. The demands grow as the baby grows

C. Major changes that take place in maternal systems include the following:

 1. The maternal respiratory rate goes up and the tidal volume increases. Mother’s lungs obtain extra oxygen and are required to remove the excess carbon dioxide by the baby.

2. The maternal blood volume increases. This increase is because (1) blood flowing into the placenta reduces the volume in the rest of her body. (2) baby changes the chemical make up of the blood. BY the end of gestation, the maternal blood volume has increased by almost 50%!

3. The maternal requirements for nutrients and vitamins climb 10-30%.

 4. The maternal glomerular filtration rate increases by 50%. (urinating)

 5. The uterus undergoes an increase in size.

6. The mammary glands increase in size and secretory activity begins. BY the end of the 6th month, the mammary glands are fully developed.

D. Structural and functional changes in the uterus.

 1. At the end of gestation, a typical uterus will have grown from 3 inches in length and 2 oz in weight to 12 inches in length and 2.4 lbs.

 2. It may then contain almost 5 liters of fluid so the organ with all contents has a total weight of about 22lbs! (without baby)

3. The uterus is muscle and stretches as it grows.

 4. In the early stages of pregnancy, the contractions are weak, painless and brief as the uterus grows.

 5. Progesterone released by the placenta prevents stronger contractions.

6. Three major factors oppose the calming action of progesterone.

 -Rising estrogen levels: Estrogen, also produced by the placenta makes contractions more likely.

 - Rising oxytocin levels: Stimulates an increase force and frequency of uterine contractions.

 -Prostaglandin production: Also stimulates uterine contractions.

7. After 9 months of gestation (36-40 weeks) multiple factors interact to produce labor contractions.

 8. Once begun, positive feedback ensures that contractions continue until delivery has been completed.

III. Labor and Delivery-forcible expulsion of the fetus is called parturition.

A. The stages of labor 

1. The dilation stage: begins with the onset of labor, the cervix dilates and the baby begins to slide down the cervical canal. This is very variable in length but takes ABOUT 8 or more hours. At the start of labor contractions are every 10-30 minutes but their frequency and intensity increases. Late in the process the amniotic sac breaks, or “water breaks.

2. The expulsion stage: begins with the cervix pushed open by the baby, dilates completely. Expulsion continues until the baby is out. Lasts usually less than 2 hours. The birth of the baby is called delivery. If the vagina is too small an episiotomy may be performed. If the baby cannot be delivered vaginally a C-section will be performed.

episiotomy

http://obgyn.healthcentersonline.com/pregnancybasics/pregnancybasics.cfm

3. Placental stage: Muscle tension builds in the wall of the empty uterus and the organ decreases in size. Usually within an hour after delivery, the placental stage ends with the ejection of the placenta, or after birth. There could be as much as 500-600ml of blood lost, but the increase of maternal blood through pregnancy helps out.

 See page 629.

B. Premature labor 

1. Premature labor occurs when labor contractions begin before the baby has completed normal development.

 2. Most babies are born between 25-27 weeks have a high risk of developmental problems.

3. A premature delivery usually refers to babies born 26-36 weeks. But these babies have a good chance of normal development.

C. Multiple births 

1. Multiple births can occur for several reasons.

2. 1 out of 89 births are multiples. 

3. About 70% of twins are fraternal or dizygotic.

4. Identical twins or monozygotic twins are less common.

Faternal Twins

IV. Postnatal development

A. In postnatal development, each individual passes through a number of life stages: neonatal, infancy, childhood, adolescence, and maturity.

 1. The neonatal, infancy and childhood period

2. The neonatal period extends from the moment of birth to 1 month thereafter.

 3. Infancy then continues to 2 years of age.

 4. Childhood lasts until puberty starts.

5. Two major events are under way during these developmental stages.

 a. The major organ systems, other than associated with reproduction, become fully operational and gradually acquire the functional characteristics of adult structures.

 b. The individual grows rapidly, and body proportions change significantly.

6. Pediatrics is medical specialty focusing on postnatal development from infancy through adolescence.

C. The neonatal period 

1. Physiological and anatomical changes occur as the fetus completes the transition to the status of a newborn infant or neonate.

 2. Typical heart rate of 120-140 beats per minute and respiratory rates of 30 breaths per minutes is normal for them.

3. The transition from fetus to neonate can be summarized as follows:

a. The lungs at birth are collapsed and filled with fluid.

 b. When the lungs expand, the pattern of cardiovascular circulation changes due to alternations in blood pressure and flow rates.

 c. Before birth, the digestive system remains relatively inactive, although it does accumulate a mixture of bile secretions, mucus and epithelial cells.

d. As waste produces build up in the arterial blood, they are excreted out the kidneys.

 e. The neonate has little ability to control body temperature, particularly in the first few days after delivery.

4. By the end of the 6th month of pregnancy the mammary glands are fully developed.

 5. The first few days after delivery the glands will secrete colostrum, this contains more proteins and less fat than breast milk. Many of the proteins are antibodies.

6. As colostrum production declines, the mammary glands will start to produce milk.

 7. The most rapid growth occurs during prenatal development, and the rate of growth declines after delivery.

8. Postnatal growth during infancy and childhood occurs under the direction of hormones.

 9. Adolescence begins at puberty, the period of sexual maturation and ends when growth is complete.

a. The hypothalamus increases its production of gonadotropin releasing hormone (GnRH)

 b. Because of GnRH, levels of FSH and LH rise.

 c. This causes the formation of gametes, production of sex hormones, and an increase of growth rate.

THE END