bio 169 human development and inheritance chapter 29

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BIO 169

HUMAN DEVELOPMENT

AND INHERITANCE

CHAPTER 29

created by Dr. C. Morgan

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TOPICS

Introduction and Overview

Fertilization

The First Trimester

The Second and Third Trimesters

Labor and Delivery

Postnatal Development

Genetics, Development, and Inheritance

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Introduction and Overview Objectives

Define development.

Review the process of differentiation.

Define fertilization.

Define growth.

Distinguish between embryonic and fetal development.

Distinguish between prenatal and postnatal development.

Define inheritance.

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Development is the gradual modification of a fertilized ovum into the functional anatomical structures present at maturity.

Differentiation is the selective activation of genes leading to cell populations that have distinct functions and form

distinct tissue types that comprise the various organs and organ systems.

Development involves generations of cell divisions and differentiation along with establishing new relationships of cells that form tissues.

Chemical signals induce the developmental changes resulting from new relationships among cells and tissues

Growth, an increase in size, is distinct from development.

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Fertilization is the fusion of the genetic material of two haploid gametes to produce a diploid cell.

Human embryological development is the time span from fertilization through 2 months of gestation.

Fetal development is the time span from the beginning of the third month until birth.

Prenatal development is the time span prior to birth.

Postnatal development is the time span from birth until maturity.

Inheritance is the transfer of genetic information from generation to generation.

Introduction and Overview (cont)

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TOPICS


Fertilization

The First Trimester


Labor and Delivery



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Fertilization Objectives

Describe what a sperm cell must do in order to fertilize an ovum.

Discuss the functional roles of the ovum and sperm.

Learn what is meant by capacitation.

Discuss the fate of sperm present in the ejaculate.

Using an illustration, describe the oocyte changes with fertilization.

Preview prenatal development.

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Fertilization

An ovulated ovum is surrounded by its plasma membrane and up to several layers of follicular cells (the corona radiata) which establish a barrier to sperm penetration.

Thus, sperm must secrete digestive enzymes that weaken the corona radiata to create an opening where the sperm cell is able to enter the ovum cytoplasm.

Since the only contribution of the sperm is its haploid set of chromosomes, all the organelles, cytosol, metabolic machinery, and nutrients along with the other haploid genome are provided by the ovum.

The cytoplasmic rich egg is 2000 times larger than a sperm.

Although sperm are motile when introduced into the vagina, they must undergo some biochemical changes (capacitation) before they are capable of fertilization.

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Fertilization (cont)

Of the approximately 100–200 million sperm in an ejaculate, only about 100 survive to make it to the ovum surface.

The collective enzymes present in the acrosome of these sperm are needed to weaken the corona radiata.

Only one sperm cell is permitted to fertilize an egg because as soon as it penetrates the cell membrane, a chemical reaction takes place in the zona pellucida to exclude additional sperm.

Fig. 1 a

Oocyte activation follows sperm entry as

metabolic activities of the ovum increase in preparation for the cleavage cell divisions.

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Fig. 1 b

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Fertilization is the first event of prenatal development also called pregnancy or gestation.

For purposes of discussion, pregnancy is considered in three month intervals or trimesters.

First trimester– embryonic and early fetal development

Second trimester– organs and organ systems complete development

Third trimester– rapid fetal growth and establishment of full functionality in organ systems

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TOPICS


Fertilization

The First Trimester


Labor and Delivery



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The First Trimester Objectives

Using an illustration, describe cleavage and blastocyst formation.

Using an illustration, describe the stages of the implantation process.

Discuss the processes that lead to the germ layers.

Describe the formation and purpose of the extraembryonic membranes.

Discuss the establishment of the placenta.

Illustrate the events during early embryogenesis.

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The First Trimester

Implantation in uterine wallFig. 2

Cleavage and blastocyst formation

cleavage implantation

placentation embryogenesis

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The First Trimester (cont)

There are several generations of cleavage divisions that occur every 10 – 12 hours after fertilization.

After 5 days, there is a hollow ball called a blastocyst that contains an inner cell mass surrounded by cells that make up the trophoblast.

The inner cell mass forms the embryo.

By day 6, the blastocyst is in the uterus and its cells are able to obtain some nutrients from the fluid secreted by the

glands of the endometrial functional zone.

By day 8, the trophoblast cells contact the endometrium, the initial step in implantation.

The surface trophoblast cells divide rapidly with some losing their cell membranes to create a multinucleate layer of cytoplasm called the syncytial trophoblast.

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The cellular trophoblast remains intact and in contact with cells of the inner cell mass.

The syncytial trophoblast digests its way into the endometrium until the entire structure is buried within the endometrium.

Implantation continues as the trophoblast digests more uterine tissue and grows around uterine capillaries, destroying capillary walls allowing maternal blood to flow through channels (lacunae) in the trophoblast.

Fig. 3 16

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Fig. 4

The inner cell mass separates from the trophoblast to create the amniotic cavity.

Villi extend further into the endometrium.

Inner cell mass forms the 2 layered

blastodisc.

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Gastrulation forms three “germ layers”

Fig. 4

TABLE 1: germ

layer fates

cells migrate

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Ectoderm:

integumentary system

nervous system

pituitary gland, adrenal medullae

mucous epithelium of nasal passages, mouth, anus

salivary glands

Mesoderm:

skeletal system

muscular system

cardiovascular system

urinary system; adrenal cortex

lymphatic system

gonads and ducts

all connective tissue linings

Endoderm:mucous epithelium of digestive tract, liver, pancreas, urinary bladder, gamete stem cells and distal reproductive ducts, thymus and thyroid glands

selected germ layer derivatives (TABLE 1)

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All organs systems are derived from cells of the germ layers.

Outside the embryo are four extraembryonic membranes which are also derived from the germ layers.

*Mesodermal cells migrate around the endodermal pouch under the blastodisc to form the yolk sac (site of early blood cell formation; becomes part of umbilical cord).

*Mesodermal cells migrate around the inner cellular trophoblast to form the chorion (forms placental tissue).

*Mesodermal cells migrate around the outside of the amnionic cavity to form the amnion (forms a cavity containing a protective fluid that surrounds embryo).

*An endodermal extension surrounds the mesoderm to form the allantois (base becomes part of bladder and remainder part of the umbilical cord).

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Fig. 5 a

nutrient source

TABLE 2: Summary of Prenatal Development

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Fig. 5 b

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Fig. 5 c

Chorionic villi with embryonic blood vessels extend

further into endometrium

placentation

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Fig. 5 d

nutrients are obtained by

transport from maternal

blood in the placenta into

the embryonic vasculature

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Fig. 5 e

Paired umbilical arteries and one vein

are part of the umbilical cord

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Fig. 6

uterus with embryo removed

Placental structure

active and passive

transport

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The placenta is also an endocrine organ with hormones secreted by the syncytial trophoblast cells into the

maternal blood.

Human chorionic gonadotropin (hCG) appears soon after implantation (basis for pregnancy test) with the role of maintaining the corpus luteum.

Human placental lactogen (hPL) and placental prolactin target mammary glands.

Relaxin, from the placenta and corpus luteum, causes the pubic symphysis to become flexible, dilates the cervix, and suppresses oxytocin secretion to delay labor.

Placental progesterone maintains the endometrium.

Placental estrogen (3rd trimester) helps trigger labor.

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vertebraemuscles

Fig. 8 a

Embryogenesis

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Fig. 8 b

fiber-optic camera image

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Fig. 8 c

fiber-optic camera image

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Fig. 8 d

Table 2

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TOPICS


Fertilization

The First Trimester


Labor and Delivery



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The Second and Third Trimesters Objectives

Characterize the fetal changes during the second and third trimesters.

Discuss the major changes involving maternal physiology.

Describe the structural and functional changes in the uterus including hormonal interactions.

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The rudiments of all organ systems are established in the first trimester so the second and third trimesters are a time of growth and establishment of physiological function.

The maternal organ systems must meet all the nutritional, respiratory, and waste disposal needs of the fetus.

To do this, maternal systems must make far reaching adjustments which include the following:

*increased respiratory rate and tidal volume

*increased blood volume

*increased nutritional and vitamin requirements

*up to a 50% increase in filtration through the kidneys

*increase in both the uterine and mammary gland size

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16 week fetus

Fig. 10 a

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displacement of visceral

organs by fetus

Fig. 10 c

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The Second and Third Trimesters (cont)

Placental progesterone inhibits contraction of the uterine smooth muscle until overpowered by estrogens.

The placental estrogens increase the sensitivity of the uterine smooth muscle.

Near the end of pregnancy, estrogen production increases.

The sensitive myometrium is stimulated to contract by rising levels of oxytocin released from the posterior pituitary.

Oxytocin release is triggered by circulating estrogens and activated stretch receptors in the cervix.

Late in pregnancy, uterine tissue produces prostaglandins which also help trigger uterine contractions.

Late in pregnancy, the fetus releases additional fetal oxytocin which probably initiates labor and delivery.

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The Second and Third Trimesters (cont)

Fig. 11

Initiation of labor

Relaxin acts on pubic symphysis and helps dilation

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TOPICS


Fertilization

The First Trimester


Labor and Delivery



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Labor and Delivery Objectives

Describe the events of the dilation stage.

Describe the events of the expulsion stage.

Describe the events of the placental stage.

Discuss premature labor.

Discuss multiple births.

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Labor and Deliver

The expulsion of the fetus is called parturition.

Smooth muscle contractions of the myometrium begin at the top of the uterus and proceed toward the cervix.

A positive feedback cycle involving oxytocin causes the contractions to become progressively stronger and more frequent to move the fetus toward the cervical canal.

Labor proceeds in three stages: dilation, expulsion, and placental.

*Contractions that last 30 seconds at 10 to 30 minute intervals cause dilation of the cervix.

It usually take 8 hours or more to complete dilation.

The amnion ruptures near the end of the dilation stage.

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Labor and Deliver (cont)

Fig. 12

Fully Developed Fetus

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Dilation StageFig. 12 a

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*Expulsion begins when the fetus has forced the cervix completely open and begins to pass through the

vagina.

Typically expulsion takes less than 2 hours.

If the vaginal opening is small, a cut through the perineal musculature (episiotomy) is made between the

vaginal and anal openings to prevent tissue tearing.

Delivery is arrival of the fetus into the outside world.

If complications are encountered or anticipated, a cesarean section surgical procedure can be performed

to remove the fetus through an abdominal incision.

*Due to the shrinking uterus, the placenta is dislodged and ejected within an hour along with substantial amounts of maternal blood (the placental stage).

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Placental StageFig. 12 c

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Premature labor occurs prior to completion of normal fetal development.

Survival of premature infants is related to their body weight.

Those weighing less than 400 g (14 oz) have little chance of survival while those weighing between 500 g and 1 kg have a 50% chance of survival (often with abnormalities).

Those weighing more than 1 kg have a very good chance of survival.

With the advent of fertility drugs, in vitro fertilization, and other mechanisms of manipulating the reproductive process, multiple births are on the rise.

Dizygotic twins are from separate eggs and sperm.

Identical twins result from separation of early cleavage cells

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TOPICS


Fertilization

The First Trimester


Labor and Delivery



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Postnatal Development Objectives

List the postnatal developmental stages.

Discuss lactation.


The postnatal developmental life stages are– neonatal, infancy, childhood, adolescence, and maturity.

*The neonatal period is from birth to one month thereafter.

Neonates have heart rates around 120 –140 and respiratory rates around 30 bpm.

Neonates who are breast fed first get a glandular secretion called colostrum which is high in protein (many are antibodies) and gradually milk after 2 to 3 days.

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Postnatal Development (cont)

Fig. 13

Milk let-down reflex

water

proteins

amino acids

lipids

sugars

salts

antibodies

750 C / liter

oxytocin triggers contraction of myoepithelial

cells

milk composition

Human placental lactogen and placental prolactin simulate mammary glands during gestation

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The most rapid growth is during fetal development and

thereafter it gradually slows.

Growth hormone, adrenal steroid hormones, and thyroid hormones control growth.

Growth does not occur uniformly in all cells and organ systems which results in a change in proportions over time.

50Fig. 14

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Infancy continues until 2 years of age.

Childhood lasts until adolescence.

Adolescence is the onset of puberty when sex hormones begin to exert their effects on target cells.

The growth rate accelerates when all of the circulating hormones appear at the same time.

Puberty occurs between the appearance of the secondary sex characteristics and completion of body growth (closure of the epiphyses).

Development ends at maturity but physiological changes occur throughout life.

Senescence is another name for aging (see TABLE 3).

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Note the changes in secretion of gonadotropins during life.

FSH and LH secretion

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TOPICS


Fertilization

The First Trimester


Labor and Delivery



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Genetics, Development, and Inheritance Objectives

Define genotype and phenotype.

Illustrate the human karyotype.

Describe the determination of sex in offspring.

Briefly discuss the origin of inherited abnormalities.

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sex

XY or

XX 55a karyotypeFig. 15

Genetics—the study of

inheritance

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Genetics, Development, and Inheritance (cont)

Inherited genetic information is in the form of DNA packaged into chromosomes of the ovum and sperm.

The sequence of nitrogenous bases that make up the DNA is the genetic code directing the synthesis of proteins.

Body cells contain a nucleus having 23 pairs (46 ) of homologous chromosomes but meiosis produces sex cells

with 23 single chromosomes.

One chromosome pair determines sex.

If that pair consists of an X and a Y, the individual is male.

If that pair consists of two X’s, the individual is female.

Since females have only X’s, all ova bear an X.

Sperm may be either X or Y bearing.

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Thus, the father’s sperm determines the sex of a baby.

The gender chance is always 50:50 at each fertilization.

A gene is a DNA segment that contains the information for the sequence of amino acids in a single polypeptide.

Although homologous chromosomes have the same genes (except for the sex chromosomes), each individual may have a slightly different base sequence at a DNA site determining a particular characteristic because one

chromosome came from the mother and the other came from the father.

Your inherited DNA from your mother and father is your genotype.

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Thus, there is variation in the expression of homologous genes that results in the phenotype (appearance) of an individual (see TABLE 4).

Consider the gene for eye color—the gene for brown eye color is dominant over the recessive gene for blue eye color.

If one chromosome contains information to synthesize brown eye pigment and the other one does not, the individual will have brown eyes.

If both chromosomes lack the gene for brown pigment, blue eye color results.

There are numerous genes on sex chromosomes, especially the X chromosome.

Thus, there are a number of sex-linked inherited disorders.


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Some X-linked disorders include color blindness, a form of hemophilia, and a form of muscular dystrophy.

Autosomal disorders do not involve the sex chromosomes.

Some inherited autosomal disorders are sickle-cell anemia, albinism, cystic fibrosis, and Huntington’s disease (a CNS disorder).

The human genome project has mapped the human genome.

The scientific field of genetic engineering seeks to repair inherited disorders or insert human genes into the genome of other organisms (especially bacteria) so the gene

product can be manufactured and then used as a drug.

Human insulin is now made using this method.

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The developmental process is guided entirely by the genome.

Progression through development occurs because of the selective expression of the genome through time.

Chemical signals select which genes are to be read (or expressed) at any moment in time.

Likewise, all of your ongoing processes depend on the presence of enzymes to catalyze chemical reactions.

Since all enzymes are proteins, it is the expression of your genome that maintains your life.

We began with chemistry—we continue with chemistry, the chemistry of life!

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TOPICS


Fertilization

The First Trimester


Labor and Delivery



bio 169 human development and inheritance chapter 29

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