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FERTILIZATION, EMBRYONICFERTILIZATION, EMBRYONICFETAL GROWTH ANDFETAL GROWTH ANDDEVELOPMENTDEVELOPMENT

CHRISTIAN ORENSE, PTRPCHRISTIAN ORENSE, PTRP

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o HUMAN GROWTH AND

DEVELOPMENT STARTS ATFERTILIZATION also knownas CONCEPTION.

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PERIODS OFPERIODS OFDEVELOPMENTDEVELOPMENT

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PERIOD OF PRENATALPERIOD OF PRENATAL

DEVELOPMENTDEVELOPMENT

PREPRE--NATAL MEANS BEFORE BIRTHNATAL MEANS BEFORE BIRTH

PRENATAL DEVELOPMENT REFERS TO ALLPRENATAL DEVELOPMENT REFERS TO ALLCHANGES FROM FERTILIZATION UP TOCHANGES FROM FERTILIZATION UP TOBIRTHBIRTH

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PRENATAL DEVELOPMENT PERIODSPRENATAL DEVELOPMENT PERIODS

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OVUM OROVUM OR

GERMINAL PERIODGERMINAL PERIOD

o starts from fertilization (also known asconception) to implantation

o shortest in duration because it lastsapproximately 2 weeks only

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PERIOD OF THE OVUM:PERIOD OF THE OVUM:

FERTILIZATIONFERTILIZATION

o occurs in the fallopian tube about 1 2-24hours after ovulation.

o m erging of the genetic m aterial fro m thesper m cell (sper m atozoon) and

secondary oocyte (ovum) into a singlenucleus

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PERIOD OF THEPERIOD OF THE

OVUMOVUM

SPERM OVUM/EGG ZYGOTE

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OVUMOVUM

ZygoteMovement

from fallopioantube to Uterus

Differentiation

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OVUMOVUMo CLEAVAGE is the term used to refer to the early mitotic

divisions that increases the number of cells of thezygote. The zygote is now called a BLASTOMERE.Mitotic divisions continue and the blastomere becomesa MORULA 96 hours after fertilization (named becausethe solid mass of cells looks like a mulberry). It is themorula that now leaves the fallopian tube and entersthe uterine cavity.

o 4 to 5 days after fertilization, the morula developsinto a hollow ball of cells called the BLASTOCYST. Theouter covering of cells is called the TROPHOBLASTwhile the inner mass is called EMBRYONIC DISK orEMBRYOBLAST.; The internal fluid-filled cavity is calledBLASTOCELE.

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PERIOD OF THE OVUMPERIOD OF THE OVUM

o Implantation- 6 days (5-7 days) afterfertilization, the blastocyst starts to attach tothe endometrium. Implantation becomescomplete on the

o Note: Viability (life span) of sperm rangesfrom 48 to 72 hours; viability of ovum is 24hours only. This means that if conception has

to take place, it should be within 24 hoursafter ovulation occurs.

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BLASTOMERE

Surrounded by ZONA PELLUCIDA

MORULA

Made up of 16-50 cellsLeaves Fallopian Tube and Enters the uterusWith fluid and cells migrate to the edges creating cavityat the center

BLASTOCYST

Hollow ball of cells with cavity at the centerZona Pellucida replaced by TROPHOBLASTInner layer of cells become the BLASTODERM

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o After a few days of cell division, a hollow ball of cells is formed and this is called a BLASTOCYST.This development of a blastocyst is significant because it is in the blastocyst form that attachesitself to the uterus.

o This attachment of the blastocyst to theendometrium of the uterine lining that occursapproximately 6 days after fertilization is calledIMPLANTATION. Implantation is the end of thisperiod of the ovum or the germinal period.

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BLASTOMERE MORULA BLASTOCYST IMPLANTATION

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DIFFERENTIATIONDIFFERENTIATION

IMPLANTATIONIMPLANTATION

OUTER LAYER OF

TROPHOBLAST

INNER LAYER OF

EMBRYONIC DISC

PLACENTA

ECTODERM

ANDENDODERM

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o The loss of the zona pellucida allows thetrophoblasts to implant into the endometrium(uterine lining) of the upper part of the uterus.The endometrial lining of the uterus will thenbegin to provide nourishment to the developing

embryo.o As the blastocyst implants, the trophoblast

develops into 2 layers thesyncytiotrophoblast ( contains no cellboundaries) and the cytotrophoblast (withdistinct cell boundaries) these 2 layers becomepart of the chorion (one of the fetalmembranes).

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OVUMOVUM

Zygote differentiates BLASTOCYST

IMPLANTATION ONTHE

ENDOMETRIUM OFTHE UTERUS

END OF THEGERMINAL/PERIOD

OF THE OVUM

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The trophoblasts secrete hCG (humanchorionic gonadotropin) which has actionssimilar to LH (luteinizing hormone). It retainsthe corpus luteu m fro m degeneration andsustains its secretion of progesterone andestrogen so menstruation doesn t occur.

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IDENTICAL AND FRATERNAL TWINSIDENTICAL AND FRATERNAL TWINS

o Dizygotic (fraternal twins) are producedwhen two secondary oocytes are fertilized by

different sperms; they may be of the same ordifferent sex.

o Identical twins develop when the fertilizedovum splits at an early age in development;the sex is the same for identical twins.

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EMBRYONIC PERIODEMBRYONIC PERIOD

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EMBRYONIC PERIODEMBRYONIC PERIOD

o embryo starts after implantation from the2nd week to end of the 2nd month. lasts for8 weeks. Differentiation continues in thisperiod that refines structures that started todevelop earlier.

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SPECIFIC STRUCTURES FORMEDSPECIFIC STRUCTURES FORMED

AT THE EMBRYONIC PERIODAT THE EMBRYONIC PERIOD1. AMNIOTIC SACThe amniotic sac is a membrane that encloses the embryo that is filled with amniotic fluid. It serves many functions- insulates

the embryo from physical trauma, serves as a temperatureregulator.2. PLACENTAallows food and oxygen to reach the embryo. The placentaserves as a substitute system for two systems that are not yet operational- gastrointestinal system and respiratory systembecause the embryo is not able to eat and breath. Describe theplacenta further____ the main fxns of the placenta are(1)exchange of metabolic and gaseous products bet maternal and fetal bloodstreams and (2)production of hormones

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SPECIFIC STRUCTURES FORMEDSPECIFIC STRUCTURES FORMED

AT THE EMBRYONIC PERIODAT THE EMBRYONIC PERIOD

3 . UMBILICAL CORDThe umbilical cord connects the placenta to the embryo.Three blood vessels are present in this cord- 2 umbilicalarteries and 1 umbilical vein.4. EMBRYONIC DISKThe embryonic disk folds in on itself to form 3 layersA. ectoderm -- becomes sensory organs, skin and NSB. mesoderm -- muscles, skeleton, and circulatory systemC.endoderm -- digestive system, lungs and glands

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ECTODERMAL DERIVATIVESECTODERMAL DERIVATIVES

o central and peripheralnervous system

o epithelium of sensoryorgans ear, mouth, anus,nasal cavity

o epidermis hair, nails,subcutaneous glands

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MESORDERMAL DERIVATIVESMESORDERMAL DERIVATIVES

o skeletal systemo muscle cellso dermis and subcutaneous tissueso urinary systemso male and female reproductive

tractso serous membranesof peritoneum,

pleura and pericardiumo cardiovascular systemo lymphatic system and spleen.o cortices of the adrenal gland

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ENDODERMAL DERIVATIVESENDODERMAL DERIVATIVES

o epithelial lining of therespiratory system

o tympanic membrane andeustachian tube

o part of the bladder andurethra

o parenchyma of the thyroido tonsil, parathyroid and

thymuso liver, pancreaso gastrointestinal system

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SIGNIFICANT EVENS OFSIGNIFICANT EVENS OF

THE EMBRYONICTHE EMBRYONIC

PERIODPERIOD

o Other significant events; heart begins to

beat, simple brain functioning

o Adverse events that result to inadequate

development of the developing embryooften results into miscarriages.

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FETUSFETUS

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PERIOD OF THE FETUSPERIOD OF THE FETUS

o This is the longest period of prenatal development and isalso called the GROWTH ANDFINISHING PHASE. It starts at the 3 rd month to the time of birth.

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SIGNIFICANT EVENTS THATSIGNIFICANT EVENTS THAT

HIGHLIGHT THE FETAL STAGEHIGHLIGHT THE FETAL STAGEo bone cells replace cartilage by 3 monthso teeth buds, limbs and fingers moveo external genitals are formed (sex of fetus is

evident)o by 4 months mother can feel movement

( quickening )o face appears human about 7 monthso age of viability -- age at which fetus can survive if

born

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DEVELOPMENT OF THESPECIFIC SYSTEMS-

NERVOUS,MUSCULOSKELETAL AND

CARDIOPULMONARY

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RESPIRATORY SYSTEM DEVELOPMENTRESPIRATORY SYSTEM DEVELOPMENT

o The respiratory system develops after the fourthweek of gestation . The respiratory system beginsas an outgrowth of the endoderm of the foregut

called the laryngotracheal bud (lung bud).o As the bud grows, it develops into the larynx,

glottis and trachea. The distal portion divides into2 lung buds which become the bronchi and the

lungs. The smooth muscle, cartilage andconnective tissue of the bronchial tubes arederived from mesoderm as well as the pleural sacsof the lungs.

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IMPORTANT NOTES ON RESPIRATORYIMPORTANT NOTES ON RESPIRATORY

SYSTEM DEVELOPMENTSYSTEM DEVELOPMENT1. The respiratory system does not carry out its physiological function of

gas exchange prenatally (before birth).

2. The lungs also continue to grow for about 8+ years postnatally (afterbirth).

3 . The respiratory tract and aveolar epithelium are endoderm in originwhile the surrounding tissues are mesoderm in origin.

4. There are four stages of lung development. The final stage (type 2surfactant secretion) only occurs in the final weeks of development.This means that premature infants have respiratory difficulties due tothe still immature respiratory system.

5. Many systems are involved in respiratory function: musculoskeletal(ribs and diaphragm) cardiovascular (pulmonary circulation). Themusculoskeletal begins functioning prenatally, the cardiovasularpulomonary circulation is activated and altered postnatally.

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THE RESPIRATORYTHE RESPIRATORYSYSTEM DEVELOPMENTSYSTEM DEVELOPMENT

HAS 4 STAGESHAS 4 STAGES

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4 STAGES OF RESPIRATORY4 STAGES OF RESPIRATORYSYSTEM DEVELOPMENTSYSTEM DEVELOPMENT

1. PSEUDOGLANDULAR STAGE (6-

16 weeks)

2. CANALICULAR STAGE ( 17 -26 weeks).3. TERMINAL SAC STAGE (26 week-birth)

4. ALVEOLAR STAGE (2 7 - 40 weeks)

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1. PSEUDOGLANDULAR1. PSEUDOGLANDULAR

STAGE (6STAGE (6--1 6 WEEKS)1 6 WEEKS)

o Formation of airways, airway branching

o Cartilage begins to developo Major lobes of the lung are recognized

o Anterior (hard) and posterior (soft) palates

develop.o Goblet cells, bronchial glands and ciliated cells

become functional.

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2 . CANALICULARSTAGE (17 -2 6 weeks)

o Terminal and respiratory bronchiolesmultiply.

oVascularization occurs.

o Beginning differentiation of type I and type IIalveolar cells.

o Capillaries in proximity to alveolar cavity but not close enough for gas exchange until 24 25 weeks gestation.

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3 . TERMINAL SACSTAGE (2 6 week-birth)

o Cells are initimately associated withnumerous blood vessels and lymphcapilliaries, surrounding spaces areknown as terminal sacs/primitivealveoli.

o Sufficient capilliaries are present to

guarantee adequate gas exchange

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4.4. ALVEOLAR STAGE(2 7 - 40 weeks)

o True alveoli appear

o Appearance of macrophage

o Type II cells proliferate- surfactant increases

o Chemoreceptors develop

o Number of alveoli increase until 8 years of age

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IN UTEROIN UTERO

o

the lungs of the fetus are filled with fetallung fluid. This fluid in the lungs isresponsible for the high degree of surfacetension existing within fetal lung.

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AT BIRTHAT BIRTH

o From the e first breath, the newborn must eliminate the fluid present in the lungs andmaintain a level of lung expansion to allow forgas exchange. Because of the presence of lung

fluid, the initial inflation of the lungs is difficult but becomes easier as the lung expands.

o The distending pressure of an alveolus(pressure required to keep the lung open) isdependent on the surface tension of the fluidwithin the alveolus and on the radius of thealveoli itself.

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SURFACTANT AND SURFACE TENSIONSURFACTANT AND SURFACE TENSION

o The Law of LaPlace suggests that the DEGREE OFSURFACE TENSION exerted by this FLUID IS INVERSELYRELATED TO THE RADIUS OF THE ALVEOLI. As theradius of the alveolus decreases, the surface tensionincreases.

o This increase in surface tension can dramaticallyincrease a newborn s work of breathing, leading torespiratory distress that may lead to respiratory failure.It is important therefore that the alveoli remaindistended to avoid respiratory distress. A newborn slungs must expand (air should reach the alveoli) to becapable gaseous exchange. This is where surfactant becomes crucial to survival of the newborn.

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o SURFACTANT disrupts the cohesive forces

within the lungs LIMITING THE INCREASE INSURFACE TENSION, and thereby reducing thenewborn s work of breathing at low lungvolumes.

o If surfactant were not present or inadequatein the lung, the smaller the volume of the lung(i.e., during exhalation), the greater the forcerequired to reopen the lung on the next inspiration.

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1. decreases alveolar surface tension and stabilizesalveoli to prevent it from collapsing

2. aids in evacuation of fetal fluid3 . enhances capillary circulation4. important for normal ventilation / perfusion ratios5. protects alveolar tissues from barotraumas

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SURFACTAINTSURFACTAINT

o The major components of surfactant includephosphatidylcholine &phosphatidylglycerol . Surfactant isPRODUCED BY ALVEOLAR TYPE II

PNEUMOCYTES, which first appear in the 17 26weeks of gestation.

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DEVELOPMENT OF THE HEARTDEVELOPMENT OF THE HEART

o The heart is derived from the mesoderm that startsfrom the formation of the two endocardial tubes.These endocardial tubes unite to form the primitive

heart tube that eventually will form a four-chamberheart. Contractions of the heart begin on the fourthweek of gestation and the direction of bloodflow isbidirectional (two-way) because the valves are not yet developed.

o Within the inferior portion of the emerging heart develops the sinus venosus, which will eventuallybecome a portion of the right atrium and vena cavae(inferior and superior).

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DEVELOPMENT OF THE HEARTDEVELOPMENT OF THE HEART

o Blood flow becomes unidirectional on the fifthweek of gestation with blood entering the sinusvenosus, traveling through the primitive atrium tothe right and left ventricles and out through

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ADAPTATIONS OF FETAL BLOOD AND

VASCULAR SYSTEM.

o The concentration of hemoglobin in fetal blood isabout 50 % greater than in maternal blood. Fetal

hemoglobin is slightly different chemically and hasa greater affinity for O2 than maternal hemoglobin.At a particular oxygen partial pressure, fetalhemoglobin can carry 20- 3 0% more O2 than

maternal hemoglobin.

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CHANGES IN THE CARDIOVASCULARCHANGES IN THE CARDIOVASCULAR

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SYSTEM AFTER BIRTHSYSTEM AFTER BIRTH

o First cry of a newborn is the initial inflation of the

lungs which causes important changes in thecirculatory system. Inflation of the lungs reducesthe resistance to blood flow through the lungsresulting to increased blood flow from the

pulmonary arteries.


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oo More blood pass thru the atrium to the More blood pass thru the atrium to the

ventricle than to the foramenventricle than to the foramen ovaleovaleoo Increased volume of blood returning to the lungs toIncreased volume of blood returning to the lungs to

the (L) atriumthe (L) atrium

oo Closure of ForamenClosure of Foramen ovaleovale separationseparation of 2 heart of 2 heart pumpspumps FOSSAFOSSA OVALISOVALIS


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o Ductus arteriosus, which connects the pulmonarytrunk to the descending aorta (systemiccirculation), closes within 1 -2 days after birth .The closed ductus arteriosus will be replaced byconnective tissue to become the LIGAMENTUM ARTERIOSUM.

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PATENT DUCTUS ARTERIOSUSPATENT DUCTUS ARTERIOSUS

o If it persists, it will increase pulmonary pressurebecause blood flows from the left ventricle to the

aorta, through the ductus arteriosus to thepulmonary arteries. This increased pul m onarypressure through ti m e (if not corrected), canlead to irreversible degenerative changes in

the.heart and lungs.

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THE UMBILICAL CORDTHE UMBILICAL CORD

oo 2 UMBILICAL ARTERIES2 UMBILICAL ARTERIES from the fetal internal

iliac arteries that carries deoxygenated blood back to the placenta where it is oxygenated

o 1 UMBILICAL VEIN carrying the oxygen blood fromthe placenta back to the fetus passing through theliver and ductus venosus

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THE UMBILICAL CORDTHE UMBILICAL CORD

o When the umbilical cord is cut, no more blood flowsthrough the umbilical arteries and vein and theydegenerate. The degenerated umbilical veinbecomes the ROUND LIGAMENT OF THE LIVERwhile the degenerated ductus venosus becomes theLIGAMENTUM VENOSUM.

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DEVELOPMENT OF THE SKELETALDEVELOPMENT OF THE SKELETAL

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SYSTEMSYSTEM

o Intramembranous and endochondral ossificationbegin when mesenchymal (mesodermal) cellsmigrate into the area where bone formation willoccur. In some skeletal structures, mesenchymalcells develop into chondroblasts that form cartilage.In other skeletal cells, mesenchymal cells developinto osteoblasts that form bone tissue byintramembranous or endochondral ossification.


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SYSTEMSYSTEM

o Limbs start as li m b buds that come frommesoderm covered with ectoderm.

o appear at the sides of the trunk at the fourth week.By the sixth week the limb buds develop aconstriction at the middle portion.

o The constriction produces distal segments of the

upper buds called hand plates and distalsegments of the lower buds called foot plates.


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SYSTEMSYSTEM

o These plates represent the beginnings of the handsand feet.

o By the seventh week, the arm, forearm and handare evident in the upper limb bud, and the thigh, legand foot appear in the lower limb bud.Endochondral ossification has begun.

oBy the eight week, the shoulder, elbow, and wrist areas become apparent.

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CONDITIONS AND FACTORS AFFECTINGCONDITIONS AND FACTORS AFFECTING

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GROWTH AND DEVELOPMENTGROWTH AND DEVELOPMENT

1. GENETIC FACTORS

2. FETAL FACTORS3 . .MATERNAL FACTORS

4. ENVIRONMENTAL FACTORS

OBSTETRIC FACTORS

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GENETIC FACTORSGENETIC FACTORS

o May lead to congenital abnormalities, chromosalaberration

o

Maternal diseaseso Infections may lead to abnormalities, stillbirth,

abortion or premature babies

o Non-infectious causes diabetes mellitus,hypo/hyperthyroidism, immunologic diseases

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MATERNAL FACTORSMATERNAL FACTORS

o age older women primigravida > 3 5 years of age areconsidered high-risk to many disorders

o physical and mental health of mother

o stresso toxemia, hypoxemiao renal diseaseso malnutritiono drugso infections

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ENVIRONMENTAL FACTORSENVIRONMENTAL FACTORS

exposure of the zygote/fetus to:

o drugs thalidomideo chemicals e.g. methyl mercury

o radiations

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OBSTETRIC FACTORSOBSTETRIC FACTORS

o labor and delivery related conditions

o birth injuries

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IN SUMMARYIN SUMMARY

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