Chapter 29: Development

BIO 211 LectureInstructor: Dr. Gollwitzer

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• Today in class we will:– Define and describe development– Trace the general processes from ovulation through

fertilization and formation of a zygote– List the stages of development– List the 3 stages of gestation and briefly describe the

major events associated with each• Distinguish between embryo and fetus• Discuss the two major roles of the placenta• Discuss the basic structural and functional changes in the uterus

during gestation– Briefly describe the events that occur during labor and

delivery– Describe lactation and milk let-down reflex

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Development

• Begins with fertilization (conception) =– When male and female gametes (sperm and

egg) unite to form single-cell zygote– Occurs in uterine tube 12-24 hr after ovulation

• Is the gradual modification from fertilization to maturity of:– Anatomical structures– Physiological characteristics

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Fertilization

Figure 29–1a, b 4

Amphimixis

• Fusion of female and male pronuclei• Moment of conception• Cell becomes zygote (46 chromosomes)• Fertilization complete

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Stages of Development

• Prenatal – before birth– Embryological development• Occurs during first 2 mo after fertilization• Organs established

– Fetal development• Begins at 9th wk and continues to birth• Organs develop

• Postnatal – after birth– Neonate = newborn

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Gestation

• Time spent in prenatal development• Consists of 3 trimesters, each 3 months long– First trimester– Second trimester– Third trimester

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First Trimester• Cell cleavage (division) and blastocyst formation• Blastocyst implantation = burrowing into uterine

wall• Placentation = formation of placenta– Temporary structure in uterine wall – Permits diffusion between fetal and maternal

circulatory systems• Embryogenesis – all organ systems begin to be

established; but nonfunctional– Embryo = organism in the developmental stage

beginning at fertilization and ending at the start of the third developmental month (weeks 1 – 8)

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First Trimester

• Most dangerous period in prenatal life• Only 40% of conceptions produce embryos

that survive past first trimester

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Cleavage and Blastocyst Formation

Figure 29–2 10

Stages in Implantation

Figure 29–3 11

Placenta

• Complex organ that permits exchange between maternal and embryonic circulatory systems

• Supports fetus in second and third trimesters

• Stops functioning and is ejected from uterus after birth

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Extraembryonic Membranes and Placenta Formation

Figure 29–5 (1, 2 of 5) 13

Extraembryonic Membranes and Placenta Formation

Figure 29–5 (3, 4 of 5) 14

Extraembryonic Membranes and Placenta Formation

Figure 29–5 (5 of 5) 15

First Trimester

Figure 29–7a, b 16

First Trimester

Figure 29–7c, d 17

First Trimester: hCG

• hCG = human chorionic gonadotropin• Produced by placenta• Appears in maternal bloodstream soon after

implantation• Used as pregnancy test/kit• Maintains CL for 3-4 months– So CL P (until placenta takes over P production)

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Second Trimester

• Fetal stage = development of all organ systems (organogenesis)

• Rapid growth of fetus– Fetus = organism in the developmental stage

lasting from the start of the third developmental month to delivery (week 9 through delivery)

• Body proportions change• Progesterone levels increase

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Third Trimester

• Organ systems fully functional• Fetal growth rate slows• Largest weight gain

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Growth of Fetus and Uterus

Figure 29–9a, b 21

Growth of Fetus and Uterus

Figure 29–9c, d 22

Overview of Prenatal Development

Table 29–2 (1 of 4) 23

Overview of Prenatal Development

Table 29–2 (3 of 4) 24

Overview of Prenatal Development

Table 29–2 (2 of 4) 25

Overview of Prenatal Development

Table 29–2 (4 of 4) 26

Third Trimester Hormones• P (placental)– Until 3rd trimester, “calms” myometrium so no

contractions

• E (placental)– Increases myometrial contractions– Sensitizes uterus to oxytocin (maternal and fetal)

prostaglandins (PGs) initiate labor

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Third Trimester Hormones• Human placental lactogen (hPL)– Helps prepare mammary glands for milk

production– Effects on other tissues comparable to GH

• Prolactin (placental)– Helps convert mammary glands to active status

• Relaxin (CL and placental) – Increased flexibility of pubic symphysis pelvis

expands– Dilation of uterine cervix so fetus can enter

vagina

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Factors Involved in Initiation of Labor and Delivery

Figure 29–10 29

Labor• False– Occasional spasms in uterine musculature– Contractions not regular or persistent

• True– Results from biochemical and mechanical factors– Continues due to positive feedback

• Premature– When labor begins before fetal development

complete; survival related to BW

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Labor and Delivery

• Goal: parturition = forcible expulsion of fetus• Stages of labor– Dilation– Explusion– Placental

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Labor and Delivery

• Dilation stage– Begins with onset of true labor– Cervix dilates– Fetus moves toward cervical canal– Frequency of contractions increases– Amniochorionic membrane ruptures (“water

breaks”)

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Stages of Labor

Figure 29–11 (1,2 of 4) 33

Labor and Delivery

• Expulsion stage– Cervix completely dilated– Maximum intensity of contractions– Continues until fetus emerges from vagina =

delivery/birth

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Labor and Delivery

• Placental stage– Uterine contractions tear connection between

endometrium and placenta– Placenta (afterbirth) ejected– Accompanied by loss of blood, usually tolerated

without difficulty

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Stages of Labor

Figure 29–11 (3,4 of 4) 36

Milk Let-Down Reflex

Figure 29–12 37