Chapter 29: Development

BIO 211 Lecture Instructor: 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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Figure 29 –1a, b

Fertilization

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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 9 th 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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Figure 29 –2

Cleavage and Blastocyst Formation

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Figure 29 –3

Stages in Implantation

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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

Figure 29 –5 (5 of 5)

Extraembryonic Membranes and Placenta Formation

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Figure 29 –7a, b

First Trimester

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Figure 29 –7c, d

First Trimester

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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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Figure 29 –9a, b

Growth of Fetus and Uterus

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Figure 29 –9c, d

Growth of Fetus and Uterus

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Table 29 –2 (1 of 4)

Overview of Prenatal Development

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Table 29 –2 (3 of 4)

Overview of Prenatal Development

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Table 29 –2 (2 of 4)

Overview of Prenatal Development

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Table 29 –2 (4 of 4)

Overview of Prenatal Development

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Third Trimester Hormones • • P (placental) – Until 3 rd 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  expands  Dilation of uterine cervix  vagina pelvis so fetus can enter

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Figure 29 –10

Factors Involved in Initiation of Labor and Delivery

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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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Figure 29 –11 (1,2 of 4)

Stages of Labor

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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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Figure 29 –11 (3,4 of 4)

Stages of Labor

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Figure 29 –12

Milk Let-Down Reflex

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