BIO 211 Lecture Instructor: Dr. Gollwitzer Chapter 29: Development BIO 211 Lecture Instructor: Dr. Gollwitzer

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

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

Fertilization Figure 29–1a, b

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

Stages of Development Prenatal – before birth Postnatal – after 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

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

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)

First Trimester Most dangerous period in prenatal life Only 40% of conceptions produce embryos that survive past first trimester

Cleavage and Blastocyst Formation Figure 29–2

Stages in Implantation Figure 29–3

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

Extraembryonic Membranes and Placenta Formation Figure 29–5 (1, 2 of 5)

Extraembryonic Membranes and Placenta Formation Figure 29–5 (3, 4 of 5)

Extraembryonic Membranes and Placenta Formation Figure 29–5 (5 of 5)

First Trimester Figure 29–7a, b

First Trimester Figure 29–7c, d

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)

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

Third Trimester Organ systems fully functional Fetal growth rate slows Largest weight gain

Growth of Fetus and Uterus Figure 29–9a, b

Growth of Fetus and Uterus Figure 29–9c, d

Overview of Prenatal Development Table 29–2 (1 of 4)

Overview of Prenatal Development Table 29–2 (3 of 4)

Overview of Prenatal Development Table 29–2 (2 of 4)

Overview of Prenatal Development Table 29–2 (4 of 4)

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

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

Factors Involved in Initiation of Labor and Delivery Figure 29–10

Labor False True Premature 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

Labor and Delivery Goal: parturition = forcible expulsion of fetus Stages of labor Dilation Explusion Placental

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”)

Stages of Labor Figure 29–11 (1,2 of 4)

Labor and Delivery Expulsion stage Cervix completely dilated Maximum intensity of contractions Continues until fetus emerges from vagina = delivery/birth

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

Stages of Labor Figure 29–11 (3,4 of 4)

Milk Let-Down Reflex Figure 29–12