Pregnancy & Human Development Chapter 29

Fertilization: It’s all in the timing! Oocyte is only viable for ~ 24 hours. Sperm is viable for 12 – 24 hours (some “super sperm” may be viable for up to 72 so be careful!) Therefore, usually, coitus must occur within a 24 hour window on either side of ovulation.

Barriers to fertilization Low vaginal pH Getting lost (50/50 chance of getting the right uterine tube) Numerous defective sperm Uterine contractions Phagocytes By the time they get to the oocyte, there are only a few dozen to a couple hundred

Capacitation Must occur before spermatozoa can fertilize secondary oocyte: contact with secretions of seminal vesicles exposure to conditions in female reproductive tract

Penetration Secondaryoocyte Head of sperm 1650X

Fertilization Figure 29–1

Fertilization Figure 29–1b (1 of 2)

Fertilization Figure 29–1b (2 of 2)

Fusion Of the pronuclei

Cleavage and Blastocyst Formation Figure 29–2

What’s this thing called, Love? Zygote – a the single cell after fusion of the pronuclei of the oocyte & the sperm. Conceptus – covers the period of develop following first cleavage and differentiation of cells into an embryo. Morula – the conceptus as a solid ball of 16 cells (about day 3). Blastocyst – a hollow ball of cells, from day 4. “Hatching” occurs at this stage, when the blastocyst emerges from the zona pellucida.

Development from zygote to implantation.

Then what ? The blastocyst differentiates into: the trophoblast, the outer ball of cells that eventually becomes the placenta and “extraembryonic” membranes. the inner cell mass (ICM) becomes the embryo. The above occurs over the course of the second week following conception. Implantation – occurs on about day 6 or so, as the blastocyst burrows into the endometrium.

Stages in Implantation Figure 29–3

Implantation Day 6

Implantation – Day 8

Implantation Days 9 - 13 and early placentation

Ectopic Pregnancy Implantation occurs outside of uterus Do not produce viable embryo Can be life threatening

The Inner Cell Mass and Gastrulation Figure 29–4

The Primary Germ Layers ECTODERM MESODERM ENDODERM All nervous tissue Muscle G.I. epithelium Epidermis & Derivatives Connective tissue Digestive glands Cornea & lens Lymphoid tissue Reproductive ducts & gland epithelium Oral, nasal & anal epithelium Endothelium of blood vessels Thyroid, thymus & parathyroid Tooth enamel Serosae Urethra & bladder epithelium Pineal, pituitary & adrenal medulla Eye’s fibrous & vascular tunics Respiratory tract epithelium Melanocytes Synovia Flat bones of cranium Urogenital organs

The Fates of the Germ Layers Table 29–1

Extraembryonic Membranes and Placenta Formation Figure 29–5 (1 of 3)

Figure 29–5 (2 of 3)

Placenta Formation Figure 29–5 (3 of 3)

View of Placental Structure Figure 29–6a

Placental Structure Figure 29–6b

Decidua: Decidua Capsularis Decidua Basalis Decidua Parietalis Thin portion of endometrium No longer participates in nutrient exchange and chorionic villi in region disappear Decidua Basalis Disc-shaped area in deepest portion of endometrium Where placental functions concentrated Decidua Parietalis Rest of the uterine endometrium No contact with chorion

Hormones of Placenta Synthesized by syncytial trophoblast, released into maternal bloodstream: human chorionic gonadotropin human placental lactogen placental prolactin relaxin progesterone estrogens

Human Placental Lactogen (hPL) Helps prepare mammary glands for milk production Stimulatory effect on other tissues comparable to growth hormone (GH) Placental Prolactin Helps convert mammary glands to active status

Relaxin Is a peptide hormone Is secreted by placenta and corpus luteum during pregnancy Increases flexibility of pubic symphysis, permitting pelvis to expand during deliveryCauses dilation of cervix Suppresses release of oxytocin by hypothalamus and delays labor contractions

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

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

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

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

Embryogenesis Body of embryo begins to separate from embryonic disc Body of embryo and internal organs start to form Folding, differential growth of embryonic disc produce bulge that projects into amniotic cavity: projections are head fold and tail fold

The First Trimester Figure 29–7a, b

The First Trimester Figure 29–7c, d

Organogenesis Process of organ formation

The Second and Third Trimesters Figure 29–8

Second Trimester Fetus grows faster than surrounding placenta

Third Trimester Most of the organ systems become ready Growth rate starts to slow Largest weight gain Fetus and enlarged uterus displace many of mother’s abdominal organs

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

Growth of the Uterus and Fetus

Progesterone Released by placenta Has inhibitory effect on uterine smooth muscle Prevents extensive, powerful contractions

Opposition to Progesterone 3 major factors: rising estrogen levels rising oxytocin levels prostaglandin production

Initiation of Labor and Delivery Figure 29–10

False Labor True Labor Occasional spasms in uterine musculature Contractions not regular or persistent True Labor Results from biochemical and mechanical factors Continues due to positive feedback

Hormone levels throughout pregnancy

Placental hormones

Contractions Begin near top of uterus, sweep in wave toward cervix Strong, occur at regular intervals, increase in force and frequency Change position of fetus, move it toward cervical canal

Stages of Labor Dilation stage Expulsion stage Placental stage

Dilation Stage Begins with onset of true labor Cervix dilates Fetus begins to shift toward cervical canal Highly variable in length: typically lasts over 8 hours

Dilation Stage Frequency of contractions steadily increase Amniochorionic membrane ruptures (water breaks)

The Stages of Labor Figure 29–11 (1 of 2)

Expulsion Stage Begins as cervix completes dilation Contractions reach maximum intensity Continues until fetus has emerged from vagina: typically less than 2 hours

The Stages of Labor Figure 29–11 (2 of 2)

Delivery Episiotomy Arrival of newborn infant into outside world Incision through perineal musculature Needed if vaginal canal is too small to pass fetus Repaired with sutures after delivery

Fetal circulation

The Beginning Next - Inheritance

Cesarean Section Removal of infant by incision made through abdominal wall Opens uterus just enough to pass infant’s head Needed if complications arise during dilation or expulsion stages

Placental Stage Muscle tension builds in walls of partially empty uterus Tears connections between endometrium and placenta Ends within hour of delivery with ejection of placenta, or afterbirth Accompanied by a loss of blood

Actual placenta

Premature Labor Occurs when true labor begins before fetus has completed normal development Newborn’s chances of surviving are directly related to body weight at delivery

Immature Delivery Refers to fetuses born at 25–27 weeks of gestation Most die despite intensive neonatal care Survivors have high risk of developmental abnormalities

Premature Delivery Refers to birth at 28–36 weeks Newborns have a good chance of surviving and developing normally

Forceps Delivery Needed when fetus faces mother’s pubis instead of sacrum Risks to infant and mother are reduced using forceps: forceps resemble large, curved salad tongs used to grasp head of fetus

Breech Birth Legs or buttocks of fetus enter vaginal canal first instead of head Umbilical cord can become constricted, cutting off placental blood flow Cervix may not dilate enough to pass head Prolongs delivery Subjects fetus to severe distress and potential injury

5 Life Stages Neonatal period - Extends from birth to 1 month Infancy - 1 month to 2 years of age Childhood - 2 years until adolescence Adolescence - Period of sexual and physical maturation Maturity

Colostrum Secretion from mammary glands Ingested by infant during first 2–3 days Contains more proteins and less fat than breast milk: many proteins are antibodies that help ward off infections until immune system is functional

Colostrum Mucins present inhibit replication of rotaviruses As production drops, mammary glands convert to milk production

Breast Milk Consists of: water proteins amino acids lipids sugars salts large quantities of lysozymes—enzymes with antibiotic properties

Milk Let-Down Reflex Mammary gland secretion triggered when infant sucks on nipple Continues to function until weaning, typically 1–2 years

The Milk Let-Down Reflex Figure 29–12

Benefits of Breast-feeding Acquired immune defenses Neutrophils, macrophages, T and B cells Immunoglobulin A Reduced incidence of later diseases in child Lymphoma, heart disease, gastrointestinal disorders, diabetes mellitus & meningitis In mother Reduced incidence of osteoporosis and breast cancer Stronger bonding, less post-partum depression, More rapid weight loss, uterine recovery

Growth and Changes in Body Form and Proportion Figure 29–13

From embryo to fetus