34.4 Fertilization and Development SC.912.L.16.1 Describe the process of human development from fertilization to birth and major changes that occur in each trimester of pregnancy.

Pregnancy Pregnancy: events that occur from fertilization until the infant is born Gestation period: time from the last menstrual period until birth (~280 days) Embryo: from fertilization through week 8 Fetus: from week 9 through birth

After 8 weeks the embryo is called a fetus conceptus Embryo Fertilization 3-week embryo (3 mm) 5-week embryo (10 mm) 8-week embryo (22 mm) After 8 weeks the embryo is called a fetus 12-week fetus (90 mm) Figure 28.1

From Egg to Zygote The oocyte is viable for 12 to 24 hours Sperm is viable 24 to 48 hours after ejaculation

For fertilization to occur, coitus must occur no more than From Egg to Zygote For fertilization to occur, coitus must occur no more than Two days before ovulation 24 hours after ovulation Fertilization: when the sperm’s chromosomes combine with those of a secondary oocyte to form a fertilized egg (zygote)

Fertilization

Embryonic Development Cleavage Mitotic divisions of zygote First cleavage at 36 hours  two daughter cells (blastomeres) At 72 hours  morula (16 or more cells) At day 3 or 4, the embryo of ~100 cells (blastocyst) has reached the uterus

Embryonic Development Blastocyst: fluid-filled hollow sphere Inner cell mass Becomes the embryonic disc ( embryo and three of the embryonic membranes)

(a) Zygote (fertilized egg) (b) 4-cell stage 2 days (c) Morula (a solid ball of blastomeres). 3 days (d) Early blastocyst (Morula hollows out, fills with fluid, and “hatches” from the zona pellucida). 4 days Zona pellucida Degenerating zona pellucida (e) Implanting blastocyst (Consists of a sphere of tropho- blast cells and an eccentric cell clus- ter called the inner cell mass). 7 days Sperm Blastocyst cavity Uterine tube Fertilization (sperm meets and enters egg) Oocyte (egg) Ovary Uterus Blastocyst cavity Ovulation Endometrium Trophoblast Inner cell mass Cavity of uterus Figure 28.4

Implantation Blastocyst floats for 2–3 days Implantation begins 6–7 days after ovulation

Placentation Formation of the placenta from embryonic and maternal tissues Placenta is fully formed and functional by the end of the third month

Placenta Organ that connection between mother and embryo Act‘s as embryo’s organ of respiration, nourishment, and excretion. Oxygen and nutrients diffuse from the mother to the embryo. Carbon dioxide and metabolic waste diffuse from the embryo to the mother. Maternal and embryonic blood supplies do not intermix

Placenta Decidua basalis Chorionic villi Yolk sac Amnion Amniotic cavity Umbilical cord Decidua capsularis Uterus Extraembryonic coelom Lumen of uterus (e) 13-week fetus. Figure 28.7e

Embryonic Development: Gastrula to Fetus Germ Layers During implantation, the blastocyst starts to convert to a gastrula Inner cell mass develops into the embryonic disc (subdivides into epiblast and hypoblast) The three primary germ layers and the extraembryonic membranes develop

Extraembryonic Membranes Amnion: transparent sac filled with amniotic fluid Provides a buoyant environment that protects the embryo Helps maintain a constant homeostatic temperature Allows freedom of movement and prevents parts from fusing together Amniotic fluid comes from maternal blood, and later, fetal urine

Extraembryonic Membranes Yolk sac: a sac that hangs from the ventral surface of the embryo Forms part of the digestive tube Source of the earliest blood cells and blood vessels

Gastrulation Occurs in week 3, in which the embryonic disc becomes a three-layered embryo with ectoderm, mesoderm, and endoderm Begins with appearance of primitive streak, a raised dorsal groove that establishes the longitudinal axis of the embryo

(e) Bilayered embryonic disc, superior view Mesoderm Endoderm Amnion Bilayered embryonic disc Head end of bilayered embryonic disc Yolk sac (b) Frontal section (c) 3-D view (a) (d) Section view in (e) Primitive streak Head end Epiblast Cut edge of amnion Yolk sac (cut edge) (f) 14-15 days Hypoblast Endoderm Right Left Ectoderm Primitive streak Tail end (e) Bilayered embryonic disc, superior view Mesoderm Endoderm (g) 16 days Figure 28.9

Cells begin to migrate into the groove Gastrulation Cells begin to migrate into the groove The first cells form the endoderm Cells that follow push laterally, forming the mesoderm Cells that remain on the embryo’s dorsal surface form the ectoderm Notochord: rod of mesodermal cells that serves as axial support

Primary Germ Layers The primitive tissues from which all body organs derive Ectoderm  nervous system and skin epidermis Mesoderm  forms all other tissues Endoderm  epithelial linings of the digestive, respiratory, and urogenital systems

Building Vocabulary Prefix and their meaning: Ecto- “outer” Meso- “middle” Endo- “inner”

Specialization of Ectoderm Neurulation First major event of organogenesis Gives rise to brain and spinal cord Ectoderm over the notochord forms the neural plate Neural plate folds inward as a neural groove with neural folds

Specialization of Endoderm Forms the lining of organs of the GI tract (gastro intestinal) Respiratory tract Excretory system

Specialization of the Mesoderm Dermis of the skin in the ventral region Parietal serosa of the ventral body cavity Bones, ligaments, and dermis of limbs The heart and blood vessels Most connective tissues of the body

Epiblast ECTODERM MESODERM ENDODERM Notochord Somite Intermediate mesoderm Lateral plate mesoderm Somatic mesoderm Splanchnic mesoderm • Epidermis, hair, nails, glands of skin • Brain and spinal cord • Neural crest and derivatives (sensory nerve cells, pigment cells, bones and blood vessels of the head) Nucleus pulposus of inter- vertebral discs • Sclerotome: vertebrae and ribs • Dermatome: dermis of dorsal body region • Myotome: trunk and limb musculature • Kidneys • Gonads • Parietal serosa • Dermis of ventral body region • Connective tissues of limbs (bones, joints, and ligaments) • Wall of digestive and respiratory tracts (except epithelial lining) • Visceral serosa • Heart • Blood vessels Epithelial lining and glands of digestive and respiratory tracts Figure 28.13

Neurulation First step in the development of the nervous system If the neural tube doesn’t close completely a serious birth defect known as spina bifida can result. Folic Acid (Vitamin B 9) can prevent spina bifida The neural tube forms before a woman knows she is pregnant. So, folic acid should be part of any woman's diet.

Pharynx Parathyroid glands and thymus Thyroid gland Esophagus Trachea Connection to yolk sac Right and left lungs Stomach Liver Umbilical cord Pancreas Gallbladder Small intestine Allantois Large intestine 5-week embryo Figure 28.12

Events of Fetal Development Fetal period: weeks 9 through 38 Time of rapid growth of body structures established in the embryo

Umbilical cord Chorionic villi Amniotic sac Umbilical vein Yolk sac Cut edge of chorion (a) Embryo at week 7, about 17 mm long. Figure 28.15

Table 28.1 (1 of 3)

Table 28.1 (2 of 3)

Table 28.1 (3 of 3)

Parturition Parturition giving birth to the baby Labor events that expel the infant from the uterus

During the last few weeks of pregnancy Initiation of Labor During the last few weeks of pregnancy Fetal secretion of cortisol stimulates the placenta to secrete more estrogen Causes production of oxytocin receptors by myometrium Antagonizes calming effects of progesterone, leading to Braxton Hicks contractions in uterus

Initiation of Labor Surfactant protein A (SP-A) from fetal lungs causes softening of the cervix Fetal oxytocin causes the placenta to produce prostaglandins Oxytocin and prostaglandins: powerful uterine muscle stimulants

Maternal emotional and physical stress Initiation of Labor Maternal emotional and physical stress Activates the hypothalamus, causing oxytocin release from posterior pituitary Positive feedback mechanism occurs

vigorous contractions of uterus Estrogen Oxytocin (+) from placenta from fetus and mother’s posterior pituitary Induces oxytocin receptors on uterus Stimulates uterus to contract Stimulates placenta to make (+) Prostaglandins Stimulate more vigorous contractions of uterus Figure 28.17

Stages of Labor: Dilation Stage Longest stage of labor: 6–12 hours or more Initial weak contractions: 15–30 minutes apart, 10–30 seconds long Become more vigorous and rapid Cervix effaces and dilates fully to 10 cm Amnion ruptures, releasing amniotic fluid Engagement occurs: head enters the true pelvis

Umbilical cord Placenta Uterus Cervix Vagina (a) Dilation (early) Figure 28.18a

Pubic symphysis Sacrum (b) Dilation (late) Figure 28.18b

Stages of Labor: Expulsion Stage Strong contractions every 2–3 minutes, about 1 minute long Urge to push increases (in absence of local anesthesia) Crowning occurs when the largest dimension of the head distends vulva Delivery of infant

Stages of Labor: Expulsion Stage Strong contractions every 2–3 minutes, about 1 minute long Urge to push increases (in absence of local anesthesia) Crowning occurs when the largest dimension of the head distends vulva Delivery of infant

Perineum (c) Expulsion Figure 28.18c

Stages of Labor: Placental Stage Strong contractions continue, causing detachment of the placenta and compression of uterine blood vessels Delivery of the afterbirth (placenta and membranes) occurs ~30 minutes after birth All placenta fragments must be removed to prevent postpartum bleeding

Uterus Placenta (detaching) Umbilical cord (d) Placental Figure 28.18d