Chapter 18 Lecture Outline See separate PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

18.1 Fertilization

Introduction Fertilization – penetration of the ovum by one sperm The zygote is formed when the genetic material of the ovum and sperm unite

Sperm and ovum anatomy Sperm Anatomy Head – contains the nucleus of 23 chromosomes and very little cytoplasm; capped by the acrosome that contains digestive enzymes Middle piece – contains energy-producing mitochondria Tail (flagellum) – for swimming

Ovum anatomy Ovum contains cytoplasm, organelles , and 23 chromosomes The plasma membrane is surrounded by the zona pellucida The corona radiata surrounds the zona pellucida

Steps of Fertilization Several sperm penetrate the corona radiata Several sperm attempt to penetrate the zona pellucida One sperm enters the ovum Acrosome releases digestive enzymes When a sperm binds to the ovum, their plasma membranes fuse and this sperm enters the ovum Sperm and ovum nuclei fuse to produce the zygote

Prevention of polyspermy Accidental entry of more than one sperm will halt development of the zygote When the one sperm fuses to the ovum cell membrane, the membrane depolarizes Release of cortical oocyte granules with lysosomal enzymes Zona pellucida becomes impermeable to any other sperm

Fertilization

18.2 Development

Processes of Development Cleavage – rapid cell division (mitosis) with no size increase Growth – size increase Morphogenesis – shaping of the embryo and movement of cells Differentiation – groups of cells take on different structures and functions

Extraembryonic membranes Are not part of the embryo and fetus Functions in humans: Chorion – develops into the fetal half of the placenta Yolk sac – first site of blood cell formation Allantois – contains blood vessels that become the umbilical blood vessels Amnion – contains fluid to cushion and protect the embryo

Extraembryonic membranes

Stages of Development Human gestation of 280 days Pre-embryonic development Events of the first week After fertilization, the zygote divides repeatedly (cleavage) A morula becomes a blastocyst Inner cell mass surrounded by the trophoblast; becomes the embryo The trophoblast will become the chorion Each cell has the genetic capability of becoming any tissue

Pre-embryonic development

Embryonic development Second week Embryo usually begins the process of implantation If implantation is successful, the female is clinically pregnant An ectopic pregnancy occurs if the embryo implants in the uterine tube The trophoblast begins to secrete HCG Acts like LH Stimulates corpus luteum to secrete progesterone and the endometrium is maintained

Second week, cont The inner cell mass separates from the trophoblast The yolk sac and amnion form Gastrulation occurs Inner cell mass becomes the embryonic disk Primary germ layers (ectoderm, mesoderm, and endoderm) form

Embryonic germ layers

Third week The nervous system becomes evident Neural plate Neural groove Neural folds Neural tube forms – later becomes the brain and spinal cord If the neural tube fails to close completely, a neural tube defect will result – spina bifida or anencephaly

Third week, cont Development of the heart begins Begins with right and left heart tubes Tubes fuse and begin to pump blood Arteries and veins grow from the heart tube Tube twists so that the vessels are all anterior

Embryonic development

Fourth and fifth weeks Exchange of gases, nutrients, and wastes can take place across the capillary network within the chorionic villi Blood vessels of the allantois become the umbilical blood vessels The umbilical cord completes formation Limb buds appear The head enlarges and the sense organs become more prominent

Sixth through eight weeks Embryo is easily recognized as human The head achieves its normal relationship with the body with the development of the neck Nervous system has developed enough to allow reflex actions Embryo is about 1.5 inches long All organ systems have formed

Human embryo at 5 weeks

Placenta Usually fully functional by the end of the embryonic period Two portions: Fetal portion composed of chorionic tissue Maternal portion composed of uterine tissue Placental membrane – epithelium of embryonic capillary and epithelium of a chorionic villus Placenta supplies the fetus with its nutritional and excretory needs – digestive system, lungs, and kidneys do not function

Placenta, cont The umbilical cord transports fetal blood to and from the placenta Two umbilical arteries carry fetal blood to the placenta for gas and waste removal One umbilical veins carries oxygen and nutrients from the placenta to the fetus The placenta begins to produce progesterone and estrogen to maintain the pregnancy

The placenta

Fetal development Third and fourth months Head growth slows and the rest of the body increases in length Epidermal refinements appear – fingernails, nipples, eyelashes, eyebrows, and hair Cartilage begins to be replaced by bone Possible to distinguish males from females During the fourth month, the fetal heartbeat can be auscultated About 6 inches long and weighs 6 oz.

Fetal development, cont Fifth through seventh months Fetal movement can be felt by the mother Lanugo and vernix caseosa Eyelids are fully open About 12 inches long and weighs 3 lbs. Eighth through ninth months Fetus usually rotates so that the head is pointed towards the cervix Weight gain due to an accumulation of fat About 20½ inches long and weighs 7½ lbs.

Human Development Summary

Five to seven month fetus

Development of male and female sex organs Gender is determined at the moment of fertilization At six weeks, both males and females have Wolffian and Müllerian ducts Gonads start developing in the seventh week Genes on the Y chromosome cause the production of testosterone Causes testes to develop from indifferent tissue Stimulates the Wolffian ducts to become male genital ducts and enter the urethra Testes secrete anti-Müllerian hormone that causes the Müllerian ducts to regress

Development of male and female sex organs, cont. If there is no Y chromosome present Ovaries develop instead of testes Müllerian ducts develop into the uterus and uterine tubes and Wolffian ducts regress At 14 weeks, both ovaries and testes are in the abdominal cavity During the last trimester, ovaries and testes descend into the pelvic cavity Testes then descend into the scrotal sacs via the inguinal canal

External genitalia At six weeks – a small bud of indifferent tissue is seen between the legs At nine weeks – urogenital groove appears By fourteen weeks The groove has disappeared in males and the scrotum has formed In females the groove becomes the vaginal opening and the labia majora and labia minora form

Male & female reproductive organs

18.3 Birth

Introduction Labor is marked by regular, long-lasting contractions A positive feedback mechanism causes the onset and continuation of labor Stretching of cervix stimulates oxytocin release Oxytocin stimulates the uterine muscles Uterine contractions push the fetus forward causing the cervix to stretch more Expulsion of the mucous plug is usually the first sign that birth is imminent

Stages of Birth (parturition) Effacement Amniotic membrane will probably ruptured if it has not already Ends when the cervix has dilated completely (about 10 cm)

Stages of Birth, cont Stage 2 Uterine contractions occur every 1-2 minutes and last about 1 minute each Baby’s head descends into the vagina The baby is delivered Stage 3 The placenta is delivered About 15 minutes after delivery of the baby, the placenta is dislodged from the uterus and expelled into the vagina

Stages of parturition

Effects of Pregnancy on the Mother Early pregnancy Nausea and vomiting Loss of appetite and fatigue Then weight gain due to: Breast and uterine enlargement Weight of the fetus Amount of amniotic fluid Size of the placenta Increase in her own body fluid Storage of proteins, fats, and minerals

3. Physiological changes due to placental hormones Progesterone Relax smooth muscle Reduced uterine motility Reduced maternal immune response to fetus Estrogen Increased uterine blood flow Increased renin-angiotensin-aldosterone activity Increased protein biosynthesis by liver Peptide hormone – increased insulin resistance

Other maternal changes Increase in pulmonary values Stress incontinence Edema and varicose veins Possibility of pregnancy-induced diabetes Striae gravidarum (stretch marks)