Human Development and Birth Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lecture Outline

Fertilization oThe zygote is formed when the ovum and sperm unite oSperm Anatomy Head – contains the nucleus Middle piece – contains energy- producing mitochondria Tail (flagellum) oOvum Anatomy Zona pellucida Corona radiata

Fertilization oSteps 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 Accidental entry of more than one sperm will halt development of the zygote

Development oProcesses of Development Cleavage Growth Morphogenesis Differentiation oExtraembryonic 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 – the allantois blood vessels become the umbilical blood vessels  Amnion – contains fluid to cushion and protect the embryo

Development oStages of Development Pre-embryonic development  Events of the first week  After fertilization, the zygote divides repeatedly  A morula becomes a blastocyst  Inner cell mass surrounded by the trophoblast  The trophoblast will become the chorion  Each cell has the genetic capability of becoming any tissue

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  The inner cell mass separates from the trophoblast  Gastrulation occurs ­Inner cell mass becomes the embryonic disk ­Primary germ layers (ectoderm, mesoderm, and endoderm) form

Development  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  Development of the heart begins

Fig 18.5

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 eighth weeks  Embryo is easily recognized as human  The head achieves its normal relationship with the body  Nervous system has developed enough to allow reflex actions

Development 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  Placenta supplies the fetus with its nutritional and excretory needs  The umbilical cord transports fetal blood to and from the placenta  The placenta begins to produce progesterone and estrogen

Development Fetal development and birth  Third and fourth months  Head growth slows and the rest of the body increases in length  Epidermal refinements appear  Cartilage begins to be replaced by bone  Possible to distinguish males from females  During the fourth month, the fetal heartbeat can be auscultated  Fifth through seventh months  Fetal movement can be felt by the mother  Lanugo and vernix caseosa  Eyelids are fully open  Eighth through ninth months  Fetus usually rotates so that the head is pointed towards the cervix  Weight gain due to an accumulation of fat

Development Development of male and female sex organs  Gender is determined at the moment of fertilization  Gonads start developing in the seventh week  Genes on the Y chromosome cause:  The production of testosterone ­Causes testes to develop ­Stimulates the wolffian ducts to become male genital ducts  If there is no Y chromosome present  Ovaries develop instead of testes  Müllerian ducts develop into the uterus and uterine tubes  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

Birth oLabor is marked by regular, long-lasting contractions oA 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 oExpulsion of the mucous plug is usually the first sign that birth is imminent

Birth oStage 1 Effacement Amniotic membrane will probably ruptured if it has not already Ends when the cervix has dilated completely

Birth oStage 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 oStage 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

Birth oEffects of Pregnancy on the Mother Early pregnancy  Nausea and vomiting  Loss of appetite and fatigue Weight gain  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 Physiological changes due to placental hormones Increase in pulmonary values Stress incontinence Edema and varicose veins Possibility of pregnancy-induced diabetes Striae gravidarum (stretch marks)