Human Development and Birth
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Lecture Outline
Fertilization
o The zygote is formed when the ovum and sperm unite
o Sperm Anatomy• Head – contains the nucleus• Middle piece – contains energy-
producing mitochondria• Tail (flagellum)
o Ovum Anatomy• Zona pellucida• Corona radiata
Fertilization
o 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
• Accidental entry of more than one sperm will halt development of the zygote
Development
o Processes of Development• Cleavage• Growth• Morphogenesis• Differentiation
o 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 – the allantois blood vessels become the
umbilical blood vessels Amnion – contains fluid to cushion and protect the
embryo
Development
o Stages 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
o Labor is marked by regular, long-lasting contractions
o 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
o Expulsion of the mucous plug is usually the first sign that birth is imminent
Birth
o Stage 1• Effacement• Amniotic membrane will probably
ruptured if it has not already• Ends when the cervix has dilated
completely
Birth
o 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
o 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
Birth
o Effects 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)