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)