From Egg to Embryo

Pregnancy – events that occur from fertilization until the infant is born

Conceptus – the developing offspring

 Gestation period – from the last menstrual period until birth

 Preembryo – conceptus from fertilization until it is two weeks old

 Embryo – conceptus during the 3rd through the eighth week

 Fetus – conceptus from the 9th week through birth

Fertilization:
Viability for oocyte is 12 to 24 hours & sperm for 24-72 hrs

 For fertilization to occur, coitus must occur no more than: 3 days before ovulation, 24 hrs after ovulation

 Fertilization – when a sperm fuses with an egg to form a zygote

 Fates of ejaculated sperm – not always favorable

 Sperm must undergo capacitation before they can penetrate the oocyte

Capacitation — a series of processes a sperm must go through to undergo fertilization

Placentation:

• Placenta formed from: embryonic trophoblastic tissues & maternal endometrial
  tissues and fully functional by 3rd month

 The chorion develops finger-like villi, which extend to the embryo as umbilical arteries and veins

 Placental barriers are: the chorionic villi & endothelium of embryonic capillaries

 The placenta also secretes – human placental lactogen, human chorionic thyrotropin, and relaxin Figure 28.7 p.1100

Decidua basalis – endometrium that lies between the chorionic villi and the stratum basalis

Decidua capsularis – endometrium surrounding the uterine cavity face of the implanted embryo

Gastrulation:

 In the 3rd week, the blastocyst develops into a gastrula with three primary germ layers: ectoderm, endoderm, and mesoderm

 The inner cell mass subdivides into the upper epiblast and
lower hypoblast forming 2 of the 4 embryonic membranes

 Amnion – epiblast cells form a transparent membrane filled with amniotic fluid providing a buoyant environment, protection & helps maintain homeostasis

 Yolk sac – hypoblast cells that form a sac on the ventral surface of the embryo that forms part of the digestive tube, produces blood cells, blood vessels & is the source of primordial germ cells

 Allantois – outpocketing at the caudal end of the yolk sac is the base for the umbilical cord & becomes part of the urinary bladder

 Chorion – helps form the placenta that encloses the embryonic body and all other membranes

 Primitive streak – raised dorsal groove that establishes the longitudinal axis of the embryo

As cells begin to migrate:

• The first cells that enter the groove form the endoderm which gives rise to the epithelial linings of the digestive, respiratory & urogenital systems.

• The cells that follow push laterally form the mesoderm which forms all other tissues

• The cells remaining on the dorsal surface form the ectoderm which forms the structures of the nervous system and skin epidermis
  

Notochord – rod of mesodermal cells that serves as axial support & serve as primitive tissues from which all body organs derive

Organogenesis:

 Gastrulation sets the stage for organogenesis, whereby the 8th week all organ systems are recognizable

Neurulation – the first event of organogenesis gives rise to the brain and spinal cord

 Ectoderm over the notochord thickens, forming the neural plate that folds inward as a neural groove with prominent neural folds

 By the 22nd day, neural folds fuse into a neural tube, where the anterior end becomes the brain; the rest becomes the spinal cord

 Associated neural crest cells give rise to cranial, spinal, and sympathetic ganglia

Soma — Means body

Specialization of the Mesoderm:

 First evidence is the appearance of the notochord

 Three mesoderm aggregates namely, somites, intermediate mesoderm, and double sheets of lateral mesoderm appear lateral to the notochord

• 40 pairs of somites have three functional parts:
  Sclerotome – produce the vertebrae and ribs
  Dermatome – form the dermis of the skin on the dorsal part of the
  body
  Myotome – form the skeletal muscles of the neck, trunk, and limbs
  

 Intermediate mesoderm - forms the gonads and the kidneys

 Lateral mesoderm - consists of somatic and splanchnic mesoderm

Somatic mesoderm forms:

• dermis of the limbs

• skin in the ventral region

• parietal serosa of the ventral body cavity

• bones

• ligaments
  

Splanchnic mesoderm forms:

• heart and blood vessels

• connective tissues

Effects of Pregnancy: Metabolic Change

 The placenta secretes human placental lactogen (hPL), also called human chorionic somatomammotropin (hCS), which stimulates the maturation of the breasts, promotes growth of the fetus & exerts a maternal glucose-sparing effect; human chorionic thyrotropin (hCT) increases maternal metabolism; parathyroid hormone levels are high, ensuring a positive calcium balance

 Digestive system - morning sickness occurs due to high levels of estrogen & progesterone

 Urinary system – urine production increases

 Respiratory system – nasal congestion may occur &

• Dyspnea (difficult breathing) may develop late in pregnancy
  

 Cardiovascular system – blood volume increases 25-40% & venous pressure from lower limbs is impaired, resulting in varicose veins

Parturition: Initiation of Labor

 Estrogen reaches a peak during the last weeks of pregnancy, causing myometrial weakness and irritability

 Weak Braxton Hicks contractions may take place

 As birth nears, oxytocin and prostaglandins cause uterine contractions

 Emotional and physical stress:

 Activates the hypothalamus

 Sets up a positive feedback mechanism, releasing more oxytocin

Extrauterine Life:

 At 1-5 minutes after birth, the infant’s physical status is assessed based on five signs: heart rate, respiration, color, muscle tone, and reflexes & each observation is given a score of 0 to 2

 Apgar score – the total score of the above assessments; 8-10 indicates a healthy baby

 Once carbon dioxide is no longer removed by the placenta, central acidosis excites the respiratory centers to trigger the first inspiration which requires tremendous effort – airways are tiny and the lungs are collapsed

 Once the lungs inflate, surfactant in alveolar fluid helps reduce surface tension

Transitional Period:

 Unstable period lasting 6-8 hours after birth

The first 30 minutes the baby is alert and active

• Heart rate increases (120-160 beats/min.)

• Respiration is rapid and irregular

• Temperature falls
  

 Activity then diminishes and the infant sleeps about three hours

 A second active stage follows in which the baby regurgitates mucus and debris

 After this, the infant sleeps, with waking periods occurring every 3-4 hours

Breastmilk:

 Colostrum: Solution rich in vitamin A, protein, minerals, and
IgA antibodies is released the first 2–3 days & then followed by
true milk production

• After birth, milk production is stimulated by the sucking infant

Advantages of breast milk for the infant

 Fats and iron are better absorbed

 Its amino acids are metabolized more efficiently than those of cow’s milk

 Beneficial chemicals are present – IgA, immunoglobulins, complement, lysozyme, interferon, and lactoperoxidase

 Interleukins and prostaglandins which prevent overzealous inflammatory responses

 Its natural laxatives help cleanse the bowels of meconium