Chapter 29: Human Development

Sperm migration path

Vagina → cervical canal → uterus → uterine tube → isthmus of tube

Fertilization site

Upper 1/3 of uterine tube

Sperm count per ejaculation

~300 million

Fate of most sperm

Die in vagina, fail to reach cervix, attacked by WBCs, or enter wrong tube

Sperm reaching egg

Only ~200 reach vicinity of egg

Fertilization window

12–24 hours after ovulation

Capacitation

Leaching of cholesterol and dilution of inhibitors from sperm membrane

Corona radiata

Layer sperm must clear to reach zona pellucida

Zona pellucida

Glycoprotein layer around egg; sperm binds to this

Acrosomal reaction

Exocytosis of hyaluronidase and acrosin to penetrate egg

Sperm entry

Only head and midpiece enter the egg

Fast block to polyspermy

Depolarization of egg membrane

Slow block to polyspermy

Cortical reaction; fertilization membrane forms

First trimester

Fertilization through 12 weeks; highest embryonic mortality

Second trimester

Weeks 13–24; organ development and human appearance

Third trimester

Weeks 25–40; cellular differentiation and viability outside womb

Preembryonic stage

Days 1–16; includes cleavage, implantation, embryogenesis

Embryonic stage

Day 16–week 8; organogenesis

Fetal stage

Weeks 9–40; organ growth and differentiation

Cleavage

First mitotic divisions producing blastomeres

Morula

Solid ball of ≥16 cells formed ~72 hrs after fertilization

Blastocyst

Hollow structure with trophoblast, embryoblast, blastocoel

Implantation timing

~6 days after ovulation

Trophoblast

Outer layer of blastocyst; contributes to placenta

Embryoblast

Inner cell mass of blastocyst

Blastocoel

Fluid-filled cavity in blastocyst

Embryonic disc

Flattened embryoblast with epiblast and hypoblast layers

Amniotic cavity

Forms between trophoblast and epiblast

Yolk sac

Formed by hypoblast cells; involved in early nutrition

Gastrulation

Epiblast cells migrate to form endoderm and mesoderm

Three germ layers

Ectoderm, mesoderm, endoderm

Ectoderm

Gives rise to skin and nervous system

Mesoderm

Forms muscle, bone, and blood

Endoderm

Forms gut lining and internal organs

Prenatal nutrition 1

Lipid droplets in egg cytoplasm

Prenatal nutrition 2

Uterine milk secreted by endometrium

Prenatal nutrition 3

Trophoblastic nutrition from decidual cells

Prenatal nutrition 4

Placental transfer of nutrients

Umbilical arteries

Two vessels carrying deoxygenated blood to placenta

Umbilical vein

Carries oxygenated blood to fetus

Chorionic villi

Fetal projections into maternal blood in placenta

Placental sinus

Space where maternal blood bathes chorionic villi

Lanugo

Fine, downy fetal hair

Vernix caseosa

Waxy coating protecting fetal skin

Meconium

First neonatal feces; black, tarry, made of bile, cells, amniotic fluid

Foramen ovale

Fetal heart shunt: right atrium → left atrium

Ductus arteriosus

Fetal shunt: pulmonary trunk → aorta

Ductus venosus

Bypasses fetal liver; blood to inferior vena cava

Neonate period

First 6 weeks post-birth

Neonatal adaptations

Cardiopulmonary changes, immune development, thermoregulation

Neonatal jaundice

Due to immature liver processing bilirubin

Premature infant weight

<5.5 lb or <2.5 kg

Respiratory distress syndrome (RDS)

Common in premature infants due to insufficient surfactant

Premature infant thermoregulation

Poor temperature control due to underdeveloped hypothalamus

Premature infant liver problems

Includes hypoproteinemia, edema, clotting issues, jaundice

Teratogens

Environmental agents that can cause birth defects