Chapter 28 Pregnancy and Human Development Notes

Pregnancy and Human Development

Introduction

Pregnancy and human development encompass the transformation of a single-celled, fertilized egg into a fully developed individual. This process involves embryology (the study of prenatal development) and developmental biology (the study of changes in form and function from fertilization through old age).

• Embryology: Focuses on the first eight weeks after fertilization, when the major organ systems form.

• Developmental Biology: Examines changes throughout the lifespan, including growth, differentiation, and aging.

Sperm Migration

Sperm migration is essential for fertilization, requiring sperm to traverse the female reproductive tract to reach the egg.

• Egg must be fertilized within 12 to 24 hours of ovulation to survive.

• Most sperm do not reach the egg due to acidic vaginal environment, immune responses, and cervical mucus.

• Only about 200 out of millions of sperm reach the vicinity of the egg.

• Sperm movement is aided by tail motion and female physiological factors (e.g., uterine contractions, cervical mucus).

Sperm Capacitation

Capacitation is the process by which sperm become capable of fertilizing an egg, involving changes in the sperm membrane and increased motility.

• Takes about 10 hours in the female reproductive tract.

• Cholesterol is removed from the sperm membrane, and inhibitory factors are diluted.

• Membrane becomes fragile, allowing Ca2+ influx and increased motility.

Fertilization

Fertilization is the union of sperm and egg, resulting in the formation of a diploid zygote.

Fertilization 1: Acrosome Reaction

• Sperm are viable for up to 6 days after ejaculation.

• Acrosomal enzymes digest the zona pellucida, allowing sperm to penetrate the egg.

Fertilization 2: Enzymatic Penetration

• Acrosomal enzymes (hyaluronidase, proteases) break down barriers around the egg.

• Only one sperm typically succeeds in fertilizing the egg; others are blocked by changes in the egg membrane.

Fertilization 3: Fusion of Genetic Material

• Combines haploid (n) sets of chromosomes from sperm and egg to form a diploid (2n) zygote.

• Polyspermy (fertilization by more than one sperm) is prevented by fast and slow block mechanisms.

Haploid (n)	Haploid (n)	Diploid (2n)
Sperm	Egg	Zygote

Fertilization 4: Prevention of Polyspermy

• Fast block: Depolarization of the egg membrane prevents further sperm entry.

• Slow block: Release of cortical granules hardens the zona pellucida.

Meiosis II

The secondary oocyte completes meiosis II only if fertilization occurs, resulting in the formation of the ovum and a second polar body.

• Pronuclei from sperm and egg fuse, restoring the diploid chromosome number.

The Pre-Embryonic Stage

The first two weeks of development are called the pre-embryonic stage, involving cleavage, implantation, and embryogenesis.

• Cleavage: Rapid mitotic divisions producing a multicellular blastomere.

• Morula: Solid ball of 16 cells resembling a mulberry.

• Blastocyst: Hollow sphere with an inner cell mass (embryoblast) and outer layer (trophoblast).

Migration of the Conceptus

The blastocyst migrates to the uterus and implants in the endometrium, usually 6 days after ovulation.

• Trophoblast differentiates into two layers: syncytiotrophoblast (invades endometrium) and cytotrophoblast (retains cell boundaries).

• Secretes human chorionic gonadotropin (hCG) to maintain the corpus luteum.

Implantation

Implantation is the process by which the blastocyst attaches to and embeds in the uterine wall.

• hCG maintains the corpus luteum, which secretes estrogen and progesterone to support early pregnancy.

• Placenta eventually takes over hormone production.

Embryogenesis

Embryogenesis involves the formation of the three primary germ layers (gastrulation):

• Ectoderm: Forms the nervous system, skin, and related structures.

• Mesoderm: Forms muscle, bone, blood, and connective tissues.

• Endoderm: Forms the lining of the digestive and respiratory tracts, and associated organs.

Ectopic Pregnancy

An ectopic pregnancy occurs when the blastocyst implants outside the uterus, most commonly in the uterine tube (tubal pregnancy).

• Can result in serious complications and requires medical intervention.

The Embryonic and Fetal Stages

After the germ layers form, the embryo undergoes organogenesis, and by the end of week 8, all organ systems are present. The fetal stage is characterized by growth and maturation of these systems.

• First trimester: Most critical for organ development; most birth defects originate here.

• Second trimester: Organs continue to develop; fetus grows rapidly.

• Third trimester: Organs mature to support life outside the womb.

Embryonic Folding and Organogenesis

Embryonic folding transforms the flat embryonic disc into a cylindrical body, forming the body cavities and organ systems.

• By week 8, all organs are present in rudimentary form.

Embryonic Membranes

Accessory organs develop with the embryo, including the placenta, umbilical cord, and four embryonic membranes: amnion, yolk sac, allantois, and chorion.

• Amnion: Transparent sac that encloses the embryo, filled with amniotic fluid for protection.

• Chorion: Outermost membrane, forms part of the placenta.

• Yolk sac: Early blood cell formation.

• Allantois: Forms the umbilical cord and urinary bladder.

The Placenta and Umbilical Cord

The placenta facilitates nutrient and gas exchange between mother and fetus. The umbilical cord contains two arteries and one vein, connecting the fetus to the placenta.

• Placental barrier is thin, allowing exchange but protecting the fetus from some harmful substances.

• Some drugs and pathogens can cross the placenta, causing birth defects.

Fetal Development

By the end of 8 weeks, all organ systems are present. The fetus grows rapidly, with most weight gained in the last 10 weeks of gestation.

• Fetal circulation includes shunts (foramen ovale, ductus arteriosus) to bypass the lungs.

Blood Circulation in the Fetus and Newborn

• Most blood bypasses the pulmonary circuit via the foramen ovale and ductus arteriosus.

• At birth, these shunts close, and normal adult circulation is established.

Birth Defects

Birth defects (congenital anomalies) are structural or functional abnormalities present at birth. They may result from genetic, environmental, or unknown causes.

• Teratogens: Agents that cause birth defects (e.g., drugs, alcohol, infections).

• Fetal alcohol syndrome: Characterized by growth retardation, facial abnormalities, and nervous system defects.

Twins

• Dizygotic (fraternal) twins: Two eggs fertilized by separate sperm; genetically distinct.

• Monozygotic (identical) twins: One egg fertilized by one sperm, then splits; genetically identical.