Development and Heredity: Stages, Physiology, and Adaptations

Development and Heredity

Stages of Development

Human development proceeds through a series of well-defined stages, each characterized by specific cellular and physiological events. These stages ensure the transformation from a single fertilized cell to a fully formed infant.

• Pre-Embryonic Stage (Weeks 1–2):

  • Fertilization: Fusion of sperm and oocyte forms a zygote.

  • Cleavage: Rapid mitotic divisions without growth, producing smaller cells called blastomeres.

  • Morula: A solid ball of 16 or more cells.

  • Blastocyst: A fluid-filled sphere with an inner cell mass (future embryo) and trophoblast (future placenta).

  • Implantation: Blastocyst embeds into the uterine wall.

• Embryonic Stage (Weeks 3–8):

  • Gastrulation: Formation of three germ layers: ectoderm, mesoderm, and endoderm.

  • Organogenesis: Development of major organs such as the heart and brain.

  • By week 8, the embryo has a basic body plan.

• Fetal Stage (Week 9–birth):

  • Growth in size and mass.

  • Functional maturation of organ systems.

  • Third trimester: rapid brain development and fat accumulation.

Fertilization

Fertilization is the process by which male and female gametes unite to form a diploid zygote, initiating development.

• Occurs in the ampulla of the uterine tube.

• Capacitation: Sperm membrane proteins are modified to enable fertilization.

• Acrosome Reaction: Enzymes (hyaluronidase, acrosin) digest the zona pellucida surrounding the oocyte.

• One sperm fuses with the oocyte membrane.

• Cortical Reaction: Zona pellucida hardens to prevent polyspermy (entry of additional sperm).

• The secondary oocyte completes meiosis II, forming the ovum and a polar body.

• Male and female pronuclei fuse, restoring the diploid chromosome number (46 chromosomes).

Implantation

Implantation is the embedding of the blastocyst into the uterine endometrium, establishing the foundation for placental development.

• Begins around day 6–7 after fertilization.

• Trophoblast differentiates into:

  • Cytotrophoblast: Inner layer.

  • Syncytiotrophoblast: Outer, invasive layer that secretes enzymes to embed the embryo.

• Syncytiotrophoblast secretes hCG (human chorionic gonadotropin), which maintains the corpus luteum and prevents menstruation.

• By day 12, the blastocyst is fully embedded, and early maternal blood supply connects with placental structures.

Fetal Circulation

Fetal circulation includes specialized shunts that bypass the nonfunctional fetal lungs and liver, ensuring efficient oxygen and nutrient delivery from the placenta.

• Umbilical vein: Carries oxygenated blood from the placenta to the fetus.

• Ductus venosus: Connects the umbilical vein to the inferior vena cava, bypassing the liver.

• Foramen ovale: Allows most blood from the right atrium to flow directly into the left atrium, bypassing the lungs.

• Ductus arteriosus: Connects the pulmonary trunk to the aorta, further bypassing the lungs.

• Umbilical arteries: Carry deoxygenated blood from the fetus back to the placenta.

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

Summary Table: Fetal Circulatory Shunts

Shunt	Location	Function	Fate After Birth
Foramen ovale	Between right and left atria	Bypasses pulmonary circulation	Fossa ovalis
Ductus arteriosus	Between pulmonary trunk and aorta	Bypasses pulmonary circulation	Ligamentum arteriosum
Ductus venosus	Between umbilical vein and IVC	Bypasses liver	Ligamentum venosum

Stages of Labor

Labor is the process of childbirth, divided into three main stages, each regulated by hormonal and mechanical factors.

• Triggered by estrogen, oxytocin, and prostaglandins.

• Positive feedback: Cervical stretch stimulates oxytocin release, leading to stronger contractions.

• Dilation Stage: Longest stage (6–12+ hours); cervix effaces and dilates to 10 cm; amniotic sac ruptures.

• Expulsion Stage: From full dilation to delivery of the infant; strong contractions and crowning; lasts 20 minutes to 2 hours.

• Placental Stage: Delivery of the placenta within 30 minutes; uterine contractions constrict blood vessels to prevent hemorrhage.

Neonatal Adaptations

After birth, the newborn must rapidly adapt to life outside the womb, with significant changes in respiratory, circulatory, and metabolic systems.

• First breath requires surfactant to reduce alveolar surface tension.

• Closure of fetal shunts (foramen ovale, ductus arteriosus, ductus venosus).

• Brown fat is metabolized to generate heat.

• Kidney and liver functions remain immature, requiring special care.

Lactation

Lactation is the process of milk production and secretion, providing nutrition and immune protection to the newborn.

• Prolactin: Stimulates milk production in mammary glands.

• Oxytocin: Triggers the milk letdown reflex.

• Colostrum: The first secretion, rich in antibodies and nutrients.

• Breast milk: Provides immunity, supports digestion, and helps establish the infant's microbiome.