Developmental Biology: Fertilization to Birth - In Depth Notes

Learning Objectives

• Understand the final stages of sperm maturation
• Understand the process of fertilization
• Understand early embryonic development
• Understand how hormonal support is maintained through pregnancy

Sperm Structure

• Development Stages:
  • Round spermatids are embedded in the apical membrane of Sertoli cells until they transform into sperm.
• Components:
  • Midpiece: Contains mitochondria that produce energy for movement.
  • Tail: Microtubules extend into the tail for motility.
  • Nucleus: Condenses by replacing histones with protamines.

Final Stages of Maturation

• Occur outside the seminiferous tubules:
  • Spermatozoa wash into the rete.
  • Flow through vasa efferentia (efferent ductules) into the epididymis.
  • In the epididymis, fluid is absorbed and sperm are concentrated.
  • By the cauda epididymis, sperm can swim; the process is androgen-stimulated.

Male Accessory Glands

• Location: Male reproductive tract.
• Function: Secrete fluid mixtures that mix with sperm at ejaculation.
  • Composition of Semen: Consult resources (e.g., table 26-3 in the textbook).
  • Secretions protect the reproductive tract against infections and include antioxidants that protect sperm DNA.

Components of Semen

• Fluid Mixture: Comprised of spermatozoa mixed with secretions from accessory glands.
  • About 3 ml of fluid in human ejaculation.
• Seminal Vesicles: Contribute up to 85% of the seminal fluid; secretion includes:
  • Fructose, fibrinogen, prostaglandins, vitamin C, other proteins.

Female Reproductive Structures and Fertilization

• Environment: Low pH (~3.8 - 4.5) is generally hostile to sperm.
• Mechanisms of Selection:
  • Chemical and physical barriers include a muscular sphincter and mucus barriers that allow permissions during fertile windows.
  • Transport of sperm is enhanced by peristaltic waves.

The Endocervix

• Function: Secretes mucus with cyclical variation that guides spermatozoa.
  • Oestrogen promotes watery mucus; progesterone inhibits secretion.
• Sperm Interaction: Sperm can penetrate from day 9; 99% may get stuck or expelled.
  • Provides a protective environment and initiates capacitation.

Capacitation Process

• Duration: Sperm does not fertilize immediately post-ejaculation; capacitation takes place.
  • Components of capacitation:
  • Loss of cholesterol.
  • Membrane destabilization allowing exposure of phospholipids.
  • Activation of phospholipases.
  • Change in membrane phospholipid composition, causing hypermotility.
• Purpose: Prepares acrosome to release enzymes for fertilization.

Fertilization Mechanics

• Timing: Takes place 12-24 hours after ovulation in the distal fallopian tube.
  • Only about 100 sperm reach the egg; they must penetrate the zona pellucida, trigger acrosome release, fuse with the egg membrane, and resume female meiosis.

Timing of Female Meiosis II

• Historical findings:
  • Loeb (1913) emphasized sperm's dual role in egg development and effect on heredity.
• Trigger Mechanism: Sperm causes Ca2+ concentration spikes, inducing oocyte division and hardening the egg coat against polyspermy.
  • Sperm releases an enzyme, PLCζ, responsible for triggering Ca2+ oscillations.

Human Preimplantation Embryo Development

• Timeline:
  • Day 1: Fertilization
  • Day 3: Third cleavage division, cells begin to compact.
  • Day 4: Formation of morula; solid mass of cells.
  • Day 5: Development of blastocyst, consisting of an inner cell mass and trophectoderm, enters the uterus for implantation.

Embryo Transfer in IVF

• Embryos are graded and the best are selected for transfer, usually 2, 3, or 5 days post-egg collection.

Implantation Process

• The trophectoderm becomes chorion, developing into the placenta.
  • Chorionic cells secrete enzymes to penetrate the endometrium, establishing nutrient and gas exchange with maternal blood without mixing.
  • Inner cells form the fetus and associated membranes.

Hormonal Support During Pregnancy

• Key Hormones:
  • hCG helps maintain corpus luteum, preventing pregnancy loss by sustaining progesterone and estrogen levels.
  • Human placental lactogen (hPL) modifies maternal metabolism and supports fetal growth.
  • Estrogen and progesterone are continuously produced to maintain pregnancy and support breast development.

Parturition Process

• Labor involves rhythmic contractions influenced by signals from the fetus and mother.
  • Multiple hormonal and physical feedback mechanisms contribute to the process, with oxytocin and prostaglandins playing crucial roles in muscle contraction and delivery.