General Embryology and Gametogenesis

General Embryology

• Images show the stages of fetal development at (A) 3rd month, (B) 5th month, and (C) at birth.

Male and Female Gametes

• Gametes (sperm and ova) are reproductive cells formed from primordial germ cells.
• Spermatogenesis occurs in males, starting at puberty and continuing throughout life.
• Oogenesis starts in females during prenatal life, is arrested, and continues from puberty to menopause.
• Ovulation occurs roughly on day 14 of the ovarian cycle.
• Sperm is approximately 55 microns long and has a head, neck, middle piece, and tail. The head contains the condensed nucleus with hereditary material and acrosomal cap with enzymes for fertilization.
• The middle piece contains mitochondria for energy.
• The tail directs movement towards the ovum.
• The mature ovum is about 120 microns in diameter and contains the ootid (mature oocyte) with the nucleus carrying the mother's hereditary material (22 autosomes and X), cytoplasm as a nutrition source, zona pellucida (glycoprotein coat for sperm attraction), and corona radiata (outer layer formed of follicular cells).

First Week of Development

• Fertilization is the union of a sperm and egg to form a zygote. It occurs in the ampulla of the uterine tube.
• Sperm capacitation involves removing the glycoprotein coat from the acrosomal region of the sperms, taking about 7 hours to be completed
• Sperm penetration through the corona radiata and zona pellucida occurs.
• The sperm fuses with the oocyte's cell membrane and releases its contents (nucleus, neck, middle piece, and axial filament), while the sperm cell membrane remains on the oocyte surface.
• Cortical and zona reactions prevent polyspermy.

Second Week of Development

• Implantation is the embedding of the blastocyst within the endometrium, starting from day 7 and completing by day 11.
• The endometrium is in the secretory phase during implantation, characterized by increased thickness, increased number and size of endometrial cells, and spiral arteries.
• Abnormal implantation sites include ectopic pregnancies (tubal, ovarian, or omental).
• Placenta previa is a pathological condition involving implantation of the fertilized ovum in the lower segment of the uterus, potentially posing risks to both mother and child.

Third Week of Development

• Gastrulation: Blastocyst undergoes restructuring
• Formation of primitive streak triggers the formation of 3 primary germ layers (endoderm, ectoderm, and mesoderm).
• Formation of the chorion, a fetal membrane, begins
• Formation of the chorionic villi, which are crucial for placental function, begins.

Fourth to Eighth Week of Development

• Organogenesis: The formation of most major organs begins.
• Neural crest cells generate a variety of tissues and organs.
• Neural tube development leads to the central nervous system.
• Formation of the brain, spinal cord, and peripheral nerves.
• Formation of the heart and blood vessels.
• Formation of limbs and other anatomical structures.

Fetal Membranes

• The chorion, amnion, and yolk sac are fetal membranes providing nourishment, gas exchange, and protection.
• The placenta, an organ composed of both maternal and fetal tissue, provides metabolic exchange that includes nutrients, oxygen, and waste products.
• Formation of umbilical cord connecting fetus and placenta.

Umbilical Cord

• The umbilical cord has three structures (two arteries and one vein) enclosed in a gelatinous matrix called Wharton's jelly.
• Crucial for fetal development, nutrition, and waste removal during pregnancy.

Yolk Sac

• Blastocyst undergoes a transformation from an inner cell mass to three germ layers.
• The yolk sac produces primordial germ cells for the development of various structures and organs.

Fetal Period and Birth Defects

• Fetal period encompasses the period extending from week 9 of gestation to birth and marked by the rapid maturation of tissues and organs.
• Birth defects result from environmental factors, chromosomal abnormalities, and genetic conditions.
• Different types of twins have differing placental and developmental issues.

Environmental Factors and Chromosomal Abnormalities

• Environmental factors during pregnancy, such as infections, radiation, drugs, and chemicals, can cause birth defects and harm the developing fetus.
• Chromosomal abnormalities like Down syndrome can affect the growth and function of various organ systems.

Fetal Therapy

• Medical interventions during pregnancy can provide treatment for fetal abnormalities or disorders.
• Stem cells therapy and gene therapy are experimental techniques for treating defects and disorders.

Folding

• During the third and fourth weeks of development, the embryo undergoes essential folding for creating its three-dimensional form.

This quiz covers key concepts in general embryology, focusing on fetal development stages and the processes of male and female gamete formation. Learn about spermatogenesis, oogenesis, and the characteristics of sperm and ova. Test your knowledge on the developmental timeline and cellular structures involved in reproduction.