Human Embryology Overview

Human Embryology

• Definition: Study of human development from zygote to birth.
• Fertilization: Union of sperm and ovum forms a zygote, initiating human development.
• Puberty: Defined by the onset of secondary sex characteristics.
  • Females: 12-15 years old
  • Males: 13-16 years old
  • Menarche: First menstruation in females.
• Gametogenesis: Process of creating gametes (sperm and oocytes).
  • Spermatogenesis: Begins at puberty (males).
  • Oogenesis: Begins during fetal development in females, pauses at meiosis I, resumes at puberty.
  • Gametes: Haploid (1n) containing 23 chromosomes.
  • Sperm: Male germ cells produced in the testes.
  • Oocyte: Female germ cells produced in the ovaries.
    • Secondary oocyte: Mature oocyte.
• Embryo: Developing human in the early stages.
• Zygote: Diploid (2n) cell formed from sperm and ovum (the beginning of a new human).
• Fertilized ovum: Impregnated secondary oocyte.
• Conceptus: All products of conception (embryo + membranes like placenta).
• Primordium: First indication of an organ/structure
• Embryonic period: Weeks 1-8
• Fetal period: Week 9 to birth
• Differentiation and growth: Prominent during fetal period, high organ function maturation in months 3 & 4, notable weight gain in final months.
• Trimester: Three-month period of pregnancy (nine months divided into three).
• Abortion: Expulsion of a non-viable embryo/fetus.
• Viable: Mature enough to survive outside the uterus.
• Significance: Understanding normal development, causes of anomalies
• Human embryonic stem cells: Pluripotent (capable of differentiating into many cell types)
• Anatomical terms:
  • Anterior/Posterior, Superior/Inferior, Lateral/Medial, Caudal/Cranial, Dorsal/Ventral
  • Coronal (frontal), Transverse (horizontal), Sagittal (longitudinal) planes
• Somatic/Germ Cells
  • Somatic cells: 2n=46 (diploid), divide by mitosis.
  • Germ line cells: 2n=46 (diploid), divide by meiosis
  • Gametes (sex cells): 1n=23 (haploid)

Cell Division

• Mitosis: Division of somatic cells, producing two identical cells.
  • DNA replication precedes mitosis.
  • Stages: Prophase, Metaphase, Anaphase, Cytokinesis.
• Meiosis: Division of germ line cells, producing gametes (haploid).
  • Two rounds of division (Meiosis I & Meiosis II)
  • Crossover: Exchange of chromatid segments during Meiosis I, crucial for genetic variation.

Gametogenesis

• Spermatogenesis (males): Transformation of spermatogonia (primitive germ cells) to spermatozoa.
  • Occurs in seminiferous tubules of the testes.
  • Takes about 64 days.
  • Three stages: spermatocytogenesis (growth to spermatocytes), meiosis (spermatocytes to spermatids), spermiogenesis (spermatids to sperm).
  • Mature Sperm: Head (nucleus & acrosome), neck (mitochondria), tail (for motility)
• Oogenesis (females): Production of female gametes (ova).
  • Starts in fetal period, pauses at meiosis I.
  • Resumes at puberty, produces one viable ovum per primary oocyte.
  • Developing follicles (primary, secondary, mature) surround the developing oocytes.

Ovulation and Fertilization

• Transport: Oocytes travel through the fallopian tubes, sperms through the male tract.
• Capacitation: Maturation of sperms enabling fertilization. Glycoprotein coat and seminal proteins are removed from the acrosome surface.
• Viability: Oocytes viable for fertilization for about 12 hours; sperms are viable for longer periods
• Fertilization: Fusion of sperm and oocyte to form zygote. Occurs mostly in the ampulla of the uterine tube.
  • Stages of fertilization: penetration of corona radiata, penetration of zona pellucida, fusion of oocyte and sperm membranes, completion of oocyte's second meiotic division, fusion of male and female pronuclei, breakdown of pronuclear membranes.

Cleavage of The Zygote

• Mittic Divisions: Repeated mitotic divisions of the zygote.
• Blastomeres: Smaller cells produced after cleavage.
• Morula: Solid ball of 12-32 cells; develops from zygote
• Blastocyst: Fluid-filled hollow ball, inner cell mass (embryo), trophoblast (placenta).

Implantation

• Attachment: Blastocyst attaches to the uterine wall.
• Trophoblast: Outer layer, forms part of the placenta.
  • Two layers: cytotrophoblast (inner) and syncytiotrophoblast (outer)

Explore the fascinating stages of human development from fertilization to birth. This quiz covers key concepts such as gametogenesis, puberty, and the formation of zygotes and embryos. Test your knowledge on the intricacies of human embryology.