Untitled Quiz

Embryology Overview

• Embryology is the study of the development of an organism from fertilization to birth.
• It involves the formation of tissues, organs, and systems from a single fertilized cell (zygote).
• The process involves intricate and miraculous cellular differentiation.

General Embryology

• A single-celled zygote develops into a complex multicellular human being.
• The process is continuous, with the zygote differentiating into definitive organ systems and their early functional stages.

Common Terms in Embryology

• Oocyte (Ovum): A mature secondary oocyte, ready for fertilization.
• Sperm (Spermatozoa): Male gamete.
• Zygote: A diploid fertilized cell, with potential to form an embryo.
• Cleavage: Rapid mitotic cell divisions immediately following fertilization.
• Morula: A solid ball of cells formed by cleavage, resembling a mulberry.
• Conceptus: The developing embryo or fetus along with its associated membranes.
• Embryo: The developing human from the first eight weeks, with primordia of all organs and systems formed.

Embryogenesis Stages

• Fertilization: Formation of a zygote.
• Fragmentation: Formation of a blastula.
• Gastrulation: Formation of germ layers and axial organs.
• Histogenesis: Tissue formation.
• Organogenesis: Organ formation.
• System genesis: System formation.

Progenesis

• Period of germ cell (egg and sperm) development.
• Closely related to embryogenesis and the early postembryonic period.

Gametogenesis (Gamete Formation)

• The production of mature male (sperm) and female (ova) gametes.
• Spermatogenesis: Process of sperm formation from primordial germ cells (spermatogonia).
• Oogenesis: Process of ova (egg) formation from primordial germ cells (oogonia).

Meiosis

• A type of cell division that reduces the chromosome number from diploid (46) to haploid (23).
• Essential for the production of gametes.
• Consists of two divisions: meiosis I and meiosis II.

First Meiotic Division (Prophase I)

• DNA replication occurs.
• Each chromosome replicates into sister chromatids.

Mitosis vs. Meiosis

• Mitosis is a form of cell division for growth and repair.
• Meiosis is a specialized type of cell division for gamete formation.

Spermatogenesis details

• Total time taken: 64 days.
• Starts after puberty (13-16 years).
• Comprises: Spermatocytogenesis (mitosis), Meiosis, and Spermiogenesis.

Spermiogenesis

• Transformation of spermatids into mature sperms.
• Changes in shape and structure of spermatids.

Male Reproductive Cells

• Formed in the convoluted seminiferous tubules of the male reproductive gland.
• The sperm has a head, midpiece (containing mitochondria) and tail (for movement).

Sperm Tail

• Several parts contribute to its shape and structure.
• The tail aids in movement.

Female Reproductive Cells

• Ovum is a large, immobile cell.
• Number of oocytes is significantly less than sperm.
• Oocyte has a single X chromosome in its nucleus.
• Cytoplasm contains SER, CG, ribosomes, mitochondria, cortical granules and vitelline layers.

Oogenesis/Ovulation/Ovarian Cycle

• Stages of follicle development, ovulation, and the formation of the corpus luteum.

Yolk Granules in Oogenesis

• Contains proteins, phospholipids and carbohydrates.
• Forms oocyte membranes (oolemma).
• Forms extracellular glycoprotein layer (zona pellucida).

The Egg (and Corona Radiata) at Ovulation

• Structure at the time of ovulation.

Oogenesis vs. Spermatogenesis Similarities

• PGCs originate from the same source and at the same time.
• Occur in sex cells.
• Both undergo two reduction divisions—meiosis.
• Cells from columnar epithelium contribute to form supportive cells—Sertoli cells in males and follicular cells in females.

Oogenesis and Spermatogenesis Differences

• Spermatogenesis begins at puberty, oogenesis begins in prenatal life.
• Meiosis is completed in oogenesis only if fertilization occurs.
• Spermatogenesis has a duration of 64 days, in oogenesis cells may remain dormant for years.
• Cytokinesis is unequal in oogenesis, leading to a single mature ovum and polar bodies.
• Spermatocytes produce two types of cells (23x & 23y), oocytes produce one type (23x).

Fertilization Overview

• Sperm penetrates corona radiata and zona pellucida.
• Acrosomal reaction releases enzymes for penetration.
• Fusion of sperm and egg membranes occurs.
• Prevention of polyspermy (multiple sperm fertilizations).

Fertilization: Generalized Acrosomal Process

• Stages involved in sperm interaction with the egg.
• Membrane fusion, entry of sperm nucleus, fusion of pronuclei.

Cortical Reaction

• Prevents polyspermy after fertilization by changing egg surface.
• Preventing multiple sperm entrance, maintaining genetic integrity.

Fertilization: Formation of male and female pronuclei

• Stages of forming male and female pronuclei after fertilization.

Transport through the Oviduct

• Process of the zygote travels through various stages of the oviduct to the uterus.
• Cleavage (early cell divisions), development into morula, blastocyst, and implantation.

Cleavage Overview

• Series of rapid mitotic divisions in a fertilized ovum, without significant cell growth.
• Forms blastomeres.

Cleavage Types

• Complete and incomplete
• Uniform and uneven
• Synchronous and asynchronous

Embryo Development After 8-cell stage: Compaction

• Embryo undergoes compaction after the 8-cell stage
• Increased cell adhesion leads to compaction.
• Differentiation of trophoblast and inner cell mass.

Formation of the Blastocyst

• Fluid-filled cavity (blastocoele) forms inside the embryo.
• Outer layer of cells (trophoblast) and inner cell mass (ICM) are established.
• Embryo is still enclosed within the zona pellucida.

Inner Cell Mass (ICM)

• Inner cell mass will form the embryo proper.
• It contains the cells destined for becoming the body proper of the embryo.

Hatching of the Blastocyst

• Trophoblast secretes proteases that digest the zona pellucida.
• Allows the embryo to escape and prepare for implantation.

Implantation

• Processes and stages in which the blastocyst adheres to the endometrium of the uterus.
• Several days of adhesion, proliferation and formation of the embryo's structure.

Implantation and Placentation (days 8-13)

• Further differentiation and invasion of maternal tissues.
• Formation of the placenta and the early stages of the embryo
• Key parts such as trophoblast, epiblast, hypoblast are formed.

Gastrulation (Week 3, Days 14-21)

• Transformation of the two-layered epiblast into three primary embryonic germ layers (ectoderm, mesoderm, endoderm).
• Forming the rudimentary structures for organs.