Introduction to Embryology and Human Development

Introduction to Embryology and the First Week of Human Development

• Embryology is the study of the development of gametes (sex cells), embryos, and fetuses, including congenital disorders (birth defects).
• Key stages of human development, from fertilization to the blastocyst are illustrated. These include the 2-cell, 4-cell, 8-cell, and 16-cell stages.
• The development of embryos and fetuses over time is shown graphically.
• Recommended reading materials include Langman's Medical Embryology (15th ed., Chapters 2 & 3) and Embryology by Ronald Dudek (6th ed., Chapter 2).

Learning Objectives

• The concept of embryology is discussed.
• Oogenesis (female gametogenesis) and spermatogenesis (male gametogenesis) are described.
• The first week of human development, from the ovarian cycle to blastocyst formation is detailed.
• Examples of birth defects of the first week of development are provided.

Clinical Correlates: Numerical Chromosomal Abnormalities

• Normal maturation division, compared to nondisjunction during the 1st and 2nd meiotic divisions are shown diagramatically.
• An example of a chromosome # is 23.
• Euploid vs aneuploid, trisomy, and monosomy are described for chromosome 23.
• This section identifies specific trisomies, like Down syndrome (Trisomy 21), Edward's syndrome (Trisomy 18), and Patau syndrome (Trisomy 13). Each trisomy has varying associated risks (birth defects) with differing maternal ages.

Clinical Correlates: Sex Chromosome Trisomies

• Klinefelter syndrome (XXY), Trisomy X, and Jacob's syndrome (XYY) are presented with their common traits and frequency per gender.

Clinical Correlates: Sex Chromosome Monosomy

• Turner syndrome (45,X) is an example, highlighting its characteristics.
• The main cause of spontaneous abortion—nondisjunction in the male gamete accounts for a large percentage of pregnancy termination cases.

The risk of aneuploidy increases with maternal age

• This is presented in graphical form, highlighting the relationship between maternal age and the increased risk of aneuploidy (abnormal number of chromosomes), specifically in relation to Down Syndrome.

Development of the male gametes: Spermatogenesis

• Diagrams are given of the key steps, from spermatogonium (germ cell) to spermatids to spermatozoa (sperm cells).
• Key stages of spermatogenesis before birth, through adolescence, and are explained, with the cell division stages highlighted.

Fertilization

• The process of fertilization in the human reproductive system is described. This includes the movements of sperm through the female reproductive tract and the importance of muscular contractions to move sperm cells into the uterine tube.
• The time taken for sperm to reach the fallopian tube can range from 30 minutes to 6 days.
• Fertilization occurs in the ampullary area of the uterine tube.

Spermatozoa undergo processing to become competent for fertilization

• Capacitation is crucial for sperm to be ready for fertilization.
• Acrosome reaction occurs when the sperm reaches the unfertilized egg's surface and allows enzymes to be released to penetrate layers of the egg.

Fertilization takes place in three phases

• Phase one is the penetration through the Corona Radiata of the egg.
• Phase two is the penetration through the Zona Pellucida
• Phase three is the fusion of the sperm and the oocyte membranes in the egg.
• Three ways the egg responds to fertilization described.
• A key element of the egg response in phase three is preventing the fertilization by multiple sperm.

Fertilization results in the formation of the zygote

• The result of the process is a zygote, and this is a genetically unique organism.
• Identical twins from a split zygote are part of the description.
• Fraternal twins are from two separate zygotes.

Cleavage

• This stage is a rapid series of cell divisions that lead to multicellular embryos without changing the overall size. The cells are called blastomeres.
• Compaction segregates the inner and outer cells.
• The morula, and blastula stages are described as key stages of cleavage.

Blastocyst Formation

• Fluid enters the morula.
• The inner cell mass produces the embryo.
• The outer cell mass forms the trophoblast.
• The zona pellucida dissipates to allow for implantation and development.

Blastocyst Implantation

• Trophoblast cells secrete enzymes to penetrate the uterine lining, the endometrium.
• The immune system adjusts to allow the "foreign" embryo to be accepted.
• The inner cell mass (embryoblast) and the outer cell mass (trophoblast) along with the uterine epithelium and stromas are key structures involved during this stage.
• Days 4.5 to 6 are highlighted in terms of the blastocyst implantation process.

The uterine wall consists of three layers

• These layers are illustrated.
• The layers are: endometrium, myometrium, and perimetrium.

The endometrium undergoes dramatic changes during the menstrual cycle

• The endometrium adjusts for pregnancy.
• The cycle occurs from puberty to menopause and is hormonally driven, cycling around 28 days.
• The phases within a given cycle are illustrated.

During a menstrual cycle, the endometrium passes through three phases

• The phases within a given cycle: menstrual, follicular/proliferative, and secretory/progestational, are illustrated and described.
• The endometrium undergoes changes depending on whether or not fertilization occurs.
• The phases are tied directly to hormone action in the body by estrogen and progesterone.

Clinical correlates: Abnormal Implantation

• Ectopic pregnancies occur when the embryo implants outside the uterus.
• Locations include the fallopian tube (90%).
• These pregnancies are life-threatening and require termination.

Clinical correlates: Assisted Reproductive Technology (ART)

• ART procedures are used to treat infertility.
• Examples of these procedures are highlighted including IVF, and in vitro fertilization (IVF).
• Complications are described that may occur from various procedures.

IVF success depends on the maternal age

• Older or younger women are more or less likely to conceive after IVF treatment.
• Risks exist for IVF pregnancies, and these are presented.
• Issues of multiple births resulting from IVF procedures are addressed, noting that laws are in place concerning implantation to reduce these occurrences.

This quiz explores the essential aspects of embryology, focusing on the first week of human development. It covers processes such as oogenesis and spermatogenesis, along with the key stages of embryonic growth. Delve into congenital disorders and clinical correlates tied to chromosomal abnormalities for a comprehensive understanding.