Basics of embryology

Summary

Embryology is the study of the origin and prenatal development of an organism, comprising the germinal (first two weeks from conception), embryonic (third to eighth week from conception), and fetal period (ninth week from conception until birth). Gestation in embryology refers to conceptional age, i.e., the time period from conception to birth, and should be distinguished from gestational age, an obstetrics term, which is measured in weeks following the first day of the mother's last menstrual period. Primordial cells develop during the embryonic period and migrate to the developing gonads. Further maturation into fertile oocytes and spermatozoa occurs via meiosis during different stages of male and female organism development. Embryonic stem cells are pluripotent and can differentiate into almost any type of tissue, thus being of great interest in the research and development of new therapeutic approaches. For more information on the morphogenesis of organ systems, see the article on embryogenesis.

Overview

Periods of embryonic development [1]
Period Conceptional age Events
Germinal period 1–2 weeks
Embryonic period

3–8 weeks

Fetal period 9 weeks to birth
  • Growth and differentiation of tissue and organs formed during the embryonic period

Organ system development begins during the embryonic period, while organ maturation occurs during the fetal period!

Germ cell development (gametogenesis)

Primordial germ cell development

Germline

  • Definition: the cells that develop into mature germ cells (gametes), which pass genetic material on to progeny
  • Process: involves meiosis of primordial germ cells within the gonads

Four functional spermatids or an oocyte with three polar bodies are produced from one primordial germ cell.

All of the oocytes that will ever be produced are formed during the fetal period, whereas sperm production begins at puberty and never stops!

Meiosis

Overview [2][3]

Stages of meiosis I

Stage Number of cells Chromosome set DNA content
Prophase I
  • 1
  • 2n
  • 4C
Metaphase I
Anaphase I
  • Homologous chromosome pairs separate via the disintegration of the chiasmata.
Telophase I
  • 2
  • 1n
  • 2C

Stages of meiosis II

Stage Description Number of cells Chromosome set DNA content
Prophase II
  • Chromosomes condense.
  • Formation of the spindle apparatus occurs.
  • 2
  • 1n
  • 2C
Metaphase II
Anaphase II
Telophase II
  • 4
  • 1n
  • 1C

Stem cells

  • Definition: cells capable of differentiating into specialized cells (potency) as well as replicating while maintaining their undifferentiated state (self-renewal)
  • Cell division
  • Characteristics
    • Totipotency (omnipotency): the ability of a cell to differentiate into all cell types, including extraembryonic (placental) cells
    • Pluripotency: the ability of a cell to differentiate into all cell types, with the exception of extraembryonic cells (for example, the cells of the ectoderm, which can differentiate into nerve or skin cells, but not into other germ layer cells)
    • Multipotency: the ability of a cell to differentiate into more than one related cell type (for example, hematopoietic stem cells, which can differentiate into myeloid or lymphoid cells)
  • Classification

Hematopoietic stem cell transplantation is used to treat hemato-oncologic conditions, e.g., leukemias.

Molecular biology of embryogenesis

Molecules involved in embryogenesis [4][5][6]
Types of molecules Subgroup/components Function
Transcription factors
  • Craniocaudal organization of embryo
  • Positioning of limbs
  • Development and patterning of the skin and limbs
  • Activation or inhibition of genes for growth and differentiation
  • Regulation of muscle development
Growth factors
  • Transforming growth factor β (TGF-β) superfamily
  • Differentiation of the nervous system
  • Produced at the apical ectodermal ridge
  • Regulates development of the limbsby enhancing mitosis of the mesodermal cells
  • Skeletal and muscle growth
  • Nerve growth factor (NGF) family
  • Neuronal survival and differentiation
  • Is highly expressed at the zones of polarizing activity at the base of limb buds
  • Regulates differentiation of somites along the anteroposterior axis and the neural tube
  • Mutations cause holoprosencephaly (due to failed forebrain cleavage)
  • Wnt (e.g., Wnt-7 gene)

Cell adhesion molecules

  • Binding of a neuron to a neighboring cell
  • Cell-to-cell adhesion (e.g., as a component of desmosomes)
    • Increased expression of E-cadherin, for example, mediates binding between blastomeres.
    • Decreased expression of E-cadherin enables cell migration during embryogenesis.
Intercellular channels
  • Communication between neighboring cells
  • Facilitation of signaling molecules (e.g., retinoic acid) channel crossing to regulate gene activity.
Extracellular matrix
  • Binding, transportation, and distribution of signaling molecules
Relevant hormones involved in embryogenesis
  • Differentiation of sexual organs

The FGF gene triggers the Fetal Growth of Fingers.

Cellular processes of development

Cellular developmental processes [4]
Process Description Examples of the process Examples of associated pathology
Apoptosis

Programmed cell death

Fragmentation and, thus, the separation of the webbed embryonic fingers and toes

Syndactyly

Differentiation

The process by which primitive (e.g., stem cells) become specialized cells

Spermatogenesis and oogenesis

Infertility

Fusion

Joining of two or more cells, epithelium, or tissue

Linea alba (abdominal wall) is formed by by fusion of the aponeurosis of abdominal muscles

Gastroschisis

Membrane of the oocyte fuses with a spermatozoid during fertilization

Infertility

Medial facial prominences are fused to form the midline of the nose and philtrum of the upper lip

Cleft lip

Anterior ⅔ and posterior ⅓ of the tongue

N/A

Urethral folds of the penis

Hypospadias

Heart septae are formed by fusion of endocardial cushions

Endocardial cushion defects, e.g., persistent foramen ovale

Migration

Movement of the cells to a specific destination

Neural crest cells along the gastrointestinal tract

Hirschsprung disease

Neural crest cells to the heart

Conotruncal cardiac abnormalities

Primordial germ cell from the yolk sac to the undifferentiated gonads

Disorders of sex development

Neuronal cells during fetal brain development

Lissencephaly: underdevelopment of cerebral grooves and folds

Proliferation

Increase in the number of cells (mediated by cell division)

Gastrulation

Conjoined twins

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  • 2. Jorde L, Carey J, Bamshad M. Medical Genetics . Elsevier; 2015.
  • 3. Dudek. High-Yield Embryology. Wolter Kluwers.
  • 4. Sadler TW. Langman's Medical Embryology. LWW; 2018.
  • 5. Gartner LP. Textbook of Histology. Elsevier; 2017.
  • 6. Theunissen TW, Jaenisch R. Mechanisms of gene regulation in human embryos and pluripotent stem cells. Development. 2017; 144(24): pp. 4496–4509. doi: 10.1242/dev.157404.
  • Kaplan. USMLE Step 1 Anatomy Lecture Notes 2016. Kaplan Publishing; 2015.
  • Le T, Bhushan V,‎ Sochat M, Chavda Y, Abrams J, Kalani M, Kallianos K, Vaidyanathan V. First Aid for the USMLE Step 1 2019. New York, NY: McGraw-Hill Medical.
last updated 11/22/2020
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