Fetal Development

Introduction

The fetal period (9-36 weeks) is about continued differentiation of organs and tissues, most importantly this period is about growth both in size and weight.

The long Fetal period (4x the embryonic period) is a time of extensive growth in size and mass as well as ongoing differentiation of organ systems established in the embryonic period and do so at different times. For example, the brain continues to grow and develop extensively during this period (and postnatally), the respiratory system differentiates (and completes only just before birth), the urogenital system further differentiates between male/female, endocrine and gastrointestinal tract begins to function.

Fetal length and weight changes

Textbooks

Human Embryology (3rd ed.) Larson Ch15: Fetal development and the Fetus as Patient p481-499
The Developing Human: Clinically Oriented Embryology (6th ed.) Moore and Persaud
Color Atlas of Clinical Embryology (2nd ed.) Moore, Persaud and Shiota Ch3: 9th to 38th weeks of human development p50-68

Fetal Size

Fetal length change is greatest in the middle period (second trimester).

Fetal Weight

Fetal Neural

During the fetal period there is ongoing growth in size, weight and surface area of the brain and spinal cord. Microscopically there is ongoing: cell migration, extension of processes, cell death and glial cell development.

Cortical maturation (sulcation and gyration) and vascularization of the lateral surface of the brain starts with the insular cortex (insula, insulary cortex or insular lobe) region during the fetal period. This cerebral cortex region in the adult brain lies deep within the lateral sulcus between the temporal lobe and the parietal lobe.

sulcus - a depression or fissure in the surface of the brain. gyration - rotation.

Insular Gyral and Sulcal Development

13-17 gestational weeks - appearance of the first sulcus
18-19 gestational weeks - development of the periinsular sulci
20-22 gestational weeks - central sulci and opercularization of the insula
24-26 gestational weeks - covering of the posterior insula
27-28 gestational weeks - closure of the laeteral sulcus (Sylvian fissure or lateral fissure)

(Data from: Afif A, etal., 2007)

Three-dimensional magnetic resonance imaging and image-processing algorithms have been used to quantitate between 29-41 weeks volumes of: total brain, cerebral gray matter, unmyelinated white matter, myelinated, and cerebrospinal fluid (grey matter- mainly neuronal cell bodies; white matter- mainly neural processes and glia). A study of 78 premature and mature newborns showed that total brain tissue volume increased linearly over this period at a rate of 22 ml/week. Total grey matter also showed a linear increase in relative intracranial volume of approximately 1.4% or 15 ml/week. The rapid increase in total grey matter is mainly due to a fourfold increase in cortical grey matter. Quantification of extracerebral and intraventricular CSF was found to change only minimally.

(Text - modified from Huppi etal., (1998) Quantitative magnetic resonance imaging of brain development in premature and mature newborns. Ann Neurol 43(2):224-235.)

Neural development will continue after birth with substantial growth, death and reorganization occuring during the postnatally (MH - postnatal not described in this current lecture)

Neural Development - Fetal

Fetal Cardiovascular

Fetal Genital

Abnormalities

Critical Periods

Fetal Origins Hypothesis

Maternal derived abnormalities relate to lifestyle, environment and nutrition and while some of these directly effect embryonic development. There is also growing evidence that some effects are more subtle and relate to later life health events. This theory is based on the early statistical analysis carried out by Barker of low birth weight data collected in the early 1900's in the south east of England which he then compared with these same babies later health outcomes. The theory was therefore originally called the "Barker Hypothesis" and has recently been renamed as "fetal origins" or "programming".

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Course Content 2009

Cite this page: Hill, M.A. (2024, May 6) Embryology 2009 Lecture 22. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2009_Lecture_22

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