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Prenatal Form and Function – The Making of an Earth Suit


Unit 3:   2 to 3 Weeks

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Play Movie - Amnion and Germ Layers
Movie 3.1 - Amnion and Germ Layers

Germ Layers

By about 15 days (2 weeks, 1 day) following fertilization, stem cells have divided and differentiated into three different germ layers called ectoderm, endoderm, and mesoderm. Each gives rise to major components of specific body structures and organs.1 (See Figure 3.2 below.)

Ectoderm derivatives include the skin, nails, hair follicles, sweat glands, and nerves within the lungs.2 Another specialized layer of cells appearing at this time is the neuroectoderm, which gives rise to the brain, spinal cord, and peripheral nerves, as well as many of the muscles and bones in the face.3

Endoderm forms the lining of the respiratory4 and gastrointestinal tracts and gives rise to major portions of internal organs including the lungs, liver, pancreas, and intestines.5

Mesoderm derivatives include the heart, kidneys, bones, muscles, and blood vessels as well as portions of the reproductive and urinary systems.6

Mesoderm also gives rise to specialized cells called somites (so’mits).7 These cells form most of the skull and ribs as well as the vertebral column or backbone.8

All of these cell layers and cell types work in concert forming the increasingly complex embryo.

somites, upper skeleton, skull
Figure 3.1 - Upper Skeleton
Somites give rise to the skull and upper skeleton.
Copyright © 2002 Lippincott, Williams & Wilkins.
All rights reserved.
These young ballerinas dance in part thanks to their bones and muscles originally formed from their mesoderm.
ectoderm, head, hair, massage
The cells of the skin, nails, and hair follicles are all derived from ectoderm.

Open PDF version of FIG 3.2, Dorsal View of Presomite Embryo (18 Days)
Figure 3.2 - Dorsal View of Presomite Embryo (18 Days) [PDF version of FIG 3.2]
From Gasser RF, 1975, 19. Atlas of Human Embryos. Copyright © 1975 RF Gasser, PhD.
All rights reserved.

Organ and Body System Formation
Figure 3.3 - Organ & Body System Formation
Copyright © 2007 EHD, Inc. All rights reserved.

Open PDF version of FIG 3.4, Right Half of the Late Trilaminar Blastocyst (16-17 Days)
Figure 3.4 - Right Half of the Late Trilaminar Blastocyst (16-17 Days) [PDF version of FIG 3.4]
From Gasser RF, 1975, 14. Atlas of Human Embryos. Copyright © 1975 RF Gasser, PhD. All rights reserved.

brain, spinal cord
Figure 3.5 - Brain and Spinal Cord
Copyright © 2002 Lippincott, Williams & Wilkins.
All rights reserved.

By day 17 the central portion of the thyroid gland appears.9 This important gland will soon regulate the rate of metabolism throughout the rest of the human life cycle.

The embryo’s respiratory system begins to develop by 3 weeks with an outgrowth of the foregut which will form the windpipe or trachea (tra’ke-a).10


The Brain & Spinal Cord - Early Emergence

The site of future brain development is first recognizable with the appearance of the neural plate by 2 weeks, 4 days. By 3 weeks the neural plate thickens first at the head end of the embryo and folds into the neural tube which will form the brain and the spinal cord.11 By 3 weeks, the 3 primary sections of the brain are identifiable. These sections are called the forebrain, midbrain, and hindbrain.12


Vital Systems – Circulation

The complexity achieved by the embryo in just the first 3 weeks of development is incredible. Considering the importance of distributing nutrients to the emerging brain and spinal cord, as well as the rest of the embryo, the early development of the circulatory system is not surprising.

red blood cells
Figure 3.6 - Early Blood Cells
Early red blood cell precursors are present in the yolk sac just three weeks after fertilization!
Copyright © 2002 Lippincott, Williams & Wilkins.
All rights reserved.
Play Movie - Heart and Circulation
Movie 3.2 - Heart and Circulation

Yet the early completion of this body system – the first system to begin functioning – is remarkable.13 By 3 weeks early blood cell precursors appear in the yolk sac.14 This process of blood cell formation is called hematopoiesis. Also by 3 weeks, early blood vessels form throughout the embryo as the network of the early circulatory system begins to take shape.15

In the middle of week 3, only 18 days after fertilization, the embryo’s heart appears.

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1 Gasser, 1975. 7; Moore et al., 2000. 13; O'Rahilly and Müller, 2001. 14, 135.
2 Moore et al., 1994. 9-10.
3 DiFiore and Wilson, 1994. 221.
4 DiFiore and Wilson, 1994. 221.
5 Grand et al., 1976. 796; O'Rahilly and Müller, 1996. 123, 226, 243.
6 Moore and Persaud, 2003. 60; Moore and Persaud, 2003. 80, 83; Sadler, 2005. 9.
7 Moore and Persaud, 2003. 69.
8 Fuse, 1996. 1.
9 Fuse, 1996. 1.
10 DiFiore and Wilson, 1994. 221; Fowler et al., 1988. 793.
11 Moore and Persaud, 2003. 67.
12 Bartelmez, 1923. 236; Moore and Persaud, 2003. 439-440; Müller and O'Rahilly, 1983. 419-420, 429; O'Rahilly and Gardner, 1979. 123, 129; O'Rahilly and Müller, 1984. 422; O'Rahilly and Müller, 1987. 90; O'Rahilly and Müller, 1999a. 47, 52.
13 Moore and Persaud, 1993. 55; Moore and Persaud, 2003. 70.
14 Carlson, 2004. 117; Guyton and Hall, 2000. 958; Metcalf and Moore, 1971. Table 4.1, 173; Moore and Persaud, 2003. 70; Palis and Yoder, 2001. 932.
15 Gilmour, 1941. 28; Navaratnam, 1991. 147-148; O'Rahilly and Müller, 1987. 86.