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Prenatal Form and Function – The Making of an Earth Suit

  
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Unit 1:   The First Week

  Applying the Science:  Ectopic Pregnancy
 

Fertilization – Forming a Single Cell Embryo

Biologically speaking, fertilization (or conception) is the beginning of human development.1 Fertilization normally occurs within several hours of ovulation2 (some authors report up to 24 hours3) when a man’s sperm, or spermatozoon, combines with a woman’s egg, or secondary oocyte, inside a woman’s uterine tube (usually in the outer third of the uterine tube called the ampulla).4

Fertilization begins with the spermatozoon contacting the cells surrounding the oocyte and ends with the mixing of the 23 male and 23 female chromosomes.5 [More about fertilization] The result is a single-cell embryo called a zygote,6 meaning "yoked or joined together,"7 and it is the first cell of the human body.

The zygote, like the oocyte, is encased by its protective covering, the zona pellucida, [More about the zona] 8 and contains 46 unique chromosomes with the entire genetic blueprint of a new individual. Chromosomes contain tightly packed, tightly coiled molecules called DNA.9 [More about DNA] Amazingly, DNA contains all the instructions needed for this single-cell embryo to develop into an adult.

oocyte, sperm, spermatozoa, fallopian tube, egg
Figure 1.1 - An Oocyte and Spermatozoa
LifeART
Copyright © 2002 Lippincott, Williams & Wilkins.
All rights reserved.
 

The First Cell Division

The final steps in zygote formation include replication of the male and female DNA and the alignment of chromosomes in preparation for the first cell division through mitosis (mi-to’sis).10 The chromosomes assume a formation called a cleavage spindle, which is a phase of mitosis.

As the 2 sets of chromosomes migrate to opposite ends of the zygote, a crease begins to form along the equator marking the impending line of division.11 (Fig xx) The zygote or single-cell embryo completes the first cell division approximately 24 to 30 hours after fertilization.12 The process of repeated cell division is called cleavage.13




 

The Closer Look Series:

  

Fertilization




Play Movie - Cell Division
Movie 1.2 - Cell Division
 

Mitosis – Dynamic Division, Magnificent Multiplication

After the first cell division, these 2 daughter cells are called blastomeres and proceed with mitosis independently. Two cells become four, four become eight, and so on as these and subsequent daughter cells divide repeatedly.36 The first days of cell division do not increase the size of the embryo because the blastomeres become smaller as their numbers increase. Additionally, after the third round of cell division, the cells become more tightly packed in a process called compaction. As compaction proceeds, the cells of the embryo divide into 2 populations with distinct destinies. (described below)

Subsequent cell divisions occur take about 8 hours to reach completion as will become apparent by reviewing Figure 1.8 below. Replication of DNA over that time period requires an assembly rate exceeding 208,000 nucleotides per second.37Amazingly, DNA replication is accomplished with an average of only 1 error per billion (109) nucleotides.38 190?39

zona pellucida, mitosis, cell division
Figure 1.3 - Mitosis in Action
Copyright © 2001-2002 Centro Riproduzione Assistita (CRA), Catania, Italy. All rights reserved. (www.cragroup.it)
Notice the zona pellucida surrounding the embryo in each of these pictures.
 

Early Pregnancy Factor

As early as 24 to 48 hours after fertilization begins, pregnancy can be confirmed by detecting a hormone calledearly pregnancy factor” or EPF in the mother’s blood40 (The test for this hormone, however, is not widely available). This substance helps prevent the mother’s immune system from rejecting the soon-to-be-implanted embryo and allows pregnancy to proceed.41

 

Embryo Transport in the Uterine Tube

As cell division proceeds, the embryo is on the move. The uterine tube steers the embryo toward the uterus by cilia motion of epithelial cells lining the uterine tube wall and continued mild muscle contractions.42 The cells of the corona radiata disappear within about 2 days43 while the zona persists in protecting the embryo and preventing premature implantation in the uterine tube.




 

The Closer Look Series:

  

The Step-by-Step Instruction Manual




Open PDF version of FIG 1.6, A Two-Cell Human Embryo (1.5 Days)
Figure 1.6 - A Two-Cell Human Embryo (1½ Days) [PDF version of FIG 1.6]
From Gasser RF, 1975, 3. Atlas of Human Embryos.
Copyright © 1975 RF Gasser, PhD. All rights reserved.



 

Formation of the Morula and Blastocyst

Play Movie - The Morula and Blastocyst
Movie 1.6 - The Morula and Blastocyst

By about 3 days after fertilization, while still in the uterine tube, the embryo contains 12 to 16 cells configured as a solid ball of cells and is called a morula (mor’u-la) [Fig ss] 66

Approximately 3 1/2 to 4 days after fertilization, the uterine tube relaxes under the influence of progesterone67 and the embryo completes its journey through the uterine tube and enters the uterus.68 By this time the embryo begins to develop a fluid-filled cavity with a collection of cells at one end and is called a blastocyst.69 [Fig ee]

The surface of the blastocyst adjacent to the inner cell mass is referred to as the polar end or embryonic pole of the blastocyst.70 [Fig ee]

The zona pellucida, having delivered the embryo through the maze of the uterine tube, degenerates shortly after the embryo arrives in the uterus.71 Embryos studied outside the uterus in a process sometimes called “hatching.”72 The now-free blastocyst is now ready to find a permanent home inside the wall of the uterus.

The blastocyst emerges from the zona pellucida in a process called hatching.
Figure 1.7 - Hatching
Copyright © 2001-2002 Centro Riproduzione Assistita (CRA), Catania, Italy. All rights reserved. (www.cragroup.it)



 

The Closer Look Series:

  

Why Do We Need the Zona Pellucida?




 

Implantation

The morula, blastocyst, and implantation.
Figure 1.8 - The Rapidly Changing Embryo
Copyright © 2001-2002 Centro Riproduzione Assistita (CRA)
Catania, Italy. All rights reserved. (www.cragroup.it)

Implantation is the process whereby the early embryo embeds into the inner wall of the mother’s uterus. Implantation begins about 6 days after fertilization and is complete by about 12 days.79

The first step of this process is the attachment phase, which begins about 6 days after fertilization80 [Fig rr]. The outer cells (trophoblast cells) of the blastocyst have specialized adhesion molecules81 which bind to the epithelial cells of the endometrium. The cells in the uterine wall are full of nutrients and water. The blastocyst attaches between the uterine glands, along its surface overlying the inner cell mass (the embryonic pole). Once attached. the trophoblast cells release enzymes that digest, liquify, and separate maternal cells forming an entry way inside the uterine wall. The trophoblast cells capture the local nutrients and actively share them with the inner cell mass. This will be discussed further in Unit 2.

The embryo embedding into the inner wall of the uterus
Figure 1.9 - Embryo Embedding
LifeART
Copyright © 2002 Lippincott, Williams & Wilkins.
All rights reserved.

Implantation represents a significant obstacle to the developing embryo. It is estimated that up to one-half of all embryos fail to successfully implant and die82 - often without the mother realizing she is pregnant. Many of these embryos are thought to have severe genetic abnormalities incompatible with survival.

By the end of the first week, the embryo has traveled extensively, multiplied from 1 cell to several hundred, dramatically changed its shape and complexity, and begun the process of finding permanent housing.


In Fig. 1.9 we see the embryo (at the blastocyst stage) embedding into the inner wall of the uterus in a process called implantation.






 

The Closer Look Series:

  

How Does the Uterus Prepare for Implantation?




 

Stem Cells — Unlimited Career Potential

From the first days of development, stem cells are present. These cells will differentiate into all of the 200 plus cell types contained in the human body!89 In fact, under the right conditions, an individual stem cell can separate from the embryo and form an entirely new embryo resulting in identical twins.

Embryonic Liver, Heart, and Brain
Figure 1.10 -The Embryonic Liver, Heart, and Brain
The Biology of Prenatal Development DVD.
Copyright © 2006 EHD, Inc. All rights reserved.

Stem Cells

These cells are the building blocks from which every successive cell in the human body develops. Stem cells function as a reservoir of undifferentiated cells and are capable of dividing and becoming highly specialized cells. Stem cell research involves stimulating these cells in such a way as to cause them to develop into anything from a brain cell to a heart cell or a liver cell. This incredible capability of stem cells perfectly suits them for early prenatal development.

The specialized cells in our brain, heart, liver, and the rest of our body all originate from stem cells in the embryo.



 

Visual Summary of the First Week

Figure 1.11 below provides a visual summary of the events of the first week following fertilization. Note the zona pellucida surrounding the embryo through the 58-cell stage at 4 days but not at the 107-cell stage at 4½ days.

Open PDF version of FIG 1.11, The Many Forms and Locations of the Human Embryo During the First Week
Figure 1.11 - The Many Forms and Locations of the Human Embryo During the First Week [PDF version of FIG 1.11]
From Gasser RF, 1975, 2. Atlas of Human Embryos. Copyright © 1975 RF Gasser, PhD. All rights reserved.



 

The Closer Look Series:

  

Proteins




 

Applying the Science:

  

ECTOPIC PREGNANCY

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The First
Week
Footnotes

1 Carlson, 2004. 3; Moore and Persaud, 2003. 16; O'Rahilly and Müller, 1987. 9; O'Rahilly and Müller, 1999b. 39; O'Rahilly and Müller, 2001. 8.
2 Cunningham FG et al., 2001. 86.
3 Sadler, 2005. 6.
4 Dorland and Bartelmez, 1922. 372; Gasser, 1975. 1; Mall, 1918. 421; O'Rahilly and Müller, 2001. 31; O'Rahilly and Müller, 1987. 9; O'Rahilly and Müller, 2001. 25; O'Rahilly and Müller, 2001. 23-24.
5 O'Rahilly and Müller, 2001. 31.
6 Gasser, 1975. 1; Moore and Persaud, 1998. 16, 37; O'Rahilly and Müller, 2001. 33.
7 Saunders, 1970. 1; Spraycar, 1995. 1976.
8 Guyton and Hall, 2000. 34; O'Rahilly and Müller, 1987. 9; O'Rahilly and Müller, 2001. 33.
9 Carlson, 1994. 31; Moore and Persaud, 1998. 37.
10 Guyton and Hall, 2000. 34; Lodish et al., 2000. 10.
11 Sadler, 2005. 40.
12 Hertig, 1968. 26; Hertig and Rock, 1973. 130; Moore and Persaud, 2003. 37; O'Rahilly and Müller, 1987. 12; Shettles, 1958. 400.
13 Moore and Persaud, 2003. 2.
14 Guyton and Hall, 2000. 920.
15 O'Rahilly and Müller, 2001. 31.
16 O'Rahilly and Müller, 2001. 31-33; Carlson, 2004. 32-36; Moore and Persaud, 2003. 32-33; Sadler, 2004. 38-40.
17 Carlson, 2004. 33; Bloom and Fawcett, 1975. 871.
18 Carlson, 2004. 34.
19 Gasser, 1975. 1; O'Rahilly and Müller, 2001. 33.
20 Carlson, 1994. 27.
21 Moore and Persaud, 2003. 33; Guyton and Hall, 2000. 944.
22 Sadler, 2004. 39.
23 Sadler, 2005. 39; Carlson, 2004. 36.
24 Carlson, 2004. 36.
25 Guyton and Hall, 2000. 920.
26 O'Rahilly and Müller, 2001. 31.
27 O'Rahilly and Müller, 2001. 31-33; Carlson, 2004. 32-36; Moore and Persaud, 2003. 32-33; Sadler, 2004. 38-40.
28 Carlson, 2004. 33; Bloom and Fawcett, 1975. 871.
29 Carlson, 2004. 34.
30 Gasser, 1975. 1; O'Rahilly and Müller, 2001. 33.
31 Carlson, 1994. 27.
32 Moore and Persaud, 2003. 33; Guyton and Hall, 2000. 944.
33 Sadler, 2004. 39.
34 Sadler, 2005. 39; Carlson, 2004. 36.
35 Carlson, 2004. 36.
36 Moore et al., 2000. 6.
37See Appendix A.
38 Alberts et al., 1998. 200.
39 Lodish et al., 2000. 456.
40 Blackburn, 2003. 91; Moore and Persaud, 2003. 33, 60; Morton et al., 1992. 72; Nahhas and Barnea, 1990. 105.
41 Morton et al., 1992. 72.
42 Carlson, 2004. 53; O'Rahilly and Müller, 2001. 37.
43 Carlson, 2004. 53.
44 Guyton and Hall, 2000. 24; Watson and Crick, 1953. 737.
45 Guyton and Hall, 2000. 24; Lodish et al., 2000. 103; Watson and Crick, 1953. 737.
46 Alberts et al., 1998. 66.
47 Lodish et al., 2000. 5.
48 Guyton and Hall, 2000. 25.
49 International Human Genome Consortium, 2001. 860; Venter et al., 2001. 1305.
50 Guyton and Hall, 2000. 24.
51 Lodish et al., 2000. 456.
52See Appendix A.
53 Alberts et al., 1998. 189; See Appendix A.
54See Appendix A.
55 Guyton and Hall, 2000. 24; Watson and Crick, 1953. 737.
56 Guyton and Hall, 2000. 24; Lodish et al., 2000. 103; Watson and Crick, 1953. 737.
57 Alberts et al., 1998. 66.
58 Lodish et al., 2000. 5.
59 Guyton and Hall, 2000. 25.
60 International Human Genome Consortium, 2001. 860; Venter et al., 2001. 1305.
61 Guyton and Hall, 2000. 24.
62 Lodish et al., 2000. 456.
63See Appendix A.
64 Alberts et al., 1998. 189; See Appendix A.
65See Appendix A.
66 Gasser, 1975. 1; Moore and Persaud, 2003. 37; O'Rahilly and Müller, 2001. 37; Spraycar, 1995. 1130.
68 Gasser, 1975. 2.
69 Moore and Persaud, 2003. 37; O'Rahilly and Müller, 2001. 39.
70 Carlson, 2004. 54.
71 Sadler, 2004. 43.
72 O'Rahilly and Müller, 1996. 37; O'Rahilly and Müller, 2001. 40.
73 Sadler, 2005. 23; Carlson, 2004. 9.
74 Carlson, 2004. 54; O'Rahilly and Müller, 2001. 27.
75 Guyton and Hall, 2000. 945.
76 Sadler, 2005. 23; Carlson, 2004. 9.
77 Carlson, 2004. 54; O'Rahilly and Müller, 2001. 27.
78 Guyton and Hall, 2000. 945.
79 Adams, 1960. 13-14; Cunningham FG et al., 2001. 20; Hamilton, 1949. 285-286; Hertig, 1968. 41; Hertig and Rock, 1944. 182; Hertig and Rock, 1945. 81, 83; Hertig and Rock, 1949. 183; Hertig et al., 1956. 444; Moore, 1977. inside front cover; O'Rahilly and Müller, 2001. 40; Cunningham FG et al., 2001. 66.
80 Gasser, 1975. 3; O'Rahilly and Müller, 2001. 40.
81 Carlson, 2004. 54.
82 Cunningham et al., 1997. 582; Moore and Persaud, 2003. 40.
83 Guyton and Hall, 2000. 945.
84 Bloom and Fawcett, 1975. 891.
85 Guyton and Hall, 2000. 945.
86 Guyton and Hall, 2000. 945.
87 Bloom and Fawcett, 1975. 891.
88 Guyton and Hall, 2000. 945.
89 Alberts et al., 1998. 32.
90 Lodish et al., 2000. 4.
91 Guyton and Hall, 2000. Table 3-1, 28.
92 Lodish et al., 2000. 3.
93 Lodish et al., 2000. 4.
94 Guyton and Hall, 2000. Table 3-1, 28.
95 Lodish et al., 2000. 3.