The Preembryonic Stage

The preembryonic stage consists of the first 2 weeks of development and culminates in the existence of an embryo. It involves three major processes: cleavage, implantation, and embryogenesis.

Cleavage

Cleavage refers to mitotic divisions that occur in the first 3 days. The first cleavage occurs about 30 hours after fertilization and produces the first two daughter cells, or blastomeres.1 These divide simultaneously at shorter and shorter time intervals, doubling the number of blas-tomeres each time. By the time the conceptus arrives in the uterus, about 72 hours after ovulation, it consists of 16 or more cells and somewhat resembles a mulberry—hence it is called a morula.2 The morula is no larger than the zygote; cleavage merely produces smaller and smaller blastomeres. This increases the ratio of cell surface area to volume, which favors rapid nutrient uptake and waste removal, and it produces a larger number of cells from which to form different embryonic tissues.

2mor = mulberry + ula = little

Saladin: Anatomy & I 29. Human Development I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

1092 Part Five Reproduction and Development

Anatomy And Physiology Ovulation
Figure 29.2 Events from Ovulation to Implantation. These processes occur during the first week after ovulation, approximately at the locations indicated in the female reproductive tract. Why can't the egg be fertilized in the uterus?

Insight 29.1 Clinical Application

Twins

There are two ways in which a woman can produce twins (and, by extension, other multiple births). Dizygotic (DZ) twins are produced when two eggs are ovulated and fertilized by separate sperm (fig. 29.3). They are no more or less genetically similar than any other siblings and may be of different sexes. Multiple ovulation can also result in triplets, quadruplets, or even greater numbers of offspring.

Monozygotic (MZ) twins are produced when a single egg is fertilized and the cell mass (embryoblast) later divides into two. MZ twins are genetically identical, or nearly so, and are therefore of the same sex and nearly identical appearance. Identical triplets and quadruplets occasionally result from the splitting of a single embryoblast. Reproductive biologists are beginning to question whether MZ siblings are truly genetically identical. They have suggested that one blastomere may undergo a mutation in the course of DNA replication, and the splitting of the embryoblast may represent an attempt of each cell mass to reject the other one as foreign.

Oogenesis Identical Twins
Figure 29.3 Dizygotic Twin Sisters, Kelly and Ellen.

Saladin: Anatomy & I 29. Human Development I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 29 Human Development 1093

The morula lies free in the uterine cavity for 4 to 5 days and divides into 100 cells or so. During this time, it is nourished by nutrients that were stored in the egg cytoplasm during oogenesis and by an endometrial secretion called uterine milk, which accumulates in a cavity in the morula. Meanwhile, the zona pellucida disintegrates and releases the conceptus, which is now at a stage called the blastocyst—a hollow sphere with an outer layer of squamous cells called the trophoblast3 and an inner cell mass called the embryoblast. The trophoblast is destined to form part of the placenta and play an important role in nourishment of the embryo, whereas the embryoblast is destined to become the embryo itself.

Implantation

About 6 days after ovulation, the blastocyst attaches to the endometrium, usually on the posterior wall of the fundus or body of the uterus (see insight 29.2 for exceptions). This is the beginning of implantation, a process in which the conceptus becomes buried in the endometrium (fig. 29.4). The trophoblast cells adjacent to the embryoblast secrete enzymes that stimulate thickening of the endometrium and they separate into two layers. The deep layer is called the cytotrophoblast because it retains individual cells divided by membranes. In the superficial layer, however, the plasma membranes break down and the cells fuse into a multinucleate mass called the syncytiotrophoblast4 (sin-SISH-ee-oh-TRO-foe-blast). (A syncytium is any body of protoplasm containing multiple nuclei.)

The syncytiotrophoblast grows into the uterus like little roots and digests endometrial cells along the way (fig. 29.4fc). The endometrium reacts to this injury by growing over the trophoblast and eventually enclosing it. Implantation takes about a week and is completed about the time the next menstrual period would have occurred if the woman had not become pregnant.

The trophoblast also secretes human chorionic gonadotropin (HCG). HCG stimulates the corpus luteum to secrete estrogen and progesterone, and progesterone suppresses menstruation. The level of HCG in the mother's blood rises until the end of the second month. During this time, the trophoblast develops into a membrane called the chorion, which takes over the role of the corpus luteum and makes HCG unnecessary. The ovaries then become inactive for the rest of the pregnancy, but estrogen and progesterone levels rise dramatically as they are secreted by the ever-growing chorion (see fig. 28.18, p. 1070).

Splitting The Zygote Blastocyst Stage

Blastocyst cavity Trophoblast

Embryoblast Endometrial epithelium

Endometrial gland Endometrial capillary

Blastocyst cavity Trophoblast

Embryoblast Endometrial epithelium

Endometrial gland Endometrial capillary

Cytotrophoblast And Syncytiotrophoblast

Embryonic endoderm

Cytotrophoblast Syncytiotrophoblast

Figure 29.4 Implantation. (a) Structure of the blastocyst about 6 to 7 days after ovulation. (b) The progress of implantation about 1 day later; the syncytiotrophoblast has begun growing rootlets, which penetrate the endometrium.

Embryonic endoderm

Cytotrophoblast Syncytiotrophoblast

3troph = food, nourishment + blast = produce, give rise to syn = together + cyt = cell

Figure 29.4 Implantation. (a) Structure of the blastocyst about 6 to 7 days after ovulation. (b) The progress of implantation about 1 day later; the syncytiotrophoblast has begun growing rootlets, which penetrate the endometrium.

Embryogenesis

During implantation, the embryoblast undergoes embryogenesis—arrangement of the blastomeres into the three primary germ layers: ectoderm, mesoderm, and endoderm. Embryogenesis begins with the formation of a narrow space called the amniotic cavity between the embryoblast

Saladin: Anatomy & I 29. Human Development I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

1094 Part Five Reproduction and Development

Chorion Cavity

Chorion

Amnion Amniotic cavity

Embryonic stalk Chorionic villi

Figure 29.5 The Implanted Conceptus at 2 Weeks. The primary germ layers and extraembryonic membranes have formed, and the conceptus is now an embryo.

Chorion

Amnion Amniotic cavity

Embryonic stalk Chorionic villi

Germ layers

Yolk sac

Germ layers

Yolk sac

Figure 29.5 The Implanted Conceptus at 2 Weeks. The primary germ layers and extraembryonic membranes have formed, and the conceptus is now an embryo.

and cytotrophoblast (fig. 29.5). The embryoblast flattens into an embryonic disc composed of ectoderm and endo-derm. As the disc elongates, a raised groove called the primitive streak forms along the midline of the ectoderm. Cells on the surface migrate medially toward this groove, down into it, and then laterally between the ectoderm and endoderm. This forms the middle germ layer, the meso-derm (fig. 29.6). The ectoderm and endoderm are epithelia composed of tightly joined cells, but the mesoderm is a more loosely organized, gelatinous connective tissue called mesenchyme. Its cells are more mobile and wispy than those of the ectoderm and endoderm. At the conclusion of embryogenesis, the individual is an embryo, about 2 mm long and 2 weeks old.

Insight 29.2 Clinical Application

Ectopic Pregnancy

In about 1 out of 300 pregnancies, the blastocyst implants somewhere other than the uterus and produces an ectopic5 pregnancy. Most cases begin as tubal pregnancies, in which the blastocyst implants in the uterine tube. This usually occurs because it has encountered an obstruction such as a constriction resulting from earlier pelvic inflammatory disease, tubal surgery, previous ectopic pregnancies, or repeated abortions. The uterine tube cannot expand enough to accommodate the growing conceptus for long; if the situation is not detected and treated early, the tube usually ruptures within 12 weeks. This can be fatal to the mother. If she survives, the conceptus may reimplant in the abdominopelvic cavity and produce an abdominal pregnancy. About

1 pregnancy in 7,000 is abdominal. The conceptus can grow anywhere it finds an adequate blood supply—for example, on the broad ligament or the outside of the uterus, colon, or bladder. This is a serious threat to the mother's life and usually requires abortion, but about 9% of abdominal pregnancies result in live birth by cesarian section.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

1. How soon can a sperm reach an egg after ejaculation? How soon can it fertilize an egg? What accounts for the difference?

2. Describe two ways a fertilized egg prevents the entry of excess sperm.

3. In the blastocyst, what are the cells called that eventually give rise to the embryo? What are the cells that carry out implantation?

4. What major characteristic distinguishes an embryo from a preembryo?

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • hayden
    How long atfer ovulation can the egg be fertilized?
    7 years ago
  • donata
    What happening during fertilisation and implantation in a womb?
    3 years ago

Post a comment