Cell division of the fertilized egg occurs without growth. The cells of the early embryo become progressively smaller, reaching the dimension of somatic cells after several cell divisions. The embryonic cells continue to cleave as the embryo moves from the ampulla toward the uterus (Fig. 39.2). Until implantation, the embryo is enclosed in the zona pellucida. Retention of an intact zona is necessary for embryo transport, protection against mechanical damage or adhesion to the oviduct wall, and prevention of immunological rejection by the mother.
At the 20- to 30-cell stage, a fluid-filled cavity (blasto-coele) appears and enlarges until the embryo becomes a hollow sphere, the blastocyst. The cells of the blastocyst have undergone significant differentiation. A single outer layer of the blastocyst consists of extraembryonic ectoder-mal cells called the trophoblast, which will participate in implantation, form the embryonic contribution to the placenta and embryonic membranes, produce hCG, and provide nutrition to the embryo. A cluster of smaller centrally located cells comprises the embryoblast or inner cell mass and will give rise to the fetus.
Fertilization (pronuclei stage)
Fertilization (pronuclei stage)
Transport of the developing embryo from the oviduct, the site of fertilization, to the uterus, the site of implantation.
The morula reaches the uterus about 4 days after fertilization. It remains suspended in the uterine cavity for 2 to 3 days while developing into a blastocyst and is nourished by constituents of the uterine fluid during that time. Implantation of the blastocyst, which is attachment to the surface endometrial cells of the uterine wall, begins on days 7 to 8 after fertilization and requires proper priming of the uterus by estrogen and progesterone. In preparing for implantation, the blastocyst escapes from the zona pellucida. The zona is ruptured by expansion of the blastocyst and lysed by enzymes. The denuded trophoblast cells become negatively charged and adhere to the endometrium via surface glycoproteins. Microvilli from the trophoblast cells in-terdigitate with and form junctional complexes with the uterine endometrial cells.
In the presence of progesterone emanating from the corpus luteum, the endometrium undergoes decidualization, which involves the hypertrophy of endometrial cells that contain large amounts of glycogen and lipid. In some cases, the cells are multinucleated. This group of decidualized cells is called the decidua, which is the site of implantation and the maternal contribution to the placenta. In the absence of progesterone, decidualization does not occur and implantation would fail. As the blastocyst implants into the decidualizing uterus, a decidual reaction occurs involving the dilation of blood vessels, increased capillary permeability, edema formation, and increased proliferation of en dometrial glandular and epithelial cells (Fig. 39.3). The exact embryonic signals that trigger this reaction are unclear, but histamine, catechol estrogens, steroids, prostaglandins, leukemia inhibitory factor, epidermal growth factor, transforming growth factor a, platelet-derived growth factor, placental growth factor, and several other pregnancy-associated proteins have been proposed.
Invasion of the endometrium is mediated by the release of proteases produced by trophoblast cells adjacent to the uterine epithelium. By 8 to 12 days after ovulation, the human conceptus has penetrated the uterine epithelium and is embedded in the uterine stroma (see Fig. 39.3). The trophoblast cells have differentiated into large polyhedral cy-totrophoblasts, surrounded by peripheral syncytiotro-phoblasts lacking distinct cell boundaries. Maternal blood vessels in the endometrium dilate and spaces appear and fuse, forming blood-filled lacunae. Between weeks 2 and 3, villi, originating from the embryo, are formed that protrude into the lacunae, establishing a functional communication between the developing embryonic vascular system and the maternal blood (see Fig. 17.6). At this time, the embryoblast has differentiated into three layers:
• Ectoderm, destined to form the epidermis, its appendages (nails and hair), and the entire nervous system
• Endoderm, which will give rise to the epithelial lining of the digestive tract and associated structures
• Mesoderm, which will form the bulk of the body, in cluding connective tissue, muscle, bone, blood, and lymph.
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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.