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THE FIRST EMBRYONIC STAGES
When the genetic material from a human sperm fuses with that of ovum; the sperm is only 1/2000 the size of the ova. Of the 300-500 million human sperm introduced to vagina during ejaculation, only about 1 % reach ovum. As they swim, an enzyme from the acrosome called acrosin increases the sperm’s motility while uterine contractions (stimulated by prostaglandins secreted in the semen) help propel the sperm. Sperm are not able to fertilize an ovum until they have been in the female reproductive tract for 10 hours during which time they undergo capacitation. The secondary oocyte cannot be fertilized by one sperm: the secondary oocyte is surrounded by the glycoproteins of the zona pellucida and the layers of follicular cells of the corona radiata. The enzymes of about 100 sperm are necessary to break down these layers sufficiently to allow 1 sperm to break through. Because of the small percentage of sperm which reach the ova and the number which are required in order for one to penetrate, a man producing fewer than 20 million sperm/ml of semen is functionally sterile (unless sperm are stored and subsequently concentrated).
When 1 sperm binds receptors in the zona pellucida and enters the secondary oocyte (syngamy), it causes the depolarization of the secondary oocyte. This causes the internal release of calcium which, in turn, causes the release of granules from the secondary oocyte to prevent polyspermy (fertilization by more than one sperm). The secondary oocyte completes meiosis II, dividing into ovum and a second polar body. The tail and head of the sperm are left behind and male pronucleus fuses with female pronucleus forming a new diploid cell called a zygote.
The zygote begins to divide 36 hours after fertilization to produce 2 blastomeres. These blastomeres divide to 4 by 48 hours; by the end of the 3rd day, there are 16 cells. Cell dividsion continues until a solid ball of cells, called a morula, results. The embryo will remain about the same size as the secondary oocyte until the uterine wall provides nourishment for additional growth.
These early stages of embryonic development are often not completed successfully. In optimal conditions, 15% of the oocytes are not fertilized. Between 10-15% begin cleavage but do not implant. Of the 70-75% which do implant, only 58% will survive the second week of embryonic development and 16% of those which survive will be abnormal (Sadler, p. 48).
These stages of gamete formation and early embryonic development are not unique to humans. The sequence of the zygote dividing into two cells, then four, then eight, eventually forming a solid ball called a morula and then a hollow ball of cells called a blastocyst are typical of animals. All bilateran animals form the three tissue layers of endoderm, mesoderm, and ectoderm which result from gastrulation.
STARFISH ZYGOTE AT FERTILIZATION (note the sperm in first image)
|FIRST EMBRYONIC CELL DIVISIONS|
|By end of 4th day, the human embryo has formed a hollow ball cells called a blastocyst. The outer covering of cells (trophoblast) won't make up part of embryo; inner mass cells will. The blastocyst attaches to the endometrium 6 days after fertilization. The outer trophoblast secretes enzymes that dissolve uterine lining allowing the blastocyst to burrow deeper.|
|The inner mass cells undergo mass migrations known as gastrulation and differentiate into 3 tissue layers (ectoderm, mesoderm, and endoderm). Not all the inner mass cells form embryo; some form the amnion and yolk sac. Those which form the future individual begin to undergo differentiation. These cells are no longer totipotent (capable of forming all cell types of the body) as they begin to specialize to specific cell fates. The cells closest to amniotic cavity become ectoderm, those bordering the primitive yolk sac (blastocoele) become endoderm, and those that leave the primitive streak and pass between these 2 layers are mesoderm. Ectoderm develops into the skin and nervous system, mesoderm develops into muscle, bone, and connective tissues, and endoderm develops into the epithelial lining of gastrointestinal tract, the respiratory tract, and several other organs.|
Many animals undergo a larval stage in which characteristics shared with other animal groups are more obvious than in the adult stages. The larvae of corals are more similar to worms than are the adult forms. Tunicate adults bear little resemblance to other chordates but the larvae are tadpole-like in overall form and possess the hallmark features of chordates such as a notochord and postanal tail. The larvae of Amphioxus are clearly wormlike and the larvae of jawless fish are more similar to Amphioxus than are the adults.
The amount of yolk present in the early embryo effects early development. In frogs, the cell divisions at the pole of the embryo which contain large yolk-containing cells (the “vegetal pole”) occur more slowly than at the opposite pole (the “animal pole”). The fourth picture depicts gastrlation in the frog.
|Amniotes increased the amount of yolk contained in the embryo. As a result of this large mass of yolk, the cells which later become the body of the embryo form a flat layer over the yolk (as in the chick embryo below). Even marsupial and placental mammals, which no longer involve yolk in their embryonic development, share this feature of a flat embryo (Arendt, 1999).|
THE BODY AXIS
In 1822, Geoffrey St. Hilaire proposed that some of the differences in the body plans of protostomes and deuterostomes resulted from a rotation of the body axis in ancestral deuterostomes. As a result, deuterostomes possess a dorsal nerve cord, a ventral heart, dorsolateral blocks of axial muscle, ventrolateral visceral mesoderm, and the anterior-flowing blood travels through ventral blood vessels while protostomes possess a ventral nerve cord, a dorsal heart, ventrolateral blocks of axial muscle, dorsolateral visceral mesoderm, and the anterior-flowing blood travels through dorsal vessels. Genetic evidence supports that protostomes and deuterostomes are descended from one ancestral body plan which has been rotated in one of the two descendant lineages. Some genes retain the same medial or lateral expression in both protostomes and deuterostomes (since medial/lateral position would not have been affected by body axis rotation) such as vnd/Nk2, ind/Gsh1,2, and Msh/msx1,3 in columns of nerve cells and netrin along the body midline. Other genes perform similar functions (and were presumably present in the last common ancestor of coelomates) such as tinman/csk which is expressed in heart progenitor cells in both lineages but on opposite sides of the body. Genes which seem to control the anterior/posterior axis (such as bone morphgenetic proteins/Dpp and their antagonists chordin/Sog) are shared but the compartments in which they are expressed are inverted when comparing deuterostomes to protostomes (Gerhart, 2000).
Enteropneust worms are basal hemichordate deuterostomes which are similar to protostomes in their body plan. They possess a ventral nerve cord (although there is a smaller dorsal nerve cord as well), a dorsal heart, protostome-like blood flow, and a ventral tail. Structures which resemble the liver and endostyle are dorsal rather than ventral and a support rod which resembles the notochord is located ventrally. Thus, the inversion of the body axis in the deuterostome lineage appears to have occurred after the split between hemichordates and chordate lineages (Gerhart, 2000).
THE GROWTH OF THE HUMAN EMBRYO AND FETUS DURING PREGNANCY