Embryology is the branch of biology that studies the fertilization of sperm, ovum and the development of embryo, fetus (Fig. 8-40) (Fig. 8-41).
Embryology is a very useful tool to understand anatomy.
< Mitosis, meiosis >
Cell division can be classified into mitosis and meiosis. Knowledge of chromosomes is a prerequisite for realizing their difference (Fig. 16-3).
Fig. 8-4. Chromosome change in mitosis.
Human cell nucleus has 23 pairs of chromosomes (Fig. 16-4). The above figure shows only 1 pair of chromosomes for simple delineation. During mitosis, each chromosome is duplicated and then divided in half.
As the consequence of mitosis, the number of chromosomes is not changed.
Fig. 8-6. Five phases of mitosis.
Chromosome duplication happens during synthesis (S) phase (Fig. 8-4). Not only S phase but also gap 1 (G1) phase and gap 2 (G2) phase belong to the interphase. The interphase takes longer than the other four phases (prophase, metaphase, anaphase, and telophase).
Successively, the chromosome division occurs during anaphase to yield the daughter cells (Fig. 8-4). The chromosome is divided in ANAphase as if the cadaver is divided in ANAtomy.
The five phases of mitosis can be memorized as suggested in the cartoon.
Fig. 8-8. Chromosome change in meiosis.
During meiosis, each chromosome is duplicated and then divided twice into quarters.
As the consequence of meiosis, the number of chromosomes is reduced by half. Meiosis happens in the testis (Fig. 7-1) and ovary (Fig. 7-22). The result of meiosis is the sperm and ovum with half chromosomes (23).
Fertilization is a process in which a sperm and an ovum meet to become a zygote with full chromosomes (46) (Fig. 8-13).
A pair of chromosomes in each cell are sex chromosomes. Male has two different kinds of sex chromosomes (X, Y), while female have only kind of sex chromosome (X, X). Which sperm (X or Y) meets the ovum (X) for fertilization decides the sex of a baby.
< Fertilization, placenta >
When? Fertilization happens only if ejaculation (Fig. 7-8) is done just before or just after ovulation (Fig. 7-24). Keep in mind that the sperm and ovum do not survive long.
Where? Fertilization happens in the ampulla of uterine tube (Fig. 7-26).
Fig. 8-14. Zygote becoming embryo, trophoblast.
After fertilization, the 1st cell, zygote successively divides (mitosis) (Fig. 8-4) to form both the embryo and trophoblast (Fig. 8-12).
One week after fertilization, the embryo and trophoblast attach to the uterus wall. This process is named implantation.
Half of the placenta is made of the maternal tissue (uterus wall) (Fig. 7-33), and the other half of the placenta is made of the embryonic tissue (trophoblast, and then chorion) (Fig. 8-4) (Fig. 8-21).
Fig. 8-17. Umbilical cord.
After the 8th week of fertilization, the embryo becomes the fetus (Fig. 8-41). The placenta is boundary between the mother and fetus. So, the umbilical cord connecting the placenta with the fetus is made of the fetal tissue (Fig. 8-16). In the umbilical cord, the blood vessel flowing to the fetal heart must be called the umbilical vein, not the umbilical artery (Fig. 10-1).
The umbilical vein, like the pulmonary vein, conveys oxygenated blood (Fig. 10-19) and nutrients. The fetal blood, filled with carbon dioxide and body wastes, travels to the placenta through the umbilical artery.
Fig. 8-19. Embryo between umbilical vesicle, amniotic sac.
During development, the embryo (ectoderm, mesoderm, endoderm) is sandwiched between two balloons: the (ventral) umbilical vesicle and (dorsal) amniotic sac. The amniotic sac consists of the amnion (wall) and amniotic cavity (containing amniotic fluid) (Fig. 8-14).
Besides, the trophoblast (Fig. 8-14) develops into the chorion that is external to the amnion. The chorion and chorionic cavity compose the chorionic sac.
The yolk of chicken egg that supplies nutrients is equivalent to the umbilical vesicle.
Fig. 8-21. Embryo folding, amniotic sac surrounding.
During the embryo FOlding (the FOurth week after fertilization) (Fig. 8-40), the umbilical vesicle (Fig. 8-19) is incorporated into the embryo to become foregut, midgut, and hindgut (Fig. 8-30). Simultaneously, the amniotic cavity and amnion totally surround the embryo.
A part of the chorion (fetal tissue) contributes to the placenta (Fig. 8-16). The uterus wall develops into the decidua (maternal tissue), a part of which contributes to the placenta too (Fig. 8-16). Another part of the decidua surrounds the uterine cavity. As a consequence, there exit three cavities: the amniotic, chorionic, and uterine cavities.
Fig. 8-22. Fused amnion, chorion.
The amnion and chorion get together to obliterate the chorionic cavity. Also the two layers of decidua meet each other to obliterate the uterine cavity. Only amniotic cavity remains. For delivery, the fused amnion, chorion, and decidua (simply describing, amnion) are ruptured and the fetus and amniotic fluid in the amniotic cavity are expelled (Fig. 8-42) (Fig. 8-43).
< Endoderm, mesoderm, ectoderm >
Students easily commit it to memory that 3 layers are discernable in the 3rd week.
The ectoderm contiguous to the amniotic cavity becomes nerves and skin. The intervening mesoderm becomes bones and muscles by way of the somite (Fig. 8-25). The endoderm contiguous to the umbilical vesicle becomes foregut, midgut, hindgut (digestive tract) (Fig. 8-30).
Fig. 8-25. Formation of neural tube.
The ectoderm forms the neural tube, which develops into the brain and spinal cord (Fig. 13-1); the canal of the neural tube into the ventricle (Fig. 13-37). The neural crest next to the neural tube is destined to become the spinal ganglion (Fig. 13-96) and other various structures.
The remaining ectoderm becomes the epidermis of skin (Fig. 15-1) after embryo folding (Fig. 8-29) (Fig. 8-30).
The ectoderm is dorsal to the other layers. This orientation is decided in early development (Fig. 8-19). Despite the dramatic change of the embryo, fetus, and even infant, the term “dorsal” is used constantly. A detailed explanation is given below.
Fig. 8-27. Development of upper and lower limbs.
When the limbs develop, the dorsa of hand and foot face posterior (Fig. 8-26) and then lateral. After birth, the shoulder and hip joints are extended (Fig. 2-22) (Fig. 2-27) to stand up, while the dorsa still face lateral. To shift into the anatomical position, the upper and lower limbs are rotated laterally and medially at 90 degrees, respectively (Fig. 2-14).
Accordingly, if we stand on tiptoe, the dorsa of the hand and foot face posterior and anterior, differently. Observe the dorsal venous network (Fig. 10-70a) and the dorsal venous arch (Fig. 10-74).
The corresponding muscles in the upper and lower limbs face differently. An example is the biceps brachii of the anterior arm muscles (Fig. 3-48) and the biceps femoris of the posterior thigh muscles (Fig. 3-75).
Fig. 8-28. Direction terms during head folding.
During the head folding (Fig. 8-30), the neural tube is flexed in different extents according to the brain levels. The term “dorsal” is constant regardless of the flexion angle. Thus, the term “dorsal” is preferred over other terms such as superior, superoposterior, or posterior in the nervous system. The same reasoning is applied to the terms “ventral, cranial, and caudal.”
Fig. 8-29. Lateral folding to form foregut, midgut, hindgut.
By lateral folding, the flat embryo composed of 3 layers (Fig. 8-25) becomes cylindrical. The embryo becomes to incorporate the umbilical vesicle; more specifically, the bilateral intraembryonic celoms finally cover the umbilical vesicle (Fig. 4-39). Simultaneously, the embryo becomes to be surrounded by the amniotic cavity (Fig. 8-21).
Fig. 8-30. Head and tail folding to form foregut, midgut, hindgut.
Along with the lateral folding, the head and tail folding occurs to make the umbilical vesicle perfectly get into the embryo. The umbilical vesicle develops into the foregut, midgut, and hindgut.
Repeatedly, the embryo gets surrounded by the amniotic cavity (Fig. 8-21) (Fig. 8-29). The oropharyngeal membrane between the amniotic cavity and foregut will rupture to become the fauces between the oral cavity and oropharynx (Fig. 4-15).
Fig. 8-31. Development of hindgut.
The cloacal membrane between the hindgut and amniotic cavity is divided into two by a grown tissue. Concurrently, the hindgut develops not only into a part of the digestive tract (anal canal, etc.) (Fig. 4-36) but also into a part of the urinary tract (urinary bladder, urethra) (Fig. 6-19). The two adjacent membranes rupture to allow the passage of feces and urine.
Fig. 8-32. Three arteries to foregut, midgut, hindgut.
The foregut, midgut, and hindgut are fed by the celiac trunk, superior mesenteric artery, and inferior mesenteric artery, respectively (Fig. 10-52). Two boundaries between the three guts become the middle of duodenum (Fig. 4-29) and the middle of transverse colon (Fig. 4-33).
Fig. 8-33. Development of gastrointestinal tract.
The superior mesenteric artery acts as the rotation axis of the gastrointestinal tract. The cecum in the midgut is the spearhead of the rotation. The cecum rotates at 270 degrees counterclockwise in the anterior view to give rise to the adult form of the small intestine (Fig. 4-29) and large intestine (Fig. 4-33).
Simultaneously, the stomach rotates along the cardia axis clockwise in the anterior view. This rotation situates the liver superior and right to the stomach (Fig. 4-40).
Fig. 8-34. Development of pericardium, pleura, peritonea.
During development, a single balloon is separated to become the five balloons with the same properties. The single balloon is the intraembryonic celom in the mesoderm (Fig. 8-25); the five balloons are the pericardium, two pleurae, and two peritonea. The two peritonea (Fig. 4-39) will coalesce to become one peritoneum (Fig. 4-38).
In case of the crowded tendons of the wrist, the synovial sheath containing synovial fluid relieves friction (Fig. 3-61). Likewise, in case of the heart, lungs, and gastrointestinal tract that move constantly, the pericardium (Fig. 10-3), pleurae (Fig. 5-15), and peritoneum (Fig. 4-38) are responsible for lubrication.
Cavities of the five balloons are the pericardial, pleural, and peritoneal cavities. All the cavities contain serous fluid (Fig. 4-42) which is slippery unlike mucous fluid (e.g., viscous sputum (Fig. 5-24)).
As a matter of fact, serous fluid is small in amount, so that the cavities have close to zero volume, but they can be expanded by the influx of air or blood. Thus, the cavities are potential spaces like the subdural space of the spinal cord (Fig. 13-40) and brain (Fig. 13-43).
Fig. 8-36. Pharyngeal arches.
During development of the head and neck, the pharyngeal arches, which look like fish gill, are formed. As their names imply, inside of the pharyngeal arches is the pharynx (Fig. 4-23). The pharynx belongs to the foregut, so that the oropharyngeal membrane between the amniotic cavity and foregut is found in the inside of pharyngeal arches (Fig. 8-30).
The 1st, 2nd, 3rd, 4th pharyngeal arches are innervated by V, VII, IX, X, respectively. For example, the masticatory muscles (Fig. 3-13) (Fig. 3-15) originate from the 1st pharyngeal arch, and thus are innervated by V (Fig. 13-71).
Like the pharyngeal arch, innervation is early decided during development. When a developing organ moves to its final position, the nerve always follows the organ, unlike artery and vein. An example is the phrenic nerve (from CN3–CN5) innervating diaphragm (Fig. 13-88).
It is believed that embryology and evolution are correlated. The transformation of human embryo enables us to predict how humans have evolved.
The embryonic heart initially forms as a tube just like a blood vessel. The single atrium and single ventricle (of the tube-like heart) are divided into the two atria and two ventricles (Fig. 10-10). The human primitive heart resembles the fish heart; it implies that humans have lived in water and evolved from fish.
< Delivery >
Until the end of the 8th week (after fertilization), the organs of embryo are dramatically formed.
After the beginning of the 9th week, the organs of fetus just grow in size.
At the end of the 38th week, the fetus is delivered by contraction of the uterus wall and widening of the cervix of uterus and the vagina (Fig. 7-34).
The delivery of fetus is followed by that of umbilical cord and placenta (Fig. 8-22).
Approximately, the sitting height of the newborn is 36 cm; its average weight is 3.4 kg.
In embryology, fertilization is the standard to tell the weeks of embryo and fetus (Fig. 8-15). But in obstetrics, last menstrual phase is the standard, because mother usually knows only when her last menstrual phase occurred (Fig. 8-12).
Fertilization is the standard to distinguish between heredity and environment. The life in the uterus belongs to environment (Fig. 8-40) (Fig. 8-41).