Embryology PDF

# Embryology ## Introduction Embryology is a subfield of biology that deals with the study of the growth and development of an organism. The basic goal in embryology is to describe the mechanisms that determine why certain cells develop into one set of structures and other cells into others. All t...

# Embryology ## Introduction Embryology is a subfield of biology that deals with the study of the growth and development of an organism. The basic goal in embryology is to describe the mechanisms that determine why certain cells develop into one set of structures and other cells into others. All the cells in your body originate from an egg cell that has been fertilized by a sperm and has become a zygote. Mitotic divisions of the zygote result in the billions of cells that you have now. This cell division is also termed cleavage, resulting to a mass of cells called blastomeres. Most animals have eggs with unequal distribution of yolk in the animal and vegetal poles. Due to the unevenness in the distribution of yolk, the rate of cell division also varies in the two poles, with the animal pole having a faster rate of cell division than the vegetal pole. Closely related animals usually have similar eggs, cleavage patterns, and developmental stages. Early cleavage patterns can also be used in connecting evolutionary relationships. In a developing embryo, tissues and organs arise from layers or blocks of embryonic cells called germ layers. Their development from unspecialized to specialized structures is termed differentiation. The ectoderm, for example, gives rise to the outer body wall, endoderm the inner lining of the digestive cavity, while mesoderm the muscles, bones, and other connective tissues. The discussion in this chapter compares the embryonic development of frog and man. ## Frog's Embryology Frog's egg has moderate amount of yolk (mesolecithal) unequally distributed throughout the egg, with the vegetal pole having more yolk than the animal pole. ### Cleavage stage: The first cleavage, 2-cell stage, starts from the animal pole then slows down along the yolk-filled vegetal pole. The second cleavage, 4-cell stage, runs from pole to pole, at right angles to the first. The third cleavage, 8-cell stage, is at right angles to the first two cleavages. The 8-cell embryo is composed of four smaller cells in the animal pole and four bigger cells in the vegetal pole. Subsequent divisions yield numerous smaller cells in the animal pole and bigger cells in the vegetal pole. The division is holoblastic, meaning, the whole cell divides unequally, due to the yolk distribution. These series of divisions result in the formation of a ball of cells called a morula. ### Blastula stage: After the morula stage follows the blastula. The blastula stage is marked by the presence of a blastocoel, a cavity inside the embryo. The blastocoel is located along the animal pole of the embryo, where lesser amount of yolk is present. ## Gastrulation and Germ Layer Formation Due to the heavy yolk along the vegetal pole, gastrulation cannot occur by simple invagination, the rolling down of cells from the animal pole to the vegetal pole. Gastrulation begins along the area of the gray crescent as cells cleave and migrate inward to the inside of the embryo. This process is called involution which is responsible for the creation of a small, slit-like blastopore. As involution proceeds, the blastopore slit spreads transversely and an archenteron starts to form internally. The lining of the archenteron becomes the endoderm. As cells roll inward, the archenteron gradually increases in size, while the blastocoel, the cavity, decreases in size. As gastrulation proceeds, the embryo starts to elongate and the ectodermal cells covering the surface of the embryo begin to proliferate. As ectoderm proliferates, it speads and thins. This spreading results in its expansion toward the vegetal pole in a process called epiboly. As epiboly continues, the blastopore moves toward the vegetal pole and takes the form of a ring that surrounds the yolk-filled vegetal pole, called the yolk plug. This ring of ectoderm will eventually close to form a small blastopore or opening which will later on become the anus, prevalent among deuterostomic animals like the echinoderms and chordates. In other groups of animals, this will become the mouth (protostomic), instead of anus. Some of the last cells to move to the blastopore along the dorsal lip are expected to become the mesoderm and notochord. Originally, these cells are part of the archenteron near the blastopore; however, after sometime, they detach from the endoderm and move between the endoderm and ectoderm in the region along the dorsal lip. Some of the mesoderm cells called chordamesoderm, spreads between the ectoderm and endoderm, and differentiates into the notochord. On each side of the notochord, the mesoderm proliferates and forms segmental blocks of tissues called somites, which later on will become connective tissues, muscle tissues, and linings of the coelomic cavities. ## Formation of the Neural Tube In the later part of gastrulation, external changes along the upper side of the frog embryo start the formation of the neural tube, a process known as neurulation in embryology. The process starts in response to the formation of the notochord by the chordomesoderm. After gastrulation, an oval-shaped area on the dorsal side of the embryo marks the start of the formation of the neural tube. This region is called the neural plate. This part flattens and thickens due to the presence of microfilaments. The edges of the neural plate roll up and over the midline of the neural plate, creating ridges called the neural folds. These neural folds fuse dorsally to create the neural tube. The neural tube eventually develops into the brain and spinal cord of the embryo. | Ectoderm | Mesoderm | Endoderm | |---|---|---| | skin | notochord | gut| | spinal cord | bones | lungs| | brain | muscles | urinary bladder| | nerve cells | blood and blood vessels| liver and pancreas | | mucous glands | heart| thyroid gland | ## External Gill Stage Just after hatching, important changes take place in the tadpole. It respires mainly through its skin and continues to feed upon the yolk, since it does not have fully developed mouth and anus. After a day, the archenteron becomes the functional alimentary canal, with an anterior part, the pharynx. At this stage, the external gill usually consists of a single stalk or ramus that protrudes from a gill arch behind the head of the animal, and close to a gill slit. This stalk is lined by thin-walled filaments that contain most of the blood vessels used for gas exchange. The stalk usually contains muscle tissues, and may be moved by the tadpole as a free appendage to constantly navigate in stagnant water. Frogs usually have one external gill on each gill arch. The external gills become elongated and branched, and the tadpole can now respire using the gills and skin. As the tail grows longer, the tadpole becomes ready for active swimming. ## Internal Gill Stage After 15 to 20 hours under the external gill stage, the tadpole is now ready to enter the internal gill stage. This is characterized by the appearance of the fold of skin, the operculum, which encloses the opercular chamber. This is followed by the disintegration of the external gills and the formation of new internal gills that bear numerous delicate and highly vascularized gill filaments. About 9-10 days after oviposition, the tadpole starts respiring like a fish. It drinks water that exits to the opercular chamber through the pharyngeal gill slits and out to the spiracle. Gas exchange takes place between the capillaries of gill filaments and water. Around 20 days after oviposition, the limbs start to develop, with the forelimbs behind the head, and the hindlimbs on either sides of the cloaca. The pairs of forelimbs and hindlimbs develop simultaneously and become visible at the later stage. After about 6-9 weeks, tiny little legs come out, the head is more distinct, and the body elongates. It now feeds on dead insects and small plants. The forelimbs start to bulge and pop out. After 9 weeks, the tadpole appears like a young frog with long tail, on its way to being a full-grown frog. A summary of the growth and development of the frog is given below: |Stage|Description| |---|---| | Fertilization | Sperm and egg unite| | Cleavage | Rapid cell division | | Gastrulation | Three germ layers form | | Neurulation | Formation of neural tube| | Hatching | Tadpole hatches from the egg| | Metamorphosis | Tadpole changes form to become an adult frog| | Organogenesis | Organs form| | Maturity | Adult frog| ## Human Embryology In human embryogenesis, the first two weeks of development is called pre-embryonic period. It starts with fertilization, the fusion of the egg and sperm. It could be considered the beginning of life, popularly called pregnancy. After fertilization, the zygote undergoes a series of rapid divisions called cleavage. Around 30 hours after fertilization, the zygote starts to divide into two cells now called blastomeres. After 72 more hours, there is rapid increase in the number of cells that results in a ball of around 12-16 cells called morula. Frogs undergo the same division, and formation of morula. This division continues as the zygote passes through the fallopian tube towards the uterus. On the fourth day, the morula enters the uterus and fuse with the central cavity called the blastocoel. At this stage, the embryo is called a blastocyst. The implantation of the blastocyst usually occurs along the posterior part of the uterine wall, starting at the end of the first week and completed on the second week of pregnancy. By the end of the second week, the blastocyst is embedded in the endometrial stroma. The cells of the embryoblast differentiate into two layers, the epiblast, composed of high columnar cells and the hypoblast composed of small cuboidal cells. These two-layered plates are called the embryonic disc. The epiblast forms the floor of the amniotic cavity; while the hypoblast forms the exocoelomic membrane. It is the folding of the flat trilaminar embryonic disc into a cylindric shape that establishes the general body form of the embryo. The folding in the medial plane produces the head and tail folds resulting in the integration of part of the yolk sac into the embryo and the formation of the foregut and hindgut. The folding of the embryo along the horizontal plane produces the lateral folds and the formation of the ventral and lateral body walls. Part of the yolk sac is incorporated into the embryo as the midgut. The derivatives of the three germ layers are summarized in the following table. | Ectoderm | Mesoderm | Endoderm | |---|---|---| | brain and spinal cord | vertebral column | tympanic and auditory tube| | pituitary gland | somatic and splanchnic mesoderm | parathyroid gland, tonsils, and thymus| | peripheral nervous system | dermis of the skin | respiratory system | | sensory epithelium of the ear, nose, and eye| muscles, bones, and connective tissues| digestive tube of gut | | epidermis and its derivatives | | liver and pancreas | | enamel of the teeth | kidneys, gonads, and their ducts or accessory glands | urinary bladder and urethra | | mammary gland | | | The embryonic period is very significant since the internal and external structures develop in the embryo. From the 4th week to the 8th week, each germ layer gives rise to the different organs; the process known as organogenesis takes place. The shape of the embryo greatly changes. The fetal period starts from the 3rd month to birth. There is a rapid growth and development of body parts including the maturation of tissues. During the 4th week of the development, the age of the embryo is expressed in the number of somites. The length of the embryo between the 4th and the 8th developmental weeks is indicated as the crown-rump length (CRL) and is expressed in millimeters. During fetal development (9th-40th week) the length of the embryo is indicated as the crown-heel length (CHL) expressed in centimeters. External appearance, weight or biparietal diameter are also used as a measure during the gestation. Growth in length is most intensive during the 3rd-5th month period, while the increase in weight is most evident during the last two months of gestation. During the fetal life, head growth rate slows down. By the 5th month, the mother can recognize fetal movements. The birth occurs 266 days or 38 weeks after fertilization.