The development of the ear of rabbit embryo.

The ear consists of three parts which are different origin but function as one unit. The internal ear originates from the surface ectoderm covering the lateral sides of myelencephalon at the fourth week. This ectoderm thickens to from the otic placode and then invaginates to form the otocyst and splits from the surface ectoderm. The otocyst or otic vesicle divides into 2 parts, the ventral cochlear and the dorsal utricular portions. The cochlear gives rise to the saccule and the cochlear duct while the utricular portions gives rise to the uteicle, semicircular ducts and endolymphatic duct. These epithelial structures so formed are known as the membranous labyrinth. The bony labyrinth and the perilymphatic space originate from the mesenchy otic capsule. The middle ear, consisting of the tympanic cavity and the auditory tube, are lined with epithelium of the endodermal origin of the first pharyngeal pouch. The ear ossicles, the malleus and incus are derived from the first and the stapes from the second arch cartilages. The external auditory meatus develops from the first pharyngeal cleft, while the tympanic membrane originates from the mesenchyme between. In order to understand ear development, pig and chick embryos were used in the laboratory studies. Since the pig embryos are presently not available, this compared the ear development of the pig and rabbit embryos, which indicate that the ear of the pig and rabbit develob in the same manner and the rabbit embryos can be used in the future instead of pig embryos for studying ear development.

The development of the ear.

The ear consists of three parts which are of different origin but function as one unit. The internal car originate from the otic vesicle, which in the fourth week of development splits off from the surface ectoderm. This otic vesicle divides in a ventral component which give rise to the saccule and cochlear duct, and a dorsal component which gives rise to the utricle, semicircular ducts and endolymphatic duct. The epithelial structures so formed are known as the membranous labyrinth. The middle ear, consisting of the tympanic cavity and auditory tube, is lined with epithelium of ectodermal origin of the first pharyngeal pouch. The ear icicles, the malleus and incurs are derived from the first and the stapes from the second pharyngeal arches. The external auditory meatus develops from the first pharyngeal cleft and is lined at the bottom by the tympanic membrane. In order to understand ear development, pig and chick embryos are used in laboratory studies. However, since pig embryos are not easily available, this study compares the ear development of pig and rat embryos. The study indicates that the ear of the pig and rat develop in the same manner and rat embryos could be used in the future instead of pig embryos for studying ear development.

Development of the urinary system: Serial section study of 4 and 6 mm rat and 10 mm pig embryos.

The urogenital system develops from the intermediate mesoderm, the coelomic epithelium and the endoderm of the urogenital sinus. The urinary system of mammals is characterized by three sets of kidney: the nonfunctional pronephroi, the mesonephroi and the functional metanephroi. The metanephroi or the permanent kidneys develop from the metanephric diverticulum and metanephrogenic tissue. At first the kidneys are located in the pelvis but later gradually ascend to the abdomen. The urinary bladder develops from the urogenital sinus and surrounding splanchnic mesoderm. In studying the development of the human urinary system at the Department of Anatomy, Siriraj Hospital, we have employed serial sections of 10-15 mm pig embryos as laboratory models. This method of study is helpful in understanding and recognizing how the kidney forms. However, pig embryos are not commercially available anymore, it is necessary to study the normal development of the kidney in other mammals. This study demonstrates that 6 mm rat embryos can be used instead of pig embryos as the development of the metanephros is very similar, although, in the rat embryo, the mesonephros is much smaller than that of a pig embryo and the subcardinal vein cannot be identified. However, it should be borne in mind that the human mesonephros is also smaller than that of pigs. For this reason, the rat embryo is still suitable for use as a laboratory model.

The development of the mice embryo’s heart.

Serial sections of 4-6 mm mice embryos were carefully studied in relation to the serial sections of 10 mm pig embryos under a light microscope to compare the development of the heart and related blood vessels. The heart of a 4-6 mm mouse embryo has incompletely divide chambers comprising two atria and two ventricles. The partition of the atrium, i.e. the septum primum, begins to from at the 4 mm stage and two ventricles. The partition of the atrium, i.e. the septum primum, begins to from at the 4 mm stage and completely attaches to the endocardial cushion at the 6 mm stage. The rupture of the septum primum brings about communication between the two atria, the foramen secundum. Communication between the two ventricles is also by the interventricular foramen. The right atrium receives venous blood from the right horn of the sinus venosus, which is larger than that of the left side. The bulbus cordis communicates with the right ventricle and brings blood to the aortic sac which extends branches to the branchial arches to join the dorsal aorta. The heart of 4-6 mm mice embryos is highly similar to that of a 10 mm pig embryo. Since mice embryos are very easy to obtain for slide preparation, there would be no problem in using mice embryos instead of pig embryos for the study of the development of the heart.

Development of the face and nasal cavities.

The human face develops mainly between the fifth and eighth weeks, following conception. Its development can be traced to five facial primordial appearing around the stomodeum early in the fourth week. There are : one frontonasal, two maxillary and two mandibular prominences. By the end of the fourth week, two nasal placodes develop on each side of the lower part of the frontonasal prominences. The early development of the human face is similar to that of other mammals, such as the pig and rat, all of which acquire branchial arches in the early development stage. The rat branchial arches develop at the 4 mm stage and nearly disappear at the 6 mm stage, as the second one grows dorsally and obliterates the cervical sinus. The medial and lateral nasal prominences appear at the 4 mm stage, and at 6 mm the nasal cavities are more extensive than those of a 10 mm pig embryo ; they are at the same stage of development as a 15 mm pig embryo. Currently we use 10 and 15 mm pig embryos as models in studying facial and nasal development. However, in the future, it might be more suitable to use 4 and 6 mm rat embryos because they are easier to obtain.

The development of the eye: Study of serial sections of 4, 6, 12 mm rat embryos, 10 mm pig embryo and 36 somites chick embryo.

Serial sections of rat embryos measuring 4, 8, 12 mm were carefully studied in relation to the serial sections of 36 somites chick and 10 mm pig embryos under light microscope to compare eye development. The eyes of the 4 mm rat embryo were at a younger stage than those of the 36 somites chick embryos, but at about the same stage as the 10 mm pig embryo. At this stage, the optic cup has already been formed but the lens has incompletely undergone double fusion. The eyes of the 6 mm rat embryo can be compared with those 36 somites chick embryo, as the optic is divided into outer pigmented and inner nervous layers, and the lens is characterized by a thinner anterior lens epithelium and thicker posterior lens fibre. Nearly fully developed eyes, with the surrounding structures, are also shown in this study by the higher magnification of the eye of the 12 mm rat embryo. If rat embryos are to be used as study models for studying the development of the eye, 4-6 mm rat embryos are at the most suitable stages.

Development of the respiratory system serial section study of 6 and 12 mm rat and 10 mm pig embryos.

The human lower respiratory system begins to form about day 26-27 after conception, and is first indicated by a median larngotracheal groove in the caudal and of the ventral wall of the larynx, trachea, bronchi and the pulmonary lining epithelium. The connective tissue, the cartilage and the smooth muscle of these structures develop from the splanchnic mesoderm surrounding the foregut. In studying the development of the respiratory system at the Department of Anatomy, Siriraj Hospital, Medical students should trace the serial section of 10 mm and 15 mm pig embryos. In doing this, they be able to observe the layngotracheal groove at the floor of the pharynx and when, tracing the sections caudally, they will be able to observe the trachea, esophagus and the bifurcation of the trachea to from primary bronchi as well as the smaller branches of bronchi. The present study attempts to show the development of the respiratory system of rat embryos in order to find the most suitable stage which can be used as a laboratory model for use by students. A 6 mm rat embryo can be used instead of a pig embryo for studying the development of the respiratory system, if pig embryos are not available.

Development of the branchial apparatus in rat, pig and chick embryos.

Serial sections of 4-6 mm rat embryos were carefully studied in relation to the serial sections of 36-somite chick embryos and 10 mm pig embryos under a low-power microscope to compare the development of their branchial apparatus. Four pairs of the branchial arch could be clearly identified in the 4 mm rat embryos, the first two being more prominent than the others. The aortic sac or the ventral aorta, the continuation of the truncus arteriosus, is located between the right and left hyoid arches. The thyroid diverticulum passes the aortic sac to attain its final position at the caudal region. Each aortic arch branches from the aortic sac and passes through the mesenchyme of each branchial arch to join the dorsl aorta. The branchial apparatus of the 4-5 mm rat embryos is highly similar to that of the 36-somite chick embryos and 10 mm pig embryos. Since the rat embryos are very easy to obtain for slide preparation, there is no significant problem in the use of rat embryos to study the development of the branchial apparatus instead of the pig embryo.

The observation of serial sections of art embryo related with the pig and chick embryos.

Serial sections of 4 mm. and 6 mm. of the rat embryos were carefully studied in relation to the serial sections of 36 somites chick embryo and 10 mm. pig embryo under low power microscope to compare their organogenesis. Most of the organs developed in the same manner except for the smaller mesonephros in the rat embryo which appeared to be smaller than in the pig embryo. Neither subcardinal vein nor subcardinal anastomosis coved be observed in these two stages of the rat embryos.

The infraorbital foramen In Thais study in relation to the mesioincisal point of the central incisor, the tip of the canine, the tip of the buccal cusp of the second premolar of the ipsilateral side.

Sixty-six Thai dry skulls, 34 males and 32 females, were used in measuring the distances from the infraorbital foramen to the mesioincisal point of the central incisor, to the tip of the canine and to the tip of the buccal cusp of the second premolar of the ipsilateral side. The average distances are 5.548 + 0.036, 4.695 + 0.040 and 4.320 + 0.038 cm. respectively. The angles between the occlusal plane and the lines joining the infraorbital foramen to the mesioincisal point of the central incisor, to the tip of the canine and to the tip of the buccal cusp of the second premolar of the ipsilateral side were calculated. The value of the angles are 59.372 + 0.546, 106.538 + 0.569 and 93.0.707 degrees respectively. On each central incisor, an imaginary rectangle was made by using the width of the occlusal edge as the base and the height of the anatomical crown as the height of the rectangle. The average angle between the occlusal edge and the diagonal line is 52.454 + 0.366 degrees. The study of the direction of the opening of the infraorbital foramen revealed that most of the infraorbital foramens of the Thai skulls directed downward and medially.

The effect of testosterone propionate, durabolin and deca-durabolin on the thymus of the male rats.

Twenty-five male rats, 26-27 days of age, average weight 61.62 gm. were divided into 4 groups. Rats of each group were injected with subcutaneously every second day 1 mg. one of the following drugs: durabolin, Deca-durabolin or Testosterone propionate, and the last group, as normal control, was uninjected. All rats were sacrificed at 54-55 days of age. The thymus glands were weighed and histological appearances were observed. This study showed that thymic weights of all injected groups were less than that of the control, indicating that all 3 drugs caused the thymus gland to involute faster than normal. In comparing the weight and histological appearances of effect on the thymus gland.