Embryology of Aedes aegypti (L.) (Diptera: Culicidae): Organogenesis

Abstract

The Aedes aegypti embryo consists of 17 units (the blactocephalon, 3 gnathal units, 3 thoracic and 10 abdominal units) by 19 hr of development. Shortening of the germ band starts at 30 hr, and by 40 hr it is completely withdrawn on to the ventral side of the egg. The anterior migration of the mouthparts is also associated with germ band movement. Formation of the labrum starts at 18 hr, and by 35 hr this structure appears as a distinct lobe. Antennal rudiments are evident at 25 hr. The mandibles and maxillae are also recognizable by 25 hr and a labium can be seen at 30 hr. A vitelline membrane is present by 30 hr. Blastokinesis occurs between 35 and 40 hr. The amnion and the serosa undergo lysis between 50 and 55 hr. Dorsal closure of the embryo is completed by 65 hr. Stomodaeal and proctodaeal invaginations are observed at 19 hr as infoldings of the blastocephalon and the terminal segment respectively. The stomodaeum differentiates into the pharynx and oesophagus by 35 hr and forms the proventriculus by 65 hr. Development of the proctodaeum begins at 45 hr; the pyloric ampulla, the ileum and the colon are discernible at 65 hr. Cells at the tip of the stomodaeal-anterior midgut and the proctodaeal-posterior midgut invaginations form the midgut proper, which appears as a closed tube by 65 hr. The gastric caeca originate from the midgut at its junction with the foregut. The malpighian tubules arise from the anterior end of the proctodaeum during hours 35–40. Anal papillae arise from the posterior end of the proctodaeum at this time. Formation of the brain starts at 35 hr with proliferation of the ectodermal cells in the posterior region of the blastocephalon; by 75 hr this organ is completely developed. The suboesophageal ganglion is formed by 45 hr. The corpus cardiacum and the corpus allatum are formed from 2 masses of ectodermal cells that proliferate from the middle region of the blastocephalon at 45 hr. Formation of the frontal ganglion begins at 45 hr with the anterior migration of the ectodermal cells of the pharynx; and by 55 hr, cells at the posterior end of this ganglion have transformed into the recurrent nerve. The hypocerebral ganglion and its nerves are observed at 60 hr. The basic form of the ventral nerve cord is initiated at 15 hr with migration of the neuroblasts from the ectodermal layer. These cells form the ganglionic lobes which become fully differentiated by 75 hr and form a chain of 3 thoracic and 8 abdominal ganglia. Larval eye rudiments appear at 45 hr and become fully differentiated by 75 hr. A cuticle which surrounds all the appendages and body segments is secreted by ectodermal cells between 45–50 hr. Rudiments of the tracheal system begin to differentiate at 40 hr, becoming well defined tracheae and longitudinal trunks by hour 60. Formation of the musculature of the gnathal segments begins at 60 hr and is completed during the next 15 hr. Other muscles of the head also become fully formed by 75 hr. The trunk musculature is formed from the somatic mesoderm which also gives rise to the heart and fat body. The ventral and dorsal longitudinal muscles form at 60 and 65 hr respectively, whereas the dorsoventral muscles of these segments are observed at 80 hr. A tubular heart is observed at 65 hr. The gonadal rudiments are formed by 35 hr in the midlateral position of abdominal segments 6 and 7. Each rudiment consists of 4 to 6 pole cells which are surrounded by a layer of splanchnic mesodermal cells. The pole cells in each rudiment begin mitosis at 70 hr; by 90 hr the total number of cells in each rudiment is attained. No other parts of the reproductive system are formed during embryogeny. A comparison of segmentation, blastokinesis, and the origin of the midgut in culicids is given. Formation of the nervous system, the basic embryonic musculature, and the gonads is described in reference to other mosquito genera.