by: Bryan Perkins
The testis of the grasshopper consist of a series of lobes that contain cells in varying stages of spermatogenesis. In each testicular lobe can be found many smaller compartments, known as testicular cysts. As the sperm cells develop they travel from the apical end of the testicular lobe to the basal end. At the apical end, the cells are grouped into presumptive germ cells known as spermatogonia. These cells undergo mitosis and give rise to all of the remaining germ cells in the testis. Moving from the spermatogonia in the apical end of the testis to the vas deferens at the opposite end of the testis, the cells mature as a group inside the testicular cysts. All the cells in one testicular cyst are in the same stage of development.
Each animal, whether frog, human, earthworm, or dog, passes through similar stages of development. Fertilization (the fusion of genetic material from two gametes) starts the life cycle and stimulates the egg to begin development. The stages of development between fertilization and hatching are known as embryogenesis; and although there is an incredible variety of embryogenic types, most patterns are variations on five fundamental processes:
Cleavage consists of a series of extremely fast mitotic divisions immediately after fertilization. During cleavage the volume of cytoplasm does not grow, cells merely divide the enormous volume of zygote cytoplasm into many smaller cells called blastomeres. The amount and distribution of yolk in each cell determines the rate of cleavage and the relative size of the blastomeres. The large, yolk-rich vegetal pole cells (macromeres) divide slowly compared to the smaller, relatively low yolk animal pole cells (micromeres) causing the vegetal pole cells to become progressively larger. The micromeres of the animal pole stain darker than the macromeres of the vegetal pole due to the higher concentration of organelles in the cytoplasm. Throughout this process, a fluid-filled cavity important for allowing cell movements during gastrulation (blastocoel) forms in the animal hemisphere of the blastula.
“It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life.”
During gastrulation, cell movements result in a massive reorganization of the embryo and the differentiation of the three embryonic germ layers. The ectoderm will develop into organs such as the skin, the nervous system, and the lens of the eye. The endoderm develops into the inner linings of the digestive tract, the linings of the respiratory passages, and many glands such as the liver and pancreas. The mesoderm forms the somites, the notochord, and the mesenchyme, which give rise to the muscles, circulatory and excretory systems of the body. In amniote embryos, a structure known as the primitive streak organizes the process of gastrulation and, thus, lays the foundation of the embryonic body plan.
By 24 hours time many structures have already formed in the chicken embryo. The neural fold can be seen, which is formed of ectodermal tissue and will eventually close to form the neural tube (the precursor to the brain cavity and spinal cord). Also, the somites flanking the soon-to-be neural tube are developing and will ultimately form parts of the skeletal and muscular systems. Hensen’s node (a.ka. the primitive knot) is the anterior most portion of the primitive streak. It is the site of involution in chick gastrulation works in the same way as the dorsal lip of the blastopore in amphibian development.
The heart of the chicken embryo develops from the fusion of paired precardiac mesodermal tubes located on either side of the developing foregut. After about 25 to 30 hours of incubation, the paired heart vesicles begin to fuse at the anterior end and continue to fuse posteriorly forming a single continuous tube. After this fusion is complete, the heart tube has four distinct regions that can be identified from anterior to posterior:
- sinus venosus
After 24 hours of development the heart tube is nearly straight and blood flows anteriorly from the sinus venosus to the conotruncus. After 33 hours the heart develops a distinct “S” shape with the ventricle bulging dramatically to the right. This is the start of cardiac looping, which, when completed, will produce the basic configuration of the mature heart. By 48 hours, the heart has folded upon itself, forming a single loop. This moves the sinus venosus and atrium to a position anterior and dorsal to the ventricle and the conotruncus. The ventricle has become U-shaped and in the medial ventral position and blood now flows posteriorly and then makes a sharp turn to flow anteriorly. In the 72-hour chick the heart has just two chambers, but the atrium and ventricle have started to expand. The atrium will eventually join the sinus venosus and divide into the two atrial chambers, and the ventricle will also divide into two chambers resulting in the adult four-chambered heart.
(All images by Bryan Perkins)