Prenatal development of major lymphocenters and lymphatics in goats(Capra hircus)

Abstract

Prenatal development of the major lymphocenters and lymphatics in goat was studied using 41 foetuses of various ages from 22 to 147 days of gestation. Morphogenesis and histogenesis of lymph nodes from five major lymphocenters, viz. parotid and mandibular from head, prescapular from neck, caudal mediastinal from thoracic cavity, jejunal mesenteric from abdominal viscera and prefemoral from abdominal wall and lymphatics, viz. tracheal and thoracic ducts and cisterna chyli were studied. During the first month, the lymphatic system presented six lymph sacs. By 22 days, paired jugular sacs and unpaired retroperitoneal sac appeared whereas, the cisterna chyli and paired iliac sacs appeared only by 24 days. Jugular sacs started to split into lymphatic spaces adjacent to the internal jugular vein and vagosympathetic trunk by 40 days. Retroperitoneal sac lay ventral to aorta, close to the root of the mesentery, near the kidney anlage and underwent regression by 53 days. Iliac sacs appeared near aorta, dorsolateral to Wolffian bodies and dorsomedial to metanephric kidneys and its onset of regression was by 50 days. Cisterna chyli appeared as a lymphatic space lateral to aorta and became a spindle-shaped dilatation at the level of last thoracic to first lumbar vertebra. By 48 days, thoracic duct formed the cranial continuation of cisterna chyli, near aorta. By 50 days, the tracheal lymph duct was seen in the developing carotid sheath. Valves appeared by fourth month in these ducts. Lymph sacs showed infiltration by lymphocytes and red blood cells by 48 days. The developing lymph nodes exhibited haemopoietic areas between 53 and 60 days. There was a progressive increase in the size of lymph nodes with age. The weight of the lymph nodes exhibited positive correlation with body weight, CRL and age. The superficial lymph nodes occurred as single ones and deep lymph nodes occurred in groups, with slight variation in position in some animals and slightly higher gross values for male animals and for the lymph nodes of right side. Capsule was undifferentiated up to 53 days and presented trabeculae by 75 days in parotid lymph node. The differentiated capsule presented dense fibrous connective tissue with collagen, elastic and smooth muscle fibers and was thicker where the cortex was more developed. Earliest aggregation of lymphocytes occurred in cortex by 60 days, in the parotid and mediastinal nodes. The nodular aggregations occurred by 75 days in the parotid lymph node, but by fifth month in all other nodes. Cortex was denser and thinner than the medulla. Medulla started differentiation by 75 days but macrophages and germinal centers were not detected till the end of gestation. Parotid and mandibular lymph nodes showed a similar pattern of development, but with a denser cortex for the latter. Prescapular lymph node presented lesser cortico-medullary differentiation than the corresponding stages in the lymph nodes of head. Even though the differentiation was slower during the initial stages, during the last month it became similar to that in the lymph nodes of head. Prefemoral lymph node followed a slower pattern of development with lesser number of trabeculae than parotid and prescapular lymph nodes. All lymph nodes presented nodular aggregation of lymphocytes in the cortex by fifth month, thereby attaining a structure similar to that in adults towards the end of gestation except for the absence of macrophages and reactive centers. Lymphocytic proliferation in the thymus preceded the appearance of lymphocytes in lymph nodes indicating these cells got differentiated first in the thymus. Contribution to the body weight of spleen and lymph nodes decreased towards the end of gestation indicating a similar development pattern. Only weak reactions were exhibited by the lymph nodes towards glycogen, acid and alkaline phosphatases and lipids. The superficial lymph nodes were encapsulated by deposition of fat from fourth month onwards.