Respiratory System

Abstract

During the last 3 days of prenatal life, development of the respiratory system proceeds at an astonishing rate. In this short period of time the head and face of Didelphis are established (Krause and Cutts 1986). Early in the tenth prenatal day maxillary and mandibular swellings are apparent in Didelphis, as are the olfactory placodes. With continued development and expansion of the frontal prominence, the maxillary process fuses with the lateral region of the naso-frontal process (McCrady 1938). Lateral and medial nasal processes fuse during prenatal day 11 forming the nasal pits. As these events occur, the lateral palatine processes come together and unite at the midline forming the palate early during the 12th day of prenatal development. Concurrently, the nasal septum grows down and unites with the anterior one-third of the forming palate. The mouth is fairly well formed by prenatal day 11, as is the tongue which protrudes from the oral cavity at this time. By the close of prenatal day 12, the snout and external nares are well defined and the palate separates nasal and oral cavities. When viewed from the exterior, the external nares of the 11.5-day opossum embryo and newborn appear large and flared (Krause and Cutts 1986). After a short distance, the nares open into the large chamber of the nasal cavity. Immediately interior to the orifice of the nares, the large flat surface cells of the epitrichium are abruptly replaced by surface cells of another form of stratified squamous epithelium. These surface cells are smaller in diameter than those of the epitrichium and have numerous, short microvilli on their luminal surface.