II. EMBRYOLOGY / PLACENTATION REVIEW

See Dr. Fletcher's interactive website - vanat.cmn.umn.edu

A. GENERAL DEVELOPMENT

1. Blastocyst = 16-64 blastomeres (embryonic cells) - hollow sphere lined with blastomeres and filled with fluid. The fluid-filled cavity is the blastocoele. The blastocyst is divided into the:
     a. Inner cell mass - this grouping of blastomeres at one pole of the blastocyst will go on to form the embryo itself and two of the fetal membranes (yolk sac and allantois)
     b. Trophoblasts - these cells lining the outer surface of the blastocyst go on to form the other two fetal membranes (chorion and amnion)

2. Gastrulation = morphogenic process giving rise to three cell layers
     a. Ectoderm - goes on to form epidermis of skin, epithelium of oral and nasal cavities, nervous system, sense organs
     b. Mesoderm - goes on to form muscle and connective tissue, bone, components of circulatory, urinary and genital systems
           i. Primitive streak = 2 ridges of mesoderm, defines longitudinal axis of embryo
     c. Endoderm - goes on to form mucosal epithelium, glands

3. Formation of C-shaped embryo - This stage is uniform among almost all species of animals. There is an obvious head with the beginnings of an eye, a rudimentary nervous system, and a beating, tubular heart.
     a. Notochord formation - The notochord is an aggregation of cells at the cranial end of the primitive streak that goes on to form the head, nervous system, and somites.
     b. Somites = blocks of mesoderm located lateral to the notochord that go on to form bone, skin (dermis) and skeletal muscle
     c. Neural tube = ridges that arise and fuse above the notochord that go on to form the brain and spinal cord - Neural crest cells are pleuripotent and go on to form a wide variety of structures including pigment cells in the skin, neurons and glial cells of the peripheral nervous system, adrenal medulla cells, meninges, and teeth.
     d. Formation of a cylindrical body - The cranial end of the developing embryo grows dorsally and forward so it projects out. Lateral body folds rise on each side and fuse ventrally, with fusion moving caudally from the head process and cranially from the tail fold, stopping at the umbilicus.
     e. Formation of the cardiovascular system - This system develops subsequent to development of the early nervous system. The embryo outgrows simple diffusion as a method of gas and nutrient exchange. Blood vessel development begins in the yolk sac and vessels fuse from cranial to caudal to form a tubular heart.
     f. Pharyngeal (branchial) arches - Three arches form (1 goes on to form the upper and lower jaw, 1 goes on to form hyoid bones and facial muscles, and the last goes on to form the hyoid bones, larynx and associated muscles.

B. PLACENTATION

1. Fetal membrane formation - The placenta is the site of exchange for gases, fluid, and nutrients between the embryo and dam. Four fetal membranes form. The yolk sac and allantois both arise from the inner cell mass of the embryo and are vascular, having some capacity for gas exchange at their surface. The yolk sac acts as a temporary placenta in dogs and horses. The chorion and amnion arise from the trophoblasts of the embryo and are not vascular. Some tissues fuse in some species; you may see terms such as allantochorion in describing these placentas. The yolk sac and allantois arise from the primitive gut of the embryo. In domestic mammals, the yolk sac decreases in size during gestation. The allantois persists as a site for deposition of waste. The amnion arises as folds of chorion fuse above the embryo, forming a fluid-filled cavity within which the embryo floats, tethered to the placenta by the umbilical cord. The chorion forms the outer boundary of the entire conceptus.

2. Placental classification
     a. Number of layers of apposition - Size layers are possible between the fetus and dam (fetal chorion, mesoderm, and endoderm and maternal epithelium, connective tissue and endothelium [blood vessels]).
          i. Epitheliochorial - the fetus' outer layer (chorion) apposes the maternal epithelium - bovine, equine, porcine
          ii. Syndesmochorial - the fetus' outer layer (chorion) apposes the maternal connective tissue - ovine, caprine
          iii. Endotheliochorial - the fetus' outer layer (chorion) apposes the maternal endothelium - canine, feline
           iv. Hemochorial - the fetus' outer layer (chorion) apposes the maternal blood directly - rodents, primates
     b. Shape
          i. Diffuse - multiple, pinpoint connections - equine, porcine
          ii. Zonary - band of placental tissue with marginal hematomas associated with green pigment - canine, feline
          iii. Cotyledonary - fetal cotyledons appose maternal caruncles to form large, discrete placentomes - bovine, ovine, caprine
           iv. Discoid - disc of placental tissue - rodents, primates
     c. Amount of maternal tissue lost at parturition
          i. Deciduate - canine, feline, primate, rodent
          ii. Non-deciduate - large domestic animals

C. TIME SEQUENCE OF CANINE EMBRYOLOGY

Based on Holst PA, Phemister RD. The prenatal development of the dog: Preimplantation events. Biol Reprod 1971;5:194-206 and Evans HE. Prenatal development of the dog. Proceedings, Gaines Veterinary Symposium, Ithaca NY, 1974:18-28.

D. NORMAL OPENINGS / CONNECTIONS IN FETUS

1. Foramen ovale = opening between right and left atria. Closes when blood pressure equalizes or is higher in left atrium.
2. Ductus arteriosus = shunt to move blood from pulmonary trunk to aorta, allowing right ventricle to be exercised without overloading lungs. Constricts at birth as partial pressure of oxygen in blood rises.
3. Ductus venosus = shunt to move blood from the liver back into the system circulation without overloading liver sinusoids. Closes within days of birth.
4. Urachus = opening of urinary tract into allantois. Allantoic stalk should constrict at birth.

E. EMBRYOLOGY OF THE REPRODUCTIVE TRACT

The gonadal ridge develops ventromedial to the embryonic kidney = indifferent gonad. Germ cells migrate from yolk sac endoderm through the gut or via the bloodstream to the gonadal ridge. If no germ cells are present, the tissue will not go on to differentiate into a gonad. All vertebrate embryos have two ductal systems at this stage, the mesonephric (Wolffian) and paramesonephric (Mullerian) ducts.

If you have a Y chromosome:
Genes from the Y chromosome stimulate migration of germ cells into cords within the gonadal parenchyma - cells form seminiferous tubules with associated Sertoli cells and interstitial (Leydig) cells. Secretion of testosterone and breakdown to its metabolite, dihydrotestosterone, stimulate formation of external genitalia and development of the mesonephric ducts. Secretion of Mullerian inhibiting substance (MIS) causes dissolution of the paramesonephric ducts.

Testicular descent - The gubernaculum is a gelatinous cord connecting the caudal pole of the testis with the scrotum. The gubernaculum accumulates fluid and becomes as large as the testis. The growth of the gubernaculums and elongation of the body move the testis caudally. The swollen gubernaculums distends the inguinal canal. Dehydration of the gubernaculums and outgrowth of the scrotal sac passively pull the testis through the inguinal ring and into the scrotum.

If you have an X chromosome:
Lack of gene products from the Y chromosome permit formation of germ cells in islands of parenchymal tissue, which go on to form follicles. Secretion of estrogen stimulates formation of the paramesonephric ducts; lack of testosterone allows dissolution of the mesonephric ducts.

STUDY AND REVIEW GUIDE

1. What is the difference between an embryo and a fetus? Why does it matter?

2. What are monozygotic twins? Dizygotic twins? Can either occur in dogs and cats?

3. When are teratogens most likely to cause optic defects? Cleft palate? Explain your answer.

4. You are presented with a 6-month-old female mixed breed dog with extreme clitoral hypertrophy. Hypothesize some developmental underlying causes for this problem.

5. Where do vaginal anomalies most commonly occur in dogs? Why?

6. How does gas, fluid, and nutrient exchange occur in the embryo prior to placenta formation? Describe two processes.

7. Are abnormally shaped spermatozoa more likely to be associated with abnormal offspring? How about ova from an aged dam? Explain your answer.

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