Growth disturbances include a broad category of lesions that in general refer to deficient growth, abnormal patterns of growth, and non-neoplastic excess growth. These conditions may result from disturbances during embryonic development, prenatal growth, or postnatal life.
Aplasia, agenesis, atresia, atrophy, dysplasia, hyperplasia, hypoplasia, and metaplasia are growth disturbance discussed in this section and neoplasia will be discussed in section 11.
Congenital anomalies are extremely numerous and varied and this section of your syllabus only mentions a few important examples.
Upon completion of this section, each student should be able to perform the following tasks:
Students should be able to define, describe, spell correctly, and use the following terms as they relate to growth disturbances.
Atrophy, hypertrophy, hyperplasia, and metaplasia are growth disturbances that occur in response to alterations in the environment. Remember, when stress and/or noxious influences impinge upon a cell, it will, to a limited extent, adapt and achieve an "altered steady state", permitting it to survive within its changed environment. As discussed earlier in the section on the normal and adapted cell, atrophy, hypertrophy, hyperplasia, and metaplasia are the most important adaptive changes that occur in cells, tissues, and organs.
Atrophy is defined as a decrease in size or wasting of a body part, organ, or tissue. It involves either a decrease in the size of the cells or a decrease in the number of cells or a combination of the two phenomenon. It is an adaptive change that usually occurs in response to decreased workload, loss of enervation, decreased blood supply, inadequate nutrition, and/or loss of endocrine stimulation. It is distinguished from other similar growth disturbances (hypoplasia, aplasia, and dysplasia) in that atrophy occurs only in situations where the tissue was present, typically in normal amounts, and has subsequently been reduced. Serous atrophy of fat, also known as mucoid atrophy of fat, refers to a reduction in the amount of fat in "fat depots" in the body. In situations where inadequate nutrition occurs for several days, the fat throughout the body is mobilized and utilized for nutrients. In this situation, the adipose tissue throughout the body becomes gelatinous in consistency. This lesions is indicative of emaciation and it is usually most prominent on the heart, around the kidneys and in the mesentery. The cause of the
emaciation may be something as simple as malnutrition but it may involve a chronic infection, parasitism, neoplasia, etc.
Refers to an increase in the size of cells and subsequently, an increase in size of involved organs. As applied to tissues and organs, hypertrophy refers to an increase in size without an increase in the number of cells.
"pure" hypertrophy" (without hyperplasia) occurs only in organs whose cells have generally lost the power to reproduce themselves.
Refers to an increase in the number of cells in an organ or tissue and subsequently, an increase in size of the involved organ or tissue. In should be noted that in most organs, hypertrophy and hyperplasia occur together. Specific types of hyperplasia include the following.
Refers to increased thickness of the stratum spinosum of the skin.
Refers to increased thickness of the cornified layer of the skin.
Refers to an increase in the amount of lymphoid tissue of the body and is usually a response to chronic infection.
Myeloid hyperplasia - refers to a change from normal fatty bone marrow in the adult to normal red marrow.
Refers to an increase in the number of cells in a localized area within an organ. The lesion is common in the liver, pancreas, spleen, adrenal cortex, prostate gland, and mammary glands of old dogs.
Refers to a change in which one adult cell type is replaced by another adult cell type. Only epithelial and connective tissue have the capacity to undergo hyperplastic changes.
The term agenesis refers to a complete lack of development of an organ or tissue, whereas aplasia is defined as incomplete or faulty development of an organ or body part. The terms agenesis and aplasia are often used synonymously, however, aplasia implies that at least remnants of the organ or tissue exist (some of the organ/tissue is present but markedly reduced in size).
Agenesis is usually caused by an inherited defect but aplasia has been associated with hereditary defects, death of cells in embryonic development, infection of the dam during gestation with certain viruses (modified hog cholera virus in swine), as well as exposure of the dam to certain drugs (thalidomide in man) and toxins (e.g. Veratrum californicum in sheep) also during pregnancy.
The term hypoplasia refers to incomplete growth of an organ or part resulting in a failure of the organ to mature or reach normal size. Dysplasia refers to abnormal growth or development of an organ or part. It usually involves variation in the somatotype and results in disorganized development. A wide range of incomplete and disorganized growths occur, and one of these terms will apply to such disturbances ranging between normal development and agenesis or aplasia. It is often difficult to impossible to distinguish between hypoplasia and dysplasia grossly and therefore it is not uncommon to find dysplastic organs being referred to as hypoplastic and vice versa.
Hypoplasia and dysplasia differ from atrophy which refers to a reduction in the size of an organ or part after it has achieved normal growth. The causes of hypoplasia are similar to or identical to those listed for aplasia. In addition, hypoplasia after birth may be due to inadequate blood supply, inadequate enervation, or malnutrition. Hypoplastic lesions are common, and the best known are hypoplasia of the cerebellum, kidneys, and testes. Cerebellar hypoplasia, for example is caused by the viruses responsible for infectious feline enteritis in cats and bovine virus diarrhea in cattle.
The term teratology refers to the study of disturbances of development during embryonal or fetal life. The following terms are commonly encountered in the study of teratology.
Refers to any deviation from the normal in development of an organ or part (defects may be hereditary or non-hereditary). The terms malformation and anomaly are used synonymously.
Refers to abnormal development that has its origin in the germ cells (transmitted from parent to offspring).
Refers to an abnormal development existing at the time of birth. The anomaly may be hereditary or due to some influence during gestation (up to the time of birth).
Anomalies may occur during any stage of development. They are usually expressed in the following ways:
The causes of most congenital anomalies are basically unknown. The most important known causes are:
Numerous congenital anomalies occur in animals, but most fall within the following general types: