Chapter 10 GROWTH DISTURBANCES

 

 

 

10.1 OVERVIEW

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. 

10.2 TERMINAL OBJECTIVES

Upon completion of this section, each student should be able to perform the following tasks:

• 1. Discuss the similarities and differences between agenesis, aplasia, atresia, dysplasia, and hypoplasia.
• 2. Distinguish between atrophy, hyperplasia, hypertrophy, and metaplasia on the basis of changes observed at the subcellular, cellular, and/or organ levels.
• 3. Distinguish aplasia, agenesis, dysplasia, and hypoplasia from adaptive changes.
• 4. Outline the sequence of events for the development of hypertrophy, hyperplasia, and atrophy following stress or an injury.
• 5. Describe the circumstances under which hypertrophy and hyperplasia are expected to occur together and/or separately in organs and tissues.
• 6. Describe the differences and similarities between metaplasia, anaplasia, and dysplasia.
• 7. Discuss the differences and similarities between hereditary anomalies and congenital anomalies.
• 8. List and describe the more common general types of anomalies expected in animals.
• 9. Provide appropriate answers for the post-instructional self examination questions located at the end of this section. 

10.3 KEYWORDS

Students should be able to define, describe, spell correctly, and use the following terms as they relate to growth disturbances.

• -- Atrophy -- Keloid
• -- Hypertrophy -- "Proud Flesh"
• -- Hyperplasia -- Cystic Hyperplasia
• -- Metaplasia -- Squamous Metaplasia
• -- Hypoplasia -- Pressure Atrophy
• -- Agenesis -- Involution
• -- Aplasia -- Disuse Atrophy
• -- Dysplasia -- Denervation Atrophy
• -- Adaptive Changes -- Iatrogenic Lesions
• -- Anomalies -- Growth Disturbances
• -- Malformations -- Neoplasia
• -- Congenital Anomalies -- Hereditary Anomalies
• -- Serous Atrophy of Fat -- Mucus
• -- Mucin -- Malnutrition
• -- Emaciation -- Parakeratosis
• -- Acanthosis -- Myeloid Hyperplasia
• -- Lymphoid Hyperplasia -- Nodular Hyperplasia
• -- Aplastic Anemia -- Teratology
• -- Oncology -- Callus 

10.4 ADAPTIVE CHANGES

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.

Hypertrophy

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.

Remember,

"pure" hypertrophy" (without hyperplasia) occurs only in organs whose cells have generally lost the power to reproduce themselves.

 

Hyperplasia

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.

Acanthosis

Refers to increased thickness of the stratum spinosum of the skin.

Hyperkeratosis

Refers to increased thickness of the cornified layer of the skin.

Lymphoid hyperplasia

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.

Nodular hyperplasia

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.

Metaplasia

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. 

10.5 AGENESIS AND APLASIA

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. 

NOTE:

The term "aplastic" anemia is used to describe a decrease in the number of erythrocytes in the vascular system due to a lack of production of these cells by the bone marrow. 

10.6 HYPOPLASIA AND DYSPLASIA

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. 

10.7 GROWTH DISTURBANCES DURING EMBRYONAL OR FETAL LIFE

(Refer to pages 97-l00 of your textbook)

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.

Anomaly:

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.

Hereditary Anomaly:

Refers to abnormal development that has its origin in the germ cells (transmitted from parent to offspring).

Congenital Anomaly:

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:

• Major defects are lethal during embryonic life.
• Some defects permit survival in utero, but not postnatally.
• Some defects permit survival postnatally, but with impairment of normal existence.
• Some defects allow relatively normal postnatal life.
• Some defects are recognized only at the time of necropsy (referred to as incidental findings).

The causes of most congenital anomalies are basically unknown. The most important known causes are:

• (l) genetic factors,
• (2) prenatal infection with a virus,
• (3) intrauterine effects of poisons ingested by the mother, and
• (4) vitamin deficiencies.

Numerous congenital anomalies occur in animals, but most fall within the following general types:

• l. Arrested development in a certain part of the embryo so that a certain structure is absent, or too small (agenesis, aplasia, hypoplasia).
• 2. Failure of parts to coalesce or close (patent foramen ovale, ventricular septal defects, cranioschisis, etc.).
• 3. Persistence of vestigial structures (ductus arteriosus, thyro-glossal duct, etc.).
• 4. Supernumerary or accessory parts (polydactyly or excess digits, accessory lung, etc.).
• 5. Ectopic or heterotopic parts (pancreatic tissue in stomach wall, etc.).
• 6. Generalized anomalies of skeletal development (chondrodys-plasia or dwarfism, etc.). 

10.3 POST-INSTRUCTIONAL SELF EXAMINATION