Comparative Anatomy

Of the Vertebrates

Larry J. Sultiz

Presented to:Prof. Gloria

Galan

CHAPTER5

Early Craniate Morphogenesis

All craniates pass through similar, though

not identical, early formative stages.

Eggs are microlecithal, mesolecithal or macrolecithal, depending on the quantity of the yolk.

They are isolecithal or telolecithal, depending on the distribution of the cytoplasm and the yolk within the egg.

Microlecithal – very little yolk (e.g. placental mammal)

Mesolecithal – moderate amounts of yolk (e.g. amphibians)

Macrolecithal – massive amounts of yolk (e.g. reptiles, birds)

Isolecithal – yolk is evenly distributed Telolecithal – cytoplasm and yolk tend to

accumulateat opposite poles

Oviparity is the condition in which unfertilized or fertilized egg are shed to the exterior for development and hatching.

Oviparous – animals that spawn or lay their eggs

Viviparity is a condition in which offspring are born alive. The developing organism may or may not be dependent on the parent for nourishment before birth.Viviparous – animals that retain their eggs within the mother’s body during embryonic development

Viviparity is found in some members of every craniate class except agnathans and birds.

Euviviparity is a viviparous condition in which offspring are dependent on the parent for nourishment throughout pregnancy.Histotrophic nutrition – nutrition by glandular secretions from maternal tissues

Ovoviviparity viviparous condition in which offspring are not dependent on the parent for nourishment throughout pregnancy.

Fertilization is internal in viviparous species, in urodeles and apodans, and in species that cover egg with a shell.

Cleavage follows fertilization. It produces a blastula with a blastocel.•Holoblastic cleavage occurs when the cleavage furrows pass through the entire egg •• cleavage can either be equal, where the resulting cells contain the same amount of yolk, or unequal, in which some cells contain more yolk than others: • - equal cleavage occurs in microlecithal eggs • - unequal cleavage occurs in mesolecithal eggs

•• cleavage results in the formation of a ball of cells (blastomeres) surrounding an internal cavity (blastocoel)

•Meroblastic cleavage occurs more in macrolecithal eggs

• cleavage takes place only in a disk at the animal pole • the cleavage furrows do not extend into the yolk • results in the formation of the blastodisk that lies on the top of the yolk

Gastrulation

Characterized by cell movement and reorganization within the embryo (morphogenetic movements) to the interior of the embryo, forming three primary germ layers: ectoderm, mesoderm, and endoderm.

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In therian mammals, an inner cell mass gives rise to the embryo.Gastrulation is bydelamination or amodification thereof.

A trophoblast membrane develops from the blastula wall, providing initial embryo and uterine contact.

Neurulation results in formation of a brain or spinal cord.

Organogenesis•After the production of the nerve tube, differentiation of the germ layers occurs rapidly, and organogenesis begins, in which the primary tissues differentiate into specific organs and tissues

Morphogen are inductor (signaling) proteins that induce organization of the germ layers and participate in differentiation of specialized tissues from embryonic mesenchyme.Morphogens are the expression of specific homeotic genes.

Mesenchyme is an embryonic tissue consisting of undifferentiated, totipotent, embryonic cells thatrequire inductorfactors fordifferentiation.

Ectoderm contributes epidermis and its derivatives, the entire nervous system, epithelia of special sense organs such as lateral eyes and inner ears, and lens of paired eyes, the lining of the stomodeum and proctodeum, and a broad spectrum of miscellaneous tissues that differentiate from neural crest.

Neural crest give rise to the sensory ganglia on the roots of the cranial and spinalnerves, to autonomic ganglia,to many of the skeletalcomponents of the skull andpharyngeal arches, to mostpigment cells, and to numerousmiscellaneous tissues.

Ectodermal placodes give rise to neurosensory cells of the olfactory epithelium, to sensory epithelia of the inner ear, to a variety of mechanoreceptive and electroreceptive organs of fishes, and to some neuroblasts in the ganglia of the cranial nerves VII, IX, and X that innervate the taste buds.

Endoderm gives rise to the epithelium of the digestive tract and to the epithelioid components of the organs that evaginate from it.

Dorsal mesoderm forms mesodermal somites consisting of sclerotome , dermatome and myotome.

These give rise, respectively, to vertebrae and the ribs, the dermis of the dorsum and skeletal muscles other than those of the pharyngeal arches.

Lateral-plate mesoderm splits into somatic and splanchnic layers.

The somatopleure consists of somatic mesoderm plus ectoderm.

The splanchnopleure consists of splanchnic mesoderm plus endoderm.

The coelom is the cavity between the somatic and splanchnic mesoderm. It is initially segmented in cephalochordates.

Intermediate mesoderm gives rise to kidney tubules and to the ducts of the urogenital system.

The chief extraembryonic membranes are yolk sac, amnion, chorion, and allantois.