adaptation for terrestrial life

Both usually produce few young at one time, and ensure their survival by caring for them.

Unlike mammals, birds lay eggs and initial stages of development occur outside the mother’s body.

The gonads develop from two sources during embryonic development. When the nephric ridges are well established, genital ridges appear on their surface.

This tissue forms sex cords which will develop into testes if the bird becomes a male or into the ovary if the bird becomes a female.

Several thousand cells near the base of the yolk sac become very large, migrate individually into the gut mesoderm, up the mesenteries and invade the genital ridges. These are the primitive sex cells that will form eggs and sperm.

Each embryo starts out with two sets of ducts - Wolffian and Mullerian ducts.

If bird becomes male, Wolffian ducts develop into vas deferens or sperm duct and Mullerian ducts atrophy. The reverse occurs in the developing female.

However, only the left Mullerian duct (and usually only the left ovary) develops.

Male reproductive system is straightforward.

Testes - composed of seminiferous tubules which produce large numbers of spermatozoa

Vas deferens - spermatozoa leave testes and accumulate in the vas deferens where they are held until copulation.

Sperm is transferred to female by a pseudo penis in some birds like ostriches and ducks.

In more derived birds, copulation involves simple juxtaposition of male and female cloacas.

Female reproductive system

Ovary is located high in abdominal cavity.

Only the left ovary develops in most birds.

Ovary enlarges during breeding season and shrinks for the rest of the year.

similar pattern for testes

weight reducing adaptation

A few oocytes begin to accumulate yolk.

Yolk - water, fat, protein

As the yolk increases, the ova move towards the surface of the ovary and finally protrude from it.

Final growth of the ovum takes place here - 4 to 7 days.

Each protruding mature ovum and its thin covering of ovarian tissue is called an ovarian follicle.

Ovulation occurs when the follicle ruptures and releases the ovum into the body cavity.

Fertilization occurs in the body cavity or just after the ovum enters the oviduct. Normally only one ovum is released at a time.

The oviduct can be divided into 5 functional regions:

Contractile folds envelop the ovum as it breaks out of the ovarian follicle.

Ovum remains here for short period (20 minutes) then moves on to next section.

2nd and largest section of oviduct, lined with glandular cells that secrete albumen around the ovum.

Albumen composed of protein and water stored by use by embryo

Egg remains for 3 hours and passes on.

Narrow part of duct is isthmus.

Egg receives the shell membranes, fibrous keratin membranes.

Remains 1.25 hours

large muscular uterus or shell gland

18 - 20 hours

Calcareous shell is secreted by uterus, and is composed of a light protein framework like collagen and a heavy deposition of inorganic minerals, mainly calcium carbonate.

Minerals are arranged as vertical crystals separated by minute pores through which oxygen and carbon dioxide pass.

terminal section of oviduct

Mucus glands and muscular wall aid in laying egg.

Birds lay relatively few eggs compared to other vertebrates.

Eggs contain a considerable store of yolk (telolecithal, megalecithal eggs)

Incubate eggs - ensure rapid development and early hatching

Feed and care for young

Usually both parents involved to ensure high survival

Mammalian reproduction is very different but also produces few young with high survival.

Reproduction among all mammals is similar, in that all have internal fertilization and females nourish their young on secretions of mammary glands.

Differences in reproduction do occur and living mammals are divided into subclasses and infraclasses primarily on their method of reproduction.

lay eggs

Platypus and echidna retain their basic reptilian heritage as egg layers.

The ovaries are large and eggs are much larger than other mammals with large amounts of yolk.

Ova released into coelom and picked up by the infundibulum -> oviduct, where fertilization takes place -> uterus -> coated with leathery mineralized shell -> urogenital sinus -> cloaca (urogenital sinus and digestive tract meet) -> vented through a single opening

(Monotremata - 1 opening)

Egg laid into a nest, temporarily develop a simple pouch, incubate egg for 10 days. Mother may suckle young for 7 months.

Platypus lays 2 eggs, no pouch, lactate young once they hatch.

viviparous, placenta

2 infraclasses

1 living order - Marsupialia (opossum, kangaroo)

Some structural features shared with monotremes and eutherians, but not an intermediate group, just a different reproductive strategy

Ova release -> oviduct, site of fertilization -> ova has more yolk than Eutheria but a lot less than Monotremes -> zygote moves into uterus where blastocyst implants into wall of uterus

Uterus is made of a thick muscular wall with a special lining which enlarges during the reproductive cycle in preparation for implantation.

Thickened wall of uterus is called endometrium - highly vascularized

The embryo is retained in the uterus for a relatively short period (12 days in opossum, a few weeks in the kangaroo)

Young are born at very early stage and climb outside belly of mother to pouch.

Embryo at birth in kangaroo is only 2 - 3 cm long.

At birth, the fetus passes down the vaginal canal to the urogenital sinus. The young then climbs to the marsupium for final development. Young attaches to nipple in pouch. Nipple swells to firmly attach young -> complex development

Paired ovaries where ova and female sex hormones are produced, ova released -> oviduct site of fertilization

Very little yolk; embryo nourished by secretions and fusion from wall of uterus

Zygote moves down to uterus where endometrial wall has thickened and blastocyst implants very deeply; very close association with fetal and maternal blood supply.

extended gestation period

Female reproduction tract has quite a bit of variation re # uteri, # cervices

Major achievement of mammals is refinement of viviparity, made possible by the evolution of the placenta in therians.

Placenta is not unique to mammals. Certain fish and reptiles have placenta-like connections allowing diffusion of materials between vascularized oviduct and embryo.

Mammalian placenta is much more effective in transferring materials.

Connection between embryo and uterus is necessary if young develop in uterus without large amount of yolk supplied as in dogfish.

Connecting structure is the placenta, consisting of embryonic and maternal tissues, and allowing nutritional, respiratory and excretory exchange of material by diffusion between embryonic and maternal circulatory systems. The marsupials have a yolk sac placenta (the initial stage in the development of the placenta in placental mammals. In placental mammals, the chorion and the allantois together form the fetal side of the mature placenta.

This is a chorioallantoic placenta.

(Figure 18-2)

Yolk sac is relatively small

Chorion forms part of fetal tissue, allantois becomes greatly enlarged and highly vascularized.

Chorioallantoic placenta is designed to provide high quantities of nutrients for long periods.

Blastocyst adheres to uterus, sinks into endometrium

As implantation proceeds, extensions from fetal side - chorionic villi - grow rapidly and push deep into endometrium.

Breakdown of uterine tissue next to the blastocyst produces nutritive substances for the embryo until the villi are functional.

Uterus becomes more highly vascularized at the site of implantation in response to the blastocyst.

When the placenta is fully formed, the highly vascularized villi provide a large surface area, for rapid exchange of materials between maternal and fetal circulation.

human - total length of villi = 50 km

Eutherians vary in the degree to which maternal and fetal blood streams are separated in the placenta.

Variation in number of layers of tissue between fetal and maternal blood, with loss of some of the tissue layers during placental development.

In nondeciduous placenta, 6 layers of tissue separate maternal and fetal circulation. (lemurs, cetaceans, some ungulates)

Oxygen and nutrients pass through the walls of the uterine blood vessels, layers of connective tissue epithelium, through walls of fetal blood vessels to fetal blood.

In deciduous placenta, destruction of placental tissue takes place to reduce the number of layers.

In humans, only two layers remain.

In rabbits and some rodents, only the endothelial linings of the fetal blood vessels in the villi separate fetal blood from surrounding maternal blood sinuses.

In all mammals, there is no mixing of fetal and maternal blood.

At birth (parturition), fetal part of placenta are expelled along with the fetus. In mammals with fewer layers, the wall of the endometrium is also lost, because of extensive erosion and intermingling of uterine and fetal tissues.

In marsupials, the young are born after a very short gestation period.

At birth, the hind limbs, eyes and other regions are poorly developed.

Small, underdeveloped young - 12 newborn opossums could

fit in a tablespoon

Forelimbs are well developed and capable of grasping.

Eutherians are born at later stage of development.

Some species’ young are able to move on their own very soon after birth (precocial), some are more dependent (altricial).

After parturition or hatching, all mammals feed on milk secreted by mammary glands

Unique feature of mammals

Monotremata - two mammary gland regions in abdomen

Secrete milk and young lap up the milk

In Eutheria, glands are concentrated and young suckle on nipples or teats (nipples most common)

Nipples have numerous ducts that release milk during suckling.

Teats have one common duct - cows, deer, etc

During pregnancy, hormonal and physiological changes (increases in estrogen and progesterone) stimulate development of mammary tissue in preparation for lactation.

Suckling of young stimulates nerves in nipples which cause oxytocin release, which returns to mammary gland and releases milk from alveoli of mammary glands

Colostrum - first fluid produced by mammary glands - is rich in antibodies, too large to cross the placental barrier.

Early stages of evolution unknown; thought to have evolve from sebaceous glands associated with hair.

An abdominal incubation area probably evolved in the endothermic therapsids or early mammals.

Some nutrients may have been in secretions, increasing survival of young. Strong selection would have favored increased nutrient content of secretions.

Text discusses the origins of lactation, suggesting that occlusion and diphyodonty would have required the previous evolution of lactation. With early milk, jaw could grow without teeth and larger teeth could erupt in a closer to adult jaw. Otherwise, functional dentition is required for baby to process its own food.

Milk provides young with continuous supply of food, while allowing female to forage at optimal times.

Mammal young often helpless and female provides nutrients and parental care. Young do not need to run risks attendant with foraging themselves.

3 major adaptations allowed mammals to retain their embryos in their reproductive tract and become viviparous.

1. internal fertilization

2. specialization of female reproductive system (uterus), with secretions from uterus nourishing embryo

3. modification of extra-embryonic membranes to form the placenta, providing nutrients and disposing of wastes through the circulatory system of the mother.

Young protected within body of mother; reduced predation and other risks

Keys to success of viviparity has been lactation, which provides for extended post-natal period of growth and development.